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Historical introduction to chemistry,
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HISTORICAL INTRODUCTION TO . CHEMISTRY
MACMILLAN AND CO., . Lir^ited
LONDON . BOMBAY . CALCUTTA . ^jy.ELBpURNE
THE MACMILLAN COMPANY
NEW YORK , BOSTON . CHICAGO DALLAS . SAN FRANCISCO
THE MACMILLAN CO. OF CANADA, Ltd
TORONTO
HISTORICAL
INTRODUCTION TO
CHEMISTRY
BY
T. M. LOWRY, D.Sc, F.R.S.
MACMILLAN AND CO., LIMITED
ST. MARTIN'S STREET, LONDON
1915
COPYRIGHT
PREFACE
In the preparation of this volume the purpose has been to present an historical account of the more important facts and theories of chemistry, as these disclosed themselves to the original workers in this branch of science. No attempt has been made to write a formal History of Chemistry, either as a survey of the various periods into which the history of the science may be divided, or in the more usual biographical form. The material has been classified by subjects rather than by authors ; but it will be found that under this system the work of individual experimenters is described quite as fully as in a biographical survey, whilst in the case of certain chemists, such as Priestley, Lavoisier and Gay-Lussac, it has been possible to include detailed descriptions of experimental work which could scarcely have found a place in a brief biography.
The Biographical Index provides a key to the work of each author as it is described in the text, and contains most of the essential items for an account in narrative form of the achievements of the great pioneers of chemistry.
In a few cases this index contains dates and titles of works not included in the text, as, for example, Cavendish's work on the density of the earth, and some of Faraday's physical experiments ; but no attempt has been made either in the biographical index or in the text to record the later and more detailed developments of organic or of physical chem- istry. Such a record would be out of place in a historic?i
vi PREFACE
introduction dealing with fundamental facts and problems ; only the most incidental references will be found therefore to the work of Hofmann in organic chemistry, or of van t'Hoff in the various branches of physical chemistry. This limita- tion is of little importance, as the advanced student will find ample descriptions of the achievements and discoveries of these later workers in the two volumes of Memorial Lectures issued by the Chemical Society.
In compiling the present volume, the standard histories of Thomson and of Kopp have been invaluable as guides to the literature, but the whole story has been written afresh from the original sources. Almost without exception, every reference and quotation has been checked directly in the printed proofs against the original text ; if, however, any errors should have survived, the author would be very grate- ful to anyone who would direct his attention to them. The few statements for which dates but no references are given are made on the authority of Kopp, but most of them refer only to incidental points. Much of the narrative, even when not enclosed between inverted commas, is in the actual words of the original descriptions, a feature which the pre- sent volume shares with the text-books of loo years ago, but which has gradually disappeared as the early history of chemistry has become more and more a " twice-told tale.''
In order to present the material in the most accessible form, quotations have been taken so far as possible from modern reprints, such as those of Scheele's Essays, or the Alembic Club Reprints, or from collected Works such as those of Lavoisier, Davy, and Stas. To guard against anachronism, full use has been made of contemporary translations of such works as Bergman's .Swoyj and Berthollet's Chemical Statics ; but as the object in view was to present a picture of the development of chemistry rather than a formal history, it was thought better not to introduce unnecessary confusion by using the name " nitric acid " when nitrogen peroxide was meant, nor to obtrude at every point the various names
PREFACE vii
by which oxygen and chlorine were described up to 1787 and 1 8 10 respectively; but whenever some more familiar term has been introduced into the text, the alteration has been indicated by square brackets.
The detailed descriptions of classical experiments, which form a leading feature of the book, should be of value not only to the student but also to the teacher of chemistry, since they show not merely how the great facts of chemistry might have been discovered, but the actual course of the discover- ies themselves. Such information has proved of real value in devising courses of instruction in elementary chemistry, atid forms a sure guarantee against an incorrect or illogical sequence ; from this point of view the book may be com- mended to those who are responsible for the training of teachers, as well as to students who intend to become teachers themselves. The historical method has also been found to provide a complete solution of the difficult problem of teaching mixed classes of students, some absolute begin- ners and others with a considerable knowledge of elementary text-books of chemistry. This problem is insistent in training colleges and in medical schools, and is probably but little less urgent in other departments of teaching. The material included in the present volume has been proved, by several years of actual practice, to provide a means of interesting and instructing both types of students. Even the laws of chemical combination acquire a new interest when presented in the picturesque imagery of Proust, or with some of the glow of Berzelius's early enthusiasm.
In contrast with most of the well-known histories of chemistry, the volume is provided with many illustrations, especially of the apparatus used by the earlier workers. It is unfortunate that large-scale copper-plate illustrations can- not be reduced in exact facsimile without destroying all their technical merits ; but great care has been taken that the wood-blocks shall reproduce as faithfully as possible all the essential features of the original drawings. Thus,
viii PREFACE
Bunsen's burner (1866) has been removed, it is hoped finally, from Dumas's apparatus for the composition of water (1841), and the big spirit-lamp has been restored to its place ; and Lavoisier's red-hot gun-barrel is again sealed with clay joints instead of with rubber. The temptation to reconstruct early apparatus (which has led to the association of the Bunsen burner with Lavoisier's work) has been resisted even in the pressing case of Cavendish's experinierits on the composition -of water, where the gap has been filled by reproducing the contemporary apparatus of Monge ; only when the text and illustrations were complete was the discovery made that two of the globes used in these experiments are still preserved in the library of the Royal Institution. The single case in which a figure has been consciously modified is a small alteration in Priestley's blackboard and table (Fig. 19 ^) with the view of making the most of the limited space avail- able for reproduction.
An exact historical narrative, such as is here presented, could not have been written without free access to books and journals, many of which are rare and some almost inacces- sible. I wish to express my indebtedness to Mr. A. H. White, of the Royal Society's Library, and to Mr. F. W. Clifford, the Librarian of the Chemical Society, for their invariable courtesy and helpfulness over a period of several years, as well as to the Institution of Electrical Engineers for access to their important collection of Volta's papers. I am indebted to the late Miss Freund for a quotation from a copy of Wenzel's Theory of Affinity in the library of the University of Bonn, to Prof. Victor Henri of Paris for some information in reference to early French publications, and to Prof. Ernst Cohen of Utrecht for a number of historical details. The early chapters of the book were written in collaboration with the late Mr. G. C. Donington, whose double qualifica- tion in History and Natural Science gave special value to his opinions and criticisms. I have also derived great benefit from the expert advice which Prof. R. A. Gregory
PREFACE ix
and Mr. A. T. Simmons have generously given to me through the whole period occupied by the compilation of the book. The proof-sheets have been read by Mr. W. A. Davis, by Dr. Merriman, by Lieut. Victor Steele and by Mr. H. S. Patterson, to whom I am grateful for much valuable help and criticism.
T. M. LowRY. London : August, 191 5.
CONTENTS
CHAPTER I RAW MATERIALS- AND PRIMITIVE MANUFACTURES . I
CHAPTER II THE ACIDS . . 12
CHAPTER III
THE BURNING OF METALS AND THE DISCOVERY OF
OXYGEN ..... 26
CHAPTER IV
CHALK, LIME, AND THE ALKALIS 48
CHAPTER V
THE STUDY OF GASES . . ... 64
CHAPTER VI
THE COMPOSITION OF FIXED AIR. CARBON, CARBONIC
ACID, AND THE CARBONATES . 94
CHAPTER VII
THE BURNING OF INFL.AMMABLE AIR, AND THE COM- POSITION OF WATER . • .112
CHAPTER VIII THE BURNING OF INFLAMMAnLE GASES, LIQUIDS, AND
SOLIDS ■ -137
CHAPTER IX SULPHUR AND PHOSPHORUS 162
xii CONTENTS
PAGE
CHAPTER X NITRE, NITRIC ACID, AND NITROGEN .... l86
CHAPTER XI
MURIATIC ACID AND CHLORINE . . . . . 2IO
CHAPTER XII
THE HALOGENS , 233
CHAPTER XIII
THE DECOMPOSITION OF THE ALKALIS .... 257
CHAPTER XIV
THE ATOMIC THEORY . . .... 29I
CHAPTER XV
THE MOLECULAR THEORY . 320
CHAPTER XVI
ATOMIC WEIGHTS OF THE METALS 360
CHAPTER XVII
MOLECULAR ARCHITECTURE 383
CHAPTER XVIII
THE CLASSIFICATION OF THE ELEMENTS .... 448
CHAPTER XIX
BALANCED ACTIONS . . ... 498
CHAPTER XX
DISSOCIATION ... 514
BIOGRAPHICAL Index ..... . . 539
Subject Index . 565
LIST OF ILLUSTRATIONS
FIG. PAGE
1. Cubic Crystal of Salt ...... 3
2. Crystal of Saltpetre 4
3. Sal-ammoniac '. T 4
4. The Colenso Diamond 1 5
5. Crystals of Quartz 6
6. Arrowhead Crystals of Marcasite 8
7. Nodule of Marcasite as described by Glauber . . 9
8. Striated Cubes of Pyrites . . , . . . 9
9. Crystal of Blue Vitriol or Sulphate of Copper . 17 10. Large "Crystal of Gypsum or Selenite (Sulphate, of
Lime) . . " 19
11-14. Mayow's Apparatus .... - 33
15. Lavoisier's. Apparatus for Calcining Lead and Tin in
Air over Water or Mercury 34
16. Lavoisier's. Apparatus for Heating Mercury in a
Limited Volume of Air 41
17. Cavendish's Apparatus .for finding the Weight and
Density of Gases 68
18. Apparatus Used by Cavendish to Prepare a Gas
vifhich " Lost its Elasticity by Contact with
Water" '. . . . '. . . . 82
19 (a). Hales's Apparatus for Measuring the Volume of Gas set free by Heating Animal, Vegetable, and
Mineral Substances ...... 85
19 (i5). Hales's Apparatus (Improved Design) . . 85
20. Priestley's Apparatus (First Plate) . . . 87
xiii
xiv LIST OF ILLUSTRATIONS
FIG. PAGE
20. Priestley's Apparatus (Second Plate) .... 89
21. Lavoisier's Apparatus for Collecting the Gas produced
by Heating Red-lead and Charcoal in an Iron Retort . . 96
22. Lavoisier's large Burning Glass . . . . loo
23. Crystals of Calc-Spar .... . . 105
24. Stalactite, i.e.^ Chalk deposited by the escape of
Fixed Air from Dripping Water .... 106
25. Lavoisier's Apparatus for decomposing Steam in a
red-hot gun-barrel 119
26. Monge's Apparatus for exploding Hydrogen and
Oxygen in an exhausted globe . . . .120
27. Volta's Eudiometer 123
28. Dumas's Apparatus for determining the Composition
of Water 126
29. Morley's Apparatus for measuring the Density of
Oxygen ... . . . ■ . 129
30. Morley's Palladium-tube for weighing Hydrogen . 130
31. Morley's Combustion-tube 131
32. Morley's Apparatus for the Combustion of Hydrogen
and Oxygen 132
33. Lavoisier's Apparatus for the Combustion of Spirit
of Wine 147
34. Dumas and Stas's Apparatus for the Combustion of
Graphite and of Diamonds 149
35. Stas's ■ Apparatus for the Combustion of Carbonic
Oxide 1,52
36. Rhombic Crystals of Sulphur 163
37. Stoppered flask used by Priestley in preparing
"Vitriolic Acid Air" 165
38. Large Cube of Galena . . .... 173
39. Cavendish's Apparatus for Sparking Air over
Mercury 189
40. Gray's Apparatus for determining the Composition
of Nitric Oxide 207
LIST OF ILLUSTRATIONS xv
PAGE
41. Twinned Cubes of Fluor Spar 234
42. Moissan's Apparatus for Preparing Fluorine 252
43. The Voltaic Pile 271
44. The Voltaic Battery or " Crown of Cups " . 272
45. Boyle's Pneumatic Engine or Air Pump . . .321
46. Tube used in Boyle's Experiments on the Condensa-
tion of the Air . . . . 325
47. Gay-Lussac's Apparatus for Measuring the Expan-
sion of Gases and Vapours 328
48. Gay-Lussac's Apparatus for Comparing the Expan-
sion of Air with that of Soluble Gases and Vapours 329
49. Gay-Lussac and Thenard's Apparatus for the Com-
bustion of Organic Compounds . . 391
50. Berzelius's Apparatus for the Combustion of Organic
Compounds 392
51. Liebig's Apparatus for the Combustion of Organic
Compounds ... ... 395
52. Dewar's Atomic Heat Curve, with a Curve of Atomic
Volumes 472-473
53. High-frequency Spectra ... . . 488
54. High-frequency Spectra of the Elements . . 489
55. Deville's " Hot-cold " Tube 517
56. Pebal's Apparatus for Proving the Dissociation of
Sal-ammoniac . . S-'
57. H. B. Baker's Apparatus for Heating Dried Mixtures
of Hydrogen and Oxygen .... 532
HISTORICAL INTRODUCTION TO CHEMISTRY
PART I ELEMENTS AND COMPOUNDS
CHAPTER I
RAW MATERIALS AND PRIMITIVE MANUFACTURES
The study of Chemistry as a branch of natural knowledge may be said to begin with the work of the Honourable Robert Boyle (1627-1691). But, at the time when Boyle commenced his work, the most important chemical processes and many well-defined substances were already familiar, many of them having been used for practical purposes from very early times. Early chemical discoveries group themselves naturally into three periods :
I. The Prehistoric and Ancient Period culminated in the civilisations of Egypt, Greece, and Rome. During this period many of the raw materials, which it is the business of Chemistry to study, were collected, purified, and brought into common use in daily life ; but only in a few cases were processes discovered for the preparation of new substances by the action of these raw materials on one another. Evidence as to the substances which were known during this period is derived mainly from casual references in the writings of ancient authors rather than from systematic works of a scientific cha,racter.
2., The^'Earlier Alchemistic Period extended from the early part of the Christian era to about 1500 a.d. During £ B
2 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
this second period the search for the philosopher's stone, a substance by which baser metals could be converted into gold, led to an exhaustive study of all available materials. Their actions upon one another were also studied, and the effect of heat upon them both separately and in mixtures of varying complexity. In this way, many new pro- cesses and compounds were discovered. The physician Geber, who lived in Spain at the close of the eighth century, was perhaps the greatest of the alchemists ; the writings that have been attributed to him afford a clear picture of the progress which the science had made in the hands of the earlier alchemistic workers.
3. The Later Alchemistic Period extended roughly from 1500 to 1650 A.D. During this period the search for the philosopher's stone (then regarded as a means of healing all diseases) and for the equally imaginary elixir of life gradually gave place to deliberate investigations of the action of drugs on the human body and to the preparation of new substances for use in medicine. The writings of Glauber (1603-1668) contain a description of many sub- stances discovered during this period, and give a good idea of the state of knowledge at the time when Boyle laid the foundations of the modern science of Chemistry.
The following pages contain an account of some of the most important materials which became known during the earlier part of these three periods, but large groups, of substances (including, for instance, the acids and alkalis) are reserved for separate treatment in subsequent chapters. The various materials may be classified conveniently under the following headings :
Soluble salts. — Among the substances in common use from very early times was the salt obtained by the evapora- tion by the sun's rays of sea-water collected in shallow pools along the sea-shore. The product was a mixture of several substances, remarkable for their sharp taste and
I RAW MATERIALS AND PRIMITIVE MANUFACTURES 3
their solubility in water, and contained a large proportion of the substance now known as common salt (Fig. i). The name salt was applied subsequently to all similar solids. Thus Boyle defined a '' salt " as being characterised by two qualities, that " it is easily dissoluble in water and that it affects the palate with a savour, whether good or evil " (see Experiments and Notes about the Producibleness of Chymical Principles, 1680, p. 3; Works, 1725, III. 365).
Soda (Latin natruin), known from the earliest times as a natural deposit on the shores of the soda-lakes of Egypt, was originally called "nitre''; it was employed as a cleansing agent and in the manufacture of glass, but was almost u n- known in West- e r n Eu rope until the eight- eenth century, when it was prepared from
the ash of marine plants. Potash, or "pearl ash," a white solid closely resembling soda in many of its pro- perties, probably received its name from the fact that it was obtained by extracting the white ash of burnt wood with water in earthenware pots. During the middle ages the chief source of potash was the "lees" or sediment of wine to which the name of tartar was given; this sediment was calcined, and the potash thus prepared, the " burnt lees of wine," was known as calcined tartar, or more simply as "tartar." These two substances, soda and potash, were known as alkalis, and were remarkable for
B 2
Fig. t— Cubic Crystal of Salt. Britisii Museum (Natural History).
4 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
their power of effervescing when mixed with acids. This property served to distinguish the Egyptian nitre or soda from the common nitre or saltpetre described in the next paragraph. It is referred to in the proverb : " As he that taketh away a garment in cold weather, and as vinegar upon nitre, so is he that singeth songs to an heavy heart" (Prov., XXV. 20). In reference to this passage Robert Boyle,
IG. 2— Crystal 0
Saltpetre.
British Museum
(Natural History).
Fig. 3— Sal-ammoniac.
who had received a specimen of Egyptian " nitre " from Constantinople, wrote in 1680: —
" And here .... give me leave to take notice of a text of the holy Scripture, that has sometimes puzzled not only me, but far better Critics in the Hebrew tongue than I, . . . where to illustrate Things very incongruous to one another
I RAW MATERIALS AND PRIMITIVE MANUFACTURES 5
the disagreement of Vinegar and Nitre is mentioned, for supposing the words to be rightly translated .... it seems very hard to find what show of Antipathy there is between Vinegar, and the Saltpetre that is commonly sold in our shops for Nitre ; wherefore strongly presuming that Solomon .... made use of Egyptian Nitre .... when once I received the Nitre that I have mentioned, and saw it in signs of an Alcalizate nature, I quickly poured upon it some strong Vinegar, and found as I expected that there presently ensued a manifest conflict, with noise, and store of bubbles, with which Experiment I afterwards acquainted some Critics, and other learned men who were not ill-pleased with it " {Experiments and Notes about the Producibleness of Cliymual Principles, 1680, p. 30; Works, 1725, III. 371).
Saltpetre or nitre (Fig. 2), a salt-like substance formed by the decay of animal matter and found as an incrustation in the neighbourhood of stables, was introduced into Europe from the East. Sal- ammoniac (Fig. 3), a salt of
similar origin, was manufac- Fig. 4— The Cor.ENSo Diamond
, , , . British Museum (Natural History).
tured by heatmg camels dung ;
it differed from other salt-like substances in that it could be vaporised completely by gentle heat. The name was first applied to a mixture of common salt and soda found near the temple of Jupiter Ammon in Upper Egypt, but was transferred by the early alchemists to the volatile salt just referred to.
Earths and rocks. — A number of other substances, which may be classed together as earths or rocks, were employed for various purposes. Fuller's earth, a white, friable clay, was used as a cleansing agent prior to the manufacture of soap. Chalk, limestone, and marble were employed
6 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
largely in their natural condition as building materials, but were also converted into lime by the action of heat and used in the preparation of mortar. Sand was used with soda in the manufacture of glass.
Many crystals and precious stones were also known and valued for personal adornment and for decorative purposes. Amongst the first to attract attention were probably emerald, TOPAZ, DIAMOND tFig. 4), and quartz (Fig. s) ; the last
Fig. 5 — Crystals of Quartz. British Museum (Natural History)
substance, known also as rock-crystal, or crystal, gave its name to the whole of this class of substances.
Substances of organic origin. — Many of the substances known in ancient and mediaeval times were formed by the agency of living organisms, either animals or plants. Among these may be mentioned sugar (in the form of honey), turpentine, oils, fats, and waxes (extracted from plants and from the bodies of animals), amber (a fossil resin), and pearls. Wine and vinegar were obtained by fermentation
I RAW MATERIALS AND PRIMITIVE MANUFACTURES 7
from the juice of the grape and other fruits, whilst saltpetre and sal-ammoniac have been referred to already as products of the decay of animal matter.
Substances prepared by the action of fire. — Many other substances were prepared from natural materials by the action of fire. Lime, obtained in this manner from lime- stone or chalk, has been mentioned previously, but greater importance attaches to the use of fire as an agent for the preparation of metals. Two metals, gold and silver, are distributed somewhat widely in a native state, and were known from the earliest times. Native copper was also found and used."^ Mercury (Latin hydrargyrum, or liquid silver), which occurs in minute droplets in certain rocks, was known to the Greeks.
Other metals were obtained by smelting their ores, that is, by heating them with charcoal. Amongst these was tin, obtained by smelting tinstone and valued highly as a means of hardening copper. The hard alloy of copper and tin is known as bronze. The stage of civilisation during which this alloy came into common use has been called the " Bronze Age," although the various European peoples learnt to use it for the manufacture of weapons and imple- ments at widely different times. The smelting of iron was a more difficult process, since a much higher temperature was required than in the case of tin or copper. The use of iron, therefore, follows that of bronze in the history of each race. Thus, the Greeks, as described in the Homeric poems, were accustomed to the use of bronze weapons and imple- ments, but esteemed iron much more highly — a lump of iron being described as a valuable prize in a contest. The Romans had reached the "Iron Age" by early classical times. Four hundred years later, at the opening of the Christian era, the Germanic races still employed the earlier
' "A land whose stones are iron, and out of whose hills thou mayest dig brass " (Deut. viii. 9).
8 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
known metal, although the use of iron had superseded that of bronze partially amongst the Gauls and other Celtic races. Lead was known to the Romans at the time of their conquest of Britain ; the mining of lead ores .was carried on by them in Derbyshire and in other parts of the island. Fire was also used as an agent in the purification of the so-called " noble metals," gold and silver, from baser im- purities, such as tin and lead, the latter being converted into dross. The dross obtained by burning lead in order to separate it from the silver which it contained, received
a special name, LITHARGE, that is, the stone (Greek, Xt'^os) obtained from silver (Greek, apyvpos) ; it was valued because it could be con- verted by gentle roasting into the scarlet paint known as
Fig. 6 — Arrowhead Crystals of Marcasite
British Museum (Natural History). MINIUM Or RED
LEAD.
Substances produced by weathering or decay. — New
materials were also obtained by the natural processes of weathering or decay. Saltpetre, wine, and vinegar have been mentioned as examples of this kind, but special reference may be made to the green, glassy substance which is formed when the minerals marcasite and pyrites (Figs. 6, 7, 8) are allowed to weather. The brilliant golden nodules of marcasite, which in England are often found as " thunder- bolts," embedded in the chalk, decay and become '' rusty " almost as easily as iron. When the rusty mass is extracted
I RAW MATERIALS AND PRIMITIVE MANUFACTURES 9
with water, a soluble substance is dissolved out and separates in green crystals on allowing the washings to evaporate. The glassy appearance of the crystals won for the substance the name of vitriol (Latin vUrum, glass). It played a most impor- tant part in the early development of chemistry on account of
Fig. 7 — Nodule of Marcasite as described by Glauber.
Fig. 8 — Striated Cubes of Pyrites.
Marcasite and iron pyrites both consist of disulphide of iron (F'eSs), but they
diflfer in crystalline form. The crystals of iron pyrites are cubic whilst those of
marcasite belong to the orthorhombic system.
the discovery of an oily product, oil of vitriol, to which it gave rise when strongly heated (see Chapter II). The following description of the way in which green vitriol was prepared from nodules of marcasite was written by Glauber about 1648 A.D., and translated into English in 165 1 : —
10 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
" Commonly in all fat soils or clayey grounds, especially in the white, there is found a kind of stones, round or oval
in form Which if it is cleansed from the earth,
and beaten to pieces, looks within of a fair yellow and in streaks, like a gold Marcasite, or a rich gold Ore, but there is no other taste to be perceived in it, than in another ordinary stone ; . . . . Now this stone is nothing else, but the best and purest Mineral (or Ore) of Vitriol, . . . out of which there may be made an excellent medicine, as followeth.
Take this Ore or Mineral beaten into pieces, and for some space of time, lay or expose it to the cool air, and within twenty or thirty days it will magnetically attract a certain saltish moisture out of the air, and grow heavy by it, and at last it falleth asunder to a black powder, which must remain further lying there still, until it grow whitish, and that it do taste sweet upon the tongue like vitriol. Afterward put it in a glass vessel, and pour on so much fair rainwater, as that it cover it one or two inches ; stir it about several times in a day, and after a few days the water will be coloured green, which you must pour off, and pour on more fair water, and proceed as before, stirring it often until that also come to be green : this must be repeated so often, until no water more will be coloured by standing upon it. Then let all the green waters which you poured off run through filtering paper, for to purify them ; and then in a glass-body [ i.e., a retort ] cut off short let them evaporate till a skin appear at the top : then set it in a cold place, and there will shoot little green stones, which are nothing else but a pure vitriol : the re- maining green water evaporate again, and let it shoot as before : and this evaporating and crystallising must be continued until no vitriol more will shoot " {Philosophical Furnaces, Part II. 1651, p. 71 ; Works, 1689, I. 21).
Summary and Supplement
Scientific Chemistry begins with Robert Boyle (1627-1691), but was preceded by three periods, namely : —
(i) An Ancient Period, extending up to the Christian era, in which many primitive manufactures were developed ;
I RAW MATERIALS AND PRIMITIVE MANUFACTURES n
(2) An Earlier Alchemistic Period, extending to about 1500 A.D., during which many substances were discovered in the attempt to convert base metals into gold ;
(3) A Later Alchemistic Period, extending to about 1650 A.D., in which new substances were prepared and tested for medicinal purposes.
Important substances known at a very early date include :
1. Soluble Salts. —
Common Salt (Sodium chloride, NaCl). Washing Soda (Sodium carbonate, Na2C03,ioH20). Potash (Potassium carbonate, KjCOj). Saltpetre or Nitre (Potassium nitrate, KNO3). Sal-ammoniac (Ammonium chloride, NH4CI).
2. Earths and Bocks. —
Fuller's Earth.
Chalk, Limestone, Marble (Calcium carbonate, CaCOj).
Sand, Quartz (Silicon dioxide, SiOj).
Emerald. Topaz. Diamond.
3. Organic Products. —
Sugar. Turpentine. Oils, Fats and Waxes. Amber and Pearls. Wine and Vinegar. Saltpetre and Sal-ammoniac.
4. Igneous Products. —
Lime from Limestone (CaCOj-^ CaO-FCOj). Tin {stannujn, Sn) from Tinstone
(SnOj-l-C-^Sn-HCOa). Iron {ferrum, Fe) from Ironstone
(Fe203-l-3C-»2Fe-f3CO). Litharge from Lead (2Pb + 02->2PbO).
Also Native Metals.— Gold {aurum, Au). Silver {argentum, Ag). Copper {cuprum, Cu). Mercury [hydrargyrum, Hg).
5. Products of Decay.— Especially, Iron Pyrites (FeSj)
-» Green Vitriol (Ferrous sulphate, FeS04,7H20) -$• Oil of Vitriol (Sulphuric acid, H2SO4).
CHAPTER II
THE ACIDS
A. Discovery of the Common Acids
Vegetable acids. — The earliest acids known were of vegetable origin, but until the middle of the eighteenth century scarcely any attempt was made to isolate them from the "sour" liquids in which they occur, or even to distinguish between various acids of similar origin. The most familiar of the vegetable acids was sour wine or vinegar, which was known to have a remarkable action upon soda (Chapter I. p. 4). Its power of dissolving chalky materials is illustrated by the story of Cleopatra and the pearls which she dissolved and drank in a cup of vinegar, as well as by Livy's fantastic story of the use of vinegar by Hannibal to dissolve away the limestone rocks of the Alps. Distilled VJNEGAR was familiar to the alchemists from the time of Geber, and was frequently used as a solvent, but it was not until a much later period that the acid constituent, acetic ACID, was isolated in a pure state.
A large number of crystalline acids of animal and vegetable origin were, however, prepared at the close of the eighteenth century by the Swedish chemist, Carl Wilhelm Scheele, (1742-1786), whose work on these substances may be
CH. II THE ACIDS 13
regarded as the basis of the modern science of " Organic Chemistry." Amongst the acids discovered by Scheele were TARTARIC ACID prepared in 1770 from " tartar," in which it is present in combination with potash ; benzoic acid from benzoin (1775); uric acid from bladder-stones (1776); lactic acid from sour milk (1780) ; oxalic acid by the action of nitric acid on oils (1783); citric acid from ■lemon-juice (1784); and malic acid from apples (1786).
Oil of vitriol, or sulphuric acid.— A great advance was made by the discovery in the early alchemistic period of powerful acids of mineral origin. The first of these to be prepared was undoubtedly oil of vitriol, which the writings of Geber (800 a.d.) describe as obtained by the distillation of alum. The acid can be prepared more easily by distil- ling green vitriol, as described in the writings of Basil Valentine and of Glauber, and it was from this method of preparation that the acid obtained the name oil of vitriol. The first stage of distillation results in the formation of clouds of steam : subsequently, dense white fumes are produced, which dissolve in the condensed steam to form oil of vitriol. When the white fumes are condensed separately, a strongly fuming liquid is produced which hisses violently when poured into water : this fuming liquid is often called nordhausen oil of vitriol from the name of the German town in which it used to be manufactured. The red, rusty residue remaining behind in the retort was known as colcothar, and is now sold as ROUGE. This method of making oil of vitriol is described by Basil Valentine as follows : —
" If you get such deep graduated and well prepared Mineral, called Vitriol, then pray to God for understanding and wisdom for your intention and after you have calcined it, put it into a well coated Retort, drive it gently at first, then increase the fire, there comes in the form of a white spirit of vitriol in the manner of a horrid fume, or wind, and cometh into the Receiver as long as it hath any such material in it." {Last Will and Testament, p. 158.)
14 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
The acid is now prepared by more direct methods from iron pyrites or from sulphur. The presence of sulphur in pyrites ^ and in oil of vitriol was known in Boyle's time, but the name sulphuric acid by which the acid is now generally known was not adopted until 1787.
Aqua fortis or nitric acid. — A second acid of mineral origin was described by Geber as obtained by distilling a mixture of saltpetre with green vitriol and alum. At a later period Glauber showed that it could be prepared more easily, and in a much purer condition, by distilling a mixture of oil of vitriol and saltpetre from a glass retort heated gently in a bath of hot sand over a furnace. From its remarkable power of dissolving metals such as copper and silver, which were not readily acted on by oil of vitriol, it came to be known as aqua fortis. In Boyle's time its acid properties, its volatility, and its origin from nitre were indicated by the name "acid spirit of nitre"; this was afterwards shortened to nitrous or nitric acid, the last name being introduced by Lavoisier in 1787.
Aqua regia — By dissolving sal-ammoniac or salt in aqua fortis, Geber prepared a still more powerful acid which was capable of dissolving gold ; it was therefore called aqua REGIS, or aqua regia.
Spirit of salt or muriatic acid. — The method of prepar- ing spirit of salt by heating salt with oil of vitriol in a glass retort is due to Glauber. He had previously made it by throwing a mixture of salt, green vitriol, and alum upon the hot fuel of a charcoal fire and passing the fumes into a large glass globe. The action of oil of vitriol on salt produces a pungent fume which escapes into the air and is lost ; but Glauber found that this fume condensed readily in a receiver half filled with water, giving an acid liquid which he described
' "Vitriols are produced from the stone . . called Marchasite, and from it on the application of fire the flowers of common sulphur are elicited in considerable abundance " (John Mayow, Medico-fhysical Works, 1674 ; Alembic Club Reprints, XVII. 28).
n THE ACIDS 15
as SPIRIT OF SALT ; another common name for the acid is MURIATIC ACID (Latin, murium, brine). The gas itself was first isolated by Priestley, who showed that it could be collected quite readily ovel- mercury.
Glauber also showed that the mixture of spirit of salt and aqua fortis, which is produced by distilling salt and saltpetre with oil of vitriol, was capable of dissolving gold and had the same properties as the aqua regia prepared by Geber's method.
B, Properties of the Acids.
Taste. Action on vegetable dyes. — The most con- spicuous property possessed by all the above acids was their sour taste. To this may be added the fact that they were capable of changing the colour of various vegetable dyes. During the eighteenth century, syrup of violets, which changes from violet to red on the addition of an acid, was largely used as a test or indicator for these substances ; at a later date " tournesol," or litmus, which changes from blue to red more easily than syrup of violets, and various artificial dyes, were used for this purpose.
When concentrated, the mineral acids were found to have very corrosive properties. Glauber describes the charring of a slip of wood and the ignition of turpentine and of spirit of wine as properties of the strongest oil of vitriol. Aqua fortis was equally corrosive, but both acids become harmless when sufficiently diluted. The diluted mineral acids were at one time employed in medicine and as substitutes for vinegar and lemon-juice in the preparation of food.
Action on alkalis and chalk. — When added to soda or potash all the acids were found to produce the violent effer- vescence that was first noticed in the action of vinegar on soda (p. 4). A similar " breaking out of air " took place when chalky materials were acted on by acids : in this action
i6 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
the chalk was usually dissolved, but oil of vitriol gave a white solid which was proved by Margraaf, in 1750, to be identical with the mineral gypsum or selenite (Fig. 10, p. 19), from which "Plaster of Paris" is made by gentle burning. The origin and the nature of the "air'' which is set free in these actions are discussed in Chapters IV. and VI.
Action of acids on metals. — The acids also had the property of corroding or dissolving metals. The poisonous green powder known as verdigris was prepared at a very early date by the action of vinegar on copper, and was used as a paint during the classical period. The mineral acids were found to act on metals in a much more powerful way; by the time of Geber (circ. 800 a.d.) methods had been discovered for dissolving all the metals that were known. The action of acids on metals was often accompanied (as in the case of soda and chalk) by the breaking out of a gas, but for many centuries no method was known by which these fugitive products could be collected. The important discoveries which were made when at last it was found possible to isolate and examine them are described in Chapter V.
C. Preparation of New Salts and Nomenclature OF Salts
New salts. — One result of the discovery of the acids was to add very greatly to the number of " salts " which were known. When an acid acts on a metal, on chalk, or on an alkali, a solution is produced which no longer has the sour taste of the acid. These solutions contain a variety of salt-like substances, which can be isolated by evaporating either to dryness or until crystals begin to separate. In this way many beautiful and useful salts were obtained. At first a special name was given to each salt ; but later a
THE ACIDS
i'7
system was devised in which each salt was named after the acid and the base (metal, alkali, or earth) from which it was derived, e.g., nitrate of silver, sulphate of potash, muriate of lime. This system was initiated about the middle of the eighteenth century and completed by a group of French chemists in 1787^; it led inevitably to the inclusion in the category of " salts " of many insoluble and tasteless sub- stances. Selenite, for instance, when prepared by the action of oil of vitriol on lime or chalk, could scarcely be excluded from the category of salts merely because it was only slightly soluble in water ; for the same reason it was necessary to regard as a salt the insoluble muriate of silver, which could scarcely be separated in an arbitrary way from the soluble muri- ates of copper and gold.
Some of the more important salts' pre- pared with the help of the acids are described below.
Vitriols or sulphates. — The salts prepared by the action of oil of vitriol on metals were of a glassy crystalline character, which won for them the name of vitriols.
"Out of all Metals there can be made a Vitriol or Chrystal (Chrystal and Vitriol is taken for one)." (Basil Valentine, Last Will and Testament, p. 157.)
This name was afterwards limited to crystals prepared from, or related to, oil of vitriol. The most important of these were green vitriol, and blue vitriol (Fig. 9),
* The MSthode de Nomenclatun Chimique, by MM. de Morveau, Lavoisier, Berthollet, and Fourcroy, was published in Paris in 1787, and translated into EngUsh in 1788 and 1796.
Fig. 9— Crystal of Blue Vitriol OR Sulphate of Copper.
i8 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
obtained by the weathering of different varieties of pyrites. The blue vitriol was of special interest because iron dipped into it became coated with copper, and seemed to have been transmuted into that metal. These two vitriols were prepared artificially by the action of oil of vitriol on plates of iron or copper, by Glauber, who writes as follows : —
" Take of your heavy oil .... as much as you please, put it into a glass body together with plates of copper or iron, set it in a warm sand, and let it boil until that the oil will dissolve no more of the metal, then pour off the liquor, filter it through brown paper, and put it into a low gourd glass and set it in sand, and let the phlegm evaporate until there appear a skin at the top, then let the fire go out, and the glass grow cold, then set it in a cold place, and within some days there will shoot fair Crystals ; if of Iron, greenish ; if of Copper, something bluish ; take them out and dry them upon filtering paper, the remaining liquor, which will not shoot into Vitriol, evaporate again in sand, and then let it shoot as before ; continue this proceeding, until all the solution (or filtered liquor) be turned to Vitriol " {Philoso- phical Furnaces. Part IV; JVorks, 1. i8).
These two vitriols were described by Lavoisier and his colleagues as sulphate of iron and sulphate of copper, after the acid and metals from which they were derived.
Glauber's salt was prepared by the action of oil of vitriol on common salt. When this action was carried out in a glass retort, the salt was separated easily in a pure con- dition by crystallisation. Its discoverer attributed to it almost miraculous properties and called it "sal mirabile." It was afterwards prepared from oil of vitriol and soda, and was therefore called sulphate of soda. The corresponding salt prepared by the action of oil of vitriol on potash was known in Boyle's time as vitriolated tartar, but in Lavoisier's system became sulphate of potash.
Gypsum or selenite (Fig. lo), which could be prepared
THE ACIDS
19
2,
artificially from oil of vitriol and chalk or lime (p. 11), was called sulphate of lime. Epsom salts, a purgative salt derived from mineral springs, was shown by Black in 1755 to contain a base magnesia in combination with oil of vitriol ; it was therefore called sulphate of magnesia.
Nitres or nitrates.— Nitre or saltpetre (Fig. p. 4), for many centuries the only source from which nitric acid and the nitrates could be derived, was pre- pared artificially in 1674 by John Mayow, a friend and contemporary of Boyle, by recombining the nitric acid with potash ; its systematic name was therefore nitrate of potash, a nitrate of soda was prepared by Geber ; enormous deposits of this salt have been discovered in the desert regions of Chile, and mil- lions of tons are now ex- ported every year for use in agriculture under the nameof Chile saltpetre.
Thenitratesof the metals were well-known to the Alchemists. Geber describes lunar caustic, the nitrate of silver, as prepared in the form of " small fusible stones like crystal," by dissolving silver in aqua fortis and boiling away two-thirds of the water in a long-necked flask. The nitrates of lead, copper, iron, and mercury, were also prepared at an early date.
c 2
Fig.
10 — Large Ckystai. ok GvrsuM or Selenite (Sulphate of Lime).
20 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
Muriates. — Glauber, who discovered the first efficient method of making spirit of salt or muriatic acid, prepared the MURIATES OF IRON, COPPER, GOLD, and Other metals, by the action of spirit of salt, or of aqua regia upon the metals ; he obtained them in the form of strong solutions which he described as "oil of Mars," "oil of Venus," etc., in accordance with the alchemistic system (which survives in the case of Mercury) of calling each of the common metals after a planet.
Common salt, which can be prepared artificially by recom- bining muriatic acid with soda, is a muriate of soda. The corresponding muriate of potash, known as sal sylvii, or SYLViNE, was prepared by the action of muriatic acid on wood-ashes or potash. Extensive deposits of the salt have been found at Stassfurt in Germany ; the mineral is used on a large scale as a fertiliser in agriculture, and is one of the chief sources from which potash is derived.
Black in 1755 prepared the muriate of magnesia, and compared its properties with those of muriate of lime.
Acetates. — Of the salts derived from organic acids the most important were the acetates prepared from vinegar, or acetic acid. The acetate of soda and acetate of lime prepared by the action of vinegar on soda and on chalk (as described on p. 15), were amongst the first salts to be prepared arti- ficially. Mention may also be made of the acetate of lead which Basil Valentine prepared from vinegar and litharge, and which acquired the name sugar of lead on account of its sweet taste ; also of the acetate of copper, which he prepared by the action of vinegar on verdigris.
"There is extracted from calcined Saturn [i.e., burnt lead or litharge] with distilled Vinegar a Crystalline Salt."
"Take some pounds of Verdigris, extract its Tincture with distilled Vinegar, let it shoot, then you have a glorious Vitriol." (Basil Valentine, Last Will and Testament, pp. 349 and 351.)
II THE ACIDS 21
Black in 1755 prepared the acetate of magnesia, in addition to the muriate, nitrate, and sulphate, for comparison with the corresponding salts of lime.
Classification of salts. — The salts described above were prepared by the action of acids (i) on metals such as iron and copper ; (2) on the alkalis, soda and potash ; (3) on earths such as lime and magnesia. The distinction, which was at first made between these three classes of salts, was rendered of little value by the observation that the salts of metals could be prepared much more easily from the earthy calces which are formed when the metals are burnt : Glauber, for instance, showed that the muriate of copper was prepared easily by the action of muriatic acid on the calcined metal, although the metal itself was attacked but slowly. The distinction was broken down finally at the commencement of the nineteenth century by the discovery that the alkalis and earths were themselves calces of easily-burnt metals. When this had been proved the custom arose of describing all salts as derivatives of metals. Thus gypsum, which was called by Lavoisier " sulphate of lime," is now described as CALCIUM SULPHATE, and Glauber's salt is called sodium sulphate instead of " sulphate of soda. " The older names are, however, still used in commerce and in pharmacy.
Strength of acids, — The fact that oil of vitriol could displace muriatic acid from common salt, and nitric acid from nitre, was recorded by Glauber. Attention was directed at first mainly to the liberated acid, but it was recognised soon that a soluble substance was left behind which contained the " fixed salt " (i.e., the base or alkali) of the original substance in combination with the stronger vitriolic acid. Thus, when oil of vitriol acted upon common salt, the residue of Glauber's salt was found to be identical with sulphate of soda prepared by the action of oil of vitriol on soda ; when nitre was used, the residue was a "vitriolated tartar" or
22 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
sulphate of potash identical with that prepared from oil of vitriol and " tartar" or potash.
These actions were studied carefully by John Mayow, a London Physician (1645-1697), who pointed out in 1674 that " although [acids] and [alkalis] pass into a neutral sub- stance when they meet, yet they do not, as is generally supposed, entirely destroy each other," since when the conditions are suitable both the acid and the alkali may be recovered from the salt. Thus: —
" If oil of vitriol is poured upon nitre, which consists of an [alkali] and of a volatile [acid] (as was shown above), the fixed salt [i.e., the base] of the nitre will soon leave its own acid and will enter into union with the acid of the vitriol, which is more concordant with it ... . That the case is so, is clear, for if nitre mixed with oil of vitriol be distilled, the spirit, or [acid] of the nitre will pass under a mild heat into the receiving vessel, while yet in other circumstances that spirit will not be carried up except by a very vehement fire .... It is a corroboration of this view that the mass left in the retort after a distillation of this kind, closely resembles vitriolated tartar, and can be properly substituted for it." (" Of the combination of contrary salts," Medico-physical Works, A.C.R. XVII. 161—162.)
As oil of vitriol was found to liberate nitric acid.from any nitrate and muriatic acid from any muriate, it was regarded as stronger than either of these acids, whilst vinegar was found by similar tests to be weaker than the three mineral acids. It is recognised now that this rough and ready method is not the best test of the strength of an acid, as it depends too largely on the readiness with which the various acids can be driven off in the form of vapour. But observations such as these were of great importance because they showed clearly that all salts possessed a dual structure ; they thus prepared the way for the system of classifying salts which is described in the preceding paragraphs.
II THE ACIDS 23
Summary and Supplement, a and b. — discovery and properties of the acids.
Vegetable Acids include : — Acetic acid, CjH^Oj, from Vinegar (known to Geber) ; Tartaric acid, C4Hg05, from Tartar (Scheele, 1770) ; Benzoic acid, CjUfi^ from Benzoin (Scheele, 1775) ; Uric acid, C5H4N4O3, from Bladder-stones (Scheele, 1776) ; Lactic acid, CjHeOg, from Sour Milk (Scheele, 1780) ; Oxalic acid, C2H2O4, from Oils (Scheele, 1783) ; Citric acid, CeH807, from Lemons (Scheele, 1784) ; Malic acid, C4H8O5, from Apples (Scheele, 1786).
Mineral Acids include : —
Oil of Vitriol, Vitriolic acid, or Sulphuric acid, H2SO4, pre- pared by distilling Alum (Geber) or Green Vitriol (Glauber).
Aqua Fortis, Acid Spirit of Nitre, or Nitric Acid, HNO3, pre- pared by distilling Nitre and Alum (Geber) or Nitre and Oil of Vitriol (Glauber), 2KN03-|-H„S04-s>K2S04H-2HN03.
Spirit of Salt, Muriatic acid, or Hydrochloric acid, HCl, pre- pared by heating Salt and Oil of Vitriol, and collected in water (Glauber), 2NaCl + H2S04-> Na2S04 -f- 2HCI.
Aqua Regia, a mixture of Aqua Fortis and Spirit of Salt prepared by adding Sal-ammoniac to Aqua Fortis (Geber), NH4Cl-|-HN03-> NH4N03-t-HCI, or by distilling a mixture of Nitre and Salt with Oil of Vitriol (Glauber).
The acids as a class possess the following properties, though some of them may be lacking in the case of individual acids : — (a) General Properties : Sour taste ; acids turn syrup of violets red and redden blue litmus ; the mineral acids are corro- sive when concentrated but harmless when dilute ; ifi) Acids dissolve Alkalis and Chalk, liberating gas, e.g., " Vinegar upon Nitre (i.e., soda)" and upon Chalk, Na2C03-|-2C2H402->2NaC2H302-f HaO-l-COa CaCOs -l-2C2H402->Ca(C2H302)2-t-H20-t-C02 ; Oil of Vitriol with Soda gives " Glauber's salt," with Chalk gives " Gypsum" or "Selenite" (almost insoluble in water) NajCOj -I- H2S04-> Na2S04 -I- H2O -)- CO2 CaCOj -l-H2S04->CaS04-l-H20-)-C02;
24 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
(c) Adds dissolve metals, e.g., Oil of Vitriol and Muriatic acid dissolve Iron ; Oil of Vitriol (concentrated) and Aqua Fortis dissolve also Copper, Mercury and Silver ; Aqua Regia dissolves Gold.
The action of acids and alkalis on vegetable tinctures such as the blue-fluorescent extract of " Lignum nephriticum," syrup of violets and cornflowers, and the purple juice of ripe privet- berries was described by Boyle {Experiments on Colours, 1663).
D. Salts. Many salts can be prepared artificially by the action of Acids on Metals, Alkalis (Soda and Potash) and earths. Amongst the salts which were prepared and examined at an early date the following may be noted : —
(a) Vitriols or Sulphates. — From Metals : Green Vitriol, Sulphate of Iron, or Ferrous Sulphate, FeS04, 7H2O ; Blue Vitriol, Sulphate of Copper, or Copper Sulphate, CuS04,5H20.
From Alkalis : Glauber's Salt, Sulphate of Soda, or Sodium Sulphate, Na2S04, 10H2O ; Vitriolated Tartar, Sulphate of Potash, or Potassium Sulphate, K2SO4.
From Earths : Gypsum or Selenite, Sulphate of Lime, or Calcium Sulphate, CaS04, 2H2O ; Epsom Salts, Sulphate of Magnesia, or Magnesium Sulphate, MgS04, 7H2O ; Alum, Sulphate of Potash and Alumina, or Potassium Aluminium Sulphate, KA1(S04)2,I2H20.
(b) Nitres or Nitrates. — Nitre or Saltpetre, Nitrate of Potash, or Potassium Nitrate KNO3 ;
Chile Saltpetre, Nitrate of Soda, or Sodium Nitrate, NaNGj ; Lunar Caustic, Nitrate of Silver, or Silver Nitrate, AgNGj. Also Nitrates of Lead, Copper, Iron and Mercury.
(c) Muriates or Chlorides.— Muriates or Chlorides of Iron (FeCy, Copper (CuCy, and Gold (AuCy. Common Salt Muriate of Soda, or Sodium Chloride, NaCl ;
Sylvine, Muriate of Potash, or Potassium Chloride, KCl ; Muriate of Magnesia or Magnesium Chloride, MgClj ; Muriate of Lime or Calcium Chloride, CaClj.
{a) Acetates. — Sugar of Lead, Acetate of Lead, or Lead Ace- tate, Pb(C2H302)2 ; Acetate of Copper or Copper Acetate, Cu(C2H302)2, pre-
II THE ACIDS 25
pared from Verdigris or Basic Acetate of Copper,
Cu(C2H302)2 + Cu(OH)2; Acetate of Soda or Sodium Acetate, NaC2H302 ; Acetate of Lime or Calcium Acetate, Ca(C2H302)2 ; Acetate of Magnesia, or Magnesium Acetate, Mg(C2H302)2.
Three systems of nomenclature are seen in the names given to the salts set out above : — (i) At first each salt received a special name, in many cases recalling the origin or properties of the salt. (2) The system of naming salts after the acid and the metal, alkali or earth from which they are derived was elaborated by de Morveau, Lavoisier, Berthollet, and Fourcroy in their M^thode de Nomenclature Chimique, published in 1787. (3) This system became obsolete when Davy, twenty years later, showed that the alkalis and earths contained metals ; it was then possible to name every salt after the acid and the metal from which it was derived and to abandon the use of the alkalis and earths in naming salts.
The idea that salts still contained the acid and alkali from which they were derived was put forward by John Mayow in his Medico- Physical Works published in 1674 ; he showed that Ammonia could be displaced from its Salts by Potash, 2NH4CI + K2CO3 ->2KC1 + (NH4)2C03 and Aqua Fortis by Oil of Vitriol, 2KNO3 + HjSOi-S* K2SO4 + 2HNO3, and was impressed with tjie idea of the unequal " concordance " of the two acids with the alkali, an idea that is essentially the same as that of the unequal strengths of different acids. Rouelle, to whom we owe the terms base {Mem. Acad., 1754, 572) and water of crystalli- sation {ibid., 1744, 356) described how neutral salts had been restricted at first to " salts formed by the union of acids with alkalis, which are soluble in water, and imprint on the tongue a saline taste. . . . The number of neutral salts was at first very small, scarcely any were known but sea salt and nitre ; but the number was soon increased, above all by the work of Glauber. Others have since been added of which the bases are the volatile alkali and an absorbent earth. Finally there have been added salts formed by the union of acids with metallic substances" {Mem. Acad., 1754, 5/2). He himself, in 1744, had defined as a neutral salt, " a salt formed by the union of an acid with any substance whatever, which serves as a base for it and imparts to it a concrete or solid form " {ibid., 573).
CHAPTER III
THE BURNING OF METALS AND THE DISCOVERY OF OXYGEN
A. The Burning of Metals.
Jean Rey (1630) shows that lead and tin increase in weight when burnt — It has been known from very early times that the metals, except gold and silver, are, by heating, gradually changed to powders of various colours. These powders were called calces from the resemblance which they showed to lime (Latin, calx), and the process of burning was called calcination. A casual examination showed that the calx was a lighter material than the metal from which it was formed; it was, therefore, natural to suppose that the burning of the metal had resulted in a loss of weight, just as is obviously the case when wood or coal is burnt. The fact " that tin and lead increase in weight when they are calcined," was therefore " observed with astonishment " by those who first put the matter to the test of experiment.
The Essays of Jean Rey (1630), a French Doctor of Medicine, " On an Enquiry into the cause wherefore Tin and Lead increase in weight on Calcination" (Alembic Club Reprints, No. XI), contain an account of one of the earliest chemical researches of which a clear record has been pre- served. He records that Brun, an apothecary of Bergerac,
" having placed two pounds six ounces of fine Enghsh tin in an iron vessel and heated it strongly on an open furnace
CH. Ill THE BURNING OF METALS 27
for the space of six hours with continual agitation and without adding anything to it, he recovered two pounds thirteen ounces of a white calx ; which filled him with amazement, and with a desire to know whence the seven ounces of surplus had come " (A. C. R. XL 36).
It had been suggested that the gain in weight was due to soot or vapour from the furnace condensing on the tin, or to the disintegration of the iron vessel; Rey showed that these explanations were unlikely, and concluded (on the basis of argument rather than of experiment)
" That this increase in weight comes from the air, which in the vessel has been rendered denser, heavier, and in some measure adhesive, by the vehement and long-continued heat of the furnace : which air mixes with the calx (frequent agitation aiding) and becomes attached to its most minute particles : not otherwise than water makes heavier sand which you throw into it and agitate, by moistening it and adhering to the smallest of its grains " (A. C. R. XI. 36).
Rey supported his argument by quoting an experiment of Hamerus Poppius, who had calcined a cone of anti- mony on a marble slab by means of a burning mirror, and had found the weight to be augmented instead of diminished, in spite of the copious exhalation of vapours and fumes (A. C. R. XI. 49) ; in this case at least, the gain in weight could not be attributed to contamination of the metal by the -fiirnace or vessel, and must surely be due to the condensation of air. A similar explanation was given in the case of lead, of which it had been recorded that "it is a remarkable thing that lead on calcination increases in weight by eight or ten pounds per cent " (A. C. R. XI. 41).
Robert Boyle (1673) confirms Key's statement that metals gain in weight on calcination. —Jean Rey himself does not appear to have made any experiments on the gain in weight of tin and lead. But the fact that metals gain in weight when calcined was confirmed forty-three years later
28 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
by the careful experiments of Robert Boyle on the calcina- tion of copper and iron, as well as of tin and lead. Boyle's observations showed a gain of : —
30 to 49 grains on 480 grains of copper ^ 60 „ „ 480 „ „ tin
66 „ „ 240 „ „ iron
7 „ „ 480 „ „ lead (in spite of loss)
whilst silver showed a gain of only 2 grains on 212. Boyle's experiments were carried out as follows : —
" Into a very shallow crucible, we put an ounce of copper- plates, and set it in a cupelling furnace, where it was kept for two hours ; and then being taken out, we weighed the copper, which had not been melted (having first blown off all the ashes), and found it had gained thirty grains."
A similar experiment with an ounce of copper filings gave an increase of 49 grains.
" Upon a good cupel, we put an ounce of English tin, of the better sort ; and having placed it in the furnace, under a muffler, though it presently melted, yet it did not forsake its place, but remain upon the concave surface of the cupel, till, at the end of about two hours, it appeared to have been well-calcined ; and then being taken out, and weighed by itself, the ounce of metal was found to have gained no less than one dram."
" An ounce of lead was put upon a cupel, made of calcined hartshorn, and placed under a muffler, after the cupel was first made hot, and then weighed. This lead did not enter the cupel, but was turned into a kind of litharge on the top of it, and broke the cupel, whereby some part of the latter was lost in the furnace ; yet the rest, together with the litharge, weighed seven grains more than the lead and heated cupel, when they were put in."
" Four drams of the filings of steel, being kept two hours
^ 20 grains = i scruple 3 scruples = i dram 8 drains = i ounce.
in THE BURNING OF METALS 29
on a cupel, under a muffler, acquired one dram, six grains and a quarter, increase of weight."
" A piece of refined silver, being put upon a cupel under a muffler, and kept there for an hour and a half, was taken out, and weighed again ; and as before it weighed three drams, thirty-two grains and a quarter, it now weighed in the same scales, three drams, thirty-four grains and a quarter" {Works, 1725, II. 389-390).
Boyle calcines tin in sealed flasks. — Although Boyle was convinced that metals really gained in weight when burnt, he regarded these experiments as unsatisfactory because the metals were exposed to the smoke and dust of the furnace. To get over this difficulty he heated the metals between two crucibles cemented together with clay, and finally (in the case of the more fusible metals) made use of glass flasks, the necks of which were loosely stoppered, drawn out to a fine point, or sealed up altogether. Boyle's experi- ments on the calcination of tin in sealed flasks are of special interest as having provided the basis for Lavoisier's experi- ments on the same subject.
"To prevent all suspicion of any increase of weight, in the metals, arising from smoke, or saline particles, getting in at the mouth of the vessel, I made the experiment in glasses, hermetically sealed, as follows. Eight ounces of good tin, carefully weighed, we hermetically sealed up in a new, small retort, with a long neck, by which it was held in the hand near a charcoal fire, that kept the metal in fusion ; being now and then shaken for almost half an hour ; in which time, it seemed to have acquired, on the surface, such a dark colour, as argued a beginning calcination ; and it both emitted fumes that played up and down', and also, afforded two or three drops of liquor, in the neck of the retort. The glass was, at length, laid on quick coals, where the metal continued above a quarter of an hour longer in fusion ; but, before the time was come, that I intended, to suffer it to cool, in order to its removal, it suddenly broke into a great multitude of pieces, and with a noise, like the report of a gun."
30 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
In order to reduce the risk of explosion the flask was next heated before sealing,
" Two ounces of filings of tin, were carefully weighed, and put into a little retort, whose neck was afterwards drawn slender to a very small apex ; then the glass was placed on kindled coals, which drove out fumes at a small orifice of the neck, for a pretty while. Afterwards, the glass, being sealed at the apex, was kept in the fire for above two hours ; and then being taken off, was broken at the same apex : whereupon I heard the external air rush in, because, when the retort was sealed, the air, within it, was highly rarefied. Then the body of the glass being broken, the tin was taken out, consisting of a lump, about which there appeared some grey calx, and some very small globules, which seemed to have been filings melted into that form. The whole weighed two ounces, and twelve grains " {Works, II. 393-394).
" Fire and flame weighed in a balance " by Robert Boyle. — Although the. observations on calcination which are described under this title ' ( Works, 1725, II. 388) agreed closely with those quoted by Jean Rey, Boyle gave a different explanation of the gain in weight, which he attributed to the absorption of heat instead of to the condensation of air. Boyle's failure to recognise the essential part played by air in combustion may be attributed to his observations " Of the strangely difficult Propagation of Actual Flame in Vacuo Boyliano " {New Experimetiis, touching the Relation betwixt Flame and Air, London, 1672 ; compare Works, 1725,11. 517) in which he found that various substances including sulphur, gunpowder, and fulminating gold, could be fired, although with difficulty, by contact with hot iron in a vessel from which much of the air had been removed by means of an air-pump.
Boyle's opinion forms the basis of the "Phlogiston" theory of Becher and Stahl. — Boyle's view that fire and flame were material things which could be " weighed in a
■ The title of the original tract is "New Experiments to make the Parts of Fire and Flame stable and ponderable." London, 1673.
ni THE BURNING OF METALS 31
balance " appeared again in a modified form in the theory OF PHLOGISTON whlch dominated the science of chemistry during the next hundred years, but was finally overthrown by the work of Lavoisier from 1770 to 1787. This theory was elaborated by two German philosophers, John Joachim Becher (1635-1682), and George Ernest Stahl (1660- 1734)1 who sought to explain the phenomena of combustion as due to a fire-element, or principle of inflammability, to which Stahl gave the name phlogiston. It was supposed that phlogiston was present in all combustible substances ; the largest proportion was contained in soot, which was thought to be almost pure phlogiston, since it left only the smallest residue of ash when burnt. In the smelting of metals the phlogiston of the fuel combined with the ore to produce the metal ; when the metal was burnt it parted with the phlogiston it had taken from the fuel and was converted into an incombustible calx, or ash. It will be noticed that whilst Boyle regarded the calx as a compound of the metal with igneous particles from the fire, thus
Metal + Fire = Calx,
Stahl regarded the calx as a simple substance, and the metal as a compound of the calx with phlogiston, thus.
Calx + Phlogiston = Metal.
Metal- Phlogiston = Calx.
As the calx is heavier than the metal from which it is derived it was clear that the phlogiston which escaped during calcination must have a negative weight ; this curious conclusion, although not much considered at first, led ultimately to the destruction of the theory.
In Stahl's opinion, air was required in combustion merely to absorb the phlogiston set free by the burning substance ; air which had become exhausted by combustion was thought to be saturated with phlogiston, and was called
PHLOGISTICATED AIR.
32 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
Hooke (1665) & Mayow (1674) suggest that air contains an active principle similar to nitre. — The air, to which Jean Ray had attributed the gain in weight of lead and tin during calcination, was regarded by Boyle as having little or nothing to do with this change. It was, therefore, left to his contemporaries, Robert Hooke and John Mayow, to follow up the clue which Rey had provided and to demon- strate the part which air plays, not only in the burning of metals, but in other cases of combustion.
Robert Hooke, in his " Micrographia " (1665), called attention to the close resemblance between the action of nitre, or saltpetre, and of air in various cases of burning. He regarded air as a solvent for the burning substance, and attributed its activity to the presence in it of a substance similar to (or even identical with) melted saltpetre, but in a very attenuated condition.
The similarity between air and nitre also formed the basis of the theory of combustion put forward by John Mayow in his " Medico-Physical Works " (Alembic Club Reprints, No. XVII.), published in 1674. He showed, as Boyle had done, that air was not needed for the burning of gun- powder, since damp gunpowder would continue to burn when the tube in which it was contained was plunged under water (A. C. R. XVII. 9). There was, therefore, something in the nitre which would take the place of air in enabling charcoal and sulphur to burn. To this common principle, present in air and in nitre, he gave the name spiritus
NITRO-^REUS or NITRO-^RIAL SPIRIT (A. C. R. XVII. l).
Mayow (1674) proves that air is absorbed during combustion. — Hitherto no attempt had been made to collect and examine gases. When substances were dis- tilled the volatile products were condensed in a cold receiver, in which water was sometimes placed, but gases and vapours which were not condensed in this way had ahvays been lost. To Mayow belongs the credit of intro-
THE BURNING OF METALS
33
ducing into chemistry the method of collecting gases in flasks or bottles inverted over water, and of studying
Fir,.
Fig.
Fig. 13.
Fig. 14.
Mayow's Apparatus.
Fig. II shows a candle burning in a glass vessel inverted over water, and
also a piece of camphor being fired by a burning-glass. Fig. 12 shows the diminution of the volume of air by a mouse breathing in
it ; a stretched bladder is sucked inwards as the air diminishes. Fig. 13 shows the same experiment carried out with air contained in a glass
vessel inverted over water. Fig. 14 shows the apparatus used to collect gases prepared artificially by
the action of acids on iron.
changes of volume in the gas by noticing the position of the water in the glass vessel. Mayow's apparatus is shown in Figs. II, 12, 13 and 14 (A.C.R. XVII. Plate 5). With
34 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
this apparatus he showed that a decrease in the volume of air occurred when a candle was burned in an inverted flask, or when camphor was fired in it by means of a burning glass (Fig. ii): this decrease he attributed to the disap- pearance of the nitre-air during burning. He also showed that a decrease in the volume of air was caused by a mouse breathing in it (Figs. 12 and 13), and that the mouse died almost immediately if placed in a jar of air in which a flame was burning : the nitre-air used in burning was therefore also necessary for breathing, and was used up in just the same way as by a burning candle.
Lavoisier (1774) proves that the gain in weight when tin is calcined is due to ab- sorption of air. — The overthrow of the phlog- iston theory and the establishment of the theory of combustion, were the work of a French nobleman, An- toine Laurent Lavoisier (1743-1794)- Lavoisier met with an untimely death dur- ing the French Revolution, but himself brought about a revolution in the science of chemistry, so complete as almost to justify the dictum of a fellow countryman, " Chemistry is a French science. Its founder was Lavoisier of immortal memory." 1 His experiments on combustion
^ This is the opening passage of Wurtz's Atomic Theory. A contrary opinion was expressed by Brande, Professor of Chemistry in the Royal Institution, who wrote in 1819 : "It is, I think, among our own countrymen that we discover the fathers of chemical philosophy : for Bacon, Boyle, Hooke, and Newton, present unequivocal claims to that distinctive title" (Mamtal nf Chemistry, y. xviii.).
. t 'l-
Fig. i; — Lavoisier's Apparatus for Cal- cining Lead and Tin in Air over Water or Mercury.
Ill THE BURNING OF METALS 35
were carried out with a full knowledge of the work of his predecessors, and were modelled largely upon the work of Mayow and Boyle.
By means of a burning-glass 33 inches in diameter, Lavoisier calcined lead and tin in air enclosed in a glass vessel inverted over water or mercury^ (Fig- 15)) and found that " the volume of air diminished by about a twentieth as a result of the calcination, and that the weight of the metal is increased by an amount almost equal to that of the air destroyed or absorbed." He concluded " that a portion of the air ... . combined with the metals during their calcination, and that the increase in weight of the metallic calces was due to this cause " (" Memoir on the Calcination of Tin in Closed Vessels," 1774, Works, II. 105.) Thus in contrast to the current view that Metal = Calx + Phlogiston Lavoisier reverted to the suggestion of Jean Rey that
Metal + Air = Calx, the metal being regarded as a simple substance and the calx as a compound.
Lavoisier calcines tin in a sealed flask. — In order to test the view that the gain in weight of metals during calcination was due to the absorption of air, Lavoisier, in 1774, repeated Boyle's experiment of heating lead and tin in sealed glass vessels. But, unlike Boyle, he carried out the critical experi- ment of weighing the sealed retort both before and after heating. In making these experiments Lavoisier had the advantage of using a balance, more sensitive than any that had been constructed previously, with which he was able to detect changes in weight of -^^ of a grain.
The experiment was carried out as follows ; Eight ounces
of tin were weighed into a retort of 43 cubic inches capacity,
heated to drive out part of the air, sealed up and again
weighed. The tin was then heated in the sealed retort for
' Compare Priestley, Experiments on Air, I. 136.
D 2
36 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
an hour and a quarter, until the surface of the molten metal ceased to tarnish and a considerable quantity of a black ^ calx had collected. In spite of the calcination that had taken place the sealed retort had only changed in weight by the loss of a quarter of a grain. If, therefore, the metal had gained in weight "it was necessary to look for the cause in the interior of the retort" [Works, II. 112).
The retort was then broken into halves by cracking it with a hot coal, and the whole was weighed again. A comparison with the first weighing of the tin and retort showed a gain of 3 13 grains; further weighings showed that the tin in the retort had gained 3"i2 grains, whilst the retort itself had not changed in weight. Lavoisier was able to calculate that the weight of air which he had sealed up in the retort was 15^^ grains, and therefore concluded that one-fifth of this air had been absorbed by the tin.
In a second experiment 20 ounces of tin were heated during two and a-half hours in a large retort of 250 cubic inches capacity. After allowing air to enter through a small crack, the retort and its contents were found to be io"o6 grains heavier than when they were first weighed : the tin had gained in weight by lo-oo grains or ^ to ^ of the weight of air in the retort. The agreement between these two figures could not be predicted, as the cracked retort did not contain ordinary air ; but it is referred to below as evidence that the ordinary air entering the retort had almost the same density as the portion which had been absorbed by the tin.
Lavoisier's figures are given on p. 37.
Lavoisier concluded :
" I. That only a limited quantity of tin can be calcined in a given quantity of air.
' When tin is heated in an excess of air, a white calx is produced.
THE BURNING OF METALS
37
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38 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
" 2. That the quantity of tin calcined is greater in a large retort than in a small one. . . .
" 3. That the sealed retorts, weighed before and after the calcination of a portion of the tin which they contain, show no difference in weight, proving that the gain in weight of the metal does not come from the fire nor from anything outside the flask.
" 4. . . . That in the calcination of tin, the gain in weight of the metal is almost exactly equal to the weight of the quantity of air admitted, proving that the part of the air which combines with the metal during calcination has almost the same density as that of atmospheric air " ( Works, II. 1 1 8-1 19).
Having thus shown that air was made up of two parts, one of which was absorbed by tin, whilst the other was not acted on, Lavoisier set himself to obtain separately the part of the air which combines with metals and enables substances to burn. He was able to imagine what the properties of this gas must be : it must support com- bustion, and probably substances would burn in it with great ease. He tried to obtain it from metallic calces in which he knew it was present, but was unable to set it free by any of the methods at his disposal. The recovery of this lost air, which Lavoisier required to complete the proof of his theory of combustion, was, however, accomplished within a few months by the independent work of Priestley in England and of Scheele in Sweden.
B. The Discovery of Oxygen.^
Priestley (1774) discovers a gas richer than common air. — The facts which Lavoisier required in order to com- plete the proof of his theory of combustion were supplied
^ The discovery of oxygen followed that of niost of the gases described in Chapter V. It is described here in order to complete the story of the calcination of metals.
Ill THE DISCOVERY OF OXYGEN 39
almost immediately by Joseph Priestley (1733— 1804), a nonconformist pastor of Leeds, who was at this time making a large number of experiments with the object of finding out what "airs," or gases, were formed when various substances were heated. Having procured a new burning glass twelve inches in diameter, Priestley " proceeded with great alacrity to examine, by the help of it, what kind of air a great variety of substances, natural and factitious, would yield, putting them into vessels .... filled with quicksilver, and kept inverted in a bason of the same." [Experiments and Observations on Different Kinds of Air, 1774, II. 28; A.C.R. VII. 8.)
With this apparatus, on the ist of August, 1774, Priestley endeavoured to extract air from " mercurius calcinatus per se," the red calx of mercury prepared by heating the metal gently in air. Having " found that by means of this lens, air was expelled from it very readily," Priestley proceeded to examine the product, and discovered to his great astonishment that " a candle burned in this air with a remarkably vigorous flame .... and a piece of red-hot wood sparkled in it, exactly like paper dipped in a solution of nitre, and it consumed very fast " (A.C.R. VII. 10). He placed a mouse in a vessel filled with the gas where " it remained perfectly at its ease another full half hour," twice as long as it would have lived in ordinary air (A.C.R. VII. 17). Later he had the curiosity to breathe the gas himself, and fancied that his " breast felt peculiarly light and easy for some time afterwards " (A.C.R. VII. 54). Priestley also obtained the gas by heating other substances, including red-lead, a red powder formed by gently roasting white lead, or litharge, and capable, like the mercury calx, of being decomposed when heated more strongly.
The only explanation which Priestley could give of his discovery of a gas better and richer than air was that
40 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
ordinary air must contain some phlogiston, and that in this new gas he had, for the first time, obtained air free from phlogiston. He therefore described it as dephlogisticated AIR, and attributed its superior power of supporting com- bustion, to the fact that it was capable of receiving more phlogiston from burning substances, and could therefore maintain a flame for a longer period than ordinary air.
Lavoisier makes quantitative experiments on the calcination of mercury. — In October of the same year Priestley, when on a visit to Paris, informed Lavoisier of his experiments. Lavoisier concluded that the new gas was the active part of the air which he had tried without success to separate. In November, 1774, he repeated Priestley's experiments, and in the spring of the following year read before the Academy of Sciences at Paris a memoir " On the Nature of the Principle which Combines with the Metals during their Calcination and Increases their Weight" {Works, II. 122). In this memoir he described as vital air or eminently respirable air the gas obtained by heating the red calx of mercury, and confirmed Priestley's observations as to its properties, but without making any reference to the source from which he had obtained his first information as to the behaviour of the calx.
In his " Elementary Treatise on Chemistry,'' published in 1789, Lavoisier described a series of experiments on the calcination of mercury {Works, I. 35-38) which display to the full his genius for exact and careful measurements. In order to study the part that air played in the formation of the red calx, he placed four ounces of pure mercury in a retort, the neck of which was bent so that it passed up into a bell-jar of air inverted over mercury (Fig. 16). The total volume of air thus enclosed in the retort and the bell-jar amounted to 50 cubic inches. He then heated the retort, and observed the formation of a red scale on the
THE DISCOVERY OF OXYGEN
41
surface of the mercury contained in it. At the end of twelve days, when further heating did not cause tlie forma- tion of any more of the red scale, the retort was allowed to cool. The mercury rose in the bell-jar and the total volume of air was found to have been reduced to 42 or 43 cubic inches, a loss of 7 or 8 cubic inches. The calx on the mercury was collected and found to weigh 45 grains. " The air which remained after this operation, and which had been reduced to five-sixths of its volume by the calcination of the mercury, was no longer fit for respiration nor combustion ; since animals introduced into it perished in a few moments, and lights were extinguished in it at once, as if they had been plunged into water " (loc. cit. p. 37).
As the mercury calx (unlike the black calx from the tin) could be decom- posed by heating it, Lavoisier trans- ferred the 45 grains to a small retort and ob- tained from its de- composition 41^ grains of mercury, and 7 to 8 cubic inches of gas, which was " much more fit than atmospheric air to support respiration and combustion," since " a candle plunged into it, gave out a dazzling light; charcoal, instead of burning quietly as in ordinary air, burnt with a flame .... and a brightness of light which the eye could scarcely bear" {loc. cit. pp. 37-38).
From these experiments it was clear that the whole of the 7 or 8 cubic inches of air absorbed in the first experi- ment had been liberated in an intensely active form in
Fig. t6— Lavoisier's Apparatus for Heating Mercury in a Limited Volume of Air. No illustration is given of the retort u.sed afterwards to decompose the calx.
42 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP.
the second. A further experiment showed that a mixture of 8 cubic inches of the respirable air with 42 cubic inches of non-respirable air behaved in all respects like ordinary air. There was, therefore, no doubt that the air which disappeared during calcination had actually combined with the mercury, and had been released from it by heating it more strongly. •
In order to complete the proof it was only necessary to show that the loss in weight of the calx (45 grains), when reconverted into mercury (41 J grains), was equal to the weight of the gas which had been liberated : a knowledge of the density of the active gas showed that the quantity collected would weigh 3!^ to 4 grains, agreeing closely with the loss of 3J grains already recorded.
Lavoisier completes his " oxygen " theory of com- bustion.— Lavoisier's proof was now complete : he had shown that the calcination of a metal meant the combina- tion of the metal with an active constituent of the air, which in the case of mercury could be recovered from the calx by heating it ; the calx was, therefore, a compound, and the combustible metal a simpler substance. The same constituent of the air was required for the burning of fuel and for respiration, two processes which differed from the burning of metals mainly in giving rise to gaseous instead of solid products (see Chapter VI).
Lavoisier showed that the same constituent of air was also concerned in the burning of sulphur and of phosphorus ; but the products of combustion differed completely from the metallic calces, dissolving readily in water and producing acid solutions. Lavoisier regarded his active gas as an essential constituent of these and of all other acids. Hs therefore selected for it the name oxygen or " acid-pro- ducer" (compare German, sauerstoff), deriving it "from the Greek words o^ws, add, and yciVo/i.ai, T beget, on account of the property of this principle, the basis of vital air, to
Ill THE DISCOVERY OF OXYGEN 43
change a great many of the substances with which it unites into the slate of acid, or rather because it appears to be a principle necessary to acidity" {Chemical Nomenclature, p. 24 ; compare Works, II. 249, where the name " oxygen " is first used ) This name has been retained to the present day in spite of the discovery (see Chapter XII) of acids in which no oxygen is present. The compounds in which oxygen was present were described as oxides ; under this title were included, not only the acid oxides of sulphur and phos- phorus, but also the basic calces derived from the metals.
By making use of the discovery of oxygen, Lavoisier had been able to explain all the main facts on which the phlogiston theory was based ; the phlogiston theory was therefore no longer necessary, and gradually ceased to be used. Lavoisier's theory was completed in the year 1777. It won its first converts nearly ten years later, when it was accepted by de Morveau, Fourcroy, and Berthollet. The " System of Chemical Nomenclature " which they issued in 1787 was one of the first manifestoes of the new faith.
Scheele's discovery of oxygen. — The separation of the gases of the atmosphere was accomplished independently by the Swedish chemist Scheele in a research, " On Air and Fire " (A. C. R. VIII), completed about the same time as Priestley's work, but not published until 1777, and not known either to Priestley or to Lavoisier at the time when they were carrying out their experiments.
Scheele's experiments led him to the belief that common air consisted of two gases, one of which could be removed by various substances. Thus when damp iron filings rusted in air, in a tightly closed bottle which was afterwards opened under water, about a quarter of the air was found to be absorbed. Similar results were obtained when phosphorus was burnt in a thin flask tightly corked, or when it was simply allowed to stand for six weeks in a closed flask until it ceased to glow. The part of the air remaining at the
44 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
close of these experiments was slightly less dense than ordinary air, and did not allow a candle to burn in it, or even the smallest spark to continue glowing. This inactive portion of the air was separated by no fewer than sixteen different methods, but the recovery of the "lost air," as Scheele called it, was a problem of much greater difficulty. Scheele actually obtained it from a variety of substances, amongst which was nitric acid, thus confirming the view of Hooke and Mayow as to the close relationship existing between nitre and air. He found that it could be prepared most readily either from nitre (saltpetre) or from common RED PRECIPITATE (obtained by dissolving mercury in nitric acid and heating the residue until the product became red) which he proved to be identical with calcined mercury (red oxide of mercury).
Having separated this pure fire-air Scheele repeated the experiments which he had carried out with ordinary air and proved that the substances which diminished the volume of ordinary air when left or burnt in it, were capable of absorbing " fire-air " entirely. For example, phosphorus, heated in a closed flask filled with " fire-air," burnt with remarkable brilliancy ; on allowing the flask to cool and opening it under water, it was found that water entered the bottle and filled it almost completely. Scheele showed that his " fire-air " (Lavoisier's " oxygen " ) was slightly denser than air, and soluble in water. He also proved it to be essential to the breathing of animals, and the growth of plants. By mixing it with 3^ times its volume of " foul air," left after burning phosphorus in a closed vessel, there was obtained a gas which in every respect resembled common air. Scheele rightly concluded that common air is a mixture of " fire-air " with about four times its volume of " foul air."
Azote. — The residue left after removing the oxygen from air was described by Priestley as " phlogisticated air," by
in THE DISCOVERY OF OXYGEN 45
Scheele as "foul air" {Air and Fire, p. 54), whilst Lavoisier usually called it the "atmospheric mofette " ; the French chemists in 1787 {Chemical Nomenclature, p. 26) described it as AZOTE, " from the Greek privative a and ^cdtJ, life" in order to indicate its inability to support life. It was shown by Cavendish to consist mainly of a gas, present in nitre, to which Chaptal gave the name nitrogen. (See Chapter X.)
Summary and Supplement.
Jean Rey, in 1630, " On an enquiry into the cause wherefore tin and lead increase in weight on calcination," concluded that the gain in weight on calcining tin, lead, and antimony must be due to condensation of air.
Robert Boyle, in 1673, i" ^'^ essay entitled " New Experi- ments to make Fire and Flame stable and ponderable," proved that copper, iron, tin and lead gained in weight when burned, but attributed this to the absorption of igneous particles. I'his materialistic view of the nature of fire was elaborated in the " phlogiston theory " of Becher and Stahl ; they assumed that combustion and calcination involved an escape of phlogiston, whilst in smelting an ore, or calx, the addition of phlogiston from the fuel revivified the metal.
Robert Hooke, in 1665, in his " Micrographia," and John Mayow, in 1674, in his " Medico-physical Essays," suggested that air must contain an active principle similar to nitre ; Mayow described this as " spiritus nitro-aereus," or " nitro-aerial spirit." Mayow showed that damp gunpowder would continue to burn under water. He made experiments with air trapped over water, and showed that the volume of air was diminished by the burning of a candle or of camphor, as well as by the breathing of a mouse.
Antoine Laurent Lavoisier, in 1774, "Memoir on the Calcination of Tin in Closed Vessels," showed that a similar decrease of volume took place when lead and tin were heated by a burning-glass in air confined over water or mercury. He repeated the experiments in which Boyle had calcined tin in sealed glass flasks, but proved that no change in weight took place until the flask was opened ; as the flask did not change in
46 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
weight, the gain in weight of the tin must have been accom- panied by the disappearance of an equal weight of air. When the flask was opened a quantity of air rushed in, the weight of which was almost equal to that gained by the tin ; this showed that the part of the air absorbed by the tin (about ^) did not differ greatly in density from the air outside.
Joseph Priestley, in 1774, isolated the active part of the air by heating the red calx of mercury, and showed that it was a brilliant supporter of combustion and maintained respiration much longer than ordinary air. He regarded it as air minus phlogiston, and called it " dephlogisticated air." Lavoisier then carried out quantitative experiments on the calcination of mercury, in which he showed that the volume of gas set free when the calx was heated was identical with the volume of air absorbed in its preparation, whilst the weight of the gas set free was equal to the loss in weight of the calx when reconverted into metal.
Since sulphur and phosphorus were converted into acids by combination with the active part of the air, Lavoisier gave to it the name oxygen, i.e., acid-producer, the same idea being implied in the expressive German name, sauerstoff.
Oxygen was a:lso prepared independently about 1774 by Carl Wilhelm Scheele, who obtained it first by heating nitric acid and nitre. He proved in many ways that air was a mixture of an active and an inactive constituent, and showed that ordinary air could be reproduced by adding " fire-air," or oxygen, to the " foul air " remaining after rusting or burning had taken place in it.
The chemical changes described in this chapter may be represented by the following equations : —
Calcination of tin . Sn-H02 = Sn02 (white calx).
(Stannic oxide.)
2Sn-(-02 = 2SnO (black calx).
(Stannous oxide.)
„ ,, lead . 2Pb-f-02=2PbO (yellow calx).
(Litharge.)
5j „ copper. 2Cu + 02 = 2CuO (black calx).
(Cupric oxide.)
THE DISCOVERY OF OXYGEN 47
Calcination of iron . 3Fe + 202=Fe304 (blue-black scale or
trTagneti(
3xide of
iron.)
(Magnetic calx.) oxide of
Preparation of red oxide of mercury — {a) " Mercurius calcinatus per se "
2Hg + 02 = 2HgO,
{b) " Red precipitate "
3Hg+8HN03=3Hg(N03)2 + 2NO + 4H20.
(Mercuric (Nitric
nitrate.) oxide.)
2Hg(N03)2 =2HgO + 4NO2 + O2. (Mercuric (Nitrogen oxide.) peroxide.)
Decomposition of red oxide of mercury—
2HgO = 2Hg+02.
„ „ nitric acid —
4HN03 = 2H20 + 4N02 + 02. „ „ nitre—
2KN03 = 2KN02+02. (Potassium (Potassium nitrate.) nitrite.)
Red lead contains more oxygen than litharge, but has not a definite composition, and cannot be represented by any exact formula. Its preparation and decomposition may be shown thus —
2PbO+.r02 = 2PbOn.„
2PbOi + ;,=2PbO+;ir02,
where .*• is a fraction which is always less than 05.
CHAPTER IV
CHALK, LIME, AND THE ALKALIS
A. Chalk and Lime
The burning of chalk to lime. — The changes wrought by the action of fire have been described (Chapter I, p. 7) as leading at an early period to the preparation of tin from tinstone, iron from ironstone, and so forth. In addition to these metallic ores, a number of rocks were known which were converted by burning into a caustic substance, known as LIME, which was used in the preparation of mortar. Joseph Black (1728-1799), an Edinburgh physican who afterwards occupied the chairs of chemistry at Glasgow and at Edinburgh, was one of the first to make a careful study of these substances. He regarded chalk as the typical source of lime, and included in the calcareous class of substances " all those that are converted into a perfect quick-lime in a strong fire, such as limestone, marble, chalk '' and " those spars and marls which effervesce with aqua fortis " (A.C.R. I. 10).
The substance formed by heating chalk has remarkable properties. When brought into contact with the skin, it produces blisters and wounds resembling those caused by fire. Hence it was known as a caustic. This property has long been used for removing the hairs from hides in the manufacture of leather. The lime drawn from the kilns is
CH. IV CHALK, LIME, AND THE ALKALIS 49
described as quicklime because it becomes hot and steamy when water is poured upon it ; the soft powder formed when the hard lumps of quicklime are wetted with water, or are left exposed to the air, is known as slaked lime. Slaked lime dissolves easily in acids and to a slight extent in water ; the solution in water is known as lime-water.
At the time when Black began his investigations, it was generally believed that the caustic properties of lime were due to the absorption of igneous particles from the fire in which it had been burnt ; this view, like the phlogiston theory of calcination, was found to be untenable as soon as exact quantitative measurements were made. Black's experi- ments were described in a paper " Experiments upon Magnesia Alba, Quick-lime and some other Alkaline Substances" (A. C. R. No. I.) published in 1755. They were of importance on account of his success in solving the problem of the relationship between chalk and lime, and also because of the stimulus that they gave to exact quantitative work. The principal points of this investigation are set out in the following paragraphs.
Chalk loses in weight when burnt to lime owing to the escape of fixed air. — Black found that "a piece of perfect quick-lime made from two drams of chalk . . . weighed one dram and eight grains " (A. C. R. I. 28), i.e., 120 grains of chalk gave 68 grains of lime, a loss in weight of 52 grains, or nearly 44%. This large loss in weight could only be due to the escape of a gas, since nothing but a little water could be condensed, when chalk was burnt to lime in a retort.
" That the calcareous earths really lose a large quantity of air when they are burnt to quick-lime, seems sufficiently proved by an experiment of Mr. Margraaf. . . He subjected eight ounces of [chalk] to distillation in an earthen retort, finishing his processs with the most violent fire of a reverberatory, and caught in the receiver only two drams of
e
50 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
water, which by its smell and properties shewed itself to be slightly alkaline " (A. C. R. I. 23).
Black had himself carried out the experiment of heating magnesia (a compound resembling chalk, but more easily decomposed by heat), and had " found only five drams of a whitish water in the receiver," although the three ounces (24 drams) of magnesia in the retort " had lost more than the half of its weight " (A. C. R. I. 15).
From these experiments it was clear that the burning of chalk to lime resulted in the escape of an invisible gas ; to this gas Black applied the name fixed air. He regarded chalk as " a peculiar acrid earth rendered mild by its union with fixed air." According to his view : —
" When the calcareous earths are exposed to the action of a violent fire, and are thereby converted into quick-lime, they suffer no other change in their composition than the loss of a small quantity of water and of their fixed air. The remarkable acrimony which we perceive in them after this process, was not supposed to proceed from any additional matter received in the fire, but seemed to be an essential property of the pure earth " (A. C. R. I. 2 2).
The contrast between Black's theory and that which it replaced may be expressed by the equations :
Old theory : Limestone + phlogiston = lime. New theory : Limestone -fixed air = lime.
Lime combines with water and with fixed air. — In
the process of slaking, lime combines vigorously with water to form slaked lime ; but in presence of fixed air it releases the water, recombines with the fixed air and is reconverted into chalk. Black writes : —
" A calcareous earth deprived of its air, or in the state of quick-lime, greedily absorbs a considerable quantity of water, becomes soluble in that fluid, and is then said to be slaked : but as soon as it meets with fixed air it is supposed to quit
DR. A. J. PACINI
IV CHALK, LIME, AND THE ALKALIS 51
the water and join itself to the air, for which it has a superior attraction, and is therefore restored to its first state of mildness and insolubility in water " (A. C. R. I. 24).
The conversion of soluble lime into insoluble chalk was thus a test for the presence of fixed air. The test was most sensitive when lime-water was used, because the chalk could then be seen immediately. But in all important cases Black preferred to rely on quantitative experiments in which the chalk, after burning to lime, was recovered in such a way as to show that the original weight of chalk had been reproduced.
Fixed air is present in common air and in air dissolved by water. — By noticing its action upon lime, Black was able to show that a small quantity of fixed air was dissolved in ordinary water, since : —
'• When slaked lime is mixed with water, the fixed air in the water is attracted by the lime, and saturates a small portion of it, which then becomes again incapable of dis- solution, but part of the remaining slaked lime is dissolved and composes lime-water " (A. C. R. I. 24).
The presence of fixed air in common air was also shown by its action on lime-water, for : —
" If this fluid be exposed to the open air, the particles of quick-lime which are nearest the surface gradually attract the particles of fixed air which float in the atmosphere. But at the same time that a particle of lime is thus saturated with air, it is also restored to its native state of mildness and insolubility ; and as the whole of this change must happen at the surface, the whole of the lime is successively collected there under its original form of an insipid calcareous earth, called the cream or crusts of lime-water " (A. C. R. I. 24).
The fixed air forms, however, only a small proportion both of ordinary air and of the air which is dissolved in water, for(i) "lime-water, which soon attracts air, and forms
E 2
52 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
a crust when exposed in open and shallow vessels, may be preserved, for any time, in bottles which are but slightly corked, or closed in such a manner as would allow free access to elastic air, were a vacuum formed in the bottle," and (2) under an exhausted receiver the same quantity of air escapes from common water, and from water from which the fixed air has been removed by the addition of lime (A. C. R. I. 30).
Fixed air is liberated from chalk by the action of acids. — Although Black was able to show that the loss of weight in burning chalk to lime was due to the escape of an invisible gas, it did not occur to him to attempt to collect the gas, or even to render it visible by causing it to bubble through water. But no special methods were needed to render obvious the " violent breaking out of air " which takes place when chalk is acted on by acids, nor the absence of this effervescence when well-burnt quick-hme is substituted for chalk. Remembering that Chalk = lime + fixed air, the contrast between the action of acids in the two cases was sufficient to suggest that the gas so easily set free by acids must be the same as the fixed air which escapes when chalk is burnt to lime.
It is remarkable that Black did not attempt to test the gas by passing it into lime-water,^ but apparatus such as is now used for this purpose had not yet been devised, and the systematic study of gases did not begin until nearly twenty years later.
A very satisfactory proof of the identity of the two gases was, however, obtained by showing (r) that the weight of gas which escapes is the same, whether the chalk is decom- posed by acids or by heating it in a furnace ; and (2) that the same weight of acid is required to dissolve the chalk
' See footnote, p. 385, for Black's use of lime-water as a test for 6xed air.
IV CHALK, LIME, AND THE ALKALIS 53
before and after burning it to lime. In the first experi- ment Black saturated 120 grains of chalk with diluted spirit of salt in a long-necked Florentine flask, and found a loss in weight of 48 grains, as compared with 52 grains when an equal weight of chalk was burnt to lime. In the second experiment he found that 120 grains of unburnt chalk were dissolved by 421 grains of diluted spirit of salt, whilst the same quantity of chalk, burnt to quick-lime and slaked with an ounce of water, required 414 grains of the acid, but dissolved "without any sensible effervescence or loss of weight" (A. C. R. I. 28).
Fixed air is present in mild alkalis such as soda and potash. — It had been known from early times that the mild alkalis resembled chalk in that they effervesced when acted on by acids. This property was regarded as a chief characteristic of the alkalis, — an idea that is preserved in the use of the word " kali " to describe a mixture of sugar with a mild alkali and a solid acid, which gives an effervescent drink when added to water. As in the case of the action of acids on chalk, Black did not test the gas by passing it into lime-water, but he obtained an even more satisfactory proof of the presence of fixed air in the alkalis by showing that they could reconvert lime into chalk quantitatively. This process had the advantage that it rendered the use of an acid quite unnecessary in proving the composition of the alkalis.
" A piece of perfect quick-lime made from two drams of chalk, and which weighed one dram and eight grains, was reduced to a very fine powder, and thrown into a filtrated mixture of an ounce of a fixed alkaline salt, and two ounces of water. After a slight digestion, the powder being well washed and dried, weighed one dram fifty-eight grains.^ It was similar in every trial to a fine powder of ordinary
^ i.e. One hundred and twenty grains of chalk gave 68 grains of lime from which u8 grains of chalk were recovered by the action of an alkali.
54 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
chalk, and was therefore saturated with air which must have been furnished by the alkali " (A. C. R. I. 28).
B. The Alkalis.
Mild alkalis are made caustic by the removal of fixed air. — The preparation of caustic alkalis by the action of caustic lime on the mild alkalis, soda and potash, was described by Geber. The caustic liquid ^ thus prepared was used from an early period in the manufacture of soap by the action of the hot alkali on fat ; it was, therefore, generally called a " soap lye." Its caustic properties had been attributed to lime dissolved in it ; but Black was not able to prepare from it any trace either of chalk or of gypsum ; he therefore concluded that " the acrimony of the caustic alkali does not depend on any part of the lime adhering to it" (A. C. R. I. 26).
On the other hand, Black found that a mild alkali made caustic by lime was acted on by acids " without the least effervescence or diminution of weight" (A. C. R. I. 32) nor did it produce more than a slight cloudiness with lime-water. The caustic alkali, when properly prepared, was thus free both from lime and from fixed air.
The action of lime in rendering the alkali caustic was therefore due, not to its ability to dissolve in the liquid or to impart to it some fiery principle which it had acquired in the lime-kiln, but on the contrary to its property of remov- ing from the alkali the fixed air which had rendered it mild.
Caustic alkalis are rendered mild by exposure to air. — Black found that after a fortnight's exposure to the air in an open shallow vessel his caustic alkali " became entirely mild, effervesced as violently with acids, and had the same effect upon lime water as a solution of an ordinary alkali"
' "A very hellish spirit, in which great mysteries lie hid" (Basil Valentine, Last Will and Testafnent^ p. 302).
IV CHALK, LIME, AND THE ALKALIS 55
(A. C. R. I. 32). The caustic alkali therefore resembled slaked lime in its power of absorbing fixed air from the atmosphere, but differed from it in its much greater solubility in water.
On account of their great solubility the caustic alkalis had not been isolated previously. Black attempted to separate them by evaporating the caustic lye in an earthen- ware bowl, but found that the inside of the bowl became corroded and pitted with holes. By using a silver dish, however, he succeeded in evaporating all the water, and obtained the caustic potash as a fused mass which solidified on cooling, but dissolved again on adding a small quantity of water (A. C. R. I. 33).
At a later date, strong solutions of the caustic alkalis were used in order to absorb (and so to collect and weigh) fixed air from various sources, the greater concentration of the solutions enabling them to take up much more fixed air than in the case of lime-water; but Black, at that date, did not understand the manipulation of gases, and had no opportunity of making use of this valuable quality.
The alkalies are not easily decomposed by heat. — From the above observations it was clear that the relation- ship between the mild and caustic alkalis was much the same as that between chalk and slaked lime. Soda and potash were closely analogous to chalk, from which they differed chiefly in being soluble in water; whilst caustic soda and caustic potash were similar to slaked lime, but differed from it in their extraordinary solubility in water.
As potash parts with its fixed air so readily when mixed with lime, it seemed probable "that alkalis might be entirely deprived of their air, or rendered perfectly caustic, by a fire somewhat weaker than that which is sufficient to produce the same change " in chalk (A. C. R. I. 37). This was found not to be the case : potash was " exposed, for several hours, in a covered crucible," to the action of a
56 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
strong fire, black lead being added to soak up the potash and so prevent it from corroding the vessel ; the potash was found to " lose a part of its air, and acquire a degree of causticity," but this was only slight, and it was not found possible to convert the alkalis into a perfect caustic by the action of heat alone. Black points out, however, that " the alkali newly obtained from the ashes of vegetables is generally of the more acrid kind " (A. C. R. I. 39) owing to its partial conversion into a caustic alkali during calcination.
The caustic alkalis, prepared in the form of solutions, evidently correspond with slaked lime rather than with quick-lime. But they are even less easily decomposed by heat than the mild alkalis ; the form of the alkali corre- sponding with quick-lime is therefore very difficult to prepare, and was not discovered until Davy (Chapter XII) had succeeded in isolating from the alkalis the very refractory metals which they contain.
Sal volatile is a mild alkali : spirit of hartshorn is a caustic alkali. — It had long been known that sal ammoniac, the volatile salt described in Chapter I, could be converted into a volatile alkali, sal volatile, by heating it with soda or potash. This volatile alkali resembled the fixed alkalis in that it effervesced when acted upon by acids. In this action the volatile alkali was reconverted into a salt ; for instance, sal ammoniac could be formed from sal volatile and spirit of salt. These facts were used by Mayow in 1674 to prove that the alkali as well as the acid still exists in the salt prepared by mixing them.
"Although [acids] and alkalis pass into a neutral substance when they meet, yet they do not, as is generally supposed, entirely destroy each other. For example, when the acid spirit of salt is coagulated with a volatile [alkali] .... although the mixed salts seem to be des- troyed, yet they may be separated from each other with
IV CHALK, LIME, AND THE ALKALIS 57
their forces unimpaired, as takes place when sal ammoniac .... is distilled with salt of tartar [i.e., potash] (A. C. R. XVII. 160).
When sal ammoniac is acted on by lime a pungent gas is liberated, to which, at a comparatively late date, the name of ammonia was given. During the alchemistic period this pungent vapour was (like spirit of salt) known only in the form of a solution which was called the volatile spirit OF SAL AMMONIAC. It was also prepared by distilling horn, and was therefore known as spirit of hartshorn.^ Priestley, who was the first to collect spirit of salt as a gas over mercury, was also the first to collect ammonia in the gaseous state (see Chapter V).
Black showed that the "mild spirit of sal ammoniac" (described above as sal volatile) could be rendered caustic by removing its fixed air with the help of magnesia; it then "emitted a most intolerably pungent smell," effervesced only slightly with acids, and produced only a turbidity when mixed with lime-water. It was therefore evident that the pungent gas was a caustic alkali which could be rendered mild by union with fixed air. Final proof of these facts was supplied when Priestley showed that gaseous ammonia could be combined with fixed air to produce solid sal volatile, whilst with gaseous spirit of salt it united to form sal ammoniac.
C. Preparation of New Earths.
Earths can be prepared from salts by the action of alkalis. — At the close of Chapter II it was shown how the
' "Of the spirit and oil of Harts-hom. — Take Harts-horn, cut it with a saw into pieces, of the bigness of a finger, and cast in one at d, time into the aforesaid distilling vessel, and when the spirits are settled, then another, and continue this until you have spirits enough : and the vessel being filled with the pieces that were cast in, take them out with the tongs, and cast in others, and do this as often as is needful." — (Glauber, Philosophical Furnaces, Part II. ; Works, I. 51)
58 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
discovery of the acids led to the preparation of a number of new salts. It may now be pointed out how the alkalis were used from a very early period to prepare earths, or bases, from the salts in which they were combined. Thus it was found that gold, which could not be converted into a calx by heating, was transformed into an earth by dissolving it in aqua regia and precipitating again with an alkali. This earth was decomposed when heated, leaving behind a residue of " revivified " metal. By precisely similar methods, silver (which likewise resisted calcination) was converted into an earth from which the metal could be recovered by the action of heat. At a much later date these unstable earths were used by Scheele for the preparation of oxygen. The earth prepared by precipitating gold from its solutions by means of the volatile alkali, exploded violently when heated, and was called auru.m fulminans or fulminating GOLD ; its composition is discussed in Chapter XII.
These actions were studied by Mayow, who suggested that the metallic earth was precipitated because the acid had left it in order to combine with the stronger alkali, just as in the action of oil of vitriol on nitre the alkali of the nitre left it in order to combine with the stronger acid of the vitriol.
Preparation of magnesia from Epsom salts. — The paper "On Magnesia Alba,'' in which Black's experiments on chalk and lime are described, derived its name from a white earth, prepared by the action of alkalis on a variety of saline liquids, and used as a mild aperient. It was first made from MOTHER OF NITRE, the mother liquor left after crystallising out saltpetre ; it was here present in combination with nitric acid, which could be removed either " by the addition of an alkali which attracted the acid to itself" or "by exposing the compound to a strong fire in which the acid was dissipated" (A.C.R. I. 6). Black prepared it from "the bitter saline liquor called bittern, which remains in the
IV CHALK, LIME, AND THE ALKALIS 59
pans after the evaporation of sea-water," but he " afterwards made use of a salt called epsom salt, which is separated from bittern by crystallisation, and is evidently composed of magnesia and the vitriolic acid " (A. C. R. I. 7).
The magnesia prepared with the help of an alkali effervesced with acids in just the same way as chalk, but gave rise to a totally different series of salts ; in particular it was dissolved by oil of vitriol, reproducing the Epsom salt from which it had been prepared, instead of giving a sparingly soluble residue of gypsum.
Like chalk, it was decomposed by heat, its weight decreasing to a remarkable extent. The product resembled lime in dissolving without effervescence in acids, but differed from it in that it could not be slaked, and was not soluble in water.
" An ounce [480 grains] of magnesia was exposed in a crucible for about an hour to such a heat as is sufficient to melt copper. When taken out, it weighed three drams and one scruple [200 grains], or had lost 7/12 of its former weight."
" I repeated, with the magnesia prepared in this manner, most of those experiments I had already made upon it before calcination, and the result was as follows :
" It dissolves in all acids, and with these composes salts exactly similar to those described in the first set of experiments : but what is particularly to be remarked, it is dissolved without any the least degree of effervescence " (A. C. R. I. 14).
This decomposition could be effected in a glass retort, but although the magnesia " lost more than the half of its weight," only 5 drams of water could be collected from 24 drams of the earth.
" We may, therefore, safely conclude, that the volatile matter lost in the calcination of magnesia, is mostly air ; and hence the calcined magnesia does not emit air, or make an effervescence, when mixed with acids."
6o HISTORICAL INTRODUCTION TO CHEMISTRY chap.
The work upon " magnesia " was of special importance becaus^ it provided Black with the knowledge which enabled him to solve the more difficult problem of the relationship between chalk and lime.
Preparation of an earth from alum. — Black also made use of the alkalis in order to separate from alum an earth to which the name of alumina was afterwards given. This earth is present in the alum as a vitriol, or sulphate. It does not combine with fixed air, and this gas is therefore set free when the alum is precipitated by means of a mild alkali.
Preparation of earths from salts by heat — A second method of preparing earths from salts, which was practised from the earliest period of alchemy, depended on dissipating the acid of the salt by heat. When salts were thrown into a charcoal fire, or were distilled from a retort, it was often found that an acid vapour escaped, leaving behind an earthy residue to which the name of caput mortuum was - given. Amongst the salts distilled in this way were green vitriol and alum, both used in the preparation of oil of vitriol ; the residue of " colcothar " from the green vitriol was a red-brown earth somewhat resembling rust ; the alum left behind a whitish residue of alumina. The nitrates were also frequently decomposed, as for instance, in the preparation of " red precipitate " by heating nitrate of mercury. Black prepared magnesia in this way from mother of nitre ; and Boyle in 1680 (M^orks, 1725, III, 372) prepared an alkah from sea-salt by converting it into a nitrate and igniting this with charcoal. As a further illustration, it will be shown in a later chapter that fixed air may be regarded as an acid, and the decomposition of chalk by heat as that of a salt into acid and base.
The method of decomposing salts by heat, although very simple, was not always available. Some salts were not changed in the fire ; others (such as sal-ammoniac and salt) were dissipated without leaving any residue : in the case of
IV CHALK, LIME, AND THE ALKALIS 6i
silver and gold the residue left on heating a salt often con- sisted of the revivified metal. In cases such as these the base of the salt could only be separated with the help of an alkali.
Summary and Supplement a. chalk and lime
Joseph Black (1728-1799), in his "Experiments upon Mag- nesia Alba, Quick-lime, and some other Alcaline Substances," read in June 1755, showed
(i) That chalk loses in weight by about 44 per cent, when burnt to lime.
(2) That, as nothing but a trace of water could be condensed by cooling the vapour (Margraaf), this loss in weight must be due to the escape of gas, to which he gave the name fixed air.
(3) That lime combines with water to form slaked lime, but releases it when it re-combines with fixed air to form chalk.
(4) That fixed air is present in small quantities in common air and in air dissolved in water.
(5) That fixed air is liberated from chalk by the action of acids ; apart from this liberation of gas, the action of acids upon chalk and lime is identical both qualitatively and quantitatively.
These changes may be represented by the following equa- tions :
Buj-ning of chalk, CaCOs -> CaO-l-COj
chalk -> lime -|- fixed air
(Calcium (Calcium (Carbon
carbonate.) oxide.) dioxide.)
Slaking of lime, CaO-1- OHj -» Ca(0H)2 lime 4- water -> slaked lime
(Calcium hydroxide.)
Action affixed air on slaked lime —
Ca(OH)2 + CO2 -^ CaCOj-l-OHa slaked.lime-l-fixed air -> chalk -1- water
Action of acids on chalk, slaked lime, and quicklime —
j- CaCOj 4- HjSOi -» CaS04 -I- HjO -I- COj I. Oil of vitrioll Ca(OH)2-fH2S04 -» CaS04-l-2H20 ICaO -I-H2SO4
62 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
In each case, if dilute acid is used, solid gypsum or selenite, CaS04,2H20, is produced.
rCaCOa +2HCI -> CaClj + HjO+CO, 2. Muriatic acid\z^{On\-^^Yi.C\ -> CaClj + aHzO ICaO +2HCI -> CaClj+HjO
In each case a solution of muriate of lime (calcium chloride, CaCy is produced.
B. THE ALKALIS Boyle recognised an alkali by the facts that " it had a fiery taste upon the tongue," that it would " make an ebullition with acid spirits and precipitate diverse spirits made with them " and would " turn syrup of violets green " {Producibleness of Chymical Principles, 1680, pp. 35 and 37 ; Works, 172c,, III. 372-373).
Black's observations on the alkalis are set out with modem equations in the following paragraphs :
1. Mild alkalis contain fixed air since they are able to re-convert lime into chalk —
K2CO3 + Ca(OH)2 -> 2KOH + CaCOj potash + slaked lime -> caustic potash-)- chalk
(Potassium (Calcium (Potassium (Calcium
carbonate.) hydroxide.) hydroxide.) carbonate.)
2. The caustic alkali which is produced in this action con- tains neither lime nor fixed air. It is very corrosive, but can be separated by evaporating in a silver dish.
3. The caustic alkali absorbs fixed air from the atmosphere, and is rendered mild thereby —
2KOH-I-CO2 -^ KjCOa-FHjO.
4. The caustic alkalis, unlike the mild alkalis, do not effer- vesce with acids, e.g.—
KOH -t- HCl -^ KCl -fHjO
caustic potash -f muriatic acid -> sal sylvii-f water K2CO3-I- 2HCI -» CO, -I- 2KCI + H2O potash -^muriatic acid -s>fixed air-t-sal sylvii + water
5. Sal volatile is a mild alkali, which can be rendered caustic by means of lime —
(NH3)2C02-l-CaO-> 2NH3 -l-CaCOs sal volatile -I- lime -> ammonia-)- chalk
(Ammonium carbamate.)
IV CHALK, LIME, AND THE ALKALIS 63
C. PREPARATION OF EARTHS.
Black prepared magnesia alba (magnesium carbonate, MgCOj) by the action of alkalis on —
1. Mother of nitre (containing magnesium nitrate),
Mg(N03)2+K2C03 -$• MgCOj + aKNOs
2. Bittern (containing magnesium chloride),
MgClj+KjCOs -» MgC03 + 2KCl.
3. Epsom salts (magnesium sulphate),
MgS04+K2C03 -» MgC03 + K2S04.
This magnesia is easily decomposed by heat, liberating fixed air and losing more than half its weight,
MgC03 -> MgO + COa. It effervesces with acids, but after it has been burnt it dis- solves without effervescence,
MgC03+H2S04 -> MgSOi+HjO + COj MgO + H2S04 -» MgSOi + HjO
Magnesia, free from fixed air, was also prepared by igniting the nitrate,
2Mg(N03)2 -^ 2MgO-|-4N02-l-Oa.
CHAPTER V
THE STUDY OF GASES
A. Fixed Air and Inflammable Air
Van Helmont recognises the existence of gases diffe- ring from ordinary air. — The fact that metals, insoluble in most liquids, are dissolved by acids, was known and used from very early times. The liberation of gases during this process must have been noticed from the first, but it was not until the latter part of the seventeenth century that any attempt was made to collect and examine these volatile products. Van Helmont (1577-1644), to whom we owe the name of " gas," recognised the existence of a poisonous GAS SYLVESTRE, i.e. " wood-gas,'' which possessed the power of extinguishing a lighted candle ; he detected it in the air of a cavern, in the fumes from a charcoal fire, and as a product of the fermentation of wine and beer ; he also recognised what he thought to be the same gas as a product of the action of nitric acid on silver, and of distilled vinegar on chalk. In contrast to this, he found in the large intes- tine, and as a product of the fermentation of dung, an inflammable gas to which he applied the name gas pingue. The names used by van Helmont were applied broadly to two types of gas, analogous with the " choke-damp '' and " fire-damp " of miners ; no attempt was made to distinguish
64
V THE STUDY OF GASES 65
between one inflammable gas and another, or between one poisonous gas and another.
Gases collected over water by Mayow (1674).— A few years later, about 1674, the familiar method of collecting gases over water was described by John Mayow, who used it both for investigating the reduction in volume of air during burning and breathing, and for examining gases prepared artificially.^ For the latter purpose Mayow used a flask inverted in a trough filled with diluted oil of vitriol (Fig. 14, p. 33). The gas was produced by the action of the acid on two or three iron spheres in the neck of the flask. When nitric acid was used, much of the gas dissolved in the liquid, and repeated action of the acid on the metal was needed to fill the flask with gas. With oil of vitriol no such contraction occurred. Mayow proved that these gases possessed the same elastic properties as air,^ and that, when added to a limited volume of ordinary air, they did not in any degree prolong the life of a mouse confined in it ; but no other tests were made, and Mayow could not be certain "whether air of this kind is really common air or not" (A. C. R. XVII. 113).
Cavendish prepares " Factitious Air '' (1766). — Nearly one hundred years intervened between the experiments of Mayow and the appearance in the Philosophical Transac- tions of the Royal Society ior 1766 of a remarkable series of three papers by the Honourable Henry Cavendish (1731-1810) entitled "Experiments on Factitious Air." Black had already investigated the part played by " fixed air " in the burning of chalk to lime, and in other related processes. But although he showed that fixed air was present in the atmosphere, he did not attempt to collect or
• As described by Boyle in 1660 {Works, 1725, II. 432).
" Boyle's tract " Touching the Spring of Air and its Effects" (1660) had recently appeared, so that this point was of special interest at the time (see Chapter XV. p. 320).
F
66 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
handle it. It was therefore left to Cavendish to make the first careful examination of those gases whose existence had been vaguely recognised during the preceding one hundred and fifty years. Cavendish writes :
" By factitious air, I mean in general any kind of air which is contained in other bodies in an unelastic state, and is produced from thence by art."
" By fixed air, I mean that particular species of factitious air, which is separated from alkaline substances by solution in acids or by calcination ; and to which Dr. Black has given that name in his treatise on quick-lime (Phil. Trans., 1766, 56, 141).
Describing next the action of acids on metals he writes :
" I know of only three metallic substances, namely, zinc, iron and tin, that generate inflammable air ^ by solution in acids ; and those only by solution in the diluted vitriolic acid, or spirit of salt."
" Zinc dissolves with great rapidity in both these acids ; and, unless they are very much diluted, generates consider- able heat. One ounce of zinc produces about 356 ounce measures of air ^ : the quantity seems just the same whichever of these acids it is dissolved in. Iron dissolves readily in the diluted vitriolic acid, but not near so readily as zinc. One ounce of iron wire produces about 412 ounce measures of air : the quantity was just the same, whether the oil of vitriol was diluted with \\, or 7 times its weight of water : so that the quantity of air seems not at all to depend on the strength of the acid" {ibid., p. 144).
Inflammable air was also obtained by the action of spirit of salt on iron, but the quantity was not measured. One ounce of tinfoil dissolved in strong spirit of salt yielded 202 ounce measures of inflammable air; the same gas was also produced slowly by the action of vitriol on tin.
' The name " inflammable air " is here used for the first time. ^ The "ounce-measure" as used by Cavendish was the volume occupied by an ounce of water.
V THE STUDY OF GASES 67
The identification of inflammable air and fixed air by Cavendish. — To Cavendish belongs the credit of intro- ducing the method of identifying gases by careful measure- ments of their physical properties. For this purpose he measured the density of inflammable air and of fixed air from various sources ; in the case of fixed air the solubility in water was also found. The densities were measured by filling a bladder with the gas, and finding the change in weight when the bladder was emptied. If the gas was heavier than ordinary air the bladder became lighter when emptied, but if the gas was lighter than air a gain in weight was observed. In either case the density of the gas was calculated from the known weight of the bladder-full of air, and the change of weight on emptying the bladder.
In the case of inflammable air a gain of weight was observed amounting in four experiments to 40 J, 40 J, 41 J and 41 grains, when using 80 ounce-measures of gas pre- pared by the action of zinc on vitriolic acid and on spirit of salt, of iron on oil of vitriol, and of tin on spirit of salt respectively. The gas produced by the four methods was therefore the same ; as air was 800 times lighter than water, 80 ounce-measures of common air weighedSo x 480 ' -;- 800 = 48 grains ; the same volume of inflammable air weighed 48 - 41 = 7 grains ; this gas was therefore 48 -^ 7 = 6'9 times lighter than common air or 80 x 480 -r 7 = 5490 times lighter than water.^
In the case of fixed air there was a loss in weight of 34 grains on a volume of 100 ounces; the air weighed 1 00 X 480 -T- 800 = 60 grains ; the fixed air weighed 60 + 34 = 94 grains ; it was therefore ixin; times heavier than air or SIX times, lighter than water.^ The values for fixed air were trustworthy; those for inflammable air were un-
^ 480 grains = I ounce.
^ The correct values are : for inflammable air, 14 '4 and 11600; for fixed air, I '53 and 530.
68 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP.
trustworthy, because they depended so largely on an exact knowledge of the density of the common air displaced by the bladder, e.g., taking the density of common air as being 850 (instead of 800) times less than that of water. Cavendish found inflammable air to be "9,200 times lighter than water, or lOj^ij lighter than common air."
Cavendish devises a new method of weighing gases. — In order to overcome this difificulty Cavendish devised a method of weighing directly the gas liberated by the action of acids on a known weight of metal or chalk. If the volume of the gas had been determined in a separate ex- periment the density could easily be calcu- lated. Cavendish writes :
" I endeavoured to find the weight of the air discharged from a given quantity of zinc by solution in the vitriolic acid in the manner represented in Fig. 17. A \% 2^ bottle filled near full with oil of vitriol diluted with about six times its weight of water : ^ is a glass-tube fitted into its mouth and secured with lute : ^ C is a glass cylinder fastened on the end of the tube, and secured also with lute. The cylinder has a small tube at its upper end to let the inflammable air escape, and is filled with dry pearl-ashes^ in coarse powder. The whole apparatus together with the zinc, which was intended to be put in, and the lute which was to be used jn securing the tube to the neck of the bottle, were first weighed carefully; its weight was 11930 grains. The zinc was then put in, and the tube put in its place. By this
ft"
Fig. 17 — Caven- dish's Appar- atus FOR FIND- ING THE WEIGHT AND DENSITY OF GASES.
^ "The lute used for this purpose .... is composed of almond powder, made into a paste with glue, and beat a good deal with a heavy hammer. This is the strongest and most convenient lute I know of" {loc^ cii.y p. 143, footnote).
^ I.e,i potash.
V THE STUDY OF GASES 69
means the inflammable air was made to pass through the dry pearl-ashes ; whereby it must have been pretty effec- tively deprived of any acid or watery vapours that could have ascended along with it {JPhil. Trans. 1766, 56, 153).
The loss in weight was iif grains, but one grain of this was due to the displacement of the common air in the bottle by inflammable air ; the true weight of the inflam- mable air was therefore lof grains.
The weight of zinc used in this experiment was 254 grains. A previous experiment (p. 66) had shown that one grain of zinc gave 356 grain-measures of inflammable air. The volume from 254 grains would therefore be 254x356 = 90424 grain-measures of gas.
By combining the two experiments it was seen that lof grains of the gas occupied the same volume as 90424 grains of water. The gas was therefore 8410 times lighter than water, or io\ times lighter than common air.
Cavendish determines the weight of fixed air set free by the action of acids on chalk and the alkalis.— The same method was applied to find the weight of fixed air liberated by acids from various substances, blotting- paper being substituted for the pearl-ashes because the greater weight of gas demanded less careful drying. Cavendish found that :
1000 parts of marble lost 407 parts of fixed air.
1000 parts of sal volatile lost 528 to 538 parts of fixed air.
1000 parts of pearl-ashes lost 284 to 287 parts of fixed air.
1000 parts of a crystalline salt obtained by saturating pearl-ashes with fixed air lost 423 parts of fixed air.
The quantity of acid required to liberate the fixed air from each substance was also measured, and compared with the weight of fixed air set free. It was found that the acid which liberated 100 parts of fixed air from marble set free 109 parts from pearl-ashes, 217 from sal volatile, and 2 1 1 parts from the crystals prepared by the action of fixed
70 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
air on potash ; the two latter compounds thus contained twice as much fixed air, relatively to their neutralising power, as the two former.
Cavendish measures the solubility of fixed air. — In the case of fixed air, its solubility in water was measured and used to identify the gas. Cavendish states that :
" Water when the thermometer is about 55° will absorb rather more than an equal bulk." " Water heated to the boiling point is so far from absorbing air, that it parts with what it has already absorbed " (/%//. Trans., 1766, 56, 163).
The gas was also lost by exposure to air.
By measuring the solubility, as well as the density. Cavendish was able to prove that the gas set free during fermentation was identical with fixed air prepared from chalk and the alkalis.
In his investigation of fixed air Cavendish introduced the method of storing over mercury the gas which he had col- lected by the displacement of water from an inverted bottle. This use of mercury in place of water for manipulating soluble gases proved to be of very great value in Priestley's experiments a few years later.
B. Gases Derived From Nitric Acid.
Priestley's work on gases. — Whilst Cavendish was the first to make accurate measurements of the physical pro- perties of gases, the discovery of their chemical properties was mainly the work of Priestley. The state of knowledge when Priestley began his work upon gases is described in the following paragraphs :
" Van Helmont, and other chymists who succeeded him, were acquainted with the property of some vapours to suffo- cate, and extinguish flame, and of others to be ignited . . . But they had no idea that the substances (if, indeed, they knew that they were substances, and not merely properties, and affections of bodies which produced those effects) were
V THE STUDY OF GASES 71
capable of being separately exhibited in the form of a permanently elastic vapour, not condensible by cold, to which I give the name of air, any more than the thing that constitutes stnell. In fact they knew nothing at all of any air besides common air, and therefore they applied the term to no other substances whatever.
" Mr. Boyle was, I believe, the first who discovered that what we now call fixed air, and also inflammable air, are really elastic fluids, capable of being exhibited in a state unmixed with common air ....
" Besides these two kinds of factitious air, that which I call nitrous air obtruded itself upon Dr. Hales ; but even he had no idea of there being more than one kind of air, loaded with different vapours ; and was far from imagining that they differed from one another so very essentially as they are now known to do. And though Mr. Boyle, Dr. Hales, and others, could not but be acquainted with the effluvium of spirit of salt, and also of volatile alkali, they could have no idea that the substance which had those powers was capable of being separated from common air, and of being exhibited free from moisture, in the form of a permanently elastic vapour, to appearance exactly like that which constitutes the common atmosphere ....
" Even Mr. Cavendish, whose experiments relating to air immediately preceded my own, appears not to have had so much as a suspicion of this kind. For he relates an experiment of his, on the solution of copper in the marine acid, as inexplicable, except on the hypothesis of there being a kind of air that lost its elasticity by the contact of water, which admits of the easiest solution imaginable, on the supposition of the spirit of salt emitting a vapour, which though calpable of being confined by quicksilver, and of being by that means exhibited in the form of air, was instantly absorbed by water, which would thereupon become possessed of all the properties of common spirit of salt.
" In fact, none of the chymists appear to have had the least idea of its being even possible to separate the acid or alkaline principles from the water with which they are always found combined ; and therefore, though they did suppose them capable of further concentration, they still
72 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
considered a certain portion of water as absolutely essential to them ; and consequently all the experiments that have hitherto been made on the affinities of the acids, and alkalis are, in fact, nothing more than the affinities of compound substances, consisting of adds or alkali, and water. I have been so particular in stating these historical facts, for the sake of those chymists who can see nothing new in my experiments on the several acids and alkali, divested of water, and exhibited in the form of air " (Experiments on Air, IT]"], Vol. III. pp. 325 et seq.).
Priestley (1772) prepares and examines nitrous air. —
The gas which Mayow prepared by the action of iron on nitric acid was dismissed by Cavendish with the remark that iron, zinc and tin, " dissolve readily in the acid and generate air ; but the air is not at all inflammable." Priestley, however, after examining fixed air and inflammable air, proceeded to make a detailed study of this action. By dissolving various metals (amongst them brass, iron, copper, tin, silver, mercury) in nitric acid, Priestley collected over water a colourless gas, which he named nitrous air. {Experiments on Air, 1 7 74, I. 109). The gas was noxious to animals, and extinguished a lighted taper. But it differed from fixed air in that it did not precipitate lime-water and was only slightly soluble in common water; water, he says, "absorbs one-tenth of its bulk of nitrous air." The gas is, however, freely soluble in a solution of green vitriol in water, which can be made to " absorb more than ten times its bulk of nitrous air, without any sensible approach to saturation." {Experiments and Observations, 1779, IV. 48) ; this fact was afterwards used by Humboldt and by Davy to test the purity of different samples. The solution in which the gas has been absorbed " becomes of a very dark colour ; but becomes green again on being exposed to the open air " {Experiments on Air, 1777,111. Preface p. xxxiii). Davy showed that the green colour could be restored and pure nitrous air recovered by gently heating the solution ( JVnrks, III. 99).
V THE STUDY OF GASES 73
Priestley studies the combination of nitrous air with common air and with oxygen. — The most remarkable property of nitrous air was that of combining with common air to form brown nitrous fumes ^ which dissolved at once in water. Priestley made a careful study of this action and showed that the diminution of volume occasioned by the addition of an equal volume of nitrous air could be used to measure the goodness of common air. Speaking of nitrous air he says :
" One of the most conspicuous properties of this kind of air is the great diminution of any quantity of common air with which it is mixed, attended with a turbid red, or deep orange colour, and a considerable heat "
" The diminution of a mixture of this and common air is not an equal diminution of both the kinds . . . but of about one-fifth of the common air, and as much of the nitrous air as is necessary to produce that effect ; which, as I have found by many trials, is about one-half as much as the original quantity of common air. . . ."
" If, after this full saturation of common air with nitrous air, more nitrous air be put to it, it makes an addition equal to its own bulk, without producing the least redness, or any other visible effect. ..."
" It is exceedingly remarkable that this effervescence and diminution, occasioned by the mixture of nitrous air, is peculiar to common air, or air fit for respiration ; and, as far as I can judge, from a great number of observations, is at least very nearly, if not exactly, in proportion to its fitness for this purpose ; so that by this means the goodness of air may be distinguished much more accurately than it can be done by putting mice, or any other animals, to breathe in it" (Experiments on Air, 1774,^ I. 110-115).
' These fumes were generally regarded as the vapour of nitric acid ; Priestley described them as "nitrous acid vapour," Davy (1800) as " nitrous acid gas," Gay-Lussac in 1809 as " nitric acid " and in 1816 as "nitrous acid"; in order to avoid confusion, the term "nitrous fumes" is used in the succeeding pages. The name " nitrogen per- oxide" was not introduced until about 1850.
''■ The first part of Vol. I., pubhshed in 1774, describes experiments carried out in 1772.
74 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
Priestley tests the goodness of common air by mixing it with nitrous air. — Priestley's method of testing air was described in detail a few years later as follows : —
" I first provide a phial, containing about half an ounce of water, which I call the air-measure. This I fill with air by having first filled it with water, and placed it over the opening of the funnel in my shelf, and when it is filled I slide it along the shelf, always observing that there be a little more air than I want. The phial being thus exactly filled with the air which I am about to examine, and care being taken that it be not warmed by holding in the hand, &c. I empty it into a jar about an inch and a half in diameter, and then introduce to it the same measure of nitrous air, and let them continue together about two minutes. I choose to have an overplus of nitrous air, that I may be sure to have phlogiston enough to saturate all the common air. If I find the diminution with these measures to be very considerable, I introduce another measure of nitrous air ; but the purest dephlogisticated air will not, I believe, require more than two equal measures of nitrous air."
" Sometimes I leave the common and nitrous air in the jar all night, or a whole day ; but always take care that, whatever kinds of air I be comparing together, they remain the same space of time before I proceed to note the degree of diminution."
"When the preceding part of the process is over, I transfer the air into a glass tube, about three feet long, and one-third cf an inch wide, carefully graduated according to the air measure, and divided into tenths and hundredth parts ; so that one of the latter will be about a sixth or an eighth of an inch. Then immersing the tube in a trough of water, so that the water in the inside of the tube shall be on a level with the water on the outside, I observe the space occupied by them both, and express the result in measures and decimal parts of a measure, according to the graduation of the tube " (Experitnents and Observations, 1779, IV. Introduction, pp. xxx. — xxxii.).
Using this method, Priestley in 1772 thought he perceived
V THE STUDY OF GASES 75
a slight inferiority in the air of his study as compared with that outside, and in a sample of air from York as com- pared with that of Leeds.
Two years later he discovered the gas, richer than com- mon air, to which Lavoisier gave the name oxygen. On adding nitrous air to the gas prepared from red precipitate or from mercury calx, he found that it was about five times " as good as the best common air " he had ever tested.
It was soon recognised that the diminution of volume of common air was due to the absorption of oxygen, but the variations which Priestley thought he had found in the behaviour of different samples were proved to be due to experimental errors. The careful experiments made by Cavendish in 1783 showed that the composition of air is remarkably constant, the value found for 100 volumes of air being :
Diminution by nitrous air (oxygen) 20-84 volumes. Residue (azote) 79" 16 volumes.
The graduated glass tube which Priestley used for measuring the goodness of air is still known as a EUDIOMETER (Greek ev, good, i^irpov, a measure ; see Priestley, Experiments on Air, 1777, IIL 379 and 380).
Priestley (1777) describes the properties of nitrous fumes. — On account of their solubility and corrosive pro- perties, the brown nitrous fumes produced by combination of nitrous air with oxygen, or by the action of strong nitric acid on metals, could not be collected by the methods which Priestley employed for other gases. He was, therefore, obliged to collect the gas by displacing the air from narrow- necked flasks provided with glass stoppers. This gas, as Priestley prepared it by the action of nitric acid upon bismuth, was obviously contaminated both with nitrous air and with common air. He writes :
" Being disappointed, as has been seen, in my expecta-
76 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
tions of confining the nitrous acid vapour by attwial oils, it occurred to me, that, in Heu of this, it might not be wholly without its use, if I could shut up this vapour in dry glass phials, with ground stopples. And though, in this method of procuring it, by the solution of bismuth, or other things with which it unites most rapidly, there is necessarily a mixture of ttitrous air, it is inconsiderable in proportion to the quantity of pure nitrous vapour itself. And although a mixture of common air also would necessarily remain in the phial, it could only serve to dilute the acid vapour, and could not materially alter the properties of it. Also, if the mouths of the phials were small, they might be opened, and various substances admitted to the vapour, without much loss of the acid ; especially as all acid vapours, I had reason to think, were heavier than common air" [Experiments on Air, 1777, III. 184).
The most conspicuous quality of the nitrous fumes is their brown colour, by which their presence is disclosed in many operations in which nitric acid takes part. The colour is permanent, and has the remarkable property of becoming much intensified by heating. This observation interested Priestley so much that he was in the habit of carrying a bottle of the gas in his pocket in order to show its curious behaviour to his friends. Priestley writes :
" The change of colour given to this vapour by heat is not a little remarkable, for it is altogether independent of gravity or condensation. In order to make some experi- ments of this kind to proper advantage, I procured a glass tube, three feet long, and about an inch wide, closed at one end, and fitted with a ground stopple at the other. This tube I easily filled with red vapour, in consequence of its being much heavier than common air ; and closing the open end with the stopple, observed, that that part of the tube which I held in my hand was manifestly of a deeper colour than any other part of the tube. On this I held one end of it to the fire, and found that that end grew most intensely red, three or four times more so than the rest of the tube. The direction in which the tube was
V THE STUDY OF GASES 77
held made no difference with respect to the red part of it ; the part that was hottest being always of the deepest colour, whether it was held upwards or downwards ; so that whether the heated vapour ascended or descended, it did not retain its colour in the smallest degree, after it had been opposite to the heated part of the glass.
" That this extraordinary redness was not occasioned by the vapour being more rarefied in that particular place, appeared by the whole tube assuming the same deep red colour when the whole length of it was made equally hot : for the vapour being closely confined, the density of it within the tube must necessarily have continued the same in all the variations of heat or cold. This redness, there- fore, must be the proper effect of heat, on the phlogiston, as I should imagine, of the vapour. Repeating this experi- ment very often, with the same tube, and the same vapour, it became alternately of a deeper or lighter colour, according as it was kept hot or cold, without any sensible change, except that which depended upon this single circumstance. This is really a striking experiment, and especially when the tube contains first so much vapour as to be nearly transparent when it is cold ; so that the heat alone gives it all the colour that it acquires.
" In order to observe the utmost effect of heat on this vapour, I placed the closed end of the tube near the fire, and bringing it gradually nearer and nearer, observed that the colour deepened uniformly with the increase of heat, till, the glass actually melting, the compressed vapour burst its way out" (^Experiments on Air, xTil, III. 186-188).
Priestley (1772) discovers " diminished nitrous air " or " laughing gas." — One of the agents used by Priestley for " phlogisticating " or deoxidising common air was a mixture of iron filings and brimstone, or sometimes iron filings alone. The action of this agent on nitrous air gave rise to a new gas possessing remarkable properties. Priestley describes this discovery as follows :
" The diminution of common air by a mixture of nitrous air, is not so extraordinary as the diminution which nitrous
78 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
air is subject to from a mixture of iron filings and brim- stone, made into a paste with water. This mixture, as I have already observed, diminishes common air between one-fifth and one-fourth, but has no such effect upon any kind of air that has been diminished, and rendered noxious by any other process ; but when it is put to a quantity of nitrous air, it diminishes it so much, that no more than one-fourth of the original quantity will be left."
" Nitrous air thus diminished has not so strong a smell as nitrous air itself, but smells like common air in which the same mixture has stood" (Experiments on Air, iTT4, I. 118-119).
Priestley was astonished to find that an agent which he had employed previously to remove all the goodness from common air, had actually converted the inert nitrous air into a DIMINISHED NITROUS AIR " with properties which, at the time of my first publication on this subject, I should not have hesitated to pronounce impossible, viz. air in which a candle burns quite naturally and freely, and which is yet in the highest degree noxious to animals, insomuch that they die the moment they are put into it " {Hid., p. 215).
As " diminished nitrous air " is soluble in cold water, it was best prepared over mercury ; the gas was then almost like oxygen in its power of supporting combustion. On the other hand, long contact with iron over water destroyed this property, the diminished nitrous air being either absorbed or wholly deoxidised. Priestley found that :
" A candle burned with an enlarged flame ... in nitrous air, which had been in contact with iron in quicksilver, about six months.
" Water being admitted to the remainder of this air, it began to be absorbed as usual.
" Nitrous air, which had been confined above a year in contact with iron, standing in water, was, in all respects like phlogisticated common air : it neither diminished common
V THE STUDY OF GASES 79
air, nor was diminished by nitrous air, and extinguished a candle" {Experiments on Air, 1775, II. 177).
The action of nitric acid on metals gives rise to nitrous fumes, nitrous air, and diminished nitrous air. — In the action of nitric acid on metals, diminished nitrous air is often formed, especially if weak acid be used ; being soluble in cold water, but expelled by heat, the gas may be set free by warming cold nitric acid in which iron, zinc, or tin has been dissolved.
"The solution of iron in spirit of nitre is known to produce nitrous air ; but when all the nitrous air is pro- duced in this manner, without foreign heat, if a candle be applied to the solution, more air will be procured ; and this will be possessed of the peculiar kind of inflammability above mentioned.
" I have also procured this kind of air in a direct process by the solution of zinc and tin^
" When ... I used only a very weak spirit of nitre in the solution of zinc ... I got no other than this kind of air in which a candle burned with an enlarged flame ; and the air was of the very same kind from the beginning to the end of the process " (" Of Nitrous Air in which a Candle Burns," Experiments on Air, i777> III-> PP- 133. 134, 139)-
BerthoUet (1785) prepares " diminished nitrous air " or " laughing gas " by heating nitrate of ammonia. — BerthoUet discovered in 1785 that when nitrate of ammonia is heated to about i5o°C., it is decomposed into water and " diminished nitrous air." This method of preparation was used in the year 1799 by Sir Humphry Davy (r778-i829), who found that the pure gas made by this method had the following properties :
a. A candle burnt in it with a brilliant flame, and crackling noise.
b. Phosphorus introduced into it in a state of inflamma- tion, burnt with infinitely greater vividness than before.
8o HISTORICAL INTRODUCTION TO CHEMISTRY chap.
c. Sulphur introduced into it when burning with a feeble blue flame, was instantly extinguished ; but when in a state of active inflammation ... it burnt with a beautiful and vivid rose-coloured flame.
d. Inflamed charcoal . . . burnt with much greater vividness than in the atmosphere.
e. To some fine twisted iron wire a small piece of cork was affixed : this was inflamed, and the whole introduced into a jar of the air. The iron burned with great vividness, and threw out bright sparks as in oxygen.
f. 30 measures of it exposed to water previously boiled, was rapidly absorbed ; when the diminution was complete, rather more than a measure remained.
g. Pure water saturated with it, gave it out again on ebullition, and the gas thus produced retained all its former properties.
h. It was absorbed by red-cabbage juice ; but no alteration of colour took place.
/. Its taste was distinctly sweet, and its odour slight, but agreeable.
j. It underwent no diminution when mingled with oxygen or nitrous gas (Davy's Works, III. 54).
Davy (1799) uses " laughing gas " as an ansesthetic—
Although the gas had been credited with the most deadly properties, and with the power of producing plague and other contagious diseases, Davy found from personal experiments in the spring of 1799 that it could be breathed without harm, and had indeed remarkable stimulating and exhilarating qualities. Robert Southey, one of —any friends who submitted themselves to the action of the new intoxicant, describes his feelings as follows :
" My first definite sensation was a dizziness, a fullness in the head, such as to induce a fear of falling. This was momentary. When I took the bag from my mouth, I immediately laughed. The laugh was involuntary, but highly pleasurable, accompanied by a thrill all through me ; and a tingling in my toes and fingers, a sensation perfectly new and delightful " {Works, III. 301).
V THE STUDY OF GASES 8l
Effects such as these gave to the gas the popular name of " laughing gas." Its permanent utility as an anaesthetic in dentistry is forshadowed in Davy's own experience :
" The power of the immediate operation of the gas in removing intense physical pain, I had a very good opportunity of asertaining.
" In cutting one of the unlucky teeth called dentes sapientiae, I experienced an extensive inflammation of the gum, accompanied with great pain, which equally destroyed the power of repose, and of consistent action.
" On the day when inflammation was most trouble- some, I breathed three large doses of nitrous oxide. The pain always diminished after the first four or five inspirations ; the thrilling came on as usual, and uneasiness was for a few minutes swallowed up in pleasure. As the former state of mind however returned, the state of organ returned with it ; and I once imagined that the pain was more severe after the experiment than before" {Works, III. 276).
C. Acid Air and Alkaline Air.
Cavendish (1766) collects "marine acid air." — From the time of Glauber it had been customary to prepare muriatic acid by distilling into water the pungent gas produced by the action of strong oil of vitriol on common salt (Chap. II, p. 14). The gas was first collected by Cavendish in " an experiment with design to see, whether copper proiltced any inflammable air by.solution in spirit of salt." He found that he "could not procure any inflammable air thereby," but, on the application of heat, he was able to drive off a considerable quantity of gas which had the peculiar property of " losing its elasticity when in contact with water." The gas set free in the bottle containing the copper and acid (Fig. 18) was at first separated from the water in the receiver by a "barrier of common air," but as soon as this was removed the dissolution of the gas proceeded
G
82 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
rapidly, so that the water in the receiver " rushed violently into the bottle and filled it almost entirely full " (Phil. Trans. 1766, 56, 157-158)-
Priestley (1772) isolates "marine acid air." — By making use of mercury instead of water, Priestley had no difficulty in obtaining this acid air in a permanent form. He showed that it could be driven off by heating spirit of salt alone, the presence of copper being unnecessary. Its peculiar behaviour depended entirely on its extreme solubility
in water, which he found to absorb 576 times its volume of the gas.'^ Spirit of salt was, indeed, merely a solution of acid air in perhaps twice its weight of water. Priestley also prepared the gas by the action of oil of vitriol on salt, a more efficient method
Fig. 18.— Apparatus USED BY Cavendish TO PREPARE which had the ad-
A GAS WHICH " LOST ITS ELASTICITY BY CONTACT . /-J
WITH WATER." vantage ot produc-
ing a dry gas. He found that "acid air, extinguishes flame, and is much heavier than common air" {Experiments on Air, 1774, I. 143-147)-
Priestley isolates "alkaline air."— Encouraged by his success in separating an elastic air from spirit of salt, Priestley next endeavoured to collect an alkaline air from the "volatile spirit of sal-ammoniac." In this he was successful: the gas which was set free by heating the
•Two and a-half grains of water dissolved three ounce-measures of the gas.
V THE STUDY OF GASES 83
volatile spirit remained permanently elastic when collected over mercury, but collapsed immediately when brought into contact with water. Priestley found that alkaline air was only a little less soluble than acid air, one volume of water dissolving 336 volumes of the gas.* It also differed from acid air in being lighter instead of heavier than common air. He found that the gas (to which the name of ammonia was given by Bergman in 1782), could be prepared directly by heating crystals of sal-ammoniac with slaked lime in a gun barrel : by passing the product into water he obtained a very strong " spirit " from which he could expel the alkaline air as required {Experiments on Air, 1774, I. 163-169).
Priestley prepares sal-ammoniac by mixing alkaline air with acid air. — Thinking that gases possessed of such opposite properties were likely to combine together to form a neutral substance, possibly identical with common air, Priestley brought together vessels containing alkaline air and acid air. To his great astonishment he obtained a solid product which he identified as sal-ammoniac ; this volatile salt was therefore a compound formed by the union of alkaline air with acid air.
" Having satisfied myself with respect to the relation that alkaline air bears to water, I was impatient to find what would be the consequence of mixing this new air with the other kinds with which I was acquainted before, and especially with acid air ; having a notion that these two airs, being of opposite natures, might compose a neutral air, and perhaps the very same thing with common air. But the moment that these two kinds of air came into contact, a beautiful white cloud was formed, and presently filled the whole vessel in which they were contained. At the same time the quantity of air began to diminish, and, at length, when the cloud was subsided, there appeared to be formed a solid white salt, which was found to be the common
' One and a-quarter grains of water dissolved seven-eighths ounce- measures of gas.
G 2
84 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
sal-ammoniac, or the marine acid united to the volatile alkali" [Experiments on Air, 1774, I. 169-170).
Sal- volatile formed by the union of alkaline air with fixed air. — On mixing alkaline air with fixed air Priestley again obtained a solid product which he identified as the volatile alkali " sal-volatile."
" Fixed air admitted to alkaline air formed long and slender crystals, which crossed one another and covered the sides of the vessel in the form of net-work. These crystals must be the same thing with the volatile alkalis which chemists get in a solid form, by the distillation of sal- ammoniac with fixed alkaline salts " (Experiments on Air, 1774, I. 171).
Apparatus for experiments on gases. — The first apparatus used for experiments on gases, as distinguished from condensible vapours, was that of Mayow, Figs. 11, 12, 13, 16. It should be noticed that, in preparing gases artificially by the action of acids on iron, Mayow was not able to collect the gas over water, but was obliged to fill the whole apparatus (Fig. 14, p. 33) with acid.
This disadvantage was removed by Stephen Hales (1677 — 1761), botanist, chemist, and Vicar of Tedding- ton, who was one of the first to separate the generator from the RECEIVER of the gas. His Vegetable Statics {i"] 2"]) is chiefly concerned with hydrostatic experiments on the pressure of the sap in plants. But the sixth chapter, occupy- ing nearly one-third of the book, deals with "an attempt to analyse the Air by means of a great variety of chymio- statical experiments, which shew in how great a proportion Air is wrought into the composition of animal, vegetable, and mineral Substances, and withal how readily it resumes its former elastick state, when in the dissolution of those Substances it is disengaged from them.''
In order to measure the volume of gas set free by heating animal, vegetable, and mineral substances. Hales used a
THE STUDY OF GASES
85
Fig. 19— (a) Hales's Apparatus for Measuring the Volume of Gas set free by Heating Animal, Vegetable, and Mineral Substances.
glass retort r (Fig. igs) luted securely to a gauge ab, standing in a trough of water xx. The gauge was
made from a long- necked flask, pierced at the bottom to admit a syphon-tube J, by means of which air could be drawn out and water sucked up as far as z. The fall or rise of the level of the water in the gauge, after heating the contents of the retort, showed how much air had been liberated or absorbed.
This apparatus had the disadvantage that the joint at a was liable to leak, especially when an iron retort had to be used for stronger heating, or when the gas had to be kept for several days before its volume became constant. These diffi- culties were over- come in a second form of the ap- paratus (Fig. 19^). This consisted of a retort rr made from an iron gun- fig. 19— (*) Hales's apparatus (improved design).
86 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
barrel, with a lead syphon attached to the end. The gas was carried by the syphon through a vessel of water xx into an inverted flask ab, where it could be stored for any length of time without risk of leakage.
The great importance of this new design will be realised on comparing it with the apparatus used by Cavendish (Fig. i8), by Lavoisier (Figs. i6, 21, 25), and by Priestley (Fig. 20).
Priestley's apparatus. — The apparatus which Priestley used for manipulating gases is described at the beginning of the first volume of his Experiments on Air and illustrated by two plates (numbered and lettered consecutively) which are reproduced in Fig. 20.^
The most important feature was the trough, often called a PNEUMATIC TROUGH, shown with all its accessories at i in the first plate. The original trough a was of earthenware, about eight inches deep, with thin flat stones, bb, just below the surface ; afterwards a large wooden trough was used with a shelf fixed an inch from the top. The various gases were stored in cylindrical ya^j-, cc, which could be immersed in the trough, or stood on the shelf, or lifted out in dishes as at 2 2 2. Mice were stored as at 3 in a receiver standing on a perforated tin plate and provided with a perforated cover held down by weights ; in order to test the goodness of a sample of air, a mouse was held by the back of the neck and passed through the water into a tall beer-glass, d, of two or three ounces capacity, in which a mouse could usually live from twenty to thirty minutes. The tapering cork, 4, was used to close or open a small bottle inside a jar of air, whilst the wire stand, 5, served to support small dishes as at /. The funnel, 6, was used to pour gases from a vessel with a wide neck into a vessel with a narrow neck,
' In the second plate, the blackboard has been moved and the table altered a little in order to reproduce on as large a scale as possible.
THE STUDY OF GASES
87
88 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
filled with water and supported near the surface of the trough.
The gun-barrel, 7 (second plate : compare Hales, Fig. 191^), with a tobacco-pipe stem, or glass tube luted to the open end, was used to expel gases from solid substances. To reduce the volume of air, the gun-barrel was filled up with dry sand. In the figure, the gas is being collected over mercury ; Cavendish had transferred to a basin of mercury, a bottle full of fixed air which was too soluble to be stored long over water, but this is the first use of mercury for collecting gases ; a carefully-shaped mercury TROUGH was introduced by Lavoisier (Fig. 33). An apparatus for collecting over mercury gases expelled from liquids by heat, or set free by the action of acids, is shown at 8, where a is a basin of mercury, b a tube filled with it, c the tube from which the gas is expelled and d a glass trap to condense out moisture ; the apparatus used for generating and collecting gases not freely soluble in water is shown at e in the first plate.
The bladder 9, provided with funnel and delivery tube, was used to receive gas from a jar standing in water and then to transfer it free from water to a vessel standing in mercury. A bladder is also shown in the apparatus, 10, used for impregnating liquid with gas, e.g., water with fixed air ; the gas generated in c was collected in the bladder and then squeezed out through the flexible leathern tube, d, into the flask a.
The candle, 12a (first plate), mounted on the end of a wire b, was used to test the air in the narrow tube 1 1 ; the candle, c, was used in the wider cylinders, from which it could be withdrawn through the water as soon as it was extin- guished.
The syphon, 13 (second plate), was used to suck air out from a cylinder as at /(compare Hales, Fig. iga).
The receiver, 14, exhausted by an air-pump, was used
THE STUDY OF GASES
89
90 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
when a dry powder had to be surrounded by gas drawn from a vessel standing in water.
The eudiometer,^ 15, was used for mixing minute quantities of nitrous air and common air ; two bubbles were introduced into the narrow tube a with the help of the iron-wire plunger b, then measured, mixed, and measured again.
Apparatus for sparking small quantities of gas is shown in the second plate at 16, 17, 18, 19, anticipating in part the apparatus used by Volta (Fig. 27) and by Cavendish (Fig. 39)-
Summary and Supplement a. fixed air and inflammable air
Van Helmont {b. 1577 ; d. 1644) recognised the existence of two "gases," a poisonous "gas sylvestre" and an inflammable " gas pingue," analogous with the " choke-damp " and " fire damp " of miners.
John Mayow, Medico-physical Works, 1674, collected gases prepared artificially by the action of iron on aqua fortis and on dilute oil of vitriol contained in an inverted flask ; he showed that they possessed the same elastic properties as common air, but was not sure that they were essentially different from it.
Stephen Hales, Vegetable Statics, 1727, by heating diflFerent substances, prepared many gases which he collected over water in a separate "receiver" ; he also mixed different gases and studied the changes of volume produced in them by the action of different agents ; but he did not recognise these gases as distinct substances, and supposed that all had the same density.
Joseph Black, Experiments upon Magnesia Alba, 1755, proved that chalk and the alkalis contain a gas which he called fixed air (carbonic anhydride or carbon dioxide, COj), but he did not at that time collect or handle it.
Henry Cavendish, On Factitious Air, 1766, measured the density of fixed air from a variety of sources, and of in- flammable air (hydrogen) prepared by the action of acids on different metals, thus : —
^ This name was first used in 1776 {Experiments on Air), 1777) ^^l* 379 and 380).
V THE STUDY OF GASES 91
Zitic on Vitriolic Acid, Zn + H2S04->ZnS04 + H, ; Zinc on Spirit of Salt, Zn+ 2HCl->ZnCl2 +H2; Iron on Vitriolic Acid, Fe + H2S04->FeS04+H2 ; Tin on Spirit of Salt, Sn+ 2HCl-»SnCl2 + Hj.
By these measurements he proved the identity of the different samples, and estabHshed the method of investigating gases by the exact measurement of their physical properties. He found that water dissolved an equal volume of fixed air, which escaped when the water was boiled or exposed to the air ; fixed air could, however, be stored permanently in a bottle inverted over mercury.
Joseph. Priestley, Experitnents on Air, Vol. I. 1774 ; Vol. II. 1775 ; Vol. III. 1777, prepared a large number of gases and studied their chemical properties very thoroughly. He examined fixed air and inflammable air, prepared three gases by the action of nitric acid on metals, discovered oxygen, and collected over mercury a series of gases which were too soluble to be collected over water.
B. GASES DERIVED FROM N[TRIC ACID
Nitrous air (nitric oxide, NO), was prepared by van Hel- mont, by Mayow, and by Hales, by the action of nitric acid on metals, but was first recognised as a distinct substance by Priestley {\']^^), who named it nitrous air.
It dissolves in ten volumes of water, but is freely soluble in solutions of green vitriol (ferrous sulphate), forming a dark brown liquid from which the gas can be recovered by gentle heating. The preparation of the gas from copper and nitric acid is often represented by the equation —
3CU + 8HNO3 -» 3Cu(N03)2 + 2NO + 4H20,
but many other products are formed at the same time.
Diminished nitrous air, or " laughing gas " (nitrous oxide, NoO), was prepared by Priesdey (1772) by the action of iron fihngs on nitrous air.
2Fe4-6NO -> Fe203 + 3N20. BerthoUet (1785) prepared it by heating ammonium nitrate. (NH4)N03 -^N20 + 2H20.
92 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
It is readily soluble in water, but is expelled by boiling. A candle burns in it with an enlarged flame, as in oxygen. The pure gas can be breathed with impunity (Davy), and has been used as an anaesthetic in dentistry since 1800.
The formation of soluble brown nitrous fumes (nitrogen dioxide, N02)on mixing nitrous air with common air had been noticed by Hales. In presence of water the action is more complex, both nitric and nitrous acid being produced (see Chapter X), thus :
Without water 2NO + 02 = 2N02
(Nitrogen dioxide.)
With water 4NO + 302 + 2H20 = 4HNO.,
(Nitric acid.)
4NO + 02 + 2H20 = 4HN02
(Nitrous acid.)
When common air is mixed over water with an excess of nitrous air (nitric oxide) all the oxygen is absorbed, together with as much of the nitrous air as has combined with it ; Priestley used the contraction produced by mixing equal volumes of the two gases in a eudiometer as a measure of the "good- ness" of air.
The brown nitrous fumes were prepared in an impure state by Priestley by the action of bismuth on nitric acid —
Bi+6HN03 -> Bi(N03), + 3N02 + 3H20.
(Nitric (Bismuth (Nitrogen
acid.) nitrate.) dioxide.)
But this gas is better prepared by heating lead nitrate — 2Pb(N03)2 -> 2PbO + 4N02+02.
(Lead nitrate.) (Litharge.) (Nitrogen dioxide.)
It can be purified by condensing it to a liquid. Priestley noticed that the gas darkened in colour when heated, but became light when cooled again. The loss of colour is due to the formation of a more complex compound (dinitrogen tetroxide) —
2N02^N20„
(Brown.) (Colourless.)
which can be frozen out as a colourless ice (Chapter XX, p. 525), but begins to decompose as soon as it is melted. The mixture of these two oxides is called " nitrogen peroxide.''
THE STUDY OF GASES
C. ACID AIR AND AT.KALINE AIR
93
Acid air (hydrogen chloride, HCl). Glauber prepared " spirit of salt " by the action of oil of vitriol on salt,
2NaCl + H2S04 -> NajSOi + aHa,
but he was only able to collect the gas by absorbing it in water. Priestley (i774) collected it over mercury as a "permanently elastic fluid." He found it to be extremely soluble in water, which dissolved 576 times its volume of the gas.
Alkaline air (ammonia, NHg). The pungent gas present in " spirit of hartshorn " was expelled by warming, and collected over mercury by Priestley (i774), who called it "alkaline air" ; the name "ammonia" was given to it by Bergman in 1782. Priestley also prepared it from slaked lime and sal-ammoniac,
Ca(OH)2+ 2NH4CI -» CaClj + HjO + 2NH3, Slaked + sal -> muriate + water-f ammonia lime ammoniac of lime
and obtained a very strong spirit by dissolving it in water, which absorbed 336 volumes of the gas. It differed from acid air in being lighter instead of heavier than common air. A mixture of alkaline air and acid air produced sal-ammoniac,
NHs-FHCl -> NH4CI (sal-ammoniac).
(Ammonia.) (Hydrogen (Ammonium chloride.) chloride.)
A mixture of alkaline air and fixed air produced sal-volatile, 2NH3 + CO2 -> (NH3)2C02 (sal-volatile.)
(Ammonia.) (Carbon (Ammonium dioxide.) carbamate.)
CHAPTER VI
THE COMPOSITION OF FIXED AIR, CARBON, CARBONIC ACID, AND THE CARBONATES
A. The Composition of Fixed Air.
Fixed air a product of combustion. — Van Hebnont
derived the name of his poisonous " gas sylvestre " from its presence in the fumes of a fire of wood-charcoal (Latin sylva, a wood or forest). But as the only test which he applied was that of a lighted candle, no importance attaches to the fact that he gave the same name to the gases liberated in fermentation, and by the action of acids upon chalk.
The recognition by Black, in 1755, of fixed air as a constituent of chalk provided for the first time a test by which a gas differing from common air could be detected and identified with certainty. By its power of reconverting lime into chalk, he proved the presence of fixed air in substances such as magnesia, sal-volatile, and the mild alkalis, as well as in natural waters and in common air. Cavendish, in 1766, added to these tests the method of measuring the physical constants (density and solubility) of the gas, and proved that these were the same for fixed air prepared from alkaline substances and by the fermentation of sugar. These physical tests could not be applied to the crude gas
CH. VI THE COMPOSITION OF FIXED AIR 95
obtained by burning charcoal in air ; but Cavendish showed that " a quantity of common air was reduced from 1 80 to 160 ounce measures, by passing through a red-hot iron tube filled with the dust of charcoal " and " observed, that there had been a generation of fixed air in this process, but that it was absorbed by soap leys " (quoted by Priestley, Experiments on Air, I. 129).
Fixed air is formed during the burning of charcoal (Priestley, 1772) and the reduction of metallic calces (Lavoisier, 1774). — Priestley, in 1772, by means of a burning mirror, or lens, heated fragments of charcoal in air confined over water. He found that the volume of the air was diminished by one-fifth, that the residue extinguished flame, was noxious to animals, and suffered no further diminution of volume when exposed to the action of iron filings and sulphur, or when mixed with nitrous air ; the air had therefore been deprived of all its "goodness" (the discovery of oxygen was not made until two years later) whilst the production of fixed air was shown by the cloudiness which was produced when the gas was confined over lime-water. When mercury was used instead of water, the volume of the air was not diminished by the burning charcoal until lime-water was added, when one-fifth of the air was absorbed ("Of Air infected with the Fumes of Burning Charcoal," Experiments on Air, 1774, I. 129-132).
About the same time, Lavoisier {Physical and Chemical Essays, 1774, Chapter V; Works I, 598-613) found that a large volume of " air " was produced when minium or red lead was reduced by heating with charcoal in an iron retort (Fig. 21.) Avery large quantity of gas was set free, 560 cubic inches being liberated in the production of about I cubic inch of lead, although the charcoal heated alone gave only eight cubic inches of gas in the course of two days. The gas dissolved in lime-water, and extinguished a lighted candle; a rat died almost instantly in it. The
96 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
Fig. 21— Lavoisier's .'Vpparatus for collecting the Gas produced by
HEATING ReD-LEAQ AND ChARCOAL IN AN IrON ReTORT.
The air could be drawn out from the bell-jar through the small aperture shown at the top of the jar, or it could be removed by means of an air-pump which has not been reproduced in the figure.
VI THE COMPOSITION OF FIXED AIR 97
production of fixed air during the reduction of the calx was thus clearly proved ; but it was not until Priestley had discovered oxygen, as described in Chapter III, that Lavoisier was able to give a satisfactory explanation of what had occurred in this reduction.
Lavoisier (1774) proves that fixed air is an oxide of carbon. — The crucial experiments by which fixed air was proved to be an oxide of carboii were made by Lavoisier in November, 1774, after Priestley had demonstrated to him the method of making oxygen from the red calx of mercury.
By heating an ounce of this calx with forty-eight grains of charcoal in a tiny glass retort he was able to collect, in a bell jar over water, 64 cubic inches of gas. This gas dis- solved in water, communicating to it the properties of the natural acidulated waters, destroyed animals brought into it, at once extinguished candles and other burning substances, precipitated lime-water, and combined readily with the alkalis, removing their causticity and enabling them to crystallise. "All these properties are exactly those of the kind of air known as fixed air."
Lavoisier found that the red precipitate heated alone gave 78 cubic inches of a gas which did not dissolve in water, did not precipitate lime-water, did not unite with alkalis or diminish their caustic qualities, but which could be used again for the calcination of metals. " In conclusion, it had none of the properties of fixed air ; far from being fatal, like it, to animals, it seemed, on the contrary, more proper for the purposes of respiration ; candles and burning bodies not only were not extinguished by it, but burned with an enlarged flame in a very remarkable manner ; the light they gave was much greater and clearer than in common air ; charcoal burned in it with a brilliancy almost hke that of phosphorus, and all combustible sub- stances were consumed in it with surprising rapidity. All these circumstances convinced me that this air, far from
H
98 HISTORICAL INTRODUCTION TO CHEMISTRY chav.
being fixed air, was even more respirable, more combustible, and consequently more pure even than the air in which we live."
The calx heated alone gave oxygen and mercury ; when heated with charcoal it gave fixed air and mercury. Fixed air was therefore evidently a compound of charcoal with oxygen. Lavoisier argued that —
"Since carbon disappeared entirely in the revivification of mercury from its calx, and since there result from this operation only mercury and fixed air, one is forced to conclude that the principle which has hitherto been known as fixed air is the result of the combination of eminently respirable air with charcoal" ("On the Nature of the Principle which combines with metals during their Calcina- tion and increases their Weight," Works, II. 122-128).
The charcoal (French charbon) used in experiments such as these always left behind when burnt a larger or smaller proportion of ash. Lavoisier and his colleagues, therefore, introduced in 1787 "the modified name of carbon,^ which indicates the pure and essential principle of charcoal," thus "distinguishing it from charcoal according to the vulgar acceptation " and isolating it "from the small quantity of foreign matter which it generally contains and which constitutes the ash'' {Method of Chemical Nomenclature, tr. 1788, p. 32).
In their system of nomenclature, fixed air became an OXIDE OF CARBON : its production during the burning of charcoal was represented by the equation : carbon + oxygen = fixed air
(oxide of carbon)
whilst the reduction of a calx was shown by the equation : carbon + calx = fixed air + metal.
(oxide of (oxide of
metal) carbon)
' French carbone.
VI THE COMPOSITION OF FIXED AIR 99
Diamond and graphite.— Newton had suspected {Opticks, 1704, II. 75) that the diamond, with its high refractive power, might be a combustible substance. This idea was confirmed by several notable experiments in which the combustion was effected both by powerful burning-glasses and by means of furnaces. But as the experiments were always made in the open, no idea could be formed of the nature of the products ; indeed, it was generally believed that the diamonds had merely been vaporised without burning at all.
To Lavoisier belongs the credit of having shown, in 1772, in conjunction with Macquer and Cadet, that if air be com- pletely excluded, the diamond remains unaltered at the highest temperature of the furnace ; the burning of the diamond is therefore a true combustion. In order to deter- mine the nature of the products of combustion, Lavoisier employed a very large burning glass (compare Fig. 22) to heat diamonds supported in air or oxygen contained in glass jars inverted over water or over mercury. No water, smoke, or soot was produced ; when mercury was used the volume of gas was not altered, but in contact with water the volume was somewhat diminished ; in both cases the gas in which the diamond had been burnt turned lime-water milky ("Destruction of the Diamond by Fire," 1772, Works, II. 38-88). Charcoal behaved in just the same way as the diamond. There was, therefore, no doubt that each of these substances gave rise to fixed air as the sole product of combustion. A few years later, in 1797, Tennant (an English chemist who had already shown that carbon could be recovered from fixed air by the action of phosphorus vapour on red hot chalk), burned diamonds by means of melted saltpetre, and showed that they gave rise to precisely the same quantity of fixed air as when charcoal was used. This observation was confirmed by the combustions carried out many years later by Dumas and Stas (Chapter VIII, p. 150) ;
H 2
lOO HISTORICAL INTRODUCTION TO CHEMISTRY chap.
VI CARBONIC ACID AND THE CARBONATES loi
it showed clearly that the diamond, like charcoal, must be regarded as a variety of carbon.
The mineral graphite, also known as plumbago or BLACK LEAD, was shoiyn by Scheele, in 1779, to give rise to fixedair when fused with nitre. Other workers showed that (if allowance were made for the ash which it always contains) it gave the same proportion of gas as charcoal and diamond, and must therefore be regarded as a third variety of carbon.
Such varieties of an element as charcoal, diamond, and graphite were described by Berzelius, in 1 840 {Jahresbericht, 20, Chem. p. r3), as allotropes.
B. Carbonic Acid and the Carbonates.
Bergman (1774) regards fixed air as an acid. — It is
■uncertain to what extent Black recognised that fixed air had the properties of a weak acid. He was aware that it was attracted by the caustic alkalis and by lime, and that it blunted and diminished their caustic and alkaline properties. But the small quantity of liquid which could be condensed when distilling chalk or magnesia differed so completely from the mineral acids produced by distilling green vitriol or nitre, that it is not surprising that Black hesitated to describe fixed air as an acid.
The clear recognition of its acid properties was due to the Swedish chemist Torbern Bergman (1735-1784), who in 1774 published a full description of its properties in a paper " On the Aerial Acid " (Bergman's Essays, translated by E. Cullen, 1784, pp. 1-90).
In support of this view he mentions :
(i) Its solubility in water, as measured by Cavendish (Chapter V).
(2) The acid taste which it imparts to natural and artificial aerated waters, as noticed by Brownrigg and by
I02 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
Priestley. In a paragraph headed " Fixed Air has an Acid Taste," Bergman writes :
" As this air is in form of an elastic vapour, it can hardly be tasted by itself, at least distinctly ; but if it be united with water, which is in itself void of flavour, being accumulated and rendered less volatile by this union, it readily affects the tongue with a weak but agreeable acidity. This is the real spirit of the cold mineral waters, which undoubtedly occasioned them to be called acidulous ; and by means of which, together with a due proportion of suitable salts, we may perfectly imitate the Seltzer, Spa, and Pyrmont waters. Such artificial waters I have now been using for eight vears with signal advantage" {Essays, p. 12).
(3) Its power of imparting to litmus a transient red coloration :
" Syrup of violets, and such other blue vegetable juices as I have hitherto tried are not reddened by fixed air, the tincture of tournsole [litmus] is of all known tinctures most easily acted on by acids, therefore the slightest vestiges, which cannot by any other means be discovered, are by this tincture easily detected" (JEssays, p. 16).
Thus if water " be tinged with tournsole to a perfect blue, when fixed air sufficient to fill about the 1/50 of the vessel has passed through it, it will be manifestly red ... In like manner one part of water, saturated with fixed air, makes 50 parts of the above tincture distinctly red. This change of colour, however occasioned by the fixed air, soon disappears in an open vessel, particularly if it be exposed to heat, or the rays of the sun ; a circumstance which indicates the volatile nature of the acid that produces the change {Essays, pp. 14-15).
(4) Its power of combining with mild alkalis to form neutral crystalline compounds, as discovered by Black and confirmed by Cavendish (pp. 69 and 108).
(s) Its power of dissolving chalk and magnesia, dis- covered by Cavendish (p. 103).
(6) Its power of dissolving iron (giving rise to artificial
VI CARBONIC ACID AND THE CARBONATES 103
chalybeate waters, as discovered by the English apothecary Lane in 1769), and of dissolving zinc.
(7) Its power of precipitating substances dissolved in alkalis, e.g. sulphur dissolved in lime-water.
On account of the acid properties of its solutions the French chemists in 1787 gave to the gas the name carbonic
ACID.
" As fixed air has been perceived to be produced by the direct combination of charcoal with vital air, by the assist- ance of combustion, the name of this gaseous acid can no longer be arbitrary, but necessarily must be derived from its radical, which is the pure carbonic matter ; therefore it is carbonic acid and its compositions with different bases are carbonates" {Chemical Nomenclature, tr. 1788, p. 32).
But as Bergman recognised, this acidity belongs to the solution rather than to the gas. For this reason the name " carbonic acid " is now restricted to the solution of the gas in water, whilst the gas itself ^ is called carbonic anhydride. The recognition of carbonic acid as an acid necessitated a change in the classification of the mild alkalis, which were now regarded as salts under the name of carbonates. In later years the term ' alkali ' became associated almost ex- clusively with the caustic alkalis, instead of the mild effer- vescent salts to which the name had been applied for a thousand years previously.
Cavendish (1767) discovers that chalk and magnesia are rendered soluble by fixed air. — To Cavendish belongs the credit of discovering that chalk is rendered soluble in water by the presence of fixed air. This discovery was made in the course of his " Experiments on Rathbone-Place Water" {Phil. Trans., 1767, 57, 92). He found that 494 ounces of a water, which on boiling liberated one-seventh of its volume of fixed air,^ deposited 271 grains of a calcareous
' Following Laurent (1854) ; see Chapter XII.
■^411 ounces of water liberated 65 ounce-measures of fixed air and 8J ounce-measures of common air.
104 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
earth consisting almost entirely of chalk. That chalk was actually held in solution by fixed air was proved by mixing lime-water with an excess of water saturated with the gas, when a clear solution was produced, whereas the use of less fixed air caused the lime to be precipitated in the form of chalk. Cavendish writes :
" Calcareous earths, in their natural state, i.e. saturated with fixed air, are "totally insoluble in water ; but the same earths, entirely deprived of their fixed air, i.e. converted into lime, are in some measure soluble in it ; for lime-water is nothing more than a solution of a small quantity of lime in water. It is very remarkable, therefore, that calcareous earths should also be rendered soluble in water, by furnish- ing them with more than their natural proportion of fixed air, i.e. that they should be rendered soluble, both by depriving them of their fixed air, and by furnishing them with more than their natural quantity of it. Yet strange as this may appear, the following experiments, I think, show plainly it is the real case."
" A bottle full of rain water was inverted into a vessel of rain water, and some fixed air forced up into the bottle, at different times, till the water had absorbed as much fixed air as it could readily do ; 1 1 ounces of this water were mixed with 6| of lime water. The mixture became turbid on first mixing, but quickly recovered its transparency, on shaking, and has remained so for upwards of a year."
" Lest it should be supposed, that the reason why the earth was not precipitated in the foregoing experiment, was, that it was not furnished with a sufficient quantity of fixed air, the following mixture was made, which contains the same proportion of earth as the former, but a less proportion of fixed air : 4f ounces of the above-mentioned water, containing fixed air, were diluted with 6\ of rain water, and then mixed with 6\ ounces of lime-water. A precipitate was immediately made on mixing, which could not be re-dissolved on shaking" {Phil. Trans., 1767, 57, loi and 104-105).
This observation was confirmed by Lavoisier, and also by Bergman, who noticed that if
CARBONIC ACID AND THE CARBONATES
105
"a small portion of lime-water be dropped into water impregnated with fixed air, slight clouds are immediately formed, occasioned by the saturation of the lime by the fixed air ; these clouds, however, disappear upon gently shaking the vessel, the lime being dissolved by the superabundant fixed air " {Essays, p. 34).
Bergman also recorded the fact that crystals of calcareous spar (Fig. 23) were dissolved by water containing fixed air, and that minute crystals of similar shape were sometimes deposited
Fig. 23 — Crystals of Calc-Spar. British Museum (Natural History).
when chalk was reprecipitated by the escape of the fixed air which held it in solution. These observations were important as affording a clue to the way in which crystalline calc-spar may have been produced ; they also afforded a satisfactory explanation of the formation of stalactites (Fig. 24) and stalagmites by the escape of fixed air from the dripping water of caverns in limestone districts.
The hardness of water due to salts of lime and magnesia. — The mineral matter held in solution by fixed air makes a water " hard," i.e. the water gives a curd with soap instead of a lather. As this hardness is removed
io6 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
when the water is boiled it is called temporary hardness. Cavendish showed that the whole of the chalk in such a water could be "precipitated . . by the addition of a proper quantity of lime-water" (loc. at. p. 107). This method of removing the fixed air and so precipitating the chalk from a hard water is used on a large scale in order to " soften " the water-supply in chalky districts such as that of the Caterham Valley.
Magnesia, of which Cavendish detected a small quantity
mixed with the chalk from Rath- bone Place water,! also owes its solu- bility to fixed air. It dissolves to a larger extent than chalk, and the solution is used in medicine as a gentle alkali under the name of " fluid magnesia." Like chalk, the mag- nesia produces a temporary hard-
FiG. 24 — Stalactite, /.tf., Chalk DEPOSITED BY THE neSS, which Can ESCAPE OF Fixed Air from Dripping Water. British Museum (Natural History). be remOVCd by
boiling, or by the addition of lime ; but the precipitation of the magnesia, and the softening of the water, takes place much more slowly than in the case of chalk.
Gypsum or selenite (of which Cavendish collected 39
' By acting on the earthy precipitate with oH of vitriol the chalk was converted into sparingly-soluble selenite or gypsum, and the magnesia into Epsom salts, of which 18 grains were extracted.
VI CARBONIC ACID AND THE CARBONATES 107
grains by evaporating 494 ounces of the water to a bulk of three ounces) shares with all the soluble salts of lime the property of rendering water hard. As the gypsum is not precipitated when the water is boiled, the hardness which it produces is called permanent hardness. This permanent hardness can be removed by adding to the water a mild alkali such as soda or potash, which precipitates the lime in the form of chalk and so renders the water soft. This is the method generally used for softening permanently hard waters.
Soluble salts of magnesia (of which Cavendish precipitated 36 grains by the addition of fixed alkali) also contribute to the permanent hardness of water. Sea water, which contains a little gypsum, and a relatively large proportion of Epsom Salts, is so hard that it is practically impossible to make it lather, even by using a very large amount of soap. The precipitation of soap by the mineral matter present in hard waters was discussed by Bergman ("Of the Analysis of Waters," Essays, pp. 90-192), who made use of a solution of soap in alcohol as a means of detecting such substances in water, thus,
" Soap is not soluble in every kind of water ; this is occasioned either by a disengaged acid, or by a large pro- portion of . . . salt with an earthy or metallic base . . . : such waters are generally called hard waters, and are unfit for washing cloths, as also for boihng pulse, and the harder kinds of flesh."
" If there be present in a can of water but 8 grains of alum, [muriate of] magnesia or [muriate of] lime, a single drop of this water occasions a turbidness in a solution of soap in alcohol, diluted with an equal bulk of distilled water" {Essays, p. 139).
Lane (1769) discovers that fixed air will dissolve iron. — Fixed air also possesses the power of dissolving iron, giving rise to chalybeate waters. In these waters the iron
io8 HISTORICAL INTRODUCTION TO CHEMISTRY chap.
is held in solution by fixed air in much the same way as the chalk and magnesia in the water investigated by Cavendish. This property of fixed air was discovered by T. Lane, an English apothecary, who described his experi- ments in the Fhilosophical Transactions for 1769, as follows :
"A wide-mouthed bottle, containing half a pint of distilled water and sixty grains of steel filings, was suspended forty-eight hours over some distiller's molasses, in brisk fermentation ; so as to receive the fixed air escaping from the fermenting liquor ; the surface of which was ten inches below the mouth of the bottle. Immediately after its removal, the clear water was decanted from the filings and ochrous sediment."
" This liquor had a brisk and ferruginous taste, with a flavour of the molasses. An infusion of galls, or green tea, soon changed part of it to a colour like ink. The remainder, being exposed to the open air, presently became turbid, threw up a party coloured pellicle, and deposited a yellowish sediment."
" The water now retained but little power of tinging with galls ; and in a few days lost this property entirely " (" On the Solubility of Iron in Simple water, by the intervention of Fixed Air," Fhil. Trans., 1769, 59, 218).
These simple experiments, carried out 140 years ago, disclose the two essential features of the modern theory of the rust- ing of iron, namely, (i) that iron is dissolved by carbonic acid to a colourless solution, and (2) that this solution deposits a yellow rust on exposure to the air.
Black (1755) discovers a second series of salts derived from fixed air. — Whilst there is some uncertainty as to whether Black recognised fixed air as an acid, there is no doubt as to his discovery of the existence of a salt containing a larger proportion of fixed air than ordinary potash.
" That the fixed alkali, in its ordinary state, is seldom en- tirely saturated with air, seems to be confirmed by the following experiment. I exposed a small quantity of a pure
VI CARBONIC ACID AND THE CARBONATES 109
vegetable fixed alkali to the air, in a broad and shallow vessel, for the space of two months ; after which I found a number of solid crystals, which resembled a neutral salt so much as to retain their form pretty well in the air, and to produce a considerable degree of cold when dissolved in water. Their taste was much milder than that of ordinary salt of tartar ; and yet they seemed to be composed only of the alkali, and of a larger quantity of air than is usually con- tained in that salt, and which had been attracted from the atmosphere : for they still joined very readily with any acid, but with a more violent effervescence than ordinary ; and they could not be mixed with the smallest portion of vinegar. . . . without emitting a sensible quantity of air " (A.C.R. XVII. 42).
The same salt was prepared by Cavendish and by Berg- man by the direct action of fixed air on a solution of the alkali. Cavendish, who examined it in 1766, found that in comparison with ordinary potash it contained twice as much fixed air relatively to its , power of neutralising acids. Bergman also examined this salt, and made a rough analysis, which showed that it contained both water and fixed air in combination with the mild alkali {Essays, p. 18).
Such salts, which contain a double portion of fixed air, were described by Bergman as "aerated vegetable alkali," " aerated magnesia," and so forth, but are now known as BiCARBONATES. It is remarkable (as Bergman noticed) that whilst the excess of fixed air converts the mild alkalis into /ess soluble neutral salts, the solubility of chalk and magnesia in water depends on their conversion into more soluble bi- carbonates. Similar soluble bicarbonates are probably formed when iron and zinc are dissolved by carbonic acid, since the carbonates of these metals are insoluble in water.
Summary and Supplement.
Priestley (1772) showed that fixed air is produced when charcoal is burnt in air confined over water, or over mercury ;
no HISTORICAL INTRODUCTION TO CHEMISTRY chap.
in the latter case no change is produced in the volume of the air
C + O2 -> CO2 (fixed air).
Carbon + oxygen —> carbonic anhydride.