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Full text of "A comprehensive treatise on inorganic and theoretical chemistry"

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Homird V/. Estill 

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. in 2007 with funding from 
IVIicrosoft Corporation 




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With 232 Illustrations. Crown 8vo, gs. 

With 334 Illustrations. Crown 8vo, 12s. 6^. 

reference to Practical Work. 
With Diagrams. 8vo, 21s. net. 

STEEL : an Introduction to the Study of Metallo- 
With 65 Illustrations. Crown 8vo, 8s. 6d. net. 














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With due regard to the law hwnanum errare est, this work aims at giving a 
complete description of all the compounds known in Inorganic Chemistry, and, 
where possible, these are discussed in the light of the so-called Physical Chemistry. 
The separation of Organic from Inorganic Chemistry is nothing more than a 
conventional convenience ; it is probable that the sharper the line of demarcation, 
the greater the loss which each of these divisions of chemistry will suffer. In the 
analysis of inorganic compounds, for example, some extraordinarily sensitive tests 
are available, and some extraordinarily clean separations can be effected by utilizing 
the properties of certain organic compounds of the metals. 

In the past, several complete records have been made. Starting from 
W. Nicholson's A Dictionary of Chemistry (London, 1795-1808), there have 
appeared in England : A. Ure's A Dictionary of Chemistry (London, 1821-35), and 
H. Watts' A Dictionary of Chemistry (London, 1866-68), which was later edited by 
H. F. Morley and M. M. P. Muir. There is also Sir Edward Thorpe's A Dictionary of 
Applied Chemistry (London, 1890-92), a new and revised edition of which is now 
in the press (1921). The English translation of L. Gmelin's Handbook of Chemistry 
(London) appeared in nineteen volumes between 1848 and 1872. This work 
covered both organic and inorganic chemistry. The sixth German edition appeared 
as the Eandbmh der anorganischen Chemie (Heidelberg) in 1871-86, while the 
seventh edition, commenced in 1905, is not yet complete. A number of other 
related books have appeared in Germany. The more important of these are 
A. Ladenburg's Eandworterbuch der Chemie (Breslau, 1882-89) ; H. von Fehling's 
Neues Handworterhuch der Chemie (Braunschweig), which commenced in 1874 and 
is not yet completed. It was founded on J. von Liebig, J. C. Poggendorff, and 
F. Wohler's Handivorferhitch der reinen und angeivandten Chemie (Braunschweig, 
1837-64). There is 0. Dammer's Handhuch der anorganischen Chemie (Stuttgart, 
1892-1903), and his Handhuch der chemischen Technologie (Stuttgart, 1895-98). 
R. Abegg's Handhuch der anorganischen Chemie (Leipzig), commenced in 1905, is 
not yet completed. In France there are E. Fremy's Encyclopedie chimique (Paris, 
1882-1905); C. A. Wurtz's Dictionnaire de chimie (Paris, 1868-1908); and 
H. Moissan's Traite de chimie minerale (Paris, 1904-6). In Italy, L. Guareschi's 
JVuovo enciclopedia di chimica (Torino), commenced in 1900, is still in progress. 
I have been more or less indebted for hints and ideas to all the above-named 
works, as well as to H. Kopp's Geschichte der Chemie (Braunschweig, 1843-47). 

Much of the material of this work was compiled in card-index form long before 
my Modern Inorganic Chemistry appeared ; and that work was really an abridge- 
ment of this one. The references which were not included in the scheme of that 
work will be found here. It was not originally intended to make the larger work 
assume the exhaustive character which this book has now acquired. Rightly or 
wrongly, I came to the conclusion that it is a mistake to load up a student with 



facts as if he were going to be a specialist in all branches of inorganic chemistry. 
In addition to the general principles, the salient features of certain type-compounds 
should be taught, and anything further should be left for works of reference, where 
full information may be obtained — to be absorbed or forgotten as may be expedient. 
Consequently, in the ideal case, a work of reference should not only give the 
authorities for statements of fact, but it should also indicate what knowledge has 
been gleaned on the particular subject in question. To do this in a practicable 
manner, attention must be directed to the original publications on the subject. 
This natui-ally makes the work of compilation extremely laborious ; in some cases, 
indeed, it happens that scores of independent references are involved in the state- 
ment of one particular fact. Fortunately I have rather a unique collection of 
dissertations and theses ; in a few cases, these have not appeared in the regular 
channels of publications. Where the original references are not in the libraries of 
this country, I have had to depend on an assistant in Berlin, who has generally 
been successful in tracking them where our libraries have failed. This has made 
some references very costly. A large proportion of the references will be found in 
the Abstracts of the London and American Chemical Societies, and of the Society 
of Chemical Industry. 

In the references, the usual abbreviation for the title of a periodical is given, 
then follow in clarendon type the volume number, the page or pages, and, last of 
all, the year of publication. In cases where a volume is made up from a number of 
bulletins with independent pagination, the number of the bulletin is employed 
instead of the page. In the cross-references, the first number in clarendon type 
refers to the volume, the second to the chapter, and the third to the § (section). 
It will be observed that the diagrams of the chapters have an independent 

In former times little more than a mere qualitative knowledge of the so-called 
physical and mechanical properties of elements and compounds was considered 
ample, but with the tremendous ramifications of the various industries increasing 
demands for precise data have been made from the workers in pure science. In 
reviewing the data I have been impressed with the prevailing lack of perspective in 
the measurements of physical properties, for, in some cases, these have been carefully 
measured with elaborate apparatus involving an experimental error hundreds of 
times smaller than the magnitude of the disturbing effects produced by impurities. 
It is not always enough to say that the materials were Herren X.Y.Z.'schen 
" chemically pure " preparations. For instance, in pre-war days I have had to 
make very serious complaints about the quantities of glass contained in their 
highest grade " chemically pure " potassium pyrosulphate. This would not have 
been suspected had its use not been attended by an epidemic of bad analyses. Of 
course, the best representative values of the physical constants of pure elements 
and compounds are very important, but in commercial work, materials of an 
extremely high degree of purity are regarded more as chemical curiosities, and 
larger errors may be introduced by using data — atomic weights, etc. — derived 
from pure materials, than by using data obtained with material of '* commercial " 

I think it was P. J. Macquer who apologized for the alphabetic form of the 
subject-matter of his Dictionnaire de chymie (Paris, 1766), by stating that chemistry 
was little more than a collection of facts scarcely entitled to the name of science, 
or capable either of synthetic or analytic explanation ; and hence he concluded that 
the dictionary form was the best mode of arranging the facts. The dictionary thus 


belongs to a primitive stage in the development of a science in that it is but a 
collection of facts to be employed in building up the science. 

We now flatter ourselves that the periodic law has given inorganic chemistry a 
scheme of classification which enables the facts to be arranged and grouped in a 
scientific manner. The appearance of order imparted by that guide is superficial 
and illusory. Allowing for certain lacunae in the knowledge of the scarcer elements 
prior to the appearance of that law, the arrangements employed by the earlier 
chemists were just as satisfactory, and in some cases, indeed, more satisfactory than 
those based on the periodic law. 

The arrangement of the subject-matter of inorganic chemistry according to the 
periodic scheme is justified solely by expediency and convention. It has a tendency 
to make teachers over-emphasize unimportant and remote analogies, and to under- 
estimate important and crucial differences. I imagine that when we have found 
a truer basis of classification, such differences as are displayed between, say, /err oswm 
a.nd.ffirricum compounds will be exhibited as if two different elements are involved, 
and that iron alone appears as the stable form when separated from these com- 
pounds. Similar remarks apply to other multi-valent elements. The difference 
between the higher and lower valent forms of an element with a given acid are 
often greater than between the compounds of two totally different elements with 
the same acid. 

The first volume of this work is mainly introductory, and in it the atom is 
considered to be the chemist's unit, or the unit of chemical exchange. The newer 
work on the structure of atoms, and the so-called elements with variable atomic 
weights will be introduced in the third volume, as a sequel to the radio-active 
elements. The collection in the first volume of most of the generalizations required 
for application to special cases in subsequent volumes has simplified many explana- 
tions. This applies, for example, to thermal diagrams, equilibrium diagrams for 
ternary systems, etc. The general historical sketches in this volume facilitate the 
reviews of the histories of the elements and their compounds which appear in 
subsequent volumes. 

Hydrogen and oxygen, and the compounds of these two elements, have been 
worked in with the introductory volume. The second volume includes the halogens 
and the alkali metals. The ammonium compounds are included with the com- 
pounds of the alkalies. The other elements will appear mainly in the order of the 
periodic law. The metal hydrides, oxides, halides, sulphides, sulphates, carbonates, 
nitrates, and phosphates are included with the metals ; the other compounds are 
described with the acids, or the acidic elements. With the complex salts and inter- 
metallic compounds of an element are included analogous compounds of ammonium, 
hydrazine, and hydroxylamine, as well as of all those elements which have been 
previously discussed. It should therefore be possible to locate a desired compound 
from an inspection of the backs of the volumes, which are lettered to show what 
elements are discussed inside. The indexes and cross-references are also available. 

In the 1778 edition of his Dictionnaire, P. J. Macquer referred to la nomencla- 
ture tres complete which was available. We are not so well provided to-day. Our 
nomenclature is inadequate and insufficient ; nor has it sufficient elasticity to adapt 
itself to increasing knowledge. Unfortunately, we have grown so accustomed to 
the system inaugurated near the beginning of the last century that we are afraid 
to make a drastic change. 

The systematic names of many compounds naturally depend on what view is 
taken of their constitution. Many names are thus determined by the prevailing 


by Weight (132); § 5. The Decomposition of Water by Metals (134); § 6. The 
Decomposition of Water by Electricity (136) ; § 7. Cavendish's Experiments on 
the Synthesis of Water by Volume (138). 



1. The Atmosphere (147); § 2. The Influence of Pressure on the Volume of Gases 
— Boyle's Law (150) ; § 3. Deviations from Boyle's Law (152) ; § 4. Dalton's Law 
of Partial Pressures (155); § 5. The Laws of Nature (157); § 6. The Influence 
of Temperature on the Volume of Gases— Charles' Law (158); § 7. Deviations 
from Charles' Law (162); § 8. The Critical State of Gases (164). 



1. Gay Lussac's Law of Combining Volumes (171) ; § 2. Amadeo Avogadro's Postulate 
(172); § 3. The Eelative Weights of the Molecules (174); § 4. The Formula of 
Compounds (178) ; § 5. The Relative Weights of the Atoms (179) ; § 6. Methods 
for Measuring the Vapour Densities of Gases, and of Volatile Liquids and Solids 
(181) ; § 7. The Struggle of Avogadro's Hypothesis for Recognition (186) ; § 8. 
Deviations from Avogadro's Law (192) ; § 9. Radicals or Radicles (197) ; § 10. The 
Atomic Weights of the Elements (198) ; § 11. The Relation between the Molecular 
Weights and the Volumes of Gases (201) ; § 12. Chemical Equations and Chemical 
Arithmetic (202) ; § 13. The Relation between Atomic and Combining Weights — 
Valency (204) ; § 14. The Polarity of Valency (211) ; § 15. The Association of Atoms 
in Three Dimensions (213) ; § 16. The Evolution of the Valency Concept (216) ; 
§ 17. Attempts to Explain Valency (225) ; § 18. Atomic, Molecular, and Specific 
Volxmies (228). 



1. The Classification of the Elements (248) ; § 2. Triads, and the Law of Octaves 
(252) ; § 3. The Periodic Law— D. I. Mendeleeff and L. Meyer (255) ; § 4. The 
Gaps in Mendeleeffs Tables of the Elements (260) ; § 5. The Application of the 
Periodic Law (262) ; § 6. Some Defects in the Periodic Law (263). 



1. The Occurrence of Hydrogen in particular and of the Elements in general (270) ; 
§ 2. The Preparation and Purification of Hydrogen (275) ; § 3. Chemical Affinity 
(291); § 4. The Measurement of the Affinity between the Acids and the Metals 
(294) ; § 5. Opposing Reactions. Guldberg and Waage's Law (297) ; § 6. The 
Solubility of Hydrogen (301); § 7. The Physical Properties of Hydrogen (313); 
§ 8. The Chemical Properties of Hydrogen (325) ; § 9. The Diffusion of Gases (338). 




§ 1. History of the Discovery of Oxygen (344) ; § 2. The Action of Heat on Mercuric 
Oxide (347) ; § 3. The Action of Heat on Potassium Chlorate (349) ; § 4. The 
Occurrence and Preparation of Oxygen (351) ; § 5. Catalysis (357) ; § 6. Consecutive 
Reactions (359); § 7. Concurrent or Side Reactions (360); § 8. The Physical 
Properties of Oxygen (363) ; § 9. The Chemical Properties of Oxygen (378) ; § 10. 
The Origin of the Terms: Acid, Alkali, Base, Salt (382); § 11. Acids (385); 
§ 12. Salts (387); §13. Neutralization (389),- § 14. Bases (393); § 15. Hydroxides 
and Anhydrides (395); § 16. The Polar Theory of Chemical Combination (397); 
§ 17. Binary and Unitary Theories of the Constitution of Acids and Salts (402). 



1. The Cycle of Water in Nature (405); § 2. The Purification and Distillation of 
Water (409) ; § 3. The Effect of Temperature and Pressure on the Volume of Water 
(410) ; § 4. The Vapour Pressure of Water— Fusion and Boiling (423) ; § 5. Gibbs' 
Phase Rule (444) ; § 6. Undercooling, Supersaturation, and MetastabiHty (450) ; 
§ 7. The Allotropic Forms of Water (457) ; § 8. The Physical Properties of Water 
(463) ; § 9. The Chemical Properties of Water (483) ; § 10. Hydrates and Hydrated 
Salts (498) ; § 11. The Vapour Pressure of Hydrated Salts (501). 



1. The Solubility of Solids in Water (506); § 2. The Freezing of Solutions (516); 
§ 3. The Solubility of Liquids in Liquids (522) ; § 4. The Solubility of Gases in 
Liquids— Henry's Law (527) ; § 5. The Solubility of Mixed Gases in Liquids — 
Dalton's Law (533) ; § 6. Diffusion in Gases and in Liquids (536) ; § 7. Solution 
Pressure — Osmotic Pressure (538) ; § 8. The Osmotic Pressure of Dilute Solutions 
and the Gas Laws (543) ; § 9. The Relation between the Vapour Pressure of a 
Solution and the Molecular Weight of the Solute (548) ; § 10. Distillation (553) ; 
§ 11. Other Hypotheses explaining Osmosis (557) ; § 12. The Relation between the 
Boiling Point pf a Solution and the Molecular Weight of the Solute (561) ; § 13. 
The Relation between the Freezing Point of a Solution and the Molecular Weight 
of the Solute (565) ; § 14. The Relation between the Solvent Power of a Solvent 
and the Molecular Weight of the Solute (568); § 15. Anomalous or Abnormal 
Results for the Molecular Weights of Substances in Solution (569) ; § 16. The Cause 
of Solution (574) ; § 17. The Physical Properties of Solutions (578). 



1. The Crystallization of Salts from Solutions (589); §2. Fractional CrystaUization 
(590) ; § 3. Crystals (593) ; § 4. The Crystallization of Solids en masse (602) ; 
§ 5. The Internal Structure of Crystals (607); § 6. The Seven Styles of Crystal 


Architecture (613); § 7. The Growth of Crystals (623); § 8. Analysis of the 
Structure of Crystals by X-rays (633) ; § 9. Liquid Crystals ; Crystalline Liquids ; 
or Anisotropic Liquids (645) ; § 10. Isomorphism— Mitscherlich's Isomorphic Law 
(651) ; § 11. The Eectifi cation of Atomic Weights by Isomorphism (668) ; § 12. The 
Formulae of Minerals, and of Isomorphous Mixed Salts (668) ; § 13. Index of 
Refraction and Dispersion (670). 



1. Matter and Energy (688) ; § 2. Thermochemistry (697) ; § 3. The Principle of 
Maximum Work (703) ; § 4. The Principle of Reversibility (706) ; § 5. Hess' Law 
(708) ; § 6. The Degradation or Dissipation of Energy (711) ; § 7. Bound and Free 
Available Energy (716) ; § 8. The Amount of Heat which can be Utilized for doing 
Work (719) ; § 9. Non-productive Energy. Entropy (721) ; § 10. The Work done 
by Afi&nity during a Chemical Reaction (730) ; § 11. The Effect of Temperature on 
Chemical Equilibria (732). 



§ 1. The Molecular Theory of Matter (740) ; § 2. The Kinetic Theory of Gases— Boyle's 
Law (742)^ § 3. The Kinetic Theory of Gases— Charles' Law and Avogadro's 
Hypothesis (747); § 4. Attempts to Obtain a More Exact Gas Equation (754); 
§ 5. J. D. van der Waals' Theory of Corresponding States (759) ; § 6. Summary of 
the Kinetic Theory of Molecules (765) ; § 7. Ultramicroscopic Particles — Ultra- 
microscopy (768) ; § 8. The Kinetic Theory of Atoms (782) ; § 9. The Two Specific 
Heats of Gases (786); § 10. The Relation between the Two Specific Heats of a 
Gas and the Degree of Freedom of its Molecules (790) ; § 11. The Molecular Heats 
of Gases (795) ; § 12. The Specific Heats of Elementary Solids — Dulong and Petit's 
Rule (798) ; § 13. Molecular Heats— Neumann's and Joule's Rules (805) ; § 14. The 
Meaning of Dulong and Petit's Rule (808) ; § 15. The Quantum Theory of Energy 
and Dulong and Petit's Rule (811) ; § 16. Debye's Theory of Atomic or Specific 
Heats (815); §17. The Kinetic Theory of Solids (818); § 18. Reactions between 
Solids — Spring's Experiments (824) ; § 19. The Vibration Frequency of Atoms and 
Molecules (828) ; § 20. Empirical Relations between the Properties of Solids (834) ; 
§ 21. The Kinetic Theory of Liquids (840) ; § 22. The Surface Tension and Surface 
Energy of Liquids and Solids (846) ; § 23. The Association or Polymerization of 
Liquids (860) ; § 24. Thermal Effects attending the Expansion and Compression 
of Gases (862) ; § 25. The Liquefaction of Gases (868) ; § 26. The Manufacture of 
Oxygen and Nitrogen from Liquid Air (874). 



1. The Discovery of Ozone and of Hydrogen Peroxide (877); § 2. The Modes of 
Formation and Preparation of Ozone (878); § 3. The Occurrence of Ozone and 
Hydrogen Peroxide (891); § 4. The Physical Properties of Ozone (893); § 5. 
Oxozone, Ozonides, and Oxozonides (899) ; § 6. The Chemical Properties of Ozone 
(901) ; § 7. The Constitution of Ozone (914) ; § 8. The Modes of Formation and 



Preparation of Hydrogen Peroxide (922) ; § 9. The Physical Properties of Hydrogen 
Peroxide (929) ; § 10. Quantitative Application of the Law of Mass Action (933) ; 
§11. The Chemical Properties of Hydrogen Peroxide (936) ; § 12. The Qualitative 
and Quantitative Determination of Ozone and Hydrogen Peroxide (949) ; § 13. The 
Composition and Constitution of Hydrogen Peroxide (952) ; § 14. Peroxides and 
Peracids (956). 



§ 1. The Products of Electrolysis (962); § 2. Faraday's Laws of Definite Electrolytic 
Action (963) ; § 3. The Velocity of Electrolytic Conduction (967) ; § 4. The Effect 
of the Solvent (968) ; § 5. The Ionic Hypothesis (969) ; § 6. The Electrolytic Con- 
ductivity of Solutions (977) ; § 7. The Number of Ions in a Solution (978) ; § 8. 
The Migration of Ions (983) ; § 9. The Speeds of Moving Ions— Kohlrausch's Laws 
(986); § 10. "Abnormal" Osmotic Pressures and Ionization (990); § 11. Equili- 
brium between Ionized and Non-ionized Solute (992) ; § 12. The Solubility Law 
(995) ; § 13. Acids and Bases according to the Ionic Hypothesis (1000) ; § 14. The 
Strengths of Acids and of Bases (1003) ; § 15. The Neutralization of Acids and 
Bases (1006). 



§ 1. The Factors of Energy (1011) ; § 2. Electrochemical Series of the Elements (1013) ; 
§ 3. Solution Pressure -Contact Differences of Potential (1015); § 4. The Ionic 
Hypothesis and Chemical Eeactions (1026) ; § 5. Polarization— Back Electromotive 
Force (1027) ; § 6. Decomposition Voltages (1030) ; § 7. Gas Cells (1033) ; § 8. The 
Relation between Electrical and Thermal Energy (1036) ; § 9. Fractional Electro- 
lysis— G. Magnus' Rule (1039). 

INDEX 1041 


there may be a retrograde movement by the advent of an age of intellectual dark- 
ness ; yet, in the main, these three periods characterize the growth of science as surely 
as the child, the boy, and the man characterize the development of an individual's 
mind. Chemistr}'' is a particularly happy illustration of Comte's idea. 

L The first, the msrthological, anthropomorphical, or superstitious stage.— 
This represents the childhood of chemistry, for, as man emerged from the mists of 
prehistoric antiquity, everything must have appeared to be full of wonder and 
mystery. He was overawed by the wind and the rain ; by the lightning and the 
thunder ; by the eclipse and the comet ; and by the rainbow and the clouds. The 
student of nature lived in a bewildering dreamland of mixed magic and myth which 
led him to ascribe supernatural explanations to inaccurately known facts, and 
consequently, he seemed to be surrounded on all sides by un monde invisible des 
esprits et des demons. Just as man's own actions seemed to be the result of his own 
efforts and volitions, so did natural phenomena appear to be the work of benignant 
or mahgnant spirits in air, earth, or sea ; and man accordingly made oblations to 
their residing deities to secure their kindly offices. Chemical phenomena were 
produced by spirits — the salamander or the sylph, the naiad or the nymph, the 
undine or the gnome — indwelling in different bodies, whose aid was invoked by 
incantation or charm to produce successful experiments. 

Accordingly, men who studied nature in those days were often suspected of 
tampering with the spirits of evil, and chemistrj'^ came to be known as one of the seven 
devilish arts. So too arose a childish fear and hatred of science, and the belief— 
widespread in the Middle Ages — that science is dangerous, and its votaries ought 
to be suppressed. In illustration, in 1287, the Order of Dominicans proposed to 
suppress chemical studies as had been attempted with physics in 1243 ; again, the 
Accademia dei Segreti — Academy of Nature's Secrets — founded by ,1. B. Porta in 
1602 for the discussion of scientific subjects, was dissolved by Pius III, after une 
existence courte mais glorieuse, apparently because it was believed that magic and the 
black arts were practised at its meetings. In the thirteenth century, Roger Bacon 
was arraigned at Oxford on an indictment for practising sorcery and magic ; and 
in order to disprove these accusations, he wrote his celebrated Epistola de secretis 
operihus artis et naturce et de nullitate magice to show that phenomena and appear- 
ances, then attributed to supernatural agencies, were simply due to the operation 
of natural laws. Again, in his Magice naturalis (Naples, 1558), J. B. Porta tried to 
show that the magic of nature is quite as wonderful as that of wizards and witches. 

T. Thomson opens his work, The History of Chemistry (London, 1830), by 
pointing out that chemistry sprang originally from delusion and superstition, and 
was at its commencement exactly on a level with magic and astrology. Superstition 
can flourish only where knowledge is imperfect and fragmentary. Day, adds C. J. 
Keyser (1914), is just as mysterious as night, and the mystery of knowledge and 
understanding is more wonderful and awesome than the darkness of the unknown. 
Mysterious phenomena, explained in one generation as the vagarious work of 
invisible demons or deities., appear to succeeding generations as the ordered workings 
of natural laws. The mists of superstition are always dissipated as positive know- 
ledge extends into wider and wider fields. 

The cuneiform inscriptions and the records of antiquity which have been 
transmitted to us, show that the early chemists were dominated by the gratuitous 
assumption that *' the interior agencies which keep the world in motion were personal 
forces essentially out of and above nature." The magician and the sorcerer, the 
necromancer and the wizard were the founders and keepers of the first rudimentary 
knowledge of nature. Accordingly, knowledge and superstition were interwoven 
with wondrous ingenuity and subtilty. The alchemists, following the mysticism 
introfcluced by the Alexandrian and Arabian schools, had virtually reverted to this 
stage of development when they spoke of red bridegrooms (gold) and lily brides 
(silver) ; of green dragons (mercury) and red lions (gold) ; of black crows (lead), 
and yellow scorpions (sulphur) ; and of flying eagles, fugitive stags, and inflated 



toads. One of the older chemists described the result of triturating mercuric 
chloride with mercury, resulting in the formation of mercurous chloride, in these 
pompous words : 

The fierce serpent is tamed and the dragon so reduced to subjection as to oblige him to 
devour his own tail. 

The anonymous work, Artis aurifer(B quam chemiam vacant (Basil, 1572), represents 

the dissolution of gold in aqua regia by a lion devouring the sun, as depicted in Fig. 1. 

This language persisted even as late as the 

eighteenth century. In W. Clarke's The 

Natural History of Nitre (London, 1670), for 

example, the red vapours formed when nitre 

is heated in a retort are called " the flying 


The seven metals — gold, silver, an alloy, 
copper, tin, iron and lead — known to the 
early Chaldeans, were also designated by the 
names and symbols of the seven greater 
heavenly bodies — the Sun, Moon, Mercury, 
Venus, Jupiter, Mars, and Saturn. A close 
relation was supposed to subsist between 
the metals and their respective planets so 
that nothing could happen to the one which 
was not shared by the other ; and it was 
further supposed that experiments with any 
particular metal were more likely to succeed Fia. 1.— Copied from an old Symbol repre- 
when the governing planet was in the ascend- gf j^^ *^^ Dissolution of Gold in Aqua 
ant, and near its zenith. Thus, in Para- 
celsus' directions for preparing an amalgam of lead and mercury, the two 
fluid metals are to be mixed " at the very moment of the conjunction of 
Saturn and Mercury." In some cases it is possible to see a fanciful reason why 
a particular metal was assigned to ^ particular heavenly body, but in other 
cases the connection is too remote to hazard even a guess! En passant^ it 
may be pointed out that an ingenious hypothesis to explain how the metals are 
affected by the planets was in circulation long after the original fancies had been 
forgotten. As N. Lemery expressed it in his Cours de chimie (Paris, 1675) : 

An infinite number of minute corpuscles pass to and from the metals and the planets, 
these corpuscles can easily pass through the pores of the metals and the planets they repre- 
sent, but they cannot pass into other bodies whose pores are not figured properly to receive 
them, or if they do get into other bodies, they cannot stay there to contribute any nourish, 
ment. The metals are thus perfected and nourished by the influence which comes from the 
planets and conversely. 

n. The second or philosophical stage. — At last man roused himself from his 
stupor of helpless wonder and childish guessing. He dimly realized some rnethod 
in nature's inscrutable complexity. Unfortunately, his vision was soon bedimnied 
and his mind intoxicated. Accordingly, we now find him arrogantly proclaiming 
the supremacy and omnipotence of the human reason. The majority of educated 
people of that age believed it to be undignified for a self-respecting man to make 
experiments, and they did not consider knowledge obtained by observing nature 
to be a serious subject worthy of mental occupation. Indeed, men were so proud 
of their intellectual supremacy that they persuaded themselves that their fancies 
about nature were finer, nobler, and more worthy of belief than nature herself ; 
and Plato apparently considered that the secret laws of nature could be invented 
by abstract thinking ; for, in his Republic, he said that " real knowledge is obtained 
by a simple process of reasoning independently of all information furnished by the 
senses." In his Phcedo, Plato expresses his delight with Anaxagoras' saying that 


** the mind is the cause and orderer of all things." The numerous absurdities 
obtained by the application of this principle are well exemplified in the pages of 
Plato's Timceus, where there are many illustrations of the vanity of the attempt 
to explain incomprehensible facts by nebulous words ; for example, Plato there 
states : 

The universe is a unique, perfect, and spherical production, because the sphere is the 
most perfect of figures ; and it is animated and endowed with reason, because that which 
is animated and endowed with reason is better than that which is not. 

Even Aristotle, the father of logic, reasoned that a vessel containing ashes would 
hold as much water as when the vessel contained no ashes. The conclusion is not 
true, showing that Aristotle did not always recognize the need for the discipline of 
the imagination by relentlessly checking reason against inexorable fact. 

Thus, man did not always see with Cicero that nature is a better teacher than 
the most ingenious philosopher. Prompted by a sublime imagination, R. Descartes, 
in his Principia philosophice (Amsterdam, 1644), built a hypothetical universe 
which had no substance, and is now regarded as little more than an idle dream. 
Well might T. Bergmann's essay De indagando vero (1779) claim : 

The philosophical method, by pretending to unlock the secrets of nature with ease and 
expedition, soothes a natural impulse to explain all things ; and by assimiing everything 
to be accessible to the human intelligence, administers pleasing flattery to vanity and 

The methods of thinking, the much vaunted philosophy of Plato and Socrates, 
in its attempt to proclaim the laws of nature from the throne of human reason, 
actually obscured the path of progress for many centuries, for it became the fashion 
to look with lofty scorn on knowledge gleaned by observing nature. Accordingly, 
the leading philosophers worshipped what Erancis Bacon might have called idola 
cogitationis— idols of the imagination ; they devoted themselves to fantastic and 
chimerical hypotheses about material things ; and made no earnest attempt to 
discriminate between the unreal and the real. As a result, their minds became so 
prejudiced that the facts were either denied, or else explained by extravagant 
ideas and fancies uncontrolled by truth and reality as we understand these terms 

in. The third, the scientific, or the positive era. — The marvellous Greeks gave 
promise of inaugurating this era before the advent of Christianity, but the feeble 
light kindled by Aristotle flickered and almost expired in the atmosphere of 
mysticism which prevailed in the Middle Ages. During this period, man almost 
reverted to the pandemonium of miracle and magic of his childhood days. The 
light re-appeared about the thirteenth century, and gained brilliancy during the 
succeeding centuries ; man then learned to see that nature is as she is, and is not 
subjected to the capricious will of deity or demon ; man recognized that nature 
is always conformable with herself without contradictions and without incon- 

The growth of chemistry as a science was nourished in the seventeenth century 
by the establishment of academies and societies for the cultivation of science. 
The famous Society of Rosicrucians,'^ which flourished mightily towards the end of 
the sixteenth or beginning of the seventeenth century, was perhaps an exception, 
for, judging from the many books which were poured from its presses between 1600 
and 1630, it rather fostered mysticism and obscurity, and was not favourable to 
the true scientific spirit. Long before the advent of the scientific societies, 
there were associations which fostered human knowledge, for example, the priests 
of Egypt had their temple laboratories ; and the same spirit led to the formation of 
the various schools of philosophy in Greece ; but the special feature of the later 
associations was their energetic protest against the worship of antiquity, where the 
authority of an ancient master was placed above experience. 

The Accndemia del Cimento, founded at Florence, in 1657, under the presidency 


of Prince Leopold de Medici, was the first scientific society of any importance ; its 
main object was " the repudiation of any favourite system or sect of philosophy, 
and the obligation to investigate nature by the pure light of experiment." Although 
it lived but ten years, it enriched the world by leaving a volume of important 
records of experiments, chiefly in pneumatics — Saggi di naturali esperienze fatti 
nelV Accademia del Cimento (Firenz, 1666). This work has been reprinted a number 
of times in several languages. The Royal Society of London was founded in 1660 ; 
VAcademie des Sciences of Paris in 1666 ; the Academia naturce curiosorum of 
Germany in 1652 ; and many others were founded in the eighteenth century. 
In some exceptional cases, these associations degenerated into " fastnesses from 
which prejudice and error were latest in being expelled ; and they joined in perse- 
cuting the reformers of science." The attitude of the University of Paris towards 
Galilei, and of the University of Oxford towards Roger Bacon have been cited as 
examples. In general, however, the policy of these associations was to encourage 
the investigation of nature by observation and experiment ; Arrierc les theories, 
vivent les f aits ! was their watchword ; and, instead of clothing their results in the 
enigmatical and allegorical language of the Rosicrucians, they sought to give a 
candid and straightforward account of their investigations and thoughts. In this 
way, the obscure mysticism of the Middle Ages was gradually dispelled. Man 
thus rediscovered that he does not bring any knowledge into the world with him ; 
that "the subtilties of nature far transcend the subtilties of the human reason" 
(F. Bacon) ; and that " knowledge cannot be invented, it must be discovered." 
Progress was then assured, and the manifold achievements of the observational 
and positive sciences during the past century are in striking contrast with the 
paucity of the results of philosophical thinking applied in vain for thousands of 


1 H. Martineau, The Positive Philosophy of Auguste Comte, London, 1875 ; J. S. Mill, Auguste 
Comte and Positivism, London, 1865 ; L. L. Bruhl, The Philosophy of Auguste Comte, London, 1903 ; 
S. Brown, Essays Scientific and Literary, Edinburgh, 1858 ; C. J. Keyser, Science and Rdigion, 
New Haven, 1914. 

2 A. E. Waite, The Real History of the Rosicrucians, London, 1 887 ; H. Jennings, The Rosi- 
crucians, their Rites and Mysteries, London, 1887. 

§ 2. The Observation and Record of Facts. Collecting Data 

The mind is like a blank tablet upon which experience writes that which is perceived 
by the senses.- — Aristotle (b!c. 320). 

To what can we refer for knowledge ? What can be a more certain criterion than the 
senses themselves ? If we cannot trust the senses, how is it possible to distinguish what 
is true from what is false ?• — Lucretius (b.c. 60). 

I know only that truth is in the things and not in my mind which judges them, and that 
the less I put my mind in my judgments about them, the more sure am I to come near to 
the truth. — J. J. Rousseau (1770). 

H. Poincare, in his La science et Vhyjpothese (Paris, 1904), emphasized in a very 
telling manner that true knowledge about material things can be acquired only 
through the senses — experientia docet ; there is no other way. Experience is the 
well-spring of true knowledge ; experience alone can teach something new ; it alone 
is irrefutable ; it alone can give certainty. The same idea was suggested by 
Aristotle and the peripatetical philosophers : nihil est in intellectu, quod non prius 
in senst^— nothing is in the intellect which was not first in the senses ; and by- 
Roger Bacon in his Opus majus about 1266, when he said : Sitie experientia nihil 
sufficienter sciri potest. Experience comprises all the impressions we observe and 
perceive through the various organs of sense. These impressions are recorded in 
our notebooks, dictionaries of chemistry, etc., as empirical realities or facts. 
Although knowledge cannot transcend the human faculties, much of the data of 
science is not directly furnished by the senses, for the senses are quite unable to 


discriminate the subtilties of nature. For instance, the speed of light and the 
size of atoms are magnitudes either too great or too small to be accessible to sense 
perceptions. Yet much data derived indirectly from the insensible physical world 
are assumed to be realities or facts, when actually they are known only by inference 
from data furnished by the senses. Without facts, science can do nothing ; they 
are the foundation and building stones of the whole superstructure. The edifice 
can be stable only in so far as it is founded upon the immutability of facts. The 
facts must be accurate, or the edifice will be unstable. 

Not very many years ago, an apt quotation from one of the classical writers — 
say Aristotle — was considered ample proof of the truth of any statement, and this 
in spite of repeated warnings ; even in the thirteenth century, Albertus Magnus 
could say : 

I pored over the books of all the sages from Morienus, Aristotle, and Plato downward, 
but yet I went wrong, until, by trial and mistakes, I at length discovered the truth. 

Science does not accept P. Bonus' dictum, in his Margarita novella (Basil, 1572) : 
** The mere fact that a great body of learned men believe a statement supersedes 
the necessitv for proof." To-day, science looks askance on records of mere opinions, 
and focuses its attention on records of facts. It is not always easy to record facts 
faithfully without unconscious distortion or bias. What we wish, said Demosthenes, 
that we believe ; what we expect, said Aristotle, that we find.i Things are not 
always what they seem. Seeing is not always believing. It is often difficult to 
distinguish appearances from realities for we are easily deceived by the mockery 
of sensations. The senses cannot be divorced from the mind ; neither is always 
to be trusted alone. The sun appears to rise and set ; in reality it does neither. 
So, although experience is the source of truth, it may also be a source of error. 
Superficial appearances may obscure hidden realities. Plato of old was unduly 
oppressed with the illusions and deceptions of sensory impressions, and he was 
accordingly led to deny the validity of knowledge derived from the sensations ; 
but Aristotle rightly showed that difficulties arise only when the mind wrongly 
interprets the testimony of the senses. 

In 1689, John Locke 2 emphasized the view that the senses are the tentacula of 
the mind because the mind primarily derives its knowledge of the external world 
through the senses. All our knowledge, said he, consists of a stock of ideas which 
were primarily produced in the mind by sensation, and which have remained after 
the sensation had ended. Our knowledge of chemistry, physics, etc., depends on 
the ability of the senses (i) to receive accurate impressions of the external world ; 
and (ii) to convey these impressions to the mind or brain. When the mind receives 
a sensation, it immediately begins to interpret the meaning, and it usually infers the 
existence of something outside itself which gave rise to the sensation. It may 
seem as if the mind directly perceives the external object which gives rise to the 
sensations ; but this is an illusion. The mind apprehends the sensation alone ; 
and it assumes that there exists a cause of the sensation external to itself. There 
is no doubt about the sensation, but there is less certainty about the inference ; 
the sensation must be accepted as a fact, but the inferential knowledge will be true 
or false according as the interpretation of the external cause of the sensation was 
correct or otherwise. The sensation does not err, it is the mind which fails when 
it misinterprets the material furnished by the senses. Hence, Plato could say that 
we do not see with the eyes but with our reason ; J. W. Goethe, that we see only 
what we know ; and E. Mach (1883), that the adaptation of thoughts to facts is 
the aim of all scientific research. 

It is therefore sometimes necessary to receive with caution the testimony of 
evidence derived from sensations. The mind interprets a sensation by comparing 
it with some former sensation, the source of which has been previously determined. 
Consequently, the faithfulness of the interpretation is dependent upon the memory 
of past sensations, or upon the sensitiveness of the mind to detect resemblances and 


differences. Otherwise expressed, the accuracy of an inference as to the nature of 
the objective source of a subjective sensation varies from a mere guess to virtual 
certainty. 3 The idea has been aptly illustrated this wise : just as a nimiber of bits 
of glass irregularly arranged always form symmetrical patterns when viewed 
through the kaleidoscope, so does the understanding of each man impose a pattern 
of its own upon the various sensations which it perceives. Consequently, as Robert 
Hooke ■* once said : It is necessary to be on guard against deep-rooted errors which 
may have been grafted upon science by the slipperiness of the memory, the narrow- 
ness of the senses, and the rashness of the understanding. The greatest caution 
must be exercised in accepting, on secondhand evidence, facts which cannot be 
verified. No reliance can be placed on vague impressions. Evidence must be 
clear and precise. 

Few persons can estimate and register facts impartially and fairly. As W. S. 
Jevons 5 puts it: " Among uncultured observers, the tendency to remark favourable, 
and forget unfavourable events is so great that no reliance can be placed on their 
supposed observations." T. Bergmann long ago drew attention to this very trait. 
He said : 

One observer will relate an event with the most extravagant encomiums ; another will 
detract from its real merit ; a third, by some oblique insinuation, will cast suspicion on the 
motive ; and a fourth will represent it as a crime of the blackest dye. These different 
descriptions represent the character of the respective observers. 

Untutored minds are very prone to mistake inferences for observations, and pre- 
possessions for facts ; their observations and their judgments are alike vitiated by 
dogma and prejudice ; they do not seek to investigate, they seek to prove. The 
old proverb is inverted, believing is seeing. The student of science must pledge 
himself to do his best to eliminate prepossession and dogma from his judgments, 
and he must spare no pains to acquire the habit of recording phenomena as they 
are observed ; and to distinguish sharply between what is or has been actually 
seen, and what is mentally supplied. It requires a mind disciplined like a soldier 
to avoid the natural inclination to look away from unwelcome facts. 

The purity of truth is almost certain to be corrupted when the observer is ruled 
by preconceived opinions, for, as 0. W. Holmes puts it : When we have found one 
fact, we are very apt to supply the next out of the imagination ; or as T. Bergmann 
said in his essay De indagando vero (1779) : 

An observer swayed by preconceived opinions, may be considered as one who views 
objects through coloured glasses, so that each object assumes a tinge similar to that of the 
glasses employed. He who seeks the truth must learn to observe with equal candour 
those facts which controvert his opinions, and those which favour them. 

It is only in a pseudo-science, said 0. W. Holmes, that positive evidence, or 
such as tells in favour of its doctrines, is admitted ; and all negative evidence, or 
such as tells against it, is excluded. C. Darwin, in his Autobiography (London, 1887), 
states that one of his golden rules was to make a memorandum of any fact or 
thought which he found to oppose his general results, because he noticed by ex- 
perience that such facts or thoughts were far more apt to escape the memory than 
favourable ones. Above all, said Robert Hooke (1665), a good observer needs a 
sincere hand and a faithful eye, to examine and record things themselves as they 
really appear. " The mind and the reason of the trustworthy observer must be 
trained to rebel against all desire, and to disobey all inclinations." 

The belief that bodies contained a definite quantity of heat substance or caloric 
prevented Black's successors from regarding the fact, known to every savage, that 
heat is produced by friction ; the theory of phlogiston prevented some of the early 
chemists from recognizing the increase in weight which occurs when metals are 
calcined — oculos habent et non videbunt (Psalm 116. 5) ; the assumption that air is 
absorbed when lead is roasted prevented Stephen Hales recognizing oxygen as the 
gas evolved when red lead is heated ; and, as E. Mach (1892) has pdinted out in 


his Populdre Vorlesungen (Leipzig, 1903), the undulatory theory of light prevented 
C. Huygens marking the fact of polarization which Isaac Newton, undisturbed by 
theories, perceived at once. 


* W Hamilton, Lectures on Metaphysics, Edinburgh, 1. 74, 1859. 

" J. Locke, An essay concerning human understanding, London, 1689; E. Mach, Populdre 
Vorlesungen, Leipzig, 1903. 

* E. Mach, Beitrage zur Analyse der Empfindungen, Leipzig, 1885 ; Chicago, 1897 ; A, Philips, 
Essays toirards a Theory of Knowledge, London, 1915 ; A. Rau, Empfindung und Denken, Giessen, 
1896 ; P. Carus, The Primer of Philosophy, Chicago, 1904. 

* R. Hooke, Micrographia, London, 1665. 

^ W. S. Jevons, The Principles of Science, London, 1874. 

§ 3. The Collating, Sifting, and Clarifying of Observations. Classifying Data 

History teaches that the commencement of every branch of science is nothing more 
than a series of observations and experiments which had no obvious connection with one 
another.- — J. von Liebig (1846). 

In order that the facts obtained by observation and experiment may be capable of 
being used in furtherance of our exact and solid knowledge, they must be apprehended and 
analysed according to some conceptions which, applied for this purpose, give distinct and 
definite results, such as can be steadily taken hold of, and reasoned from.* — W. Whewell. 

The record of facts obtained by observation and experiment, jper se, is empirical 
knowledge. Empirical is derived from the Greek word ifxireLpLKo^, meaning ex- 
perienced. It has just been emphasized that all knowledge is derived from 
experience, and hence empiricism would appear to be the right method of 
acquiring knowledge. The term, however, has slightly changed in meaning, for 
it is now usually applied to chance experiences which occur irregularly without any 
orderly plan of investigation. 

All true science, said T. Huxley, must begin with empirical knowledge. Nature, 
however, presents to our senses a panorama of phenomena co-mingled in endless 
variety so that we are sometimes overwhelmed and dazed by the apparent com- 
plexity of empirical knowledge. It is work for the intellect to educe the elements 
of sameness amidst apparent diversity, and to see differences amidst apparent 
identity. It is work for the judgment to reject accidental and transient attributes, 
and to consolidate essential and abiding qualities. Consequently, while the primary 
aim of science is to collect facts, the higher purpose of science is to show that, 
amidst wild and terrible disorder, order and law reign supreme. The man of 
science seeks a refuge from this bewildering complexity in unifying principles by 
which the facts can be grouped and classified into systems. As he gazes into 
nature, the man of science must be quick to discern hidden resemblances amidst 
a thousand differences ; he must be quick to disentangle natural relations 
from a medley of detail ; and quick to detect dissemblances amidst alluring 

Empirical knowledge describes facts ; science begins by comparing facts. 
Empirical facts, in consequence, can form a science only when they have been 
arranged, rearranged, grouped, or classified so as to emphasize the elements of 
similarity and identity in different phenomena. Accordingly Thomas Hobbes 
expressed the opinion that the main purpose of science is the tying of facts into 
bundles. This bundle-tying, indeed, forms no small or insignificant part in the 
development of science ; otherwise expressed, a significant advance has been made 
in the development of a science when the observed facts have been codified into a 
system so that a medley of empirical facts is systematically summarized under a 
small number of heads. This means that the facts must be arranged in a methodical 
and systematic manner until finally all the relevant facts taken together may form 
one system. The process of classification and correlation is one of the methods 


of scientific investigation. Knowledge so systematized is scientific knowledge. 
T. Bergmann (1779) illustrated the idea in his essay previously cited : 

A vast number of observations without order or regularity is not luilike a confused 
heap of stones, lime, beams, and rafters requisite for constructing an edifice, but which 
being combined with no skill fail in producing the proposed effect. 

The material framework of the world appears in a myriad different guises and 
combinations, but the chemist can resolve each combination into a few definite 
elementary forms of matter ; similarly, a multitude of forces can be resolved into 
comparatively a few primitive forms of energy. About 150 a.d., the Egyptian 
astronomer Claudius Ptolemy measured the angles of incidence and refraction of a 
beam of light passing from air into water, but more than fourteen hundred years 
elapsed before W. Snell (1621) detected the law of refraction hidden in Ptolemy's 
data. By tabulating his measurements of the volumes of air confined under different 
pressures, Robert Boyle discovered the law known by his name. Each of these 
laws summarizes in one simple rule myriads of possible measurements. 

Scientific knowledge is not necessarily more accurate than empirical knowledge. 
Empirical uncoordinated facts are no less true, definite, and real than scientific 
facts, for all facts are equally true fer se. A collection of empirical facts always 
requires some theory to serve as framework in order that the facts may be arranged, 
grouped, and pigeon-holed. According to F. Hoefer (1843) : 

II n'y a rien de plus stupide qu'un fait, quand il ne se rattache a aucune cause connue, 
a aucune loi dominante. II faut done concilier I'individualisation des faits avec leur 
generalisation. C'est la que reside le vrai crit^rium, I'avenir de la science. 

If a group of facts — scientific facts — has been organized on an erroneous system, 
the facts are no less true though the system be false. Chemistry presents a 
curious mixture of empirical facts with isolated fragments of scientific knowledge. 

§ 4. The Generalization of Observations 

Facts are the body of science, and the idea of those facts is its spirit. — S. Brown. 
It is the intuition of imity amid diversity which impels the mind to form science. — 
F. S. Hoffman. 

The correlation of empirical facts requires qualities of the mind different from 
those employed in observation and experiment. Both qualities are not always 
located in the same individual. Some excel in the one, not in the other. 
J. Priestley, C. W. Scheele, and H. Davy, for instance, were admirable observers, but 
they were not brilliant in the work of correlation ; J. Dalton and A. L. Lavoisier 
were not particularly distinguished as experimenters, but they excelled in correlating 
observed data. W. Hamilton ^ did not rate the fact-collecting faculty very highly. 
He said : 

In physical science the discovery of new facts is open to every blockhead with patience, 
manual dexterity, and acute senses ; it is less effectively promoted by genius than by 
co-operation, and more frequently the result of accident than of design. 

J. Priestley (1783) recognized his own limitations when he said : " I have a tolerably 
good habit of circumspection with respect to facts, but as to conclusions from them, 
I am not apt to be very confident." Skill in the critical analysis of observational 
data, and in collating, sifting, and clarifying records, is not a sufficient recommenda- 
tion to the adytum — the sanctorum sanctissimum — of science. There is still a 
higher type of work for but a few seekers after knowledge. It is 

To search thro' all 
And reach the law within the law. — Tennyson. 

It is the sprite imagination which usually reveals the deeper meaning of facts which 
have been diligently garnered, and laboriously sifted . ^ 


It cannot be doubted that science in its higher work, requires a supple and 
well-developed imagination 2 which T. Gomperz says is the instrument of genius, 
no less for scientific discovery than for artistic creation. The secret charm of 
scientific discovery is not in the facts per se, but rather in the extrication of natural 
relations among the facts one with another. Particular groups of facts must be 
unified or generalized into a system — the so-called law. Science begins with facts 
and ends with laws. Law is the essence of facts. As pointed out elsewhere, Newton's 
celebrated law epitomizes in one simple statement how bodies have always been 
observed to fall in the past. Immortal Newton did not discover the cause or the why 
of the falling of the apple, but he did show that it was due to the operation of the 
same forces which hold the earth, the planets, and their satellites in their appro- 
priate orbits. Newton's simple and comprehensive law epitomizes in one single 
principle the many and varied phenomena associated with falling bodies, planetary 
motions, etc., and generally, the scientific generalization explains the operations 
of nature by showing the elements of sameness in what at first sight appears to be 
a confused jumble of phenomena. Generalization is the golden thread which 
binds many facts into one simple description. That peculiar type of genius, that 
rare quality of mind required for the, work of generalization, is found only in a Newton 
or a Darwin. Plato said that if ever he found a man who could detect the one in 
inany he would follow him as a god. 

Unification is the supreme goal of modern science, or, as Heracleitus (c. 450 B.C.) 
proclaimed, the highest goal of knowledge is the one law regulating all events. 
However, with A. Comte,^ the majority will have la profonde conviction personelle, 
that the attempt to explain all phenomena by une hi unique is chimerical. Several 
natural phenomena belong to different categories, and are irreducible one to another. 
At best, man has to apply a very weak intellect to a very complicated world ; and 
the resources of the human intellect are too narrow, and the universe is too complex 
to leave any hope that it will ever be within man's power to carry scientific perfection 
to Tennyson's last degree of simplicity : 

. . . one law, one element. 


1 W. Hamilton, Discussions on Philosophy and Literature, London, 239, 1852. 

2 T. Gomperz, Greek Thinkers, London, 4. 125. 1912. 

' A. Comte, Cours de philosophie positive, Paris, 1. 44, 1864 ; H. Martineau, The Positive 
Philosophy of Av^uste Comte, London, 1. 13, 1875. 

§ 5. The Aim of Science in General, and of Chemistry in Particular 

Let us remember, please, that the search for the constitution of the world is one of the 
greatest and noblest problems presented by nature. — G. Galilei. 

The ordered beauty of the world of nature suggests an infinite inteUigence with powers 
of action such as no man possesses.' — Benjamin Moore. 

Science embraces the sum-total of human knowledge, and it ranges over the 
whole realm of nature. Science is not a mass of empirical knowledge gained by 
observation and experiment, but it is an organized body of facts which have been 
co-ordinated and generalized into a system. Science tacitly assumes that nature 
is a harmonious unity, and that rational order pervades the universe. Science seeks 
a complete knowledge of the multitude of inter-related parts of the universe which 
act and react on one another producing endless variety. In fine, science aims at 
omniscience. The target, however, appears to recede with increasing knowledge. 
As man grows in wisdom and knowledge, he begins dimly to realize that the unknown 
multiplies into boundless proportions. 

The sciences are too complex and too vast to be comprehended by one man's 

One science only will one genius fit, 
^ So vast is art, so narrow human wit. — Pope. 


Our feeble wit has rendered it necessary to rear a tree of scientific knowledge with 
many branches : astronomy, physics, chemistry, mineralogy, geology, biology, 
sociology, etc. " The divisions of the sciences," said Francis Bacon, " are like the 
branches of a tree that join in one trunk," and they are therefore more or less closely 
related with one another. The astronomer, the physicist, the chemist, each usually 
keeps to his own particular branch. This separation of the sciences is mere con- 
vention. Even in the middle of the thirteenth century Roger Bacon saw that there 
are no real lines of demarcation between the different sciences, for he pointed out 
in his Opus tertium (1267) : 

All the sciences are connected ; they lend each other material aid as parts of one great 
whole. Each does its own work, not for itself alone, but for the other parts. ... No 
part can attain its proper result separately ; since all are parts of one and the same com- 
plete wisdom. 

The science of chemistry is man's attempt to classify his knowledge of all the 
different kinds of matter in the universe ; of the ultimate constitution of matter ; 
and of the phenomena which occur when the different kinds of matter react one 
with another. The science of chemistry is itself so vast, that many branchlets are 
necessary for useful work, and thus we have : inorganic chemistry, organic chemistry, 
physical chemistry, mineralogical chemistry, bio- chemistry, agricultural chemistry, 
pharmaceutical chemistry, etc. The chemist also frequently aims at applying his 
knowledge to useful purposes in the arts and industries ; and thus arises appHed, 
industrial, or technical chemistry. 

Applied chemistry. — About the middle of the thirteenth century, Roger Bacon 
distinguished between knowledge sought for the sake of truth, and knowledge 
utilized in the practice of the various arts ; or, as I. R. Averroes expressed it a 
century earlier : In pure science, scimus ut sciamus ; and in applied science, scimus 
ut operemur. The distinction, however, was recognized in the fourth century B.C., 
for it was explicitly expounded in Aristotle's Metaphysics, and it was also intimated 
still earlier in Plato's Republic.'^ The purpose of pure science is to observe pheno- 
mena and to trace their laws ; the purpose of art is to produce, modify, or destroy. 
Strictly speaking there is no such thing as applied science, for, the moment the 
attempt is made to apply, science passes into the realm of art. It has been well 
said that " science is indebted to art for the means of experimenting, but she 
instructs art concerning the properties and laws of the materials upon which the 
latter operates." In an essay on The usefulness of experimental philosophy, Robert 
Boyle (1663) emphasized the mutual benefits which would obtain when science, 
or, as he called it, when natural philosophy is applied to the various arts and crafts ; 
and he claimed that it is prejudice, no less pernicious than general, which has kept 
science so long a stranger in the industries. Boyle's ideas have been still further 
emphasized by Lord Kelvin (W. Thomson), who said in 1883 : 

There cannot be a greater mistake than looking superciliously upon practical applica- 
tions of science. The life and soul of science is its practical application, and just as the 
great advances in mathematics have been made through the desire of discovering the 
solutions of problems which were of a highly practical kind in mathematical science, so in 
physical science many of the greatest advances that have been made from the beginning 
of the world to the present time have been in the earnest desire to turn the knowledge of 
the properties of matter to some purpose useful to mankind. 

The so-called applications of science to the industrial arts — say, applied chemistry 
— may be (i) An attempt to extend the methods of scientific investigation to the 
industrial arts ; or (ii) To adapt known operations and laws to useful purposes. 
When the chemist is occupied in the systematic observation of phenomena, and in 
tracing their laws, he is engaged in scientific investigation, no matter if the work be 
conducted in academy, in counting house, or in factory. 


^ W. Hamilton, Lectures on Metaphysics, Edinburgh, 1859; Anon., Chem. News, 18. 215,239, 
263, 1868; 19. 1, 61. 109, 1869. 


§ 6. Experiment 

Experiment is the interpreter of nature. Experiments never deceive. It is our judg- 
ment which sometimes deceives itself because it expects results which experiment refuses. 
We must consult experiment, varying the circumstances, iintil we have deduced general 
rules, for experiment alone can furnish reliable rules.- — Leonardo da Vinci. 

Nature speaks to us in a peculiar language, the language of phenomena. She answers 
all the questions we ask her, and these questions are our experiments. — J. von Liebig. 

Chemistry is largely an experimental science. Experiment is really a method 
of observation, which is employed when the facts are so masked by other conditions 
that they cannot be accurately observed unless the obscuring conditions are sup- 
pressed. The chemist would not make much progress if it were only possible to 
observe phenomena just as they occur in nature, and not possible to make observa- 
tions under determinate conditions. By experiment, it is possible to make combi- 
nations of different forces, and different forms of matter which are not known to 
occur in nature ; to eliminate complex disturbing conditions ; and to observe 
phenomena under simplified conditions. An experiment has been well defined as 
une observation provoquee. Experiment, said G. A. Reid, is useful only when there 
are conditions which obscure direct observations. The most successful experiment 
does no more than make a fact which was previously obscure as patent as one that 
was open to direct observation from the first. Chemical phenomena, per se, are 
usually too complex for our minds to grapple, and they must be simplified by 
simple experiments. Consequently, chemistry is an experimental science because 
its facts can rarely be observed in any other way. If data could be obtained by 
direct observation, there would be no need for experiment. 

It requires much acumen to determine the precise conditions under which an 
experiment shall give a successful result. Every experiment has the character of 
a specific question. The skilled questioner — the experimenter — knows what he is 
asking, and he tries his best to interpret nature's reply, be it affirmative, negative, 
or evasive. If the answer be negative or evasive, the question has not been properly 
asked, and it must be plied again and again until 

A sharphooked question baited with such skill 
It needs must catch the answer. 

Paradoxically enough, the investigator can usually say with " Dr. Moreau " : " ] 
asked a question, devised some method of getting an answer, and got — a fresh 
question." Some such ideas were in Robert Hooke's mind when he said : 

The footsteps of nature are to be traced, not only in her ordinary course, but when she 
seems to be put to her shifts, to make doublings, and turnings, and to use some kind of art 
in endeavouring to avoid our discovery. 

The more intricate the experiment, the greater the probability of an obscure 
and ambiguous result. As A. L. Lavoisier has pointed out, " it is a necessary 
principle in experimental work to eliminate every complication, and to make experi- 
ments as simple as possible." The quality of an experiment, not the quantity, is 
best adapted to throw light upon a phenomenon. Experiments carelessly performed 
may be sources of error and obscurity. Many of the results obtained by the alchemists 
in the Middle Ages show how ineffective or abortive are the results of experiments 
in incompetent hands — here, the experiments wandered into eccentric by-paths, 
and furnished preposterous conclusions. Experiment is an art, said G. A. Lewes 
(1864) and demands an artist. 

Joseph Priestley believed in making a large number of haphazard experiments, 
and said that he discovered oxygen by trying the effect of heat on many substances, 
apparently selected at random by John Warltire of Birmingham. Thomas A. 
Edison, also, appears to have discovered the phosphorescence of calcium tungstate 
when exposed to Rontgen's rays by deliberately trying the effects of these 
rays on a large collection of different substances. This old prosaic method of 


experimenting by trying everything is necessary in some cases, and, though usually 
dubbed empirical or rule-of-thumb, the process is fundamentally scientific, but it 
is not generally economical in time and labour. Discoveries are then due, as 
J. Priestley once argued, more to " chance than to any proper design or preconceived 
theory." More frequently, the track of the experimenter is blazed by means of 
working hypotheses. 

§ 7. Hypothesis, Theory, and Law 

We are gifted with the power of imagination, and by this power we can enlighten the 
darkness which surrounds the world of senses. Bounded and conditioned by co-operant 
reason, imagination becomes the mightiest instrument of the physical discoverer. — 
J, Tyndall. 

The nearer to the practical men keep, the mightier their power. The theorist who 
dreams a rainbow dream, and calls his hypothesis true science, at best is but a paper financier 
who palms his specious promises for gold.- — T. L. Harris. 

Hypotheses are cradle songs by which the teacher lulls his pupils to sleep. — L. W. 

It is a popular belief that the aim of science is to explain things ; as a matter of 
fact, the so-called explanations of science do not usually get much beyond describing 
the observed facts in the simplest possible terms so as to make their relations with 
one another clear and intelligible. i The description may emphasize the history of 
a phenomenon, or the conditions under which the phenomenon occurs : In other 
words, science may explain a phenomenon by describing how one event is determined 
by an antecedent action — sometimes called a cause; and how one particular set of 
conditions — the cause — can give rise to another set of conditions — the effect. 
Science explains a phenomenon (the effect) by showing that it is a necessary or rather 
a probable consequence of another phenomenon (the cause). 

Classical scholars tell us that Aristotle has lorty-eight, and Plato sixty-four 
meanings for the word cause. The later metaphysicians have also played a game 
of shuttle-cock with the term. The word cause is usually appHed to an event, 
action, or process which " produces " an effect ; or, with R. Shute, cause may be 
regarded as that which the mind selects as a sign of the coming of that other phe- 
nomenon which it calls the effect ; or conversely, an effect is regarded as something 
which the mind selects as a sign of the past existence of a cause. There can there- 
fore be no cause without an effect, and no effect without a cause. The one pre- 
supposes and completes the other. Hence, as P. Carus has observed, the law of 
causation describes a transformation in which form alone is changed ; and conse- 
quently, the law of causation is nothing more nor less than another aspect of the 
famous law of the conservation of matter and energy. The search for the cause of 
an event is a search for the determining factors which would produce that event. 
When the cause of an event has been discovered, the event is said to be explained 
by the cause. 

There are certain circumstances or conditions which may exercise, directly or 
indirectly, a determinative influence on the effect produced by the activity of a 
cause ; and very often certain conditions must obtain before an event can occur, 
thus the temperature of hydrogen must be raised above its ignition point before 
combustion can ensue. The effect obtained by burning hydrogen is more vigorous 
if the flame be in oxygen gas than if it be in air. Hence, an atmosphere of oxygen 
gas is a favourable condition for the combustion of hydrogen ; a reduced pressure 
is a retarding condition because it hinders the speed of combustion and reduces the 
vigour of the flame. The term cause is frequently employed when reason is mtended. 
The difference is marked in different countries by the use of different terms— Greek : 
ah-id (cause), alxv (principle, reason) ; Latin : causa, ratio ; French : cause, 
raison d'etre ; German : Ursache, Grund ; Italian : causa, ragione ; etc. Gravita- 
tion is said to be the cause of the falling of a vase from the mantelpiece, whereas 
the cause of the fall may have really been a push from the elbow. In the former 


case, the reason why the vase fell downwards is the very same reason why all masses 
gravitate, and a push was the real cause of the catastrophe. Here the reason of 
the fall is referred to an inherent quality of bodies, just as the reason why bodies 
react chemically is explained by investing matter with an inherent quality or vii< 
occulta — chemical affinity. If these distinctions be borne in mind, there is no need 
for confusing cause, reason, and condition, even if one term be used for all three 

The law of continuity — emphasized by G. W. von Leibniz (1687) — assumes that 
no interruption between cause and event is possible, and that there is a connected 
chain in the order of natural phenomena so that when several of the links are 
known, the intermediate links can be inferred. Consequently, men of science 
assume that each phenomenon is an efiect of a previous event, and is itself the cause 
of a succeeding effect, and that under like conditions, the same causes produce 
the same effects. Apart altogether from the question whether or not nature can 
do precisely the same thing again under precisely similar circumstances as she has 
done before, the principle of continuity or uniformity assumes that any phenomenon 
will be repeated if all the preceding phenomena be precisely repeated ; otherwise 
expressed : the same antecedents are invariably accompanied by the same conse- 
quents. Hence, it has been said that science does not now seek for the reason or 
the why of events, but rather for invariable relations between phenomena. The 
law of causation is taken to describe a sequence of changes starting with the cause 
and ending with the effect. G. Kirchhoff introduced the term description as a 
synonym for cause at the very beginning of his Vorlesungen uber mathematische 
PhysiJc (BerHn, 1876), where he said : " The object of mechanics is to give a complete 
description in the simplest possible manner of such motions as occur in nature." 

Although every effect may be traced to a previous event as its cause, in the 
physical world, phenomena follow one another as links in an unbroken chain of cause 
and effect. It is soon recognized that the cause of a phenomenon is an effect which 
itself needs explaining by some ulterior cause, so that causes can be traced back- 
wards in a never-ending chain of events. Owing to the limited range of man's 
understanding in a world of infinite complexity, we are far, very far, from compre- 
hending the true conditions, the true causes, or the true reasons for natural pheno- 

The mind cannot receive a long series of details without encircling and con- 
necting them by a common bond which is a kind of mental nexus ; similarly, 
in the attempt to find the causes of many phenomena, man is compelled to build 
an imaginary model showing how a given set of conditions — the hypothesis or theory 
— is always followed by particular effects. A phenomenon is then explained by 
showing that it is bound to occur by the operation of the set of conditions postu- 
lated by the hypothesis. Consequently, hypotheses are essentially guesses at truth. 
The rational observer does not trust to random guesses, but he is guided by a more 
or less vague intuitive conjecture (hypothesis) as to the meaning of the phenomena 
under investigation, and experiments are devised accordingly, for 

Man's work must ever end in failure, 
Unless it bear the stamp of mind. 
The head must plan with care and thought, 
Before the hand can execute.- — Schiller. 

The Spanish philosopher J. L. Balmes emphasized this same idea m his Filosofia 
/ondamental (Barcelona, 1846), when he said : 

Although one accepts as a real truth the most uncontested and the most certain fact, it 
remains sterile if ideal truths do not f ecimdate it. . . . To acquire scientific value, the facts 
must become objective, or, being submitted to reflection, must be impregnated by the 
mind with the light it lends to necessary truths. 

Hypotheses precede observation and prompt experiments, for they indicate the 
conditions under which the search for new facts is likely to be successful. Hence, 


when Leonardo da Vinci (c. 1500) 2 said that " hypothesis is the general, and experi- 
ments are the soldiers," he probably meant that hypotheses direct or indicate what 
experiments should be made. Accordingly, hypotheses are indispensable aids in 
the systematic quest after the secret meaning in nature's deeds. Those who refuse 
to go beyond fact, said T. H. Huxley (1887), rarely get as far as fact. It is difficult 
to believe that so astute an investigator as Joseph Priestley really overlooked this 
niode of investigation, as might be supposed from some preceding remarks — nor 
did he. On the contrary, he said : 

It is by no means necessary to have just views, and a true hypothesis, a priori, in order 
to make real discoveries. Very lame and imperfect theories are sufficient to suggest 
useful experiments which serve to connect those theories, and give birth to others more 
perfect. These then occasion further experiments, which bring us still nearer to the truth, 
and in this method of approximation, we must be content to proceed, and we ought to think 
ourselves happy if, in this slow method, we make any real progress. 

The many gaps in our knowledge are temporarily bridged by the assumptions 
called hypotheses. Hypotheses thus help to render intelligible the interrelations 
between different facts, and they are employed by men of science to extend and 
deepen their experience by predicting and disclosing new facts ; to correct and 
purify their knowledge of natural phenomena by eUminating errors and contra- 
dictions ; and to systematize their description of facts so as to obtain the greatest 
control over them with the least possible effort. 

An hypothesis contains a speculative term, an assumption which goes beyond the 
observed facts ; while a law is a generalization which does not extend beyond the observed 
facts. A law is thus limited by the facts it describes. When an hypothesis has been so 
extended that it has a wide and comprehensive scope, the hypothesis becomes a theory. 
Like the hypothesis, a theory usually contains an unproved assumption — e.g. the kinetic 
theory, the electron theory, etc. Some writers — e.g.W. Ostwald — apply the term theory to 
a generalization which does not extend beyond the observed facts, and in that case, theory 
becomes law when the generalization has a wide and comprehensive scope. There are 
several other uses of the term theory. For historical reasons the term may appear to be 
confused because the passage from hypothesis to theory, or from theory to law, has not 
always been attended by a change in the corresponding terms — e.g. Avogadro's hypothesis, 
by the definitions here given, might be called a theory. 

The verification of hypotheses. An hypothesis may seem to be the logical 

consequence of known facts, or it may be a random flash of the imagination. 
However probable an hypothesis might appear, both the hypothesis and the 
logical consequences of the hypothesis must be tested by comparison with facts. 
Aristotle (c. 320 B.C.) certainly recognized the need for basing reasoning on observed 
facts, but, as G. H. Lewes (1864) has emphasized, Aristotle did not reahze the very 
vital importance of verifying his logic by comparing its conclusions with facts, nor 
did he recognize that the true purpose of experiment is to verify the accuracy of 
data and of theoretical conclusions. We are indebted to Roger Bacon (c. 1280), 
perhaps more than to any other, for first insisting on verification as the essential 
pre-requisite for every trustworthy conclusion. He said : 

Experunental science is the mistress of speculative science. She tests and verifies the 
conclusions of other sciences. ... In reasoning we commonly distinguish a sophism from a 
demonstration by verifying the conclusion through experiment. 

Experiments have a way of giving results which differ from those which rigorous 
logic concluded must occur ; and when the prediction fails, it is necessary to fmd 
what has been overlooked. This does not mean that constant verification is needed 
to establish the validity of the process of reasoning, for that may be irreproachable 
and yet the conclusion may be false because the facts or premises upon which the 
reasoning was founded may have been interpreted to mean something very different 
from what actually obtains in nature, or because some unrecognized or undiscovered 
factor was involved. It is not wise to dogmatize when direct trial is possible : " Do 
not think," said J. Hunter, " try." 


It has been aptly said that the remarkable discoveries of modern science have 
been made by invariably sifting the truth through a fine mesh of logical experiment. 
One of C. Darwin's favourite methods of applying this method was to reason : "If 
my hypothesis be true, then certain consequences must also be true. Now let us 
find if they are true ; " and H. St. C. Deville used to say that there is no need to 
argue if an experiment can be made. In fine, it is necessary to submit all con- 
jectures to the incorruptible test of fact in order to avoid being seduced by im- 
material creations of the imagination. Faith without facts availeth nothing. The 
ad experiinentum test must be made with unremitting diligence, rigorously and 
impartially, without conscious bias. Trial by a combat of wits in disputations has 
no attraction for the seeker after truth ; to him, the appeal to experiment is the last 
and only test of the merit of an opinion, conjecture, or hypothesis. 

If one hypothesis does not fit the facts, it is discarded, and a modification of the 
old, or totally new hypothesis is tried. Thus, J. Kepler, in his De inotibus stellce 
martis (1608), is said to have made nineteen hypotheses respecting the form of 
planetary orbits, and to have rejected them one by one until he arrived at that which 
assumed their orbits to be elliptical. " To try wrong guesses," said W. Whewell, 
" is apparently the only way to hit the right ones." This method of trial and 
failure is continued until the golden guess crowns the investigation ; but one single 
real conflict between fact and hypothesis will destroy the most plausible hypothesis. 
Of fifty hypotheses, only one may prove fruitful ; the unsatisfactory ones are weeded 
out, until that particular one remains which has established its right to live by 
proving itself useful or by satisfying some need. Quoting M. Faraday : 

The world little knows how many of the thoughts and theories which have passed through 
the mind of a scientific investigator have been crushed in silence and secrecy by his own severe 
criticism and adverse examination ; that in the most successful instances not a tenth of the 
suggestions, the hopes, the wishes, and the preliminary conclusions have been realized. 

This quotation may give a wrong impression,^ for Michael Faraday displayed 
consummate skill, not only in framing hypotheses per se, but in deducing hypotheses 
that were worth testing. Without hypotheses, the experimental method may 
degenerate into empiricism ; without experiments, hypotheses may degenerate 
into speculation. 

The promulgation of immature or premature hypotheses without a substantial 
basis of fact is discouraged by most scientific societies. The celebrated nebular 
hypothesis was ushered in by P. S. de Laplace (1796) with those misgivings and 
doubts which must of necessity becripple all hypotheses which are not based upon 
observation or calculation. An hypothesis may be invaluable when it can be 
verified or refuted by a definite appeal to observation. If this check be not possible, 
the imagination riots in the wildest speculations. If the evidence of an alleged 
phenomenon cannot be tested by verification, it is outside the range of science. 
A. W. Hofmann is reported to have said that he would readily listen to any suggested 
hypothesis, but on one condition — that he be also shown a method by which it might 
be tested. Accordingly, scientific inquiry is limited to such objects and phenomena 
as admit of direct or indirect observational or experimental verification. On the 
other hand, science cannot enter into the dark territory beyond the scope of man's 
faculties, and where verification, direct or indirect, is not possible. A vivid imagina- 
tion can people this region with phantasms and be deluded with the hallucination 
that these creatures of the imagination are real, substantial, objective facts. It 
is now generally recognized that imagination, uncontrolled by facts, has produced 
all the palsying superstitions which have blinded and cursed the human race — past 
and present. 

Rival hsrpotheses. — Two or more contradictory hypotheses may be consistent 
with the facts ; both cannot be right. There is then need for an experimentum 
crucis, an experiment which will decide in favour of the one and exclude the other. 
An hypothesis is supposed to be established when it, and it alone, is in harmony 


with known facts. The hypothesis then ranks as a theory or law. In the majority 
of cases, the so-called laws of nature can be regarded as prophecies which becaui 
they have always been fulfilled in the past, are expected to be also fulfilled in innu- 
merable cases in the future. Laws, theories, and hypotheses are all on probation. 
However successful a theory or law may have been in the past, directly it fails to 
interpret new discoveries its work is finished, and it must be discarded or modified. 
However plausible the hypothesis, it must be ever ready for sacrifice on the altar 
of observation. On account of the unproved assumption embodied in all hypotheses, 
they are of necessity transient, fleeting, and less stable than theories ; and theories] 
in turn, are less stable than laws. A theory believed to-day may be abandoned 
to-morrow. New facts need new laws. An hypothesis is invalid when it fails to 
unite and coordinate facts. All our hypotheses and theories are to be superscribed 
" subject to revision," for they are continually changing. " Science in making is 
a battlefield of competing theories," the path of progress is strewn with dying and 
dead hypotheses. For example, W. Ostwald (1893) claims that the theory of chemical 
combination is a strange and contradictory conglomerate of the fossil constituents 
of earlier hypotheses. Science is not a state, but is rather a stage of progress. Even 
Isaac Newton's law of gravitation is included in this category ; and the astronomer 
R. Ball 4 could say : 

When the law of gravitation is spoken of as being universal, we are using language 
infinitely more general than the facts warrant. At the present moment we know only that 
gravitation exists to a very small extent in a certain indefinitely small portion of space. 

Ever since T. Bergmann's time (1779), science has been compared with a building 
in the course of erection, and scientific hypotheses have been compared with the 
scafEolds and ladders required by the builder in order to place the stones of ex- 
perience where they belong. The scaffolding must be rejected when it hinders 
further developments, and when the purpose for which it was erected has been 
fulfilled. Accordingly, an hypothesis is not the end, but rather the means of 
attaining that end. To think otherwise would be to suppose that the builder 
erects a mansion for the sake of showing off the ladders and scaffolds used in its 
construction. The imperfect notions and hypotheses of men of science must not 
be mistaken for descriptions of observed facts. In the chemica docens of our 
schools, the term science usually includes both the growing building and the auxiliary 
scaffolding ; otherwise expressed, the term includes the immutable facts, the 
ephemeral hypotheses, the transient theories, and the more or less incomplete 
generalizations from observations. The facts alone are certain to endure throughout 
all time. When S. Brown (1849) inquired : Is it necessary to the nature of a science 
that it be all true, and that it contains no admixture of error ? and answered : By 
no means ! Otherwise chemistry was no science during the reign of phlogiston, and 
the Lavoisierian chemistry no science so long as oxygen was taken for the principle 
of acidity — he included in the term science those transient theories which are 
necessarily employed in the erection of the temple of truth. 

Deductive and inductive induction.— The term induction is applied by the 
logician to the quest of science for generahzations, that is, for the camnes or uni- 
versales regulce of Roger Bacon. In deduction, the attempt is made to widen the 
bounds of knowledge without stepping outside known facts— the Euchdean method 
is a good illustration ; in induction, a leap is taken from the known into the ilhmit- 
able beyond. Two important methods of induction will be recognized— one may 
be called the deductive method, the other the inductive method. The former was 
favoured by Francis Bacon, the latter by Isaac Newton. 

1. Bacon's deductive method, by what he called the interpretaho naturo'. 
Here the facts are exhaustively classified until the generalization becomes clear, 
a is either M or N, or 0, or P, or . . . ; but a is not N, nor 0, nor P, nor . . . ; 
and consequently, a is 31. Thus, in the 105th aphorism of his Novum Organum 
(London, 1620), F. Bacon said : 

VOL. I. ^ 


The induction which is to be available for discovery and demonstration . . . must analyse 
nature by proper rejections and exclusions ; and then, after a sufficient number of negatives, 
come to a conclusion on the affirmative instances. 

The method appears to proceed from known facts to general conclusions, a parti- 
culari ad universale. It is based on facts already known, and has therefore been 
called a priori reasoning. The method by which Boyle's and Charles' laws were 
discovered might be cited in illustration of one form of the method of deductive 

2. Newton's inductive inethod, by what F. Bacon called the anticipatio 
naturoB. Here the attempt is made to infer the hidden generalization from the 
consequences of the assumption (hypothesis) what that generalization is. The 
process is sometimes called a posteriori reasoning. This method of investigation 
was extensively employed with glorious results by Isaac Newton, although it had 
been advocated by Aristotle tv/o thousand years earlier. Francis Bacon, indeed, 
before Newton's time, protested against anticipating nature by hypotheses, but 
the greatest triumphs of modern science have been won by the application of the 
Newtonian method while the Baconian method has been singularly unfruitful. 
Francis Bacon's failure in the practice of his own method was complete. 

The particular form which the Newtonian method takes in science is to devise 
provisional generalizations called hypotheses or working hypotheses to explain facts 
and phenomena. The appeal is then made to observation and experiment in order 
to test the validity of the proposed generalization. Examples : The cause of the 
increase in the weight of metals calcined in air ; A. L. Lavoisier's theory of com- 
bustion, and his experiments on the transformation of water into earth ; J. Mayow's 
work on combustion ; etc. The application of this method of inquiry involves 
(a) The accumulation of facts by observation and experiment ; (6) The employment 
of the imagination in framing hypotheses to explain the facts ; and (c) The appeal 
to facts to prove or disprove the hypotheses. By this procedure, said W. Whewell, 
the hypothesis becomes the guide of its former teacher — observation. There is a 
kind of cycle from facts to hypothesis, and from hypothesis to facts. 

Induction, said Aristotle, does not prove. I. Newton's phrase : Hypotheses non 
jingo — I do not frame hypotheses — is often quoted to show that he discountenanced 
the inductive method of scientific investigation. This is based upon a misunder- 
standing, for Newton here referred to hypotheses not suggested by observation. 
On the contrary, Newton's own procedure was to use hypotheses deduced from 
phenomena similar to the way science uses them to-day. Accordingly he 
asserted that " no great discovery was ever made without a bold guess," and his 
immortal Philosophice naturalis principia ^nathematica (London, 1687) is a wonderful 
record of discoveries made possible only by the exercise of the greatest freedom in 
the elaboration of hypotheses. Indeed, from the first of his communications on 
light to the Royal Society to the last revision of his Principia, Isaac Newton seems 
to have been steadily and persistently guessing. 

The method of investigation employed in scientific, positive, or modern chemistry 
thus involves four operations : (i) observation and experiment ; (ii) classification 
and comparison ; (iii) deduction, or speculation and hypothesis ; (iv) testing and 
verification. Francis Bacon did not grasp the prime importance of testing his 
induction by comparison with facts. A. de Morgan (1872) ^ puts this rather 
cleverly : According to Francis Bacon, facts are used to make theories from, and 
according to Isaac Newton, to try ready-made theories hy. Chemistry could 
progress as a science only when this method of investigation was discovered, so that, 
as S. Brown stated in 1843, before discovering chemistry it was necessary to discover 
the art of discovering chemistry. 


1 P. Carus, The Primer of Philosophy, Chicago, 137, 1904 ; Truth on Trial, Chicago, 1911 ; 
R. Shute, A Discourse on Truth, London, 103, 1877 ; K. Pearson, The Grammar of Science, 


London, li3, 1900; C. A. Mercier, On Cavsation, London, 1916; B. Russell, Myaticiem and 
Logic, London, 1919. 

2 H. Grote, Leonardi da Vinci als Ingenieur und Philosophy Berlin, 1874 ; P. Duhem, ttudea 
sur Leonard de Vinci, Paris, 1906-1913 ; W. R. Thayer, Moniat, 4. 507, 1894. 

» M. Faraday, Lectures on Education, London, 1855 ; Experimental Researches in Chemistry 
and Physics, London, 486, 1859 ; G. J. Stoney, B. A. Rep., 243, 1879. 

4 R'. Ball, Pop. Science Monthly, 23. 94, 1883. 

^ A. de Morgan, A Budget of Paradoxes, Chicago, 1. 88, 1915. 

§ 8. The History of Chemistry in China, India, and Chaldea 

It is vain and ridiculous to attempt to trace the origin of chemistry to the first men who 
worked in the metals, cut and polished stones, fluxed sand, or dissolved and crystallized 
the salts. This would be analogous to an attempt to trace the elements of geometry in 
the efforts of the savage to trim irregular fragments of rock to a more regular form in order 
to adapt them to his first needs.- — ^A. F. de Fourcroy (1782). 

There can be no doubt that the chemical arts had their origin in the darkness 
before the dawn of history ; the very etymology of the word chemistry is lost in 
obscurity. Many have been the attempts to fix a date at which chemistry began, 
and as often have these attempts proved abortive. The names of mythological, 
classical, and scriptural writers have been enrolled among the adepts, and as often 
have these names been expunged from the list. What L. Blanc (1847) said of the 
beginning of the Frencl^ Ke volution applies also to chemistry. Its history begins 
and ends nowhere. The origins are so confused and the many facts known to the 
ancients are so obscurely connected that there is no event which can be regarded 
with certainty as a first cause. 

The historians and antiquarians in chemistry now recognize how futile must be 
the attempt to fix time or place for the birth of chemistry. They see that inquiries 
can be profitably directed only in the attempt to find what particular form chemistry 
took, or what particular ideas concerning chemical phenomena prevailed during any 
given epoch. Thus, in his work Les origines de Valchimie (Paris, 1885), M. Berthelot ^ 
says : 

Chemistry is not a primitive science like geometry or astronomy, because it is constructed 
from the debris of a previous scientific formation which, half chimerical and half positive, 
is itself founded on the treasure slowly accumulated by practical discoveries in metaUurgy, 
medicine, industry, and domestic economy. 

Evidence of an old prehistoric civilization, long prior to that indicated at the 
beginning of the biblical record, has been laid bare during excavations in Egypt 
and elsewhere. The antiquities which have been unearthed are arranged by 
archaeologists in three successive periods — the stone age, the bronze age, and the 
iron age. It is assumed that stone would be used by a rude savage people before 
metal, and that copper, being oftenest found native, and readily hammered into 
shape, would come into use before iron. This view was taken by Lucretius in his 
De rerum natura (5. 1282) written about 60 B.C. He said : 

The first weapons used by man were the hands, the nails, and teeth, also stones and the 
branches of trees ; and then was discovered the power of iron and copper. The use of copper 
was known earlier than that of iron, since copper is more abundant and easier to work 
than iron. 

Long before Lucretius, Hesiod (c. 700 B.C.) stated that the earth was first tiUed 
with copper instruments because iron had not been discovered. 

The three periods do not altogether represent divisions of tmie, but rather 
stages of human culture, and they were not uniform in all parts of the world ; 
rather did they merge more or less one into the other so that stone weapons were 
used throughout the age of bronze, while bronze and iron were known m the stone 
age ; and similarly, stone and bronze were used in the iron age. Hence this 
classification is not altogether reliable historically, but it is so convenient that it 


has been adopted by the leading museums in the world for the classification of 
antiquities or ancient relics. 

The Aryans. — Comparative philologists 2 who have studied the languages of the 
different countries of Europe and Asia, have brought forward evidence in favour of 
the theory that most of the European languages were derived from a family of 
people speaking one language — now called the Aryan language—and that this 
primitive language is also the source of much of the Indian, Iranian, and Armenian 
languages. The common parentage is suggested by striking similarities in the roots 
of many words in the languages of these different peoples. The evidence further 
indicates that the primitive Aryan tongue was spoken by nomad herdsmen 
wandering over the plains of Europe during the neolithic age, that is, when man had 
learned to polish his flint weapons — very roughly about 6000 B.C. There is no 
satisfactory evidence to prove that the Aryans were a civilized people which invaded 
Europe from the East — as was once supposed. In time, the geographical continuity 
of the primitive Aryans was disturbed and local variations in speech — dialects — ■ 
began to arise which ultimately were fractionally crystallized, producing the different 
languages which now separate the different families derived from the original Aryans. 

Owing to the absence of any common root for words connected with the smith's 
craft, we are told that the arts of extracting and working the metals were developed 
after the linguistic separation ; and for similar reasons, the philologists suppose 
that the Aryans were not acquainted generally with iron, tin, or gold. Their 
common knowledge of copper is supposed to be shown by i^e relation of the different 
words — Sanscrit, ayas ; Gothic, aiz ; Latin, ces ; German, erz ; English, ore — for 
the metal or its ore. The probability is increased by the fact that copper occurs 
native in the metallic state. Some of the oldest metal implements, imitating the 
older stone implements, found in old tombs and in the remains of pile-dwellings in 
various parts of Europe, are of copper, not bronze. The knowledge of the metals 
seems to have spread in Europe from the Mediterranean northwards, and is supposed 
to have been introduced by Phoenician traders. The different stages of development 
of the people, after the differentiation of the language, were not synchronous, since, 
when one nation was in the stone age, another was in the bronze age, and a third in 
the iron age. 

Chaldea. — In Chaldea the remains of ancient cities and temples have been 
ransacked, and the existence of another civilization before that of Egypt has been 
revealed. The early Chaldeans must have been skilful workers in the metals over 
sixty centuries ago — 4000 B.C. — and, since there were no mines and very little fuel 
in the country, it is thought that the Chaldeans must have got some of their know- 
ledge from another people more favoured in this respect. There is no written record 
of early Chaldean chemistry, nor of any historical names in connection with their 
chemical arts. Zoroaster (c. 1500 B.C.) is reputed to have been the founder of the 
philosophy of the early Chaldeans and Persians. This subject, however, is very 
obscure. Zoroaster is said to have made a very special study of the movements of 
the planets. The cuneiform inscriptions show that the Chaldean wise men or 
priests were practised in the arts of astrology, incantation, divination, and conjur- 
ing. The number 7 appears to have been considered very important in their 
philosophy and religion ; and the Chaldeans recognized this number of gods, devils, 
planets, colours, metals, etc. The Babylonians established the divisions of time 
which are employed to-day ; the seven days in a week thus originated from religious 
and astrological considerations before 2300 B.C. The same number is sacred in the 
Zarathustrian faith, the Mithras religion, and among the Buddhists, Jews, and early 
Christians. A. Origen in his Contra Celsum (c. 22), says that the Persians repre- 
sented the revolutions of the heavenly bodies by seven stairs which led to the same 
number of gates each of a different metal — lead, tin, copper, iron, a mixed metal, 
silver, gold. He added : 

The leaden gate had the slow tedious motion of Saturn ; the tin gate the lustre and 
gentleness of Venus ; the third gate of copper was dedicated to Jupiter ; the fourth, iron, 


was dedicated to Mercury on account of its strength and fitness for trade ; the fifth, mixed 
metal, to Mars ; the sixth, silver, to the moon ; and the last, gold, to the sun. 

The astrological nomination of the metals has thus been traced to the Chaldeans, 
and it appears to have been used by the Hindus, for F. Philostratus said in his 
Vita Apollonii (3. 41), that the Brahmin larchas gave Apollonius seven rings named 
after the seven planets ; one ring to be worn daily — each one on the day of the 
week which bore its name. 

The characters employed by the early writers to represent the planets were also used 
for the corresponding metal, » but they were not agreed in the dedication of particular 
metals to particular planets, and the characters themselves were subjected to certain 
changes of form. Thus, G. F. Rodwell says that in a manuscript written by Antonio Neri 
before 1613, mercury is designated by no less than thirty -five different names and twenty- 
two symbols ; lead by sixteen names and fourteen symbols ; and sulphur by two names, 
and sixteen symbols. The mythological symbols used largely by the alchemists of the 
Middle Ages were : 





















J. Beckmann has suggested that these symbols are the remains of Egyptian hieroglyphics, 
or else corrupted forms of the initial letters of the names of the deities which were supposed 
to reside in particular planets ; and he claims that the symbol for copper 9 , said to 
symbolize the looking-glass of Venu3, may really be a distorted form of the initial letter 
of the Greek term ^aa-cpSpos for that goddess ; the so-called scythe of Saturn, a corruption 
of the first two letters of his Greek name Kp6vos ; the imaginary caduceus of Merciuy, a 
modified form of the initial letter of his Greek name ^Tifiwi which in the oldest manuscript 
was written C or o with the next letter added below ; the lance and shield of Mars, €ui 
abbreviation of the Greek name of the deity @ovpos, obtained by placing the last letter 
above the first ; and the symbol for the thunderbolts of Jupiter was similarly derived from 
the initial letter of the Greek equivalent Zeds for Jupiter with the last letter added below, 
as is actually done in some of the older writings. The circle, the symbol for the sun, was 
also the symbols of divinity and perfection. The semicircle for the moon is appropriate 
since it is the only one of the heavenly bodies which appears in that form to the naked eye. 
The following excerpt from K. Digby's Chemical Secrets (London, 1683) illustrates the way 
the alchemists employed the symbols : 

Take good mineral 9 , mortifie it with radicated vinegar ; then separate its 
quintessence with pure S.V. ; with that quintessence, dissolve ^ duplicatiun of 9 > 
that both become an oyl, which unite with a subtle calx of 0, and bring them to an 
incombustible oyl, which will transmute ^ into 0. 

Hence, astrology, and the emphasis which the alchemists later sometimes laid on 
the number 7, are relics of Chaldean thought. The Chaldeans supposed that the 
planets influenced the properties of the metals, the organs of the body, and the 
destiny of man. 

The Chaldeans seem to have had some knowledge of metallurgy, dyeing, weaving, 
the manufacture of colours, glass, and the imitation of gem stones. The chemical 
arts practised by the early Chaldeans were probably adaptations of chance observa- 
tions to useful purposes ; these arts gradually drifted to the early Egj^ptians. 
For instance, it is related that Abraham came from Ur in Chaldea {Gen., IL 31),* and 
he probably brought a higher civilization into Canaan, and also to Egypt. The 
Egyptians developed and improved the Chaldean arts in the laboratories and 
workshops attached to their temples. When the Babylonian empire ceased to 
exist, the Chaldean nation was dispersed, and the priests were scattered over the 
neighbouring lands, so that the term Chaldean became a by-word synonymous with 
" a wise man from the East." The scholars also tell us that the Assyrian rab-mag 
or the Semitic ma^— meaning a priest— has furnished the Latin and European 
language with the terms fnagus, magic, and magician. 

India.— India played no direct part in the development of Western science. 
It is a tradition that Hermes the Egyptian predicted that naught of the histor}^ of 
Egypt, but the letters engraved on stone, would survive. W hether this be true or 


not, scholars are now mainly dependent upon the inscriptions on tombs and monu- 
ments for their knowledge of the early Chaldeans and Egyptians. On the other 
hand, what remains of Indian thought is recorded in their books — the Vedas, the 
Charaka, and the Susruta — for the Indians were a literary people. According to 
Max Miiller, there are many points in common between the early Greek and Indian 
philosophers, and there is a historical possibility that the Greeks were influenced 
by Indian thought travelling through Persia. From this point of view, it was only 
when commerce had opened up the country that it became possible to recognize 
the debt which European science owes to India, and to find that a great deal 
formerly attributed to the Arabians was of Indian origin. The learning of Greece, 
Persia, and India is said to have been taxed to help the sterility of the Arabian 

In his History of Hindu Chemistry (London, 1902), P. C. Ray has shown that 
Indian chemistry developed largely on independent lines — medicine, not the metals , 
was mainly emphasized. The contact between the Hindus and Persians is thought 
to have given the latter a bias towards medicine which later showed itself in 
the polypharmacy of the Arabians. Very fair accounts of the philosophical views 
of the Hindus are available. H. T. Colebrooke,^ for instance, has shown that an 
early philosopher Kanaka developed an atomic theory rivalling that of Lucretius 
(60 B.C.). The Hindus also developed a five-element theory of the constitution of 
the world, but the elements of the Hindus were not the same as the quintet — air, 
earth, fire, wood, metal — of the early Chinese. The Hindu quintet embodied : Water, 
the first thing created ; the sacred fire ; the unbounded cether ; the foster-mother 
earth ; and the air which animates all living beings. The idea that water is the most 
primitive element of all is found in many of the classical books of India. 

The Vedic hymns, over 1000 B.C., personify the elements and natural phenomena 
— for instance, they raise the active principles of plants to the dignity of gods. 
The medical work Charaka, and the more recent Susruta, seem to be repositories 
of information — chemical and therapeutical — which had accumulated between the 
Vedic period and approximately the eighth century. Gold, silver, copper, iron, 
tin, lead, as well as some varieties of brass were known. The writers also mention 
a number of salts of the metals — e.g. alum, copperas, sodium and potassium carbon- 
ates, and a few products of the mineral kingdom. The Hindus developed some 
alchemical notions, but they directed their attention mainly to medicine. According 
to P. C. Ray, the practice of chemistry between the twelfth and thirteenth centuries 
was distinctly in advance of that of the same period in Europe. The Hindus 
learned about zinc and mercury ; but, as L. Hoefer has pointed out, real progress 
in chemistry in India and China was not possible so long as the preparation of the 
mineral acids was unknown. These acids are incidentally described in works 
dating from the sixteenth century. 

The arts and sciences were largely cultivated by the higher classes, but, according 
to P. C. Ray, when the caste system was established, the opinion that industrial 
work tends to lower the standard of thought, which at one time threatened Europe, 
seems to have likewise developed in India with disastrous results. The arts and 
sciences were relegated to the lower castes, the spirit of inquiry gradually died, and 
the artisan classes, guided solely by their mother wit and common sense, alone 
kept up the old traditions. The withdrawal of the intellectual community from 
active participation in the arts rendered India " unfit for the birth of a Boyle, a 
Descartes, or a Newton, and her very name was all but expunged from the map 
of the scientific world." 

China. — So far as we can gather, the Chinese were civilized, and cultivated 
the arts and crafts at a time when the European nations were barbarians. Some 
scholars claim that there is strong evidence of a Western origin of Chinese civilization, 
and that the first Chinese settlers came from a country in the far West which was 
closely connected with the founders of Babylonian culture. The earliest docu- 
mentary evidence of man's attempts to answer the question : From what are 


all things made ? occurs in the Chinese work Shoo King, which is said e to contain 
a document called The Great Plan purporting to have been given by heaven to Yii 
the Great, about 2200 B.C., and which is considered by the scholars to be older than 
Solomon's writings. Here reference is made to five elements— water, fire, metal, 
wood, earth. The Chinese element wood was never recognized as an element in the 
West. The early Chinese philosophers supposed that two elementary principles— 
yang (male or active) and yin (female or passive) — were derived from 
t'ai kih~the Great Origin of the Grand Cause. The two principles lur.^-'-.' 
yang and yin gave rise to the five elements. Fire and wood belong '■'■'^•--- 
to the yang element ; water and metal to yin ; and earth is neutral. *^"'"''-- 
The union of the five elements produces yang and yin : and the union ^•'^'*'— 
of these two principles produces the grand cause which is itself without Fio. 2.— Stupa 
cause. The Chinese Buddhists symbolized the five elements by the ^onn of the 
square (earth), circle (water), triangle (fire), crescent (air), and the gem ?^j^T^^?" 
(aether) —as indicated in Fig. 2 . In the mediaeval European symbolism, j^\, ^ ^ y^, 
the two latter figures are treated as one, and serve as the common ments. 
symbol of air. It is said that all over those parts of Asia dominated by 
Chinese civilization, stupas or monuments built in the general shape of the symbols 
of the five elements are to be found — e.g. the gateway to the Buddhist monastery 
of Pekin, etc. 

The philosopher Lao-tze, founder of the Taou religion in the sixth century B.C., 
believed that a fine essence or spirit arising from matter may become planets and 
stars ; and these speculations led to the search after the sublimated essence of 
things. The Taouists sought some flux which would purge man from the dross of 
animalism and leave the higher part of man's nature to be crystallized out and 
sublimed into some stable and eternal form. They had no success in finding an 
elixir of life, or philosopher's stone ; but they obtained a number of fairly pure 
mercurial preparations. According to J. Adkins (1855), the earliest Chinese work 
on alchemy now extant is the second-century treatise Chen tung chi, by Wei Peh 
Yang ; and two centuries later, P'au P'on Tsze wrote many works on alchemy and 
kindred subjects. Later still, a disciple of Lao-tze — Wei Poh Yang — wrote a book 
The Uniting Bond in which reference is made to a red elixir which was probably 
mercuric sulphide or vermilion, prepared from galena or lead ore — symbolized by a 
white tiger— and mercury — symbolized by a blue dragon. The red elixir was 
regarded as an elixir of life even though Wei Poh Yang appears to have been poisoned 
when he attempted to practise his own philosophy. There seems to have been 
some contact between eastern and western Asia during the seventh-century invasions 
of the Mahomedans, and the teachings of the Taouistic alchemists penetrated 
Arabia, and appear there as the philosopher's stone and the elixir of life. 

W. A. P. Martin,7 in a chapter on Alchemy in China (1901), considers that the 
alchemy of China is not an exotic but a genuine product of the soil of that country ; 
alchemy is indigenous to China, and coeval with the dawn of letters. He under- 
stands the words alchemy and chemistry to represent different stages in the progress 
of the same science, and says that the skill of the Chinese in the chemical arts and 
their knowledge of many chemical compounds give evidence of lives passed among 
the fumes of the alembic. Whatever be the true facts, there can be little doubt 
that the early Chinese practised the chemical arts somewhat extensively, and they 
knew quite a long list of chemical preparations — nitre, borax, alum, corrosive 
sublimate, arsenic, mortars, cements, oils, paper, sugar, etc. They appear to have 
invented printing, the manufacture of paper, and gunpowder— or rather a kind of 
Greek fire which was placed in vessels, ignited, and projected from a throwmg 
machine. They were acquainted with various precious stones ; some of their 
pottery has never been surpassed. Chinese porcelain seems to have originated 
about the time of the Han dynasty— 206 B.C. to 220 a.d.— and it attained its highest 
development under the Ming dynasty— 1368-1644. Glass was made m China in 
the Wu Ti dynasty~422-455— and was probably derived by contact with Western 


nations. They knew about gold, silver, mercury, lead, copper, iron, zinc, nickel, and 
various alloys. The method for making zinc was probably derived from India. 
They seem to have had ideas about the transmutation of the base metals into gold ; 
and they are credited with a knowledge of oxygen and the composition of water 
as early as the eighth century. All this, however, exerted no direct influence on 
the development of European chemistry, although there is much evidence to show 
that indirect communication between Europe and China was possible — e.g. the 
Arabian alchemist Avicenna is said to have been born at Bokhara on the borders of 
the Chinese empire. From the time of Confucius, the Chinese made little progress 
in the arts and sciences, while Europe rapidly grew in knowledge. 


* H. Kopp, Geschichte der Chemie, Braunschweig, 2. 3, 1844 ; Beitrdge zur GeschicMe der 
Chemie, Braunschweig, 40, 55, 1869 ; M. Berthelot, Introduction a V etude de la chimie des 
anciens et du moyen age, Paris, 1889 ; T. Bergmann, De primordiis chemice, Upsala, 1779 ; 

E. Cullen's trans., Edinburgh, 1791 ; H. Boerhaave, Elementa chemio',, Lugduni Batavorum, 1732 ; 
P. Shaw's trans., London, 1753 ; F. Hoefer, Histoire de la chimie depuis les temps les plus recules 
jusgu'd notre epoque, Paris, 1842 ; J. C. Brown, A History of Chemistry, London, 1913. 

2 O. Schrader, Sprachvergleichungen und Urgeschichte, Jena, 1907 ; T. Taylor, The Origin of 
the Aryans f London, 1892 ; F. M. Miiller, Biographies of Words, and the Home of the Aryas, 
London, 252, 1888. 

3 G. F. Rod well, PM. Mag. (4), 35. 1, 1868 ; J. Beckmann, Beitrdge zur Geschichte der Erfind- 
ungen, Leipzig, 1780-1805 ; A History of Inventions, London, 1814 ; P. Carus, Open Court, 15. 
335, 412, 1901. 

* F. Hommel, Der habylonische Ursprung der dgyptischen Kultur, Munich, 8, 1892 ; Z. A. 
Ragozin, Chaldea from the Earliest Times to the Rise of Assyria, London, 1886 ; G. Radet, La 
Lydie et le monde grec au temps des Mermnudes, Paris, 1893 ; I. P. Cory, Anx:,ient Fragments of 
the Phoenician, Chaldean, London, 1836 ; V. E. Johnson, Chaldean Science, London, 1896 ; 
H. V. Hilprecht, Excavations in Assyria and Babylonia, Philadelphia, 1904; A. H. Sayce, Babylonians 
and Assyrians, London, 1900. 

^ H. T. Colebrooke, Essays on the Religion and Philosophy of the Hindus, London, 1853 ; 
Ia Mavilleau, Histoire de la philosophic atomistique, Paris, 1 895. 

« J. fl. Gladstone, B. A. Rep., 448, 1883 ; J. Adkins, Journ. Roy. Asiatic Soc, 18. 1, 1856 ; 

F. P. Smith, Amer. Chemist, 4. 46, 1873 ; A, Wylie, Notes on Chinese Literature, Shanghai, 1867 : 
P. Carus, Chinese Philosophy, Chicago, 1896 ; Chinese Thought, Chicago, 1907 ; Monist, 15. 500, 

7 J. Klaproth, Mem, Acad. St. Petersburg, 2. 476, 1810 ; C. W. Duckworth, Chem. News, 53. 
260, 1886 ; H. Chatley, Journ. Alchem. Soc., 2. 33, 1913 ; W. A. P. Martin, The Lore of Cathay, 
Edinburgh, 1901 ; The Chinese, their Education, Philosophy, and Letters, New York, 1901 ; P. M. 
Cibot, Memoires concernant Vhistoire, les sciences, les moeurs, les usages des chinois, Paris, 1776- 
1814; E. Soubeiran, Journ. Pharm.Chim.,{5), 13. 213, 1866; H. J. Holgen, Che7U. Weekblad, 
14. 400, 1917; H. C. Bolton, Chem. News, 70. 53, 1894. 

§ 9. The History of Chemistry in Egypt 

Let us confess at once, without going round the subject, that practical chemistry took 
its rise in the workshops of the smith, the potter, or the glass-blower, and in the shops of 
the perfumer ; and let us agree that the first elements of scientific chemistry date no 
further back than yesterday.- — -J. B. Dumas. 

According to Diodorus Siculus' report i of his visit to Egypt — Bibliotheca 
historica (c. 30 B.C.)— during the reign of Julius Caesar, the Egyptians regarded 
Hermes Trismegistus as a man 2 to be esteemed above all others for his penetrating 
genius in discovering everything that could be useful in life ; and it was the favourite 
opinion of the Arabian and European alchemists in the Middle Ages, that this 
Hermes laid the foundations of chemistry about the time of Moses. Hermes was 
accordingly called the father of philosophy and of alchemy by the alchemists of 
the Middle Ages — e.g. by Albertus Magnus (c. 1250), Koger Bacon (c. 1250), etc. 
It was also said that before the time of Hermes, the transmission of knowledge 
from one generation to another depended upon oral tradition, but Hermes invented 
a system of recording events upon stone pillars in the same way that modern 
writers employ parchment or paper ; consequently, engraved pillars were the 


standard literature of the day. Some of those who now appear to be the more 
credulous writers of early history, state that Hermes inscribed upon an emerald 
the most essential secrets of alchemy, and presented this jewel to Sarah the wife 
of Abraham ; and that after many subsequent adventures the stone was lost ; 
they also state that a copy of the inscription survives. From the translations 
which have been made of the supposed inscription it appears that even if the 
inscription itself be not lost, its meaning has gone. The alchemists honoured Hermes 
when they spoke of the hermetic sealing of a vessel. 

Attempts have been made to identify Hermes Trismegistus with the Egyptian 
king Siphoas or Memnon, who had the surname Hermes, and also with various 
biblical celebrities — ^Adam, Cain, Enoch, Joseph, Moses, and Abraham. 
Some writers maintain that Hermes Trismegistus is a fabulous personage, 
and it is generally supposed that this Hermes was identical with the 
Egyptian god Thoth — ^literally a pillar. Thoth is represented by the 
Egyptians as an ibis-headed god with a pen in his hand, the tutelary 
deity of wisdom and letters, Fig. 3. It is further said that the supposed 
writings of Hermes really cover three successive epochs — the first 
Hermes dealt with the period down to the deluge ; the second Hermes 
was concerned with early traditions ; and the third Hermes embodied 
the full-grown science of Egypt. The whole system of Thoths or pillared 
literature was personified as Hermes Trismegistus — rpis, thrice ; /teyto-ro?, 
greatest — meaning literally thrice great interpreter. It is theref ore ^'^- 3.— 
easy to understand how Hermes might have been credited with being J^ q^ 
an extraordinarily prolific author. Thus, in his De mysteriis ^gyft (c. Thoth. 
360 A.D.), lamblichus says that Hermes was the author of 36,525 books 
— T. Bergmann (1779) laconically observes that, if so, the books must have 
been very concise after the manner of those times, and that each book could 
have contained but a few sentences. Indeed, in his Stromaia (c. 200), Clement 
of Alexandria describes imposing celebrations in which the books of Hermes were 
borne in processions. 

Most of the writings attributed to Hermes appear to have been lost at the 
destruction of the Alexandrian library ; a few passages are quoted from them by 
Zosimus (c. 400) ; and copies of some dealing with burial rites and the future life 
have been found buried with the mummies of kings and priests ; these have been 
embodied in what is now called The Book of the Dead. In general, it has been said 
that Egyptian thought was heavily hampered and severely restrained by a powerful 
priestcraft ; that the people were haunted by dread and dismal shadows from the 
underworld ; that they fostered an elaborate cult of the dead ; that their houses 
were temporary abiding places ; while their tombs were their eternal homes. 

Herodotus (c. 440 B.C.) believed that the early Egyptians were the wisest of men. 
He said that they had three communities of priests— at Heliopolis, Memphis, 
and Thebes. The priests were responsible for the preservation of such knowledge 
as was considered worthy of being retained ; this knowledge was kept secret and not 
divulged except to the elect. The sacerdotal secrets were in part described by 
hieroglyphics on stone pillars, and on manuscript papyri, but the allegorical nature 
of the symbols prevented them being read or understood by the unimtiate^. 
According to lamblichus (c. 360), every discovery which was approved by the 
priests was engraved, without the author's name, on stone pillars in the temples 
Clement of Alexandria, Plutarch, and others say that the priests possessed still 
more secret writings. No original record of the early writers is avadable, and our 
knowledge of the practice of the Egyptian arts is gleaned from fragments in the 
writings of Pliny (c. 23), Plutarch (c. 100), C. Galen (c. 190), etc. 

About 332 B.C., while Egypt was under Greek rule through conquest by Alexander 
the Great, the Greeks were received in the Academy of Alexandria ; and some ot 
the Egyptian manuscripts were translated into Greek, and later on distributed oveT 
Europe— Paris, for instance, among others, has one by Zosimus ; the bt. iMarK 


manuscript is preserved at Venice ; and a number are reported to be at the Vatican 
in Rome, the Sultan's museum in Constantinople, etc.3 

The Rhind rmiihematical papyrus in the British Museum is the main source of 
our knowledge of the early Egyptian mathematics. It is considered to be a copy 
made about 1600 B.C., by an Egyptian priest, from a document seven hundred 
years older. Researches near Memphis have given indications of Egyptian medical 
practices 4500 B.C., and pictures of surgical operations of a date not later than 
2500 B.C. have been found. The celebrated Georg Ehers' papyrus * was found in 
the winter of 1872-3, near Memphis, in a terra-cotta vessel between the legs of a 
mummy which was buried about 10 ft. deep. The papyrus is supposed to be a 
copy of one of the six medical papyri of Hermes (c. 1550 B.C.) about the time of Moses, 
and the text refers back to kings who reigned 3700 B.C. The papyrus is a kind of 
materia medica and medical treatise ; it gives some directions to the medical 
attendant of a sick person, and describes the necessary incantations and invocations 
for the co-option of the help of the gods. The papyrus also contains references to 
a number of metals and some compounds. 

A portion of one of theearliest Egyptian manuscripts is preserved in the museum at 
Leyden, and is known as the Leyden papyrus. It was found enclosed in the wrappings 
of a mummy at Thebes, and is considered to have been written about the third 
century. It was presented to the Netherlands by I. d'Anastasi, the Swedish consul 
at Alexandria in 1828. The work contains over a hundred magic formulae, and 
recipes for the preparation of alloys used in making various objects of the goldsmith's 
art. It also has drawings of some chemical apparatus. It has been investigated 
by C. J. C. Reuvens, M. Berthelot, etc.^ The grammatical errors and spelling have 
led to the opinion that the papyrus must have been the memorandum book of an 
uneducated artisan engaged in attempts to imitate gold and silver for fraudulent 
purposes — e.g. the preparation of asem, an alloy of copper and tin, occupies a 
prominent place among the recipes for imitating gold. The Royal Swedish 
Academy of Stockholm also acquired a papyrus about the same time and from the 
same source as the Leyden papyrus ; but the existence of the Stockhohn papyrus 
seems to have been overlooked until about 1906. It was translated by C. 0. 
Lagercrantz in 1913.^ It deals with the diplosis of silver, the imitation of precious 
stones, and dyes. 

These papyri are supposed to represent the class of books on the chemistry 
of gold and silver which, according to Suidas' Lexicon (c. 1000), were burned by the 
order of the Roman emperor, Diocletian, about 290 a.d., as a supposed punishment 
for an attempted rebellion, and to prevent the Egyptians making gold, and so 
acquiring wealth sufficient to enable them to oppose the authority of the Romans. 
These incendiary forays on the books of a prohibited and feared art — alchemy — 
were not infrequent in the early Christian era — witness The Acts of the Apostles 
(19. 19). This helps to explain the paucity of the early records of Egyptian science ; 
yet, in spite of various conquests of Egypt by the Persians, Babylonians, Greeks, 
and Romans, the arts were cherished by the priests with more or less vigour until 
the Saracen invasion of the seventh century, when every abode of learning, and 
every monument of science was destroyed with a ruthless hand. In 642 a.d., the 
famous Alexandrian Library, with its 700,000 books, was condemned to destruction 
by Kaliph Omar, who, in refusing a petition to spare at least a part of the library, 
is reported to have said : "If the books agree with the Koran, they are useless ; 
and if they differ from it they are dangerous." A mania for pillage and destruction 
with the idea of terrorizing the stricken inhabitants of a conquered territory, has 
long been characteristic of the temper of invading barbarians in ancient and modern 
times — witness the invasion of Europe by the Goths, the Vandals, and the Huns 
early in the Christian era, and the more recent rape of Belgium and North France 
by Teutonic hordes maddened with the lust of a world's conquest. Egypt never 
recovered from the severe blows she received, and what was presumably the greatest 
treasury of knowledge garnered by the ancient world, was used for kindling the 



fires of the baths of the invaders. l*robably a few volumes were surreptitioiwly 
preserved ; others, said to have been saved from the plunderers, are probably 

The Egyptians appear to have been acquainted with some six or seven metals 
and with some alloys. 7 Various metallurgical processes for extracting or melting 
metals have been depicted in their tombs, etc. — Fig. 4, for instance. The Egyptians 
were well versed in the arts of glass-making, potting and the manufacturing of 
precious stones and enamels ; they were familiar with the arts of dyeing, painting, 
tanning, brewing, and baking ; they were acquainted with many poisons and their 
antidotes, and with expressed and distilled oils. They were highly skilled in the use 
of antiseptics — particularly in the embalming of the dead. Among the operations 
employed in the arts and crafts of the Egyptians were : calcination, digestion, 
decoction, distillation, expression, evaporation, fusion, fermentation, levigation, 
and sublimation. So far as the available records go, there is nothing to show that 
any results were obtained by experiments directed as deliberate questions to 
nature. To know that liquids boil and evaporate, or that metals fuse and form 
calces, may indicate an unconscious sagacity in observation, but it is not scientific 
observation ; the early arts, said W. Whewell,^ were the parent, not the progeny 
of science. 

So far as we can learn, the disjointed knowledge of technical processes, so 
jealously guarded by the Egyptian priests, was purely empirical, and it required 

Fig. 4.— Gokl Washing, and the Fusion and Weighing of the Metal aa depicted in an early 

Egyptian Tomb. 

centuries of eflort before man learned to view these processes in a comprehensive 
rational way. Evidences of the practice of these chemical processes are found in 
ancient monuments and tombs of high antiquity — see, for example, R. Lepsius* 
Die Metalle in den agyptischen Inschriften (Berlin, 1872)— and Fig. 4 represents the 
washing, fusion, and weighing of gold as is reported to have been depicted on an 
old Egyptian tomb. The records are too imperfect to form a clear idea of Egyptian 
science, if they really had one such. There is some fragmentary evidence, more 
or less confused by fictions, and disguised by personifications, that the Hermetic 
writings assumed that all substances are produced from two elements: fire, the 
spirit of the world ; and tnortuum malum, the inert matter of the earth— that is, 
energy and matter ; although, according to Seneca's Qucesiiones natmales (3. 14, 
c. 63 A.D.), the Egyptians adopted an extended form of the four-element theory in 
which each element had an active (male) or passive (female) ioim—e.g. active air 
was the ivind, and passive air the at7nosphere ; flame was active fire, and hfjht, 
passive fire. According to Diodorus the Sicilian (c. 30 B.C.), the Egyptians taught 
that by some internal changes, all bodies sprang from their seeds or atotn^, 
and were changed, perfected, and then destroyed. 

Consequently, the impression that Egyptian chemistry was mamly practical 
recipe and unverified speculation, is well founded on pertinent evidence ; but 
others claim that too little inside knowledge is available to justify speaking witn 
any confidence. In any case, it will be clear that before the Christian era Egypt 
must have been a kind of focus or centre which collected, assimilated, extended, and 
developed knowledge derived from various Eastern sources ; otherwise expressed, 


the rise of chemistry in Egypt can be compared with a river which drains a large 
tract of territory, there is not one source, but many sources, each feeding a tributary 
of the river. 

Phcenicia. — It is fairly clear that the indefatigable merchant Phoenicians had 
acquired some knowledge of the so-called chemical arts during their contact with 
the Egyptians. The Phoenicians were famous for the manufacture of a purple dye 
— Tyrian purple — the special boast of Tyre ; for the manufacture of glass — Sidonian 
glass — particularly at Sidon ; for the weaving of fabrics of various kinds ; for 
working in metals; and for the engraving of gems (// Chronicles, 2. 14). The 
Phoenicians were great navigators, and it is supposed that they circumnavigated 
Africa. Strabo says that they made a special study of astronomy and arithmetic. 
Posidonius, a Greek writer of the first or second century B.C., made a special study 
of Phoenician mining, and gathered his data from the remains of the Phoenician 
mines in Spain.^ 

The biblical record. — The biblical records of the unfortunate Israelites show 
evidence of the chemical arts and crafts employed by their Egyptian masters. 
The Israelites must have carried much of this knowledge into Asia during their 
exodus from Egypt under the leadership of Moses. Even from the beginning of 
Genesis, we are told that Tubalcain (3870 B.C.), the eighth man from Adam, was a 
worker in metals {Gen., 4. 22) ; good gold is said to have been obtained at Havilah 
{Gen., 2. 11), and silver coins were in use at the time of Abraham {Gen., 23. 16) ; in 
all, about six metals — gold, silver, copper, iron, lead, and tin — were known to the 
IsraeUtes {Numb., 31. 22) ; Noah made wine from grapes {Gen., 9. 21) ; and vinegar 
was in use {Numb., 6. 3) ; bricks were burned for the building of the tower of Babel 
{Gen., 11. 3) ; weaving and dyeing were known {Exod., 26. 1) ; and oils, perfumes 
{Exod., 30. 23), and butter {Gen., 18. 8) were manufactured. 

The mechanical performance of operations essentially chemical in their nature 
is not chemistry, otherwise the first man to light a fire, boil a rabbit, or roast a pig 
was the father of chemistry. It is difiicult to see why the mere practice of these 
arts should be taken to prove that the early artisans were chemists in all but name, 
unless there is some collateral evidence of scientific procedure in the development 
of the empirical crafts — roasting and boiling, baking and brewing, or potting and 
dyeing. Moses' demonstration lo of the solubility of Aaron's golden calf {Exod., 32. 
20) has been taken to show the profundity of the chemical knowledge he must have 
acquired during his tuition by the Egyptian priests ; but, before the indignant 
prophet can be credited with any profound knowledge of chemistry, more details 
are required. Well might Francis Bacon, in his Novum Organwn (London, 1620), 
protest against the vanity of the attempt to found science upon the scriptures. 

In Exodus (31. 3) we are told that Bezaleel, the son of Uri, was endowed with 
the spirit of the Lord, and with skill to work in metals and precious stones. These 
hints of the early arts have been expanded by surmise and guess, and deformed by 
fiction and fable. For instance, it has been said that man received his first knowledge 
of the arts and sciences by divine or diabolic revelation. In his Chronicorum 
canonum (c. 300), P. Eusebius tells us that the apocryphal Enoch was taught by 
the angels, and transmitted his divine revelations orally, through Methuselah (c. 3300 
B.C.) and his descendants down to Abraham. From the writings of Zosimus (c. 400), 
it would appear as if there was once a race of amorous genii prone to fall in love 
with women, for he says that the secrets of nature were revealed by such genii to 
the daughters of men in return for love. The dowry was called Trapdoymv Oitav- the 
divine tradition ; the first account of these revelations was called xvf^"^ (chema) ; 
and the art itself, xvi^'-^ (chemia). Chema is thus an early tradition respecting the 
operations of nature taught to mankind by angels, who appear to have been damnati 
a Deo for their ill-timed loquacity. The credulous and imaginative O. Borrichius, 
in his De ortu et progress^ chemim dissertntio (Hafniae, 1668), said that the angels 
or demons here mentioned were the offspring of Seth and of Tubalcain, who had 
been instructed by their progenitors in the mysteries of nature, and who profaned 


their trust by communicating heavenly secrets to the daughters of Cain bv whose 
charms they were seduced (6^en. 6 2-4). The gynecial myth cf the origin of chemistry 
recalls the Jewish story of the fall of man, and also th6 Grecian legend of the Sibvl 
who demanded both length of years and a knowledge of the divine arcana as the 
price of her favours to Apollo the sun-god. Somewhat similar mvths are reported 
to have been current among the Phoenicians, the Persians, and the Egyptians 
They illustrate the extreme creduHty of man in the first of Comt^'s les trois etals' 
when everything that is not understood is believed to have a supernatural origin ' 


1 G. Booth, The Historical Library of Diodorus Sicvlus, London, 1721. 

2 R. Pietschmann, Hermes Trismegistus nach oegyptischen, griechischen, und orientcUischen 
Ueberheferungen, Leipzig, 1875; L. M6nard, Hermes Trismegistus, Paris, 1867; Alethophilo 
Hennetis Trismegisti, Stuttgart, 1855. ' 

3 For lists of these manuscripts, see H. Kopp, Beitrdge zur Oeschichte der Chemie, Braunschweiir 
2. 243, ] 869. *' 

^ E. 0. von Lippmann, Abhandlungen und' Vortrdge zur Oeschichte der Naturwi^senschaften 
Leipzig, 1. 1, 1913 ; H. C. Bolton, Amer. Chemist, 6. 165, 1875 ; F. H. Garrison, An Introduction 
to the History of Medicine, Fhila,de\phia, 191S ; H. Schaeffer, Z)ie Alchemie ; ihr dgyptisch-gricch- 
ischer Ursjnung und ihre weitere historische Entwicklung, Flensburg, 1887. 

5^ C. J. C. Reuvens, Lettres a M. Letronne sur les papyrus bilingues et grecs du Musee d'ArUiquitis 
de V Universite de Leide, Leide, 1 830 ; M. Berthelot, La chimie des £gyptiens d'apres les papyrus 
de Leide, Paris, 1886 ; H. Kopp, Beitrdge zur Oeschichte der Chemie, Braunschweig, 1. 97, 1869. 

^ C. 0. Lagercrantz, Papyrus Gtcbcus Holmiensis, Upsala, 1913. 

' V. E. Johnson, Egyptian Science, London, 1891. 

8 W. Whewell, History of the Inductive Sciences, London, 1. 253, 1857. 

^ G. Rawluison, Phoenicia, London, 1888. 

1" W. Herapath, Phil. Mag., (4), 3. 528, 1852 ; J. D. Smith, ib., (4), 4. 142, 1852 ; H. Kopp, 
Beitrdge zur Oeschichte der Chemie, Braunschweig, 2. 400, 1869 ; J. Napier, Manufacturing Arts 
in Ancient Times, with special reference to Bible History, Paisley, 1879. 

§ 10. The History of Chemistry in Greece and Rome 

They had visions. Oh ! They were as grand 
As ever floated out of fancy land. 

From the testimony of Diodorus the Sicilian (c. 30 B.C.), Clement of Alexandria 
(c. 200 A.D.), and lamblichus (c. 350 a.d.), it would appear that the Greeks learned 
the practice of the chemical arts and crafts largely from the Egyptians. i Diodorus 
says in his Bihliotheca historica (c. 30 B.C.) : 

Orpheus, Musaeus, Melampus, Daedalus, Homer, Lycurgus, Solon, Plato, Pythagoras, 
Eudoxus, and Democritus the Abderite all went into Egypt, and they doubtless learned 
there all those things which rendered them afterwards famous among the Greeks. For 
thirteen years Plato and Eudoxus associated with those priests in Egypt who most excelled 
in the knowledge of celestial things. They kept their knowledge in the greatest secrecy 
for a long period and would not deign to impart it to any one. At length, subdued by time 
and humble entreaty, they revealed some few things, but the greater part they concealed 
entirely from the vulgar. 

In the opinion of E. Zeller 2 there is little trustworthy evidence to support the 
assumption that the philosophy of the Greeks was derived from Oriental or Egyptian 
influences, although it is highly probable that it received some impulses from the 
East ; but whatever the Greeks borrowed from foreign sources was clarified and 
refined by the fire of their own genius. For example, it has been said that the 
Phoenicians taught the Greeks the art of writing, but that it was the Greeks who 

The knowledge of the secret arts, and the prevailing opinions of the Egyptian 
priests, as Herodotus (c. 440 B.C.) relates, must have been communicated in part to 
many vagabond Greeks during their sojourn in Egypt from about 660 B.C. The 
unrivalled Grecian artists surpassed their teachers in the beauty and elegance of 
their aesthetic productions, and also in works dependent upon imagination and 


fancy ; but artisans and craftsmen made much slower progress with the philoso- 
phical Greeks than with the more practical Egyptians. An Alexandrian Society 
is reported to have been formed among the Greeks in Alexandria about the third 
century, but, so far as we can gather, the knowledge which they are supposed to 
have acquired mainly from the Egyptians, was confused with metaphysical 
fancies ; and its expression was obscured by ambiguous allegories and cabalistic 
symbols — possibly aping the hieroglyphics of Hermes — so that their writings now 
appear to us as if the authors tried to conceal their own ignorance in a cloud of 
words and symbols. , 

The Greeks did not contribute much to the chemical arts, but they furnished 
chemistry with a science of method applicable to all the sciences. The Egyptians 
accumulated facts and invented useful arts ; the Greeks discovered the laws of 
investigation, the principles of discovery, and the laws of thought. The most 
important result of centuries of Grecian effort was consummated in the mighty 
Organon of Aristotle (c. 320 B.C.). This organon of deductive and inductive method- 
ology should have inaugurated the third of Comte's les trois etats, but it did not. 
The facts had not been determined with sufficient accuracy. Isaac Newton could 
not have discovered the gravitational law if accurate data had not been prepared 
for him by J. Kepler and G. Galilei. Aristotle's organon came too early. In 
any case, the method of investigation so gloriously established by Aristotle was 
unproductive ; it was degraded, misunderstood, and perverted by his disciples, 
who, instead of applying the great principles of the organon, worshipped their master's 
opinions on a host of special subjects as if they were oracles divine. Thus, I. R. 
Averroes, about the middle of the twelfth century, went so far as to say, " The 
doctrine of Aristotle is the perfection of truth, for his understanding attained the 
utmost limit of human ability." 

The method of Aristotle was rediscovered and restated by Francis Bacon in his 
Novum organum (London, 1620). The two methodologies are substantially the 
same. To some. Bacon's organon appears to have inaugurated a kind of Lutherian 
reformation in science ; rather did the Baconian organon grow tardily from seeds 
planted by Aristotle and his predecessors in the unproductive soil of metaphysical 
speculation. According to G. H. Lewes' Aristotle (London, 1864), the main 
cause of the sterility of the method of Aristotle and Francis Bacon was their 
failure to appreciate the need for unremitting verification, so well emphasized 
by Roger Bacon (c. 1280) ^ — Bacon the First — in order to vindicate the 
principles deduced from the available facts. The same idea was emphasized by 
Albertus Magnus, about the same time as Roger Bacon : 

A principle which does not agree with experimentali cognitione (experimental knowledge 
acquired by the senses) is no principle, but rather the opposite. 

Aristotle himself frequently emphasized the danger of relying on mere guesses as 
if they were observed facts, but he so often departed from his own precepts that he 
was frequently inveigled by the perils of his own speculations. The illustrious 
Francis Bacon likewise completely failed in vigilance when he attempted to apply 
his own method because he did not practise the very principles he had expounded 
so well. Bacon the Second even went so far as to say that if his methods were 
adopted, little would depend upon the acuteness of the intellect, for the varied 
talents of all men would be reduced to one common level. He said : 

Our method of discovering the sciences is one which leaves not much to the acumen and 
strength of wit, but nearly levels all wits and intellects. 

Although the principles of Francis Bacon's organon have been available for nearly 
three hundred years, they have proved quite inadequate, and there are no signs 
of this socialistic levelling, for the interval between mediocrity and talent is as great 
as ever it was. Francis Bacon himself can scarcely be considered to have been a 


scientific man, or even to have possessed the scientific instinct. Science may have 
been about him but it certainly was not in him. 

The Greeks seem to have generally emphasized subtilty in speculation and 
debate rather than accuracy in observation and experiment. In theoretical work, 
they had an overweening tendency to extreme abstraction, and they were careless 
and credulous in observation ; otherwise expressed, they founded arguments too 
confidentl} on unproved statements, and seem to have regarded logical consistency 
of greater weight than accuracy in the statement of facts. This characteristic was 
well summed up by the saying which Plato, in his Timceus, ascribes to the Egyptian 
priest of Sais : "Ye Greeks will be always children ... for though wisdom falls 
from your lips, your actions are weak and puerile ; " or as S. Brown expressed it : 
" In the art of experiment, the Greek was as feeble as a child ; but in the sphere 
of ideas and vast conceptions it is not a paradox to say that he was sometimes 
stronger than a man." 

The Ionian doctrine of one primal element.— The Theogony of Hesiod (c. 700 b.c.) 
assumed that " the earth is the unmovable basis of the cosmos," but the poem is 
rather a record of mythic cosmology, and anthropomorphism, characteristic of the 
first of Comte's les trois etats, and it had no influence on the development of 
philosophical opinions.'* Similar remarks apply to Pherecydes (c. 600 B.C.), who 
followed Hesiod with an improved mythology. Pherecydes made a definite attempt 
to distinguish between the material constituents of the universe — e.g. between the 
earth and its atmosphere, and also between matter and force. He regarded force 
as a mysterious power exerted by the god Zeus. 

The Ionian philosophers — Thales, Anaximander, and Anaximenes — still further 
substituted impersonal causes, acting uniformly and continuously, for personal 
causes acting capriciously and arbitrarily. At this time, therefore, the Greeks 
were in a transitional stage between the first and second of Comte's les trois etats. 
Thus, the early philosophers of Greece soon recognized that a belief in superhuman 
gods was not sufficient to explain the complex phenomena in the physical world. 
They then promulgated hypothesis after hypothesis to explain how the universe 
grew from some simple principle — earth, water, air, fire. The new explanations 
proved just as unmanageable as those which regarded natural phenomena as the 
work of supernatural agencies. These early speculations of the Greeks do certainly 
testify to the vigour and activity of their questioning spirit,^ but their ardour and 
confidence were untamed by labour or reverses. It required centuries of chasten- 
ing discipline for man to learn that " he must acquire, slowly and patiently letter 
by letter, the alphabet in which nature writes the answers to such inquiries." 

The first of the seven wise men of Greece, Thales of Miletus (c. 600 B.C.)— a 
contemporary of Solon — made one of the earliest protests against the personifica- 
tion of nature by assuming natural phenomena to be produced by capricious designing 
agencies— diabolic or divine. Three centuries later, Epicurus likewise protested 
emphatically against referring natural phenomena to the deliberate interventions 
of gods. Thales believed that natural phenomena are due to the operation of 
invariable laws to be discovered by a proper appHcation of the human intellect. 
According to Thales, all the various forms of matter are different manifestations of 
one underlying essence or prima materia. The universe, to him, was made from 
water, which he regarded as the primal element. The same idea occurs m many 
of the sacred books of the Hindus— e.^r. The Institutes of Manu—&hout the nmth 
century B.C. There is really nothing to show how Thales was led to make the 
assumption. It has, however, been noted that it is typical of systematic thinkers 
to reduce to one general proposition that characteristic which is possessed m common 
by a number of simple facts ; and it is therefore hinted by Aristotle that 1 hales, 
meditating on the constitution of the universe, saw that water or moisture is 
omnipresent ; that Thales was impressed by the marvellous transformations ot 
water in the form of rain and dew, snow and hail, river and sea ; and that the eartn 
appeared to be floating in an ocean of water. All things also seemed to be nourished 


by water, and he accordingly assumed that water is the sole primal element which 
is convertible into all the manifold varieties of matter— mineral, vegetal, and 
animal — -found in the world. W. Whewell, however, emphasizes his belief that the 
opinions of the philosophers of this period are based on vague suggestions and 
casual analogies, rather than on reasons which will bear examination. It is very 
remarkable, said A. Comte (1839), that the most inaccessible problems, such as the 
origin and cause of phenomena, should be the very ones which first occurred to 
students of nature, while those which were within their reach were considered to 
be unworthy of meditations serieuses. 

Another Miletian, Anaximenes (c. 500 b.c), is considered to have been the pupil 
of Anaximander, who, in turn, was the disciple of Thales. Anaximenes sought for 
the first principle of things in the omnipresent yet invisible air, which he regarded 
as the equivalent of life because all living beings were nourished by air. Air 
embraces the whole world, said Anaximenes, and he regarded air as the one eternal 
essence, more primitive than Thales' water. He called air to aTreipv — the infinite 
— and considered it to be devoid of any material differentiation. Even as late as 
the eighteenth century, some chemists accepted Anaximenes' air as the primordial 
element. Thus S. Hales, in his Vegetable Staticks (London, 1727), supposed that 
atmospheric air deprived of its elasticity entered in a solid form into the composition 
of most substances, and that air is the universal bond in nature. G. E. Stahl also 
wrote to the same effect in his Experimenta,ohservationes,animadversionesCCC nuynero 
chymiccB et physicce (Berlin, 1731). We have no record how the lonians — Thales and 
Anaximenes — accounted for the formation of the different forms of matter from 
their primitive elements, since matter by itself can only be matter. 

The Ephesian Heracleitus (c. 450 b.c.) expressed himself in such enigmatical 
terms that he has been called the Obscure Philosopher.^ A few fragments of his 
writings have survived. Heracleitus appears to have maintained that all ideas are 
derived from sensations, and he was the author of the celebrated doctrine that all 
things are perpetually in a state of motion or flux, and that there is no rest or quietude. 
Strife between opposite tendencies is the parent of all things. All life is change, 
and change is strife. The living and moving element in nature seemed to him to 
be an setherial exhalation, or fire. All things in nature are formed of the principle 
of fire, which, in turn, is composed of small indivisible parts, i/^ty/xara or atoms, 
which are in perpetual motion. If all things are conceived to be in perpetual 
motion or change, then all things are fire. Never-resting fire rules all. Every- 
thing has arisen from fire by condensation or rarefaction, and all things resolve 
themselves back into fire. This idea is but a modification of the water and air 
elemental of the Ionian philosophers. Obviously, Heracleitus' elemental fire was not 
ordinary fire ; he probably understood fire to mean that which by constant trans- 
mutation causes all the varied changes seen in the universe, and which itself remains 
unchangeable. This idea of a primum mobile comes as near to the modern doctrine 
of energy as was possible with the facts then available. 

The Grecian Hippocrates (c. 400 b.c.) was not exactly a follower of Heracleitus, although 
there is a strong resemblance between the views of both. Hippocrates specialized in medicine, 
and he has been called the oracle of medicine ; he expressly rejected the use of hypothetical 
philosophy in medicine ; he did not altogether rely on empirical experience, but attempted 
to formulate general rules and principles derived from experience and knowledge. From 
the chemist's point of view, a small treatise, On airs, waters, and sites, is considered to be 
the most interesting of the works attributed to him. As might have been anticipated it 
contains many errors and inexactitudes. 

The Ionian doctrine of one primitive element was abandoned by Anaxagoras 
in a work On nature (c. 450 b.o.).'^ He assumed that every difference in the sensible 
qualities of bodies is fundamental, original, and inalienable ; and that there are 
so many elements as there are simple substances ; no means were known at that 
time for breaking down the majority of substances, and they were accordingly 
assumed to be simple or elemental. The number of elements was therefore supposed 


to be very large, or infinite. By repeated subdivision, Anaxagoras argued that all 
natural things could be resolved into ultimate particles which were later on termed 
homoBomericB — ofjioios, like ; fxipos, a part. The homoeomerise were supposed to be 
eternal, unchangeable, infinitely divisible, and capable of continuous extension. 
Like homceomerise act on like, and so form matter. If the qualities of the homoeo- 
merise are assumed to be developed only when the particles are in combination with 
others, Anaxagoras' homoeomeriae are not very different from the atoms of Leucippus, 
Democritus, and Lucretius. Anaxagoras' atoms are the same in kind as the 
substance itself ; Leucippus' or Democritus' atoms are indivisible particles of one 
kind of matter. Anaxagoras also introduced the idea of a motive principle, which 
he called vov<s, as the cause of all changes. Democritus called this principle dvayicrj ; 
Heracleitus, avaOvfiiacn^ or fire ; and Aristotle, at^^p, aether. 

The four and five element theories. — The four and five element theories are 
among the oldest attempts to classify the protean and multitudinous forms of 
matter which make up the world. The five-element theory seems to have been 
favoured by the Chinese and Hindu philosophers. The Greeks reduced the number 
of elements to four. Diogenes Laertes (c. 250) tells us that the five-element theory 
was first promulgated by the Pythagoreans, and that Empedocles (c. 500 B.C.) first 
advocated the four-element theory as a consistent doctrine. Empedocles cited the 
burning of wood in favour of his hypothesis. When wood burns, srrwke or air rises 
upwards, and this is followed by flame or fire ; moisture or water is deposited upon 
any cold surface in the vicinity ; and ash or earth remains behind. Empedocles' 
simple statement seems to be the first record of a chemical analysis. Wood is 
resolved into its supposed elementary constituents — fire, earth, water, air. True 
enough, modern methods can probe much deeper, but Empedocles' analysis is excel- 
lent for its time. The doctrine of the four elements thus appears as a methodical 
deduction from facts observed during the analysis of wood, by burning it in air. 
This analysis has been claimed as " the starting-point of chemistry in history." 

Empedocles also formulated the germinating conception of chemical afl&mty, 
for lie said that the cause of the various combinations and separations of these four 
elements is love (<f>t\ia) and hate (vcikos), which are exerted as active forces pro- 
ducing the union or decomposition of substances. The four elements of Empedocles 
soon lost their material character, and grew into abstract principles. It was then 
fancied that the whole world was compounded of four distinct principles or entities 
—the earth typified all solids ; water, liquids ; air, the winds, clouds, and the breath ; 
and lastly, fi/re, which was symbolized by the sun, and worshipped by many as 
a god. Hence, in the writing of the alchemists of the Middle Ages, there is usually 
a chapter devoted to this quartet — earth, water, air, and fire. In J. Lacinius' 
alchemical treatise Pretiosa fnargarita novella de thesauro (Venice, 1546), fire is 
symbolized by an angel, air by a bird, water by a dragon, and earth by a bull. 
Aristotle added a fifth element, at%, aether, more divine than the others, and which 
pervaded all things, and was in perpetual motion. Later, Aristotle's aether became 
the quinta essentia—a. kind of primal matter, a divine subtle extract, the qumtessence 
of the other four. The ancient Hindu philosophers also had a fifth element, which, 
in their system, was wrongly supposed to be a medium for propagating sound, etc., 
and which, in consequence, had something in common with the modern concept 
of an aether pervading all space. The Institutes ofManu regarded the subtle aetHeT 
as being first created ; and from this, by transmutation, sprang air, which changed 
into light or fire, and thence into water, and finally earth. f a o 

Aristotle assumed that the one primitive quintessence of matter can act as a 
vehicle or carrier for four primitive qualities : hot or cold, wet or dry. ii tnese 
four qualities or elements are united with inert passive matter in pairs tne lour 
primary forms of matter— air, earth, water, fire— are produced ; for i^fj^n^' J^^^ 
has hot and dxy ; t^a^er, cold and wet ; air, hot and wet ; ^nd eart/^ cold ana a^^^^ 
The different varieties of matter arise when different degrees of these ^ourelementa 
qualities are impressed on matter. Aristotle denied that the four elements ot 


Empedocles are really elements because they are mutually convertible one into 
another. Empedocles' elements, however, may represent the four primary forms 
of matter perceived by the senses, and into which the four qualities appear to be 
resolvable : 

For hot, cold, moist, and dry, four champions fierce. 
Strive here for mast'ry, and to battle bring 
Their embryon atoms. — J. Milton. 

The alchemists of the Middle Ages supposed that the elements were formed 
when the primal essence was clothed with three principles — tria 'prima — which they 
called respectively salt, sulphur, and mercury. In the quaint words of Paracelsus : 

Eisen, stahel, bley, smaragd, sappir, kieszling, nichts anders seind denn schwefel, salz, 
und mercm*ius. 

Salt represented the earth or the principle of fixity and solidity ; mercury represented 
air and water, or the principle of liquidity and gaseity ; and sulphur typified fire or 
the principle of combustion. Thus, said Paracelsus, " whatever fumes and evaporates 
in the fire is mercury ; whatever flames and is burnt is sulphur ; and all ash is salt.''' 
Albertus Magnus typified the three principles by arsenic, sulphur, and mercury, for 
he supposed the metals were compounded of these elements. 

The three principles of the alchemists were not substances or corpora, but rather 
principia or qualities ; they were representative types of qualities or classes. 
Sometimes the tria prima were confused with the four elements of the Greeks, and 
it is difficult to understand clearly what was gained by the invention of the three 
principles. The mystic alchemists went even further and imagined that all material 
things were composed of a trinity : "A body and a soul held together by a spirit 
which is the cause and the law." They believed the soul of matter to be the trans- 
forming principle which they tried to extract in a pure form, and which they 
expected would enable them to transform the baser forms of matter into the purer 
forms, of which gold was the best type. 

The four-element theory was demolished when water, air, and the earths were 
decomposed into still simpler bodies ; and when fire was shown to be a manifestation 
of energy. The term " element " was obviously not intended to be used in the same 
sense as it is to-day. The four and five elements of the ancients were not con- 
sidered to have aH independent natural existence, but to be derivatives of one 
another ; the earlier notion of an element rather referred to the genesis of matter 
than to its ultimate analysis, for the distinction between simple and compound 
substances does not seem to have entered their minds. Whatever the idea involved 
in the three, four, or five element theories, it was believed by many different races 
in different parts of the globe ; it has pervaded the philosophy of all thinking races ; 
it has been sung by the poets of every land ; and it has had a longer life than any 
succeeding philosophy. The theory was living three centuries ago ; it is now dead. 
The Greek philosophers. — Three gigantic spirits have dominated Grecian thought 
— Pythagoras, Plato, and Aristotle. Each one in his turn exerted a profound 
influence on his contemporaries, and on subsequent thinkers. Thomas Carlyle has 
well said that all history revolves around certain famous personages. The records 
of Pythagoras (c. 500 b.c.) and of his school are overgrown with myths and fictions ; 
and, as with the records of other influential men of old, the older the records, the 
greater the tendency to associate miraculous and extraordinary events with the 
men's lives.^ The Pythagoreans formed primarily a moral, religious, and political 
association, although the sect early gave a definite trend to philosophical thought. 
The scholars are now mainly dependent upon more or less untrustworthy reports 
for their knowledge of the physical tenets of the Pythagoreans. It is generally 
agreed, however, that the Pythagoreans believed that number is the essence of all 
things. It is difficult to gather what was meant by this high-sounding phrase, for 
number appears to be merely a relation, or the expression of certain facts. One 
section of the Pythagoreans — e.g. lamblichus — held number to be the substantial 


element of corporeal things ; otherwise expressed, Uke Plato's ideas, numbers are the 
eternal archetypes of thmgs ; another section— e.^f. Hippasus— held that all things are 
formed, not out of number, but after the pattern of numbers— otherwise expressed 
number is the pattern or model from which things are copied, meaning that all things 
bear the same fixed relation that a series of whole numbers bears to unity ; or again 
as expressed by Philo, or whoever wrote the Booh of Wisdom, " God ordained all 
things in measure, number, and weight." According to E. Zeller's account of the 
Pythagoreans, the idea must have arisen as man dimly realized the definite and 
mathematical order in natural phenomena. 

From the more or less legendary accounts of the Pythagoreans, ft appears that 
they reduced all things ultimately to one incorporeal monad, and assumed that all 
things are compounded of monads with dissimilar and opposite natures, the uniting 
bond being harmony. Later writers — e.g. Ecphantus — appear to be in error when 
they state that Pythagoras' monads were corporeal. The Pythagoreans attached 
special importance to the number 4, the quarternion, which was said to be the source 
and root of eternal nature ; and the later Pythagoreans — e.g. Philolaus — were fond 
of arranging things in series of four. Philolaus considered that the elementary 
nature of bodies depended upon their form, and it was assumed that the smallest 
constituent parts of the earth had the form of a cube ; air, an octahedron ; fire, a 
tetrahedron ; water, an icosahedron ; and the fifth dodecahedral element represented 
the universe, and embraced all the others. The diagrams. Fig. 5, explain the idea. 
The historical evidence has not enabled the scholars to decide whether Empodocles 
adopted four from Pythagoras' five elements, or whether Pythagoras added a fifth 

Tetrahedron Octahedron Icosahedron Cube 

(Fire) (Air) (Water) (Earth) 

Fig. 5. — ^Primitive Particles of Pythagoras' Elements 


element to Empedocles' four. It is thought that the Pythagoreans probably 
derived the five-element theory from the Hindus. According to Max Miiller, the 
coincidences between the teachings of Pythagoras and Hindu learning are so 
numerous as to make it highly probable that Pythagoras obtained his leading 
tenets by contact with the Indians in Persia. 

The celebrated Athenian pupil of Socrates, Plato, expounded his views on 
natural phenomena in his Tiynoeus (c. 360 B.C.). He assumed that all things 
and all phenomena are transitory and unreal, but the abstract idea of them is 
alone eternal and real. Hence, the aim of philosophy is to discover the ideas or 
abiding principles of which the phenomena of the material world are but the 
image. I. Kant (1790) described Plato's hypothesis by the celebrated metaphor : 
Just as a flying dove, feeling the resistance of the air, might wrongly 
suppose it would be able to fly faster in airless space, so did Plato, feeling 
the limits which the sensuous world opposed to his understanding, assume 
that by abandoning the sensuous world, he would be more successful in the void 
space of pure intellect. Plato asserted as an a priori truth that the principle of 
matter was infinite, eternal, and deprived of all qualities ; that matter is converted 
into bodies by being impressed with some occult moving power ; and that matter 
may possess particular qualities— hotness, dryness, coldness, and wetness. He 
considered that there are four elements— air, water, fire, earth— and assunied that 
these elements can never be destroyed. The elements can be divided into infinitely 
smaU particles incapable of further subdivision ; the ultimate particles of the 
elements have definite forms analogous to those suggested by Philolaus, Fig. 5. 


The differences between the various kinds of the same elements are due to differences 
in the bounding planes of the constituent particles. Fire, air, and water can be 
transformed into one another by the coalescence of the primitive particles into forms 
peculiar to these substances. Earth cannot be converted into any of the other 
three elements because its cubical particles have no mathematical relation with the 
forms of the other three. ^ 

The influence of Plato's pupil, Aristotle, on the world of thought has been 
rivalled only by the founders of the great religions. Aristotle lived between 384 
and 322 b.c. Excluding his Organon, to which reference has already been made, 
Aristotle's most interesting contribution to natural science is entitled Meteorology, 
and it deals with astronomical, chemical, and geological subjects ; his views on the 
constitution of matter are expounded in his Generation and Corruption. There is 
a work on Physics containing unfruitful disquisitions on abstract space, motion, 
infinity, etc., and also a kind of sequence to this work entitled The Heavens. 
Aristotle also wrote some biological works. There has been some discussion as to 
whether a work on Mechanical Problems attributed to him is really the one to which 
he sometimes refers. lo 

Greek was not a familiar language to the philosophers of the Middle Ages, and 
Aristotle's writings in the original Greek do not appear to have been known in 
Western Europe prior to the thirteenth century. Aristotle's works were translated 
into Syriac, thence into Arabic, and carried to Spain by the Moors. About the 
fourteenth century Latin translations, made direct from the Greek manuscripts, were 
read in Europe, and soon got a remarkable hold on European thought. In a general 
way, it has been said that although Aristotle professed to rely on experience and 
induction as the sources of true knowledge, he often went astray ; his treatment 
of natural philosophy displays a capable mind, hampered by unsuspected super- 
stitious prejudices, wrestling with problems beyond its strength. Aristotle rejected 
Plato's idea-hypothesis and Pythagoras' number-hypothesis. He supposed matter 
to be capable of infinite division, and he objected to Democritus' idea of atoms, 
although he admitted that matter may be made up of particles which are actually 
though not potentially indivisible. Aristotle did not agree with Pythagoras' 
and Plato's hypotheses that the elemental monads have definite geometrical forms. 
He said that the attempt to bestow an intrinsic figure on the elements — Fig. 5 — is 
absurd, an element cannot have one. Elementary matter must be formless and 
amorphous, ready to take on any form according to circumstances, but itself possess- 
ing no particular form. 

A famous disciple of Aristotle, Theophrastus (c. 372-287 B.C.) of Lesbos, suc- 
ceeded his master at the Lyceum.ii Theophrastus wrote two works on botany 
which were standard even throughout the Middle Ages ; a history of physics, a 
work on natural science, and several other fragments — some writings ascribed to 
him are no doubt spurious. Theophrastus followed the philosophy of Aristotle 
rather closely in his Treatise on Fire (c. 315 B.C.) — Trepl nvpos. Theophrastus removed 
fire from the list of elements ; and he recognized that air plays an important part 
in the maintenance of a flame, and in the development of plants. In a fragment 
On Odours, he adds that the odour of a substance is due to its volatility. The more 
important parts of Theophrastus' writings, from the chemists' point of view, dealt 
with minerals — irepl KiOoiv. Here he mentions coal, cinnabar, orpiment and 
sandarach (arsenic sulphides) for the first time, and he also describes the prepara- 
tion of white lead, red lead, verdigris, colcothar, chrysocolla, etc. 

A number of writings and fragments of Archimedes of Syracuse (287-212 B.C.) 
has been preserved. 12 They deal with some mechanical and hydrostatical problems. 
The discovery of the celebrated principle of Archimedes — if a solid be weighed in 
air, and then immersed in water, the apparent loss of weight is equal to the weight 
of a volume of water equal to the volume of the solid — is described in M. P. Vitru- 
vius' De architectura, published near the beginning of the Christian era. Al-Khazini's 
account in the twelfth century lacks the piquancy and interest of that of Vitruvius. 


It has been remarked that although Archimedes had fairly entered upon the right 
path of his department of experimental science, no further advance was made for 
nearly two thousand years, when Galilei and Stevinus took up the work. The 
celebrated Hero flourished about 117 b.c. In his work on Pneumatics, he described 
the principal physical properties of air known to the ancients, and indicated some 
ingenious mechanical contrivances operated by means of rarefied or compressed air ; 
ho also wrote a treatise on the properties of reflected light ; and two treatises on 
the mechanical powers.^^ 

The main contributions of the Greeks to chemical science are the prima Tnateria 
hypothesis ; the four-element theory ; the atomic theory ; the idea of the trans- 
mutation of matter from one form to another by some agent or principle ; and more or 
less vague notions of an active principle causing combination or change. There is 
also Aristotle's work on methodology. i* The Greeks were not generally guided by 
observation and experiment either in founding or in verifying their hypotheses. 
Consequently, their great conceptions were wondrous feats of the imagination ; but 
Lord Macaulay would have none of it, for, in his essay on Lord Bacon (London, 1837), 
he claimed that the Greeks aimed at the stars, and through no want of skill the shot 
was thrown away. The arrow was indeed followed by a track of dazing radiance, 
but it struck nothing. Their philosophy began in words and ended in words. 

Rome. — The Romans acquired some knowledge of the chemical arts after they 
had conquered the Egyptians and the Greeks. The Romans had no philosophy of 
their own, but they borrowed ideas and learned lessons at the feet of conquered 
Greece.15 War was the strength of the Romans, and they favoured the arts and 
crafts which made good soldiers. The works of art which the Romans acquired as 
loot from conquered nations attracted much attention, and stimulated some of their 
artisans to imitate these productions. The Romans, however, displayed but little 
inventive genius, and it is probable that what successes they obtained were largely 
due to the work of imported craftsmen. The early Romans developed a code of 
civil law which has been a pattern for succeeding nations. The doctrine of the 
supremacy of law inculcated by the Romans probably exerted some influence on 
man's subsequent attitude towards external nature, and some confusion has resulted 
from the assumption that a law of nature represents an obligation on the part of 
natural phenomena analogous to the obligations of a people to its civil law. The 
modern view of a law of nature is very difierent from this. 

The poem of Lucretius (95-52 b.c), De rerum natura, is much admired, and is 
intimately associated with the history of the atomic theory. It has been considered 
curious that, with the exception of a few fragments and letters, the works of the 
three founders of the Grecian atomic theory — Leucippus, Democritus, and Epicurus 
— should have been lost, and that we have to rely upon the Roman's eloquent poem 
for a clear and concise account of Epicurus' doctrine. There is nothing to show if 
Lucretius added anything new to what he found in Epicurus' two works — Concern- 
ing nature, and On atoms and voids — which have been lost. 

The principal writings dealing with the physical arts and crafts of the Romans 
are the works of Vitruvius, Galen, Dioscorides, Varro, Seneca, and Pliny. M. P. 
Vitruvius was an engineer and architect in the service of the Roman state at the 
time of Augustus— near the beginning of the Christian era. He wrote the cultured 
work, De architectural^ in which he gives many indications of the learning of his time, 
viewed more particularly from the point of view of the practical application of 
theoretical knowledge. Another celebrity— Dioscorides— was born in Asia Minor 
and flourished in Rome about 75 a.d.i^ contemporaneously with Pliny. His Z)c 
tnateria inedica was a standard work for many years. In this book, Dioscorides 
describes the art of distillation for the first time, although Aristotle (c. 320 B.C.) 
seems to have had this operation in his mind when he wrote in his Meteorology (2. -) : 

Sea water is rendered potable by evaporation ; wine and other liquids can be submitted 
to the same process, for, after having been converted into vapours, they can be condensea 
back into liquids. 


Dioscorides also describes other chemical operations — e.g., the extraction of mercury 
from cinnabar, by heating a mixture of the latter with carbon. He also mentions 
lime-water, zinc oxide, blue vitriol, white lead, etc. 

Another Greek, C. Galen (131-201 a.d.), regarded himself as a disciple of Hippo- 
crates. He practised medicine in Rome about 160 a.d. He was an experimental 
physiologist, and wrote on human anatomy, physiology, and botany. M. T. Varro 
(116 B.C.-28 A.D.) wrote on agriculture, law, mensuration, etc. A. Seneca wrote 
a work, Qucestiones naturales (c. 63 a.d.), which appears to have been largely 
drawn from Aristotle's Meteorology.'^^ It deals mainly with astronomy, meteorology, 
and physical geography ; and it was the authority on science in the Middle Ages 
up to the fourteenth century, when it was largely supplanted by Aristotle's works, 
which then became accessible in Europe through the Latin translations of the Greek 
texts. Caius Plinius Secundus, or Pliny the Elder,2o wrote the Historia nuturalis 
about 77 A.D. It deals with an enormous variety of subjects and is a congested and 
uncritical compilation from credible and incredible authorities and popular beliefs. 
E. Gibbon, in his Decline and Fall of the Roman Empire (London, 1789), called it 
" an immense register of the discoveries, arts, and errors of mankind." The works 
of but a few of the authorities quoted by Pliny are known. 

The prosperity of the Roman Empire — which included England, France, Spain, 
and all the countries about the littoral of the Mediterranean Sea — was on the wane 
about 180 A.D. The Romans, satiated with conquest, became indolent and corrupt, 
and their intellectual activity slackened. Their empire was invaded by the un- 
civilized northern races — Goths, Vandals, and Huns. The destructive impulses of 
the invaders led to the complete disintegration of the empire ; and about the fifth 
century, culture and civilization in Rome were crushed in a few dark years. Many 
of the records of science, literature, and art were deposited in monasteries, where 
they were preserved as sacred trusts until civilization again revived in Western 
Europe. Fortunately, however, Constantine transferred the Roman capital to 
Byzantium (Constantinople) in the fifth century, and the New Rome maintained a 
continuity of government and of civilization until the raid of the fourth crusaders 
in 1204. It is considered that more destruction and damage to ancient records 
were wrought in the sack of Constantinople by these Crusaders than by the Mahom- 
edan conquest in 1453.21 


1 G. Grote, A History of Greece, London, 1. 355, 1869. 

2 E. Zeller, A History of Greek Philosophy, London, 1. 26, 1881 ; S. H. Butcher, Some Aspects 
of the Greek Genius, London, 1891 ; L. von Schroder, Pythagoras und die Inder, Leipzig, 1884 ; 
M. B. St. Hil&ire, Premier memoire sur le Sankhya, Paris, 1852; R. Gar be, Monist, 4. 176, 1894 ; 
W. Jones, Works, London, 3. 236, 1799 ; H. T. Colebrooke, Miscellaneoiis Essays, London, 1837. 

' Roger Bacon, Opera inedita, London, 1860; Opiis majus, London, 1773; G. H. Lewes, 
The History of Philosophy, London, 2. 77, 1871 ; B. R. Rowbottom, Journ. Alchem. Soc, 2. 75, 
1914 ; S. Brown, Essays, Edinburgh, 1858; J. E. Sandys, Boger Bacon, London, 1914. 

* E. Zeller, A History of Greek Philosophy from the Earliest Period to the Time of Socrates, 
London, 1. 211, 266, 1881 ; F. Bacon, De principiis atque originibus, London, 1612 ; J. Burnet, 
Early Greek Philosophy, London, 1908. 

* W. Whewell, History of the Inductive Science, London, 1. 20, 1857 ; J. H. Bridges, Essays 
and Addresses, London, 143, 1907. 

* F. Lassalle, Die Philosophic Herakleitos' des Dunkeln, Berlin, 1858; T. Gomperz, Sitzber. 
Akad. Wien, 997, 1886 ; Greek Thinkers, London, 1. 69, 1901 ; J. Burnet, Early Greek Philosophy, 
London, 143, 1908 ; G. Gladisch, Herakleitos und Zoroaster, Leipzig, 1859. 

' T. Gomperz, Greek Thinkers, London, 1. 223, 1901 ; J. Burnet, Early Greek Philosophy, 
London, 290, 1918. 

® E. Zeller, A History of Greek Philosophy from the Earliest Period to the Time of Socrates, 
London, 1. 306, 1881 ; J. Miiller, Naturwiss. Ver. Innsbruck, 23. 1897 ; T. Gomperz, Greek Thinkers, 
London, 1. 99, 1901 ; J. Burnet, Early Greek Philosophy, London, 319, 1908. 

' E. Zeller, Plato and the Older Academy, London, 1876 ; T. H. Martin, fjtudes sur le Timee de 
Platon, Paris, 1841 ; J. S. Konitzer, Ueber Verhdltniss Form und Wesen der Elementarkorper n/ich 
Plato's TimcBus, Neu Ruppin, 1846 ; E. O. von Lippmann, Journ. prakt. Chem., (2), 76. 513, 1907 ; 
AbJiandlungen und Vortrdge zur Geschichte der Naturwissen-scJuiften, Leipzig, 2. 28, 1913; F. W. Bain, 
On the Realization of the Possible and the Spirit of Aristotle, London, 1899. - 


1" E. T. Poselger, Aristotle's Mechanische PrdbUme {Quoestiones mechanica), Hanover, 1881. 
^^ E. Zeller, Aristotle and the Earlier Peripatetics, London, 2. 348, 1897. 

12 T. L. Heath, Archimedes, Cambridge, 1897; Archimedes, Opera, Basil, 1544; (Euvres 
Paris, 1807. 

13 W. Schmidt, Hero's Werke, Leipzig, 1899 ; B. Woodcroft, The Pneumatics of Hero of 
Alexandria, London, 1851 ; T. H. Martin, Hero, Paris, 1854. 

1* E. Zeller, Aristotle and the Earlier Peripatetics, London, 1897 ; G. H. Lewes, Aristotle, a 
Chapter from the History of Science, London, 1864; C. Daubeny, An Introduction to the, Atomic 
Theory, Oxford, 1850 ; M. B. St. Hilaire, La physique d'Aristote et la science contemporaine, 
Paris, 1863 ; T. E. Jones, Aristotle's Researches in Natural Science, London, 1912 ; J. Lorscheid, 
Aristotles' Einfluss auf die Entwicklung der Chemie, Miinster, 1872 ; E. O. von Lippmann, Arch. 
Geschichte Naturwiss. Technik., 233, 1910 ; Abhandlungen und Vortrdge zur Geschichte der Natur- 
uissenschaften, Leipzig, 2. 64, 1913. 

15 A. Terquem, La science romaine a Vepoque d'Aiiguste, Paris, 1885. 

1^ J. Gwilt, Vitruvius, London, 1826 ; M, H. Morgan, Vitruviu^, London, 1914 ; A. J. Brock, 
Galen, 1916. 

1' E. 0. von Lippmann, Zeit. angew. Chem., 18. 1209 ; 1905 ; Abhandlungen und Vortrdge zur 
Geschichte der Naturwissenschaften, Leipzig, 1. 47, 1906. 

1^ H. Kopp, Beitrdge zur Geschichte der Chemie, Braunschweig, 217, 1869 ; E. 0. von Lippmann, 
Chem. Ztg., 26. 629, 1189, 1911 ; Abhandlungen und Vortrdge zur Geschichte der Naturwissen- 
schaften, Leipzig, 2. 157, 162, 1913. 

1* J. Clarke, Physical Science in the time of Nero, being a translation of the Quoestiones naturales 
of Seneca, London, 1910 ; A. Terquem, La science romaine a Vepoque d^Auguste, Paris, 1885. 

^^ E. 0. von Lippmann, Zeit. an^ew. Chem., 6. 383, 1893 ; Abhandlungen und Vortrdge zur 
Geschichte der Naturwissenschaften, Leipzig, 1. 1, 1906. 

21 E. Pears, The Fall of Constantinople in the Fourth Crusade, London, 1885 ; J. B. Bury, The 
Roman Empire, London, 1910 ; H. Gelzer, Byzantin Kulturgeschichte, Tiibingen, 1909. 

§ 11. The History of Chemistry in Syria, Persia, and Arabia 

Very little advance in culture could be made even by the greatest man of genius if he 
were dependent for what knowledge he might acquire merely on his own personal observa- 
tions. Indeed it might be said that exceptional mental ability involves a power to absorb 
the ideas of others, and even that the most original people are those who are able to borrow 
the most freely. — W. Libby (1917). 

About the third, fourth, and fifth centuries, the Neo-platonic schools at Alexan- 
dria and at Athens included Ammonius Saccas, Plotinus, Porphyry, lamblichus, 
Proclus, etc. These schools cultivated mysticism and magic. As with the Pytha- 
goreans, they taught that the air is full of spirits and demons which control health 
and disease, and natural phenomena in general. It was said : 

God rules the world. He has demons imder his control, some of which govern animals, 
some vegetables, and others minerals. . . . One demon governs the liver and another the 

When animals or vegetables were destroyed by fire, the gases which escaped were 
supposed to be subtle spirits returning to the air. With beliefs like these, natural 
phenomena could be investigated only by contact with the supreme divinity, and 
this could be attained only by certain mysterious ceremonies involving the use of 
secret symbols, incantations, and prayers. A knowledge of these ceremonies was 
regarded as a divine gift particularly reserved for the priests and the mitiat^d. 
Somewhat similar ideas were later incorporated in the mystical forms of alchemy 
of the Middle Ages. 

In the period between the first and fifth centuries, alchemy attracted the 
attention of many learned men, and authentic writings on alchemy began to appear. 
The first, Zosimos of Panopolia, lived in the third century, and most of his writings 
seem to have been lost. Some fragments attributed to him have been collected 
from Greek papyri, and he is often quoted by later alchemists. Zosimos described 
various forms of apparatus and furnaces, minerals, and alloys and he frequently 
refers in more or less obscure language to the transmutation of the metals J^rag- 
ments of the writings of Zosimos, Africanus, Synesius, Olympiodorus, (pseudo) 
Theophrastus, (pseudo) Democritus, and several other Greek alchemists i-a bout 


150 in all — were preserved in European museums — Venice, Rome, Paris, Munich, 
etc. — whither they drifted after the conquest of the Turks in 1453. The essays 
reproduced in M. Berthelot's Collection des alchimistes grecs are all composed in an 
enigmatical style with obscure chemical terms used in many different ways ; they 
discuss magical and astrological formulae ; and give citations from mythical authors. 
The writers were acquainted with many ores, minerals, earths, salts, and animal 
and vegetable substances ; there is no evidence of a scientific classification ; and the 
writers were in ignorance of the mineral acids and their important derivatives. 
They were chiefly concerned with the operations of solution, distillation, and heating. 

The conquests of Rome brought the Orient and the Occident, the East and the 
West, into close communication. At the beginning of the Christian era, Alexandria 
was the asylum of Eastern traditions, the centre of medical, alchemical, and philoso- 
phical culture ; and the sanctuary of the world's learning. The Roman depreda- 
tions in the fourth century led to a rapid decline ; and as a result of the Mahomedan 
conquest of Egypt in the sixth and the seventh centuries, the Alexandrian philoso- 
phers and teachers were scattered, and some refugee Byzantine alchemists travelled 
to Constantinople ; others settled in Persia and Syria, where they introduced the 
Greek and Egyptian philosophies. Some of the writings of the Greek philosophers 
were translated into Syrian. 

In the seventh century, the Arabians overran Syria and Persia ; and the Syrian 
schools languished and died. The Arabians then began to cultivate those very 
arts which they had done so much to destroy. Syrian scholars were employed by 
the rulers for positions demanding wisdom, knowledge, and judgment. Copies of 
two Syrian manuscripts are preserved in the British Museum ; one is translated in 
M. Berthelot's La chimie au tnoyen age (Paris, 1893). It contains various technical 
recipes, discussions on magic and mystic doctrines, the elixir of life, the adulteration 
of gold, and descriptions of some chemical apparatus. An Academy was founded 
at Bagdad about 800 a.d., and the Arabians began to collect and translate books 
from various countries — East and West. The works of Aristotle were translated 
from the Greek into Syrian, and re-translated from Syrian into Arabian. Conse- 
quently the alchemists of Arabia derived their ideas and knowledge from those of 
Syria ; the Syrians in turn were largely dependent on the Greek works of the 
pseudo-Democritus, Zosimos the Panopolite, the pseudo-Cleopatra, and others who 
flourished at Byzantium. Historians generally consider that the Greek writers of 
this period, in turn, derived their ideas from the Egyptians. ^ In any case, the 
Arabians, like the Greek writers of the Alexandrian school, imparted mysticism into 
their versions of Hellenistic philosophy, so that there was a partial reversion to 
the first of Comte's three states. Instead of regarding natural phenomena as the 
workings of natural law, they were inclined to consider them to be subject to the 
capricious wills of superior intelligences, and creatures of an imagined demonology. 
As a result, physical scdence reverted to magic, astronomy to astrology, and philo- 
sophy to theosophy. The alchemical operations were described in mystic language. 
Hence too arose the philosopher's stone, the elixir of life, etc. The Arabians had 
a bias in favour of medicine and pharmacy rather than metallurgy, and they appear 
to have interpreted the alchemical writings from the Egyptians, in terms of medicine 
and pharmacy — a bias possibly derived from the Hindus. Consequently, the 
philosopher's stone of the Alexandrian school became the Arabian elixir of life. 

The reputation of one Geber, an Arabian writer of the eighth century, loomed 
mightily in the alchemical world about the later half of the Middle Ages. He is 
credited with having been the first to give chemical knowledge a systematic form by 
publishing the first extant system of chemistry. It is very true that the ideas 
expressed in these writings are the earliest to stand in historical continuity with those 
of the present day. This fact has invested the writings of Geber with a special 
interest, and this int-erest is only quickened by a knowledge of their contents and 
style. The fragmentary information which is available respecting Geber is most 
disappointing ; there is no agreement among the^historians concerning his birth-place, 


his parents, his social or political relations, his rank, the events of his life or 
his death.3 There are, however, quite a number of Latin treatises alleged to be 
translations of Arabic texts of Geber's writings. For example, up to recent years, 
the Summa perfectionis magisterii was credited to Geber, and it was said to have 
been the first work exclusively devoted to chemistry. The book is an attempt to 
summarize what was then known or believed with respect to chemical operations 
and processes. It is, however, disfigured by unintelligible matter which has wrongly 
led some to the idea that the term " gibberish " for unintelligible words, is a tribute 
to Geber's style of writing. 

The Latin Geber was acquainted with alum, copperas, saltpetre, sal ammoniac, 
aqua fortis, oil of vitriol, aqua regia, etc. ; he described the action of mercury on 
gold, and of sulphur on red-hot iron ; and he supposed that there are three elementals 
— mercury, sulphur, and arsenic. The metals, said the Latin Geber, are compound 
bodies which are extracted from their earthy ores when the latter are mixed with 
carbonaceous materials and heated in a furnace in the absence of air. It seemed 
to him as if the calx got something from the furnace and so became a metal. Ac- 
cording to the Latin Geber, the metals are compounds of the same substances — 
mercury and sulphur — united in different proportions. Geber also accepted as 
dogmas of his faith, the transmutation of the metals, and the influence of the 
planets on the metals — although he said : 

It is as impossible to transform the metals into one another as it is to turn a bull into a 
she-goat ; for it has taken nature thousands of years to make the metals, and we cannot 
hope to effect the transformation when we rarely live a hundred years. 

Many grave doubts have arisen as to the genuineness of the Latin writings which 
have been attributed to the eighth-century Geber. M. Berthelot has compared 
the texts of the Latin works, and translated the known Arabic texts preserved in 
the Museums at Paris and Ley den. He has also compared these works with those 
of contemporary writers. The style and standards of the Latin and Arabian works 
are altogether different ; and, as a result, Berthelot concludes that the Latin works 
attributed to Geber were the composition of one or more writers about the thirteenth 
century, who forged the name of the Arabian Geber to crown the book with veneration 
and respect. The Latin version of Geber is not to be regarded as a translation 
from Arabic texts. The Latin versions, on which Geber's reputation rests, are some- 
times called the thirteenth-century works of the pseudo-Geber to distinguish them 
from some Arabic texts which were probably the work of an unimportant Geber, or 
of some writer between the eighth and eleventh centuries. The Arabian Djaber 
(Geber) is reputed to have been the pupil of a Khaled ben Yezid ibn Moaouia, the 
first Mahomedan writer on alchemy. M. Berthelot's translation of the works of 
the Arabian Geber show that Geber use(f the hydrostatic balance ; attempted to 
classify minerals ; discussed the changes in volume which occur when substances 
are heated ; and stated that he had seen many persons ignorantly attempting to 
manufacture gold and silver by wrong methods, and added : "I perceived these 
workers were divided into two categories, the dupers and the duped. I had pity 
for both of them." m j • 

A debate among the Arabians as to the possibility of alchemy is described m 
the writings of the Arabian E. S. Avicenna (980-1037). The doctrine was defended 
by A. M. Rhases (840-940), or Rhazes,whose writings are often quoted by meditevai 
alchemists. The Arabic physician Avicenna wrote on chemistp^ and medicine, and 
he also wrote commentaries on the works of Aristotle. Judging from the reports 
of his Porta elementorum and his Dictiones, his philosophical ideas closely followed 
those of Aristotle ; his medical work. The Canon, was mainly a compilation ol 
Hippocrates and Galen ; and his general knowledge was but little in advance ot 
the Greeks. Notwithstanding this, Avicenna's medical works were long revered 
as a code of science ; but they sank into almost complete obhvion about the end 
of the seventeenth centurj^ Similar remarks apply to the commentaries ot 


I. R. Averroes (1126-1198) upon the works of Aristotle ; in fact, it was mainly 
through the commentaries of Averroes that Aristotle's scientific work became 
known in Europe in the Middle Ages. 

There is a very important treatise, The hook of the balance of wisdom, written 
in the twelfth century by the Arabian optician and physiologist Al-Khazini, or 
AlhazanA It contains a memoir on the use of the balance for the determination 
of specific gravities, and is supposed to have been based upon a work by Abu-r-Raihan 
written about 1000. Al-Khazini said : 

The water-weight of a body visibly changes according to the difference between the waters 
of different regions in respect to variety and density, together with incidental difference due 
to variety of seasons and uses. ... In winter one must operate with tepid, not very cold, 
water on account of the inspissation and opposition to gravity of the latter, in consequence 
of which, the water- weight of the body conies out less than it is found to be in summer. . . . 

The temperature was apparently estimated by the distance a kind of hydrometer 
sank in water. The specific-gravity bottle was described, and an improvement on 
the floating hydrometer of Pappus (c. 400 B.C.) indicated. Gravitation seems to 
have been regarded as a force directed to the centre of the earth, and which diminished 
proportionally with the distance ; it remained for Newton to show that it diminishes 
as the square of the distance. Both Abu-r-Raihan and Al-Khazini compiled tables 
of the specific gravities of various solids and liquids with which they were acquainted ; 
and the numbers agree closely with those adopted to-day. 

During the period of the intellectual darkness which prevailed in Europe after 
the decline and fall of the Roman Empire, the torch of learning was borne by the 
Arabians, but there is little to show that the Arabian alchemists — Avicenna, Anven- 
zoar, Averroes, etc. — who flourished between the eleventh and thirteenth centuries 
— did much to extend the chemical knowledge which they derived mainly from their 
contact with the Egyptians, Greeks, and Hindus. The Arabians borrowed freely ; 
but they showed little genius for independent thought. In his posthumous A 
History of Chemistry (London, 1913), J. C. Brown sums up by saying : "far from 
crediting the Arabians with being the originators and improvers of chemistry, as 
stated by E. Gibbon (1789),^ much of their knowledge was not understood, and they 
involved it in mystical confusion which hindered the progress of science for cen- 
turies ; " and W. Whewell, by saying : 

The Arabians cannot claim in science or philosophy, any really great names, they pro- 
duced no men and no discoveries which have materially influenced the course and destinies 
of human knowledge, they have tamely adopted the intellectual servitude of the nation 
which they conquered by their armies ; they joined themselves at once to the string of 
slaves who were dragging the car of Aristotle and Plotinus. 

About the eighth century, the Arabians amalgamated with the European settlers 
in Egypt, and under the name Moors, crossed into Spain, where they founded 
Academies at Cordova and Granada. These Moorish universities flourished between 
the eighth and eleventh centuries, and furnished the schools of Europe with many 
learned teachers. The power of the Moors in Spain was destroyed with the conquest 
of Granada by the Christians under Ferdinand and Isabella in 1492. The Arabian 
centre of learning at Bagdad was captured in the eleventh century by the Turks, a 
tribe which separated from the Mongols in the sixth century, and settled in Asia 
Minor. The Turks gradually extended their power westwards, and formed the 
Ottoman or Turkish Empire under the leadership of Othman (born 1258). The 
Turks crossed into Europe in 1356, and about a century later, 1453, captured 
Constantinople. The learned men congregated in that city then slowly drifted 
westwards with their manuscripts and learning. 

The authors of the earlier Arabian alchemical books were directly or indirectly 
associated with the famous schools of Alexandria, the last resting-place of the secrets 
of the Egyptian priests. There can be no doubt that the chemical arts were well 
developed in old Egypt. The Egyptian origin of the term chemistry would harmonize 


with the prefixing of the article al (the) to the word Khem (Egypt) when the Arabians 
overran Egypt, and thus learned many of the secrets of the temple laboratories of 
the Egyptian priests. No doubt also the contact of the Arabians with Persia made 
them acquainted with some chemical knowledge derived by the Persians from 
India. The Arabians also learned from the Grecian philosophers through the Syriac 
translations. The learning derived by the Arabians from East and West was 
probably distorted, modified, and adapted to suit their own particular dogmas, 
and carried to Europe partly by the currents of returning crusaders, and partly 
by the Moors via North Africa and Spain. 

The origin of the term chemistry.— Chemistry had no special name prior to the 
sixth century, before which it was variously known as the art of Hermes, the Hermetic 
art, the Sacred art, the Occult art, or the Black art. Many have tried to trace the origin 
of the name chemistry, and the quest has led etymologists to suggest several 
different hypotheses ; accordingly, the student has the choice of a number of 
plausible guesses at his disposal.^ 

(1) The various attempts which have been made to make the root a Greek word have 
not been veiy successful. H. Barbarus, in his Compendium acientice naturalia (1547), and 
A. Libavius, in his Alchymia (Francofurti, 1606), consider it possible that the term is derived 
from x^l^os- — -a juice or menstruum- — 'in reference to the use of various solvents by the 
early alchemists ; or from x^'w — to fuse or melt ; and J. A. Quercetanus, in his De pris- 
corum medicina (c. 1600), uses the term halchymiam for a fused salt — iAs, salt ; €ind 
Alexander the Aphrodisian (c. 200 a.d.) speaks of the use of x^'f^ opyava — a kind of crucible 
for melting substances. While this derivation of the word was in fashion, alchemy was 
spelt alchymy, and chemistry, chym,istry ; but this spelling was dropped when it wsus 
recognized that the Greeks had neither the name chemia nor the science ; it was only 
near the beginning of the Christian era that the new science began to attract attention in 
Europe. The scholars tell us that the word alchemy does not occur in Greek writings 
earlier than the third or fo\u*th century, when J. F. Matemus mentioned the acientia 
alchemice in an astrological work entitled Mathesis, written about 337 a.d. He says, in 
the jargon of astrology : "If man be bom in the house of Mercury, he will devote himself 
to astronomy ; if in Venus, he will be fond of singing and pleastu-e : if in Mars, he will 
apply himself to arms- ; if in Jupiter, he will follow religion and law ; if in Saturn, he will 
devote himself to alchemical knowledge. ..." Zosimos of Panopolis (Egypt), a writer 
possibly contemporaneous with, or possibly earlier than, Mattmus, refers to x'/M^a. 
chemia — or xvi^^'ta, chemeia — as the art of making gold and silver. We are also" told that 
the term was seldom or never used by subsequent writers before the ninth century, but 
thereafter somewhat frequently. 

(2) It has been argued that the word is derived from the Hebrew word Chatnan or 
haman, meaning a mystery or secret, in which case, chemistry would mean the secret art ; 
and Zosimos (c. 400) considers that chemistry shoxild be treeisured as a religious secret to 
be known and jealously guarded by the priestcraft. S. Bochart (c. 1660) favours a de- 
rivation with a similar connotation, for he refers the word to the Coptic kema or kemo, 
obscure or hidden, or the Arabic chem/i, to hide. Hencje the old designation the occult 
science, and the Arabic book of secrets called Kemi. 

(3) It has been suggested by S. Bochart, in his Oeographios sacrce (Cadomi, 1646), that 
the word may be derived from Noah's son Cham, whom he thinks was identical with 
Zoroaster the 'founder of the Magi. According to Diodorus Siculus' Bibliotheca historica 
(c. 30 B.C.), the word chemistry is derived from the name of an Egyptian king named 
Chemnis or Chemhes ; and, according to H. Goring's De hermetica n/iedicina (Helmestadii, 
1648), the god Chemnis was worshipped in the city of Thebes, which was famous for its metal 
and colour industries. 

(4) Plutarch, in his De Iside et Osiride (c. 100 a.d.) implies that the word comes- from 
the Egyptian Kham or Khem (Psalms, 105. 27)— meaning black or dark— because the same 
word was applied to the country of Egypt. The term thus refers to the art of the black 
coimtry, or the Egyptian art. The trend of opinion seems to favour this suggestion. 


1 H. Kopp, Beitrdge zur Geschichte der Chemie, Braunschweig, 1. 9^' ^869; 2. 1, 1869 . 
M. Berthelot and C. E. Ruelle, Collection des anciens alchimistes grecs. Pans, 1887-8. 

2 H. W. Schaefer, Die Alchemic, Flensburg, 1887 ; M. Bemiam, SenUnttis sacro medvcw, 
Hamburg, 1640 ; A. J. Pernety, Les fables igyptiennes et grecques detmleesjt reduites au mime 
principe avec une explication des hieroglyphes et de la guerre de Troye, Pans, 1 /o8. xj^^*^, 

« H. ^mgst^ll,Literaturgeschichte der Araher, Wien, 2. 185, 1851 ,;^ 3. 293, 1851 ; F. Hoefer, 
Histoire de la chimie, Paris, 1. 308, 1842 ; H. Kopp, Geschichte der Chemie, Braunschweig, 1. 61, 


1843 ; T. Thomson, The History of Chemistry, London, 1. 119, 1830 ; K. C. Schmeider, Geschichte 
der Alchemie, Halle, 86, 1832 ; M. du Fresnoy, Histoire de la philosophic hermeiique^ Paris, 1. 29, 
1842 ; 0. Sprengel, Histoire de la medecin, Paris, 2. 263, 1815 ; J. Ferguson, Laboratory, 1. 71, 
1867 ; Bibliothcca Chemica, Glasgow, 1. 299, 1906; Geber, Works, Gedani, 1682. 

* J. W. Draper, A History of the Intellectual Development of Europe, London, 2. 45, 1876 ; 
H. C. Bolton, Am^r. Chemist, 6. 413, 1876 ; N. Khanikoflf, Joum. Amer. Oriental Soc., 1. 1859 ; 
J. J. Clement-MuUet, Joum. asiatique, (5), 11. 379, 18. 

5 W. Whewell, History of the Inductive Sciences, London, 1. 211, 1857 ; E. Gibbon, The History 
of the Decline and Fall of the Roman Empire, London, 1789. 

« G. Hoffman, Ladenburg's Handworterbuch der Chemie, Breslau, 2. 516, 1884 ; A. F. Pott, 
Zeit. deut. morg. Ges., 30. 6, 1876; E. Wiedemann, ib., 32. 575, 1878; C. Schorlemmer, Chem. 
News, 40. 309, 1879 ; R. A. Smith, ib., 42. 68, 244, 1880 ; E. O. von Lippmann, Chem. Ztg., 38. 
685, 1914. 

§ 12. The History of Chemistry during the Middle Ages. Alchemy and 
Medico- or latro-chemistry 

The applications of chemistry to various kinds of industries are all buried in the tombs 
of many generations of artists who have left no other traces of their existence than a few 
of their productions.- — P. Lacroix (1869). 

The Middle Ages are sometimes taken to extend from about the seventh to the 
seventeenth centuries. During the fourth century Western Europe was ravaged 
by Teutonic barbarians— the Goths and the Vandals. The Koman Empire trans- 
ferred its capital to Byzantium (Constantinople), on the banks of the Bosphorus, 
where Greek metaphysics mingled with Oriental mysticism ; and intellectual 
Europe there managed to exist until the Turkish conquest of Constantinople in 
the fifteenth century. The traditions of the Greek philosophers were preserved 
in the schools of Alexandria and Byzantium,^ and there was a succession of real 
though feeble students of philosophy, physical and natural science, mathematics, 
and medicine. Byzantium thus kept alive the thought and knowledge of the 
ancient world during a period when Western Europe was submerged in turmoil 
and strife. 

During the fifth century, the Huns, under Attila, devastated the fairest provinces 
in the West about the time the Anglo-Saxons were conquering England. Natural 
science could make no progress under these turbulent conditions ; and ignorance 
and superstition prevailed in the West. There was a gradual infiltration of ideas, 
knowledge, and art from Byzantium, the Greco-Roman Empire, into Western 
Europe between the fifth and the fifteenth centuries. The fall of Byzantium 
(Constantinople) in 1453 led to the westward migration of the scholars of the 
Eastern Empire. Europe also gained some hints of the chemical lore of the 
Arabians from the returning crusaders ; and after the Moors had carried Arabian 
literature into Western Europe vid Spain in the tenth century, some progress was 
made. The works of the Grecian and Egyptian writers were not directly known in 
the West until after the thirteenth century, although Latinized versions of Arabian 
translations, preserved in the Mahomedan libraries in Spain, were available. 
This gave rise to the erroneous impression that chemistry originated in Arabia. 

Some Latin translations of the Arabic writings were collected and printed in 
the seventeenth century — for instance, the Theatrum chemicurn (Argentorati, 1613- 
22) and J. J. Manget's Bibliotheca chemica curiosa (Genevse, 1712). M. Berthelot 
found in these works whole passages taken from the older Greek alchemists. The 
meaning of the original writings seems to have been distorted and perverted during ^^ 
the many translations and re-translations ; as a result, the mediaeval chemists oi^H 
alchemists started their work with mutilated and incoherent descriptions of the ^ 
technical and philosophical works of the Greeks and the Egyptians ; and the literary 
productions of the alchemists of this period are characterized by much obscurity, 
either in unconscious mimicry, because their mutilated models were similarly 
tainted, or else to hide their real meaning from a hostile community, or from the 
vulgar. It was said : "A profound secret should not be revealed in the vulgar 


tongue, the true adept can sufficiently comprehend the mystical language, and it 
would not be right that it should be understood by the people." 

Historians tell us that the tardy growth of science in the early Middle Ages was 
largely due to the constitution of society. The chief elements of feudal society were 
the barons and the priests. The barons were perpetually at war, and the study of 
natural science and philosophy was eminently distasteful to them. The priests were 
often men of great learning, but they devoted their energies mainly to theology. They 
possessed great power over society, for on them devolved both the spiritual and the 
temporal teaching of thepeopJe. Except inrare cases, the priests did not devote special 
attention to the physical and chemical sciences. Aristotle's works were considered 
sufficient for all purposes, and speculations in reference to natural phenomena were 
discountenanced, and in some cases forbidden. Ignorance appeared to be a sacred 
duty. It was generally thought impious to attempt to draw aside the veil enshroud- 
ing nature's mysteries, and man shrank from all inquiry into the perplexed ways 
of the universe. What a reversion from the intellectual fearlessness with which 
the Greek un weary ingly interrogated nature, and wrestled with her secrets ! What 
a contrast with Euripedes' hymn (c. 450 B.C.) : 

Happy is the man who has learned to search into the reasons of things, and to discern 
the deathless and ageless order of nature — whence it arose, how, and why. 

The alchemical SChooL — The most celebrated alchemists during the twelfth and 
the thirteenth centuries were Albertus Magnus, Thomas Aquinas, Roger Bacon, Arnold 
Villanovanus, and Raymond LuUy. Their works serve as milestones indicating the 
state of alchemy at that period. Very few important additions to chemical knowledge 
were made, since the general tendency of the age was towards magic, sorcery, and 
the* transmutation of the base metals into gold. Albertus Magnus and Thomas 
Aquinas were Dominican friars ; Roger Bacon was a Franciscan monk ; and 
Arnold Villanovanus was a university professor at Barcelona. Some of the works 
attributed to these men are no doubt spurious. 

Some religious orders sought to spread a knowledge of the arts and sciences, 
but they unfortunately also attempted to control the progress of science in pre- 
determined channels ; and the promulgation of hypotheses, or the discovery of facts 
which did not harmonize with accredited authorities, or orthodox beliefs, was re- 
garded as a serious offence against the State or Church. The students of alchemy 
were believed to be magicians, and were supposed to be in communication with 
beneficent or malignant spirits ; and, although Albertus Magnus denied this 
assumption when he declared that " all those stories of demons prowling in the 
regions of the air, and from whom secrets of futurity may be ascertained, are absurdi- 
ties which can never be admitted by sober reason," yet, the fear and dread of magic 
took complete possession of the popular mind ; even " the church service books gave 
agonizing petitions for averting these dire influences, and prescribed impressive 
exorcisms for thwarting the occult powers." 2 The first step to be taken by a student 
of nature was thought to be to league himself with Satan by bartering his soul fo^ 
knowledge and occult power, and whenever a mediaeval thinker appeared to be 
inspired by a love of knowledge and freedom of thought, the disease was ascribed 
to diabolic agency. 

One of the oddest and oldest tricks of the human mind, in ancient and modern 
times, is to invoke spirits, in time of need, to explain ill-understood phenomena 
In accord with the beliefs and customs of the times both Roger Bacon and 
Arnold Villanovanus were prosecuted for being in league with demons, and in 
1317, the inquisition of Tarragona condemned the writings of Arnold to be burned 
on account of their heretical sentiments. Both Albertus ^^^gnus and Ihomas 
Aquinas were astute enough to escape the severe persecution which betell man> 
of their brother monks who studied the alchemical arts. ^ 

It must be confessed that the authorities probably had some justification tor 
their attitude against the " unholy quest of alchemy," just as to-day it is necessary 


to limit the activity of fortune-tellers, etc., by legislation. In the fifteenth century 
severe interdicts against the practice of alchemy were issued in the Roman provinces, 
in England, and elsewhere ; indeed, Duke Frederick I of Wiirttemberg is said to 
have kept a special gallows for hanging the alchemists 3 — but the alchemists still 
continued their labours. 

The views of the eminent German alchemist, Albert of Bollstddt, or Albertus 
Magnus (1193-1280) ,4 were mainly derived from those of Aristotle. The alchemical 
writings attributed to Albertus Magnus have been shown by the scholars to be in 
the main compilations from Arabian sources, although he introduced several 
novelties. Albertus Magnus specially studied the union of sulphur and the metals ; 
and, like the Arabian Rhases, he considered the metals themselves to be compounds 
of difierent proportions of the three principles or elementals : arsenic, mercury, and 
sulphur. Sulphur, said he, " blackens silver and burns the metals on account of 
the affinity which it has for these substances.'- The term affinity was thus used for 
the first time to designate the unknown cause of chemical action. Silver was 
supposed to be the metal most closely allied to gold, so that he considered the trans- 
mutation of silver into gold would be the easiest to realize. Albertus Magnus knew 
how to separate the noble from the base metals by fire, and how to separate gold 
from silver by aqua regia. Some suppose that the treatise on alchemy ascribed to 
Albertus Magnus is spurious. The canonized scholar, Thomas Aquinas (1225-1274),5 
was a pupil of Albertus Magnus. It has been said that while the master was a 
student of nature and philosophy, the pupil was a student of man and society. 
Both are considered to have excelled as exponents of theology rather than as students 
of natural science. From the little knowledge which is available concerning the 
alchemical labours of Thomas Aquinas, he would appear to have been particularly 
attracted by the action of mercury on the metals — lead, tin, etc. — and he applied 
the term a7nalgam to the liquid or paste which is formed when these metals are 
opened up with mercury. 

Among the foremost in substantial knowledge in the thirteenth century stood 
Roger Bacon (1214-1294). He saw far beyond his age ; and his reputation among 
his contemporaries was so great that he was styled Doctor Mirabilis. His knowledge 
was thought to be uncanny; his insight was mistaken for wizardry. Roger Bacon's 
knowledge of physical science was probably derived from Arabian and Greek sources,^ 
for no new principle has been traced to Bacon himself. S. Vogl has pointed out that, 
during a great part of his life, Roger Bacon was practically without the means of 
prosecuting experimental research ; and he was thwarted in his aspirations at 
every turn by his superiors. It is therefore not surprising that he failed to enrich 
science by any striking original discoveries. Nevertheless, his critical examination 
of the science of his time was conceived in a broad philosophical spirit which showed 
that he had made a great advance in the methodology of science. Bacon was 
not exactly an admirer of Aristotle, for he said : 

If I had all the books of Aristotle in my power, I would cause every one of them to be 
burnt, because studying them is only a loss of time, and a cause of error, and a multiplica- 
tion of ignorance, beyond what can be explained. 

In his Ofus majus, R. Bacon emphasized very clearly the importance of scientia 
experimentalis, which, in his opinion, is the mistress of all the sciences — domina est 
omnium scientiarum. Indeed, he actually claimed an equal rank for observation 
and experiment. True enough, towards the end of the Grecian epoch, there dawned 
an era of experiment, for C. Galen experimentally investigated the nerve system, 
and C. Ptolemy, the refraction of light ; consequently the experimental method 
was not a new thing. R. Bacon's merit lies in having explicitly indicated the im- 
portance and bearing of experiment as a universal instrument of research. The 
more important scientific works of Roger Bacon are : the Opus majus written in 
1266, and the supplementary Opus minor, with its introductory Opus tertiurn,, com- 
pleted within a year of the publication of the Opus majus. The last-named work 


contains very little about alchemy, but much more occurs in the two subsidiarv 
works. • ^ 

Alchemy, said R. Bacon, falls into two divisions— speculative and operative 
Operative alchemy includes the practical and industrial processes pursued, with 
more or less wisdom, by men who have a definite purpose in view. Alkimia 
speculativa treats of the transformations of matter from its simplest to its most 
complicated form, and in this sense the problem of R. Bacon's speculative alchemy 
approaches that of modern chemistry. Roger Bacon was necessarily ignorant of 
the fundamental truths of chemical science, and he could do little more than compile 
a number of empirical facts. He believed that air is the food of flame, for if a lighted 
lamp be placed in a closed vessel, the flame is extinguished. Like Albertus Magnus 
he supposed the best and basest of metals to differ only in the relative proportions 
of their constituent parts — mercury and sulphur — and their degree of purity. He 
also devoted special attention to the^ properties of saltpetre and gunpowder. Arnold 
Villanovanus (1234-1312) 7 specially studied distillations, ^nd he prepared many 
essential oils — turpentine, rosemary, etc. The fanatical Raymond Lully (1235- 
1315) enjoyed an ephemeral reputation ; he led a turbulent restless life, and although 
an enormous number of books have been attributed to him, it is certain most are 
spurious.8 There is also the probability that there are two different Raymond 
Lullys — one the fanatic, one the alchemist. Lully is reputed to have made spirit of 
wine which he called aqua vita ardens, and he seems to have rectified it by distillation 
from potassium carbonate. 

This quintet may be taken to represent characteristic types of alchemists during 
the twelfth century. Arnold Villanovanus ascribed any successes which he 
obtained in his experimental work to the favourable position of the planets and 
stars, and to suitable prayers ; these conditions seemed to him to be more im- 
portant than a mastery of the controllable conditions under which the operations 
were performed. This was rather unsatisfactory because no science is possible if 
the phenomena under consideration are subject to the capricious wills of beneficent 
or malignant spirits, for science postulates that natural phenomena are but linka 
in an endless chain of cause and effect, and that in experimenting " the same 
antecedents are invariably followed by the same consequents." The intellect of 
man now began to assert its claim for independent thought ; and a general yearning 
for progress was apparent. Learning revived in Italy, the land whence it had been 
almost blotted out of existence a thousand years before. A few literary societies 
appeared during the fifteenth century, and in the sixteenth century these societies 
became quite numerous. Their chief work was the study of the philosophy of 
Plato, and the development of the Italian language. Scientific societies were also 

The invention of printing, about the middle of the fifteenth century, gave an impetus 
to the pursuit of literature. There was also a spirit of social unrest. The voyage of Colum- 
bus opened up the New World for those who sought new fields of discovery, fortune, or 
adventure. Martin Luther's revolt was inaugurated in 1517 by the posting of his thesis 
upon the church door at Wittemburg. That versatile genius Leonardo da Vinci ( 1 452-1 51 9), 
whose compendious manuscripts were so long thought to be written in secret script because 
written backwards, has been but recently appreciated, and his notes in part transcribed tind 
edited. He was a pioneer of the modern spirit of investigation and practised the inductive 
method a century before Francis Bacon. The foundations of astronomy, mechanics, and 
physics were laid about this time ; Nicolas Copernicus had published his De revoltUionibus 
orhium ccelestium in 1543. During the next fifty years fuller and more accurate data were 
compiled by Tycho Brahe (1546-1601). About 1608, the astronomical Don Quixote, 
Johann Kepler (1571-1630), published voluminous works » which have been styled " a most 
singular medley of soimd thoughts and vmmitigated nonsense." Kepler, however, did submit 
his ridiculous conceptions to the test of observation, and rejected those which did not stand 
the trial. Among the wildest of guesses on the motions of the planets and their satelhtes, 
he discovered those truths which have long been known as Kepler's laws. Galileo Galilei 
made important experiments on the laws of motion, towards the end of the sixteenth 
century ; and a century later, Isaac Newton demonstrated the aU-embracmg law of 
gravitation in his epoch-makmg Philosophia naturalis prmcipta mathematica (London, 1685). 


A Latin compilation on technological chemistry, entitled : Compositiones ad 
tingenda, was published towards the end of the eighth century, and about the 
tenth century one entitled : Mappce clavicula. These works contain recipes for 
industrial processes closely resembling those of the ancient Greek papyri. The 
term vitriol for impure ferrous sulphate was used for the first time in the eighth- 
century work. Reference is made to the use of the hydrostatic balance in the 
analysis of alloys of gold, and this has been taken to show that the knowledge of 
this instrument did not pass through Arabian channels to Western Europe, but 
came direct from the writings of Archimedes of Syracuse (287-212 B.C.), which 
were carried west by the fugitives from Constantinople after its capture by the Turks 
in U53. 

A large number of alchemists — P. Bonus, N. Flamel, Isaac of Holland, G. Ripley, 
T. Norton, T. Charnock, E. Kelley, John Dee, M. Sendibogius, M, Maier, J. Boehme, 
T. Vaughan — who wrote under the nom de plume, Eupenius Philathes — and another 
— who wrote under the pseudonym, Erenaeus Philathes — laboured with some skill, 
between the fourteenth and seventeenth centuries, although the alchemical school 
was perhaps at its zenith in the fifteenth century. About this time there were three 
different types of alchemist. The first or bookish type spent his time commenting 
upon, elucidating, or unconsciously obscuring the views of the earlier writers ; this 
type might also include the mystical chemists who hinted at a secret doctrine of 
a spiritual order. The second or mercenary type hoped to find unlimited riches 
when he had succeeded in converting the base metals into gold ; and the third or 
investigating type sought to discover the properties and combinations of the metals, 
and the best means of extracting them from their ores. The last formed the 
prototype of the modern chemist, although representatives of all three types still 
survive. The majority of the alchemists were diligent experimenters, and although 
they worked in a stupendous chaos of phenomena, their indefatigable zeal will 
long be remembered for the multitude of primary facts which they discovered, 
even though the names of the discoverers are forgotten. The alchemists crystallized 
and calcined, digested and distilled, filtered and fused, just as chemists do to-day. 

Auguste Comte i^ has said that it is difficult to understand how the early investi- 
gators could have had the energy and perseverance to discover the chief chemical 
phenomena had they not been constantly incited by unbounded hopes arising from 
their chimerical notions of the constitution of matter. The alchemists were indeed 
stimulated and guided in their work by a logical system of hypotheses. For 
instance, they accepted the older prima materia hypothesis of the ultimate constitu- 
tion of matter. The changes which were observed in the different forms of matter 
appeared as the outer clothes of an unchangeable all-pervading essence. The 
qualities of the elements, not their essences, are changeable ; some of these qualities 
are more easily removed than others, thus the four elements were regarded as 
firmly clinging coverings, while heat and cold, moistness and dryness, were more 
easily removed. The different varieties of matter were the different vestments 
or wrappings of the one universal entity, the quintessence of things. The universal 
essence was regarded as the perfect thing — The One Thing. This one thing was 
given many different names — e.g. the stone of wisdom or the philosopher's stone,ii 
a term which, according to M. Berthelot, appeared in alchemical writings about 
the seventh century, although the central idea is much older. 

The property of matter which enabled it to withstand the action of fire was 
attributed to its possessing the quality of fixidity later symbolized by salt ; if it 
possessed the principle of volatility — later symbolized by mercury — the substance 
would volatilize ; if it possessed the principle of combustibility — later symbolized 
by sulphur — the substance would burn ; the principle of redness gives matter a 
red colour ; and so on. To the Romans, lead and tin were differently coloured 
varieties of the same metal, and called dark and light lead respectively. Thus, the 
variations in the different forms of matter were supposed to depend on the qualities 
or principles with which it was endowed. The chemical properties of matter were 


but dimly recognized even in the Middle Ages ; and the differences between bodies 
were considered to depend essentially on their physical qualities. Hence, it was 
assumed that the properties of a body could be modified by the abstraction or 
addition of qualities and forms. It was argued that just as the hardness, colour 
fusibility, and other properties of certain metals can be altered, so must it be possible 
to change all the properties of one metal into those of another, and thus produce 
a veritable transmutation. Consequently, the alchemists believed in the transmu- 
tation of the metals. 

The idea of transmutation occurs in the pre-Christian Greek writings, but the 
idea of transforming the base metals into gold developed near the beginning of the 
Christian era when the Egyptian goldsmiths seem to have carefully studied the 
diplosis — 8t7rA(oo-t? — or doubling of gold ; in other words, the art of increasing 
the weight and bulk of gold by adulteration with cheaper metals. In M. Berthelot's 
Collection des anciens alchimistes grecs (Paris, 1887-8) quite a number of works on 
this subject are cited — one by Moses (not the law-giver of Israel, though possibly 
by one who adopted this name as a nom de plume) is entitled Trcpi StTrAwo-ews xP^a-ov, 
or The diplosis of gold, is preserved in the collection of alchemical writings at Venice ; 
another by Cleopatra (not the celebrated queen), entitled KXcoTrarpa? xP^a-oTroaa 
{c. 50 B.C.) or The chrysopoeia of Cleopatra, is in the collection at Ley den ; etc. 
The last-named manuscript deals with the preservation of beauty ; with weights 
and measures ; and with the making of gold. In the Collection des anciens alchimistes 
grecs there are drawings of digesters, aludels, alembics, and a variety of apparatus 
for distillations, and of water baths i^ for heating in the laboratory. It may be 
added that the water bath was in use, 500 B.C., in Egypt, and was caUed the hath of 
Isis : the name was later altered to the bath of Mary — or the bain marie, as it is 
still called in France — after an Egyptian Jewess, Mary, a writer on alchemical 

In the opinion of M. Berthelot 13 the idea of alchemy, as a method for trans- 
muting the base metals into gold, was a development from the fraudulent practices 
of the goldsmiths in Egypt as an accidental accretion to chemistry, either from a 
misreading or misunderstanding of ancient manuscripts. As a result, the working 
recipes for adulterating gold were regarded as directions for the transmutation of 
the metals. This is shown by the fact that some of the Egyptian papyri — e.g. the 
Leyden papyrus — contain elaborate prescriptions for the falsification of the precious 
metals, and these recipes reappear later obviously copied as formulae for the trans- 
mutation of the base metals into gold. Hence H. Kopp could say : Die Geschichte 
der Alchemic ist die Geschichte eines Trrtums. 

According to the transmutation hypothesis, the baser metals were diseased and 
imperfect ; gold was the most perfect of the metals. The process of transmutation 
consisted in healing and ennobling the diseased metals. It was postulated that a 
stone of wisdom, or philosopher's stone, could be found which would heal the 
diseased metals, for, said W. Salmon, in his Bibliotheque des philosophes chimiques 
(Paris, 1672-8), the philosopher's stone is " the universal medicine for all imperfect 
metals, it fixes that which is volatile, purifies that which is impure, and gives colour 
and lustre more brilliant than nature herself." This hypothesis is quite legitimate, 
but the questions which the alchemists asked from nature appear to have been too 
profound; they could not understand her responses. The idea of a universal 
'medicine for diseased metals was extended and the philosopher's stone was invested 
with all kinds of mystic properties by extravagant visionaries. The Arabian 
pharmaceutists supposed it to have the power of elevating man's diseased and sickly 
body into a state of golden health, and thus arose the idea of an elixir of life i* or 
elixir vitce—oi universal medicine capable of curing all curable diseases, and which 
later developed into an elixir of immortality. Still later, in the old age or dotage 
of alchemy, the alchemists sought a philosopher's stone which would preserve 
health, raise the dead, rejuvenate the old, make cowards brave, etc. The en- 
thusiastic visionaries gave still further play to their fancies, and Paracelsus miagmed 

VOL. I. * 


an alkahest or universal irresistible solvent which would dissolve every substance 
with which it came into contact ; there was also the perpetual lamp which would 
burn for ever ; 15 perpetual motion ; etc. The series of facts which nature revealed 
to the first experimenters in chemistry were so unlike anything already known that 
the ordinary principles of belief were shaken or subverted ; and their mind became 
so exceedingly credulous that J. Play fair, in an essay On the progress of mathematical 
and physical science (Edinburgh, 1853), could say that one who professed to be in 
search of truth ever wandered over the regions of fancy in paths more devious and 

The medico-chemical or iatro-chemical school.— In the sixteenth century, 
alchemy received an impetus in another direction — medicine. Philip Hoehener, 
who, on commencing his professional career, styled himself PhiUppus Aureolus 
Theophrastus Paracelsus Bombastus, was born at'Zurich in 1493,i6 and he seems to 
have developed the amazing arrogance, insolent presumption, and swelling vanity now 
implied by the term " bombast." It has been pointed out that it is not generally 
the calm, cautious, common-sense men who do the new and great things of the 
world, for it seems to require vigorous impulses and certain extravagances of 
character to institute drastic reforms. W. Ostwald, in his Grosse Manner (Leipzig, 
371, 1909), attempted to arrange men of genius in two classes which he called respec- 
tively romanticists and classicists. The classification is based on mental reaction 
velocity — or mental temperature, so to speak. The romanticist has a high and 
the classicist a low mental reaction velocity. The latter is inclined to be phlegmatic 
and melancholic, and the former sanguine and choleric. The romanticist with his 
agile mind reads everything, he is interested in everything and everybody, and, 
as a result of his enormous consumption of facts, he writes a great deal. On the 
other hand, the classicist works more silently and more alone, and he writes com- 
paratively little. W. Ostwald would undoubtedly have classed the wayward erring 
Paracelsus among the romanticists. Paracelsus seems to have combined in himself 
the personality of two men : there is the daring reformer and incessant observer, 
and there is also the mystic hypnotized by conceit who claimed that he was privileged 
above all others, and received knowledge direct from God or by inspiration from 
the Divine. 

The works of Paracelsus embrace many subjects—chemistry, botany, philosophy, 
physics, astrology, theosophy, magic, and most important of all, medicine. His 
style is generally clear, and characterized by energy and vigour, but suffused with 
mysticism. Paracelsus maintained that each disease has its own specific symptoms 
and cause, and must be combated by specific remedies— every disease, said Paracelsus, 
must have a remedy. The development of this idea led to his being called the 
Luther of medicine since, previously, all diseases were considered to result from an 
excess of phlegm, bile, or blood. Paracelsus introduced many new remedies, and 
he directed the attention of medical men to the importance of chemical preparations 
and medicines ; he taught that the direct object of chemistry is not to make gold, 
but to cure disease ; and he gave a bias to the quest for the essences or quintessences 
of things — e.g. he investigated the active principles of plants which he used medicin- 
ally in the form of tinctures, extracts, essences, etc. — and thus he prepared tincture 
of opium or laudanum. 

There is little evidence to show that Paracelsus contributed any important 
discovery to chemical science. There are, however, references in his writings to 
zinc and bismuth which he characterized as bastard metals because, though resem- 
bling the metals in general appearance, they lacked the characteristic ductility and 
malleability of the seven metals known from ancient times. Paracelsus prepared 
arsenic acid by the action of nitre on arsenious oxide ; he discriminated between 
the alums and vitriols by showing that the former had an earth and the latter a 
metal as base ; he prepared copper amalgam by the action of mercury on copper 
precipitated from its sulphate by iron ; he noted the development of a gas during the 
action of oil of vitriol on iron ; he used an infusion of nut-galls for detecting iron 


in mineral waters ; he mentioned the bleaching action of the fumes from buming 
sulphur on red roses ; and he described the separation of hydrochloric from nitric 
acid by means of silver. Paracelsus promulgated some astounding, even childish 
hypotheses on the slenderest of evidence, so that the wildest vagaries were 
promulgated by the followers of the mystic Paracelsus during the succeeding 
century. The great merit of Paracelsus lies in his having undermined faith in the 
traditions which had previously corrupted and demoralized the thought and works 
of most of the earlier alchemists. 

Soon after Paracelsus' degraded death in 1541, the alchemists seem to have 
parted ways. The palseo-alchemical school— TraAaios, ancien1>— still pursued the 
transcendental and ever-vanishing images of alchemy which could not be brought 
into harmony with the inflexible world of fact. It is characteristic of a science in 
its early stages, said S. Brown (1843) and A. N. Whitehead (I916),i7 to be both 
ambitiously lofty in its aims, and trivial in its handling of details. This statement 
is very true of the mediaeval alchemists, and " their successors still tried to scale to 
heavenly heights ; but their vitality was gone and they degenerated into fanatical 
inanities of no historical significance ; and their compilations are usually mystical 
anonymities fathered on to the potentates of old." The neo-alchemical school — 
v€ops, new — soon renounced the unattainable sublimities of the earlier alchemists, 
they dropped the Arabian al, and sagaciously pursued the sober and attainable 
aims of a truer chemistry. They sought knowledge, not gold ; they confined their 
attention to phenomena and reactions which could be realized experimentally ; 
and they assiduously devoted themselves to the discovery of primary' facts, without 
dissipating much energy on attempts at transmutation. In fine, they were 
undoubtedly the working chemists of their day, and they laid the foundations of 
experimental chemistry. 

Masses of information were rapidly accumulated by George Agricola (1491-1555) 
— the father of metallurgy, and author of the painstaking De re metallica (Basil, 
1556), 18 on mining and metallurgy ; by Andreas Libavius (1540-1616), i^ the dis- 
coverer of tin tetrachloride or liquor fumans Libavii ; and by AngeloSala (1575- 
1640), 20 who severely criticized the old mystic hypotheses, and who would have 
chemists cease from trifling with sublimities. To the alchemists who professed 
to extract from antimony a mercury which would effect the great transmutation, 
A. Sala said : " Show me only one drop of your wonderful mercury and I will 
believe you ; but meanwhile I am deaf to your nonsensical claims." A. Libavius 
proved that the acid obtained by distilling alum and green vitriol (ferrous sulphate) 
is the same as that obtained by burning sulphur with saltpetre ; he studied the action 
of nitric acid on sulphur ; and prepared artificial gems by tinting glass with metal 
oxides. A. Sala specially studied ammonia ; and he synthesized ammonium 
chloride by treating ammonium carbonate with muriatic acid. A. Sala recognized 
that iron is not changed to copper when dipped in a solution of blue vitriol, for he 
saw that the copper comes from the blue vitriol. Paracelsus had given a bias to 
alchemy which led its followers to study diligently the preparation of medicines 
rather than pursue an emasculated alchemy in the quest for the unattainable. The 
new school of medico-chemists and pharmaceutists made a mistake in attempting 
to explain the changes and processes which occur in the human organism by fanciful 
hypotheses founded upon their ignorance of the facts. Paracelsus himself seems to 
have made the childish assumption that a demon named Archseus resided in the 
stomach, and changed bread into blood, etc. 

The talented J. B. van Helmont (1577-1644) of Brussels, began his career an 
enthusiastic alchemist, and ended a worthy chemist ; he also speciabzed m medicme, 
and helped to carry on the medical reform inaugurated by Paracelsas. Conse- 
quently, his posthumous collected works ^i—Ortus medicin(B (Amsterdam, 1648)— 
appear to be both alchemical and chemical. J. B. van Helmont is particularly 
noted for distinguishing clearly between air and gases ; for his work on carbon 
dioxide which he did not distinguish sharply from sulphur dioxide, ammonia, and 


nitrogen peroxide ; for wholeheartedly advocating Thales' doctrine that water is 
the 'prima materia out of which all things are made — although Paracelsus had some- 
thing to say in the same direction ; and for his denying the elemental nature of fire 
which he considered was not a material substance at all. J. B. van Helmont is also 
noted for first using melting ice and boiling water as fixed points in thermometry ; 
for his use of the term saturation to signify the combination of an acid with a base ; 
for emphasizing the imperative claims of the balance for a premier place in the 
chemical laboratory ; and for showing that although a metal can enter into many 
combinations, yet it does not lose its own peculiar nature since it can always be 
again separated unchanged— no metal can be obtained from a salt if it is not already 
present therein. The clear recognition of this fact was a necessary condition for 
progress in chemistry. It was previously supposed that a change in the appearance 
of a metal constituted a veritable transmutation. It was not until the chemical 
properties had been studied that it became possible to realize that the differences 
between the various kinds of matter depend on differences in their chemical com- 
position, and are not produced solely by the addition or abstraction of certain 
qualities or principles. 

The famous J. R. Glauber (1604-1668) was a laborious and diligent chemist 
who studied the preparation and properties of several salts — e.g. he prepared blue 
vitriol by the action of sulphuric acid on verdigris ; various acetates by the action 
of wood vinegar on alkalies, earths, or metals ; ammonium sulphate, or as he called 
it secret sal ammoniac, by the action of sulphuric acid on sal ammoniac ; ammonium 
nitrate which he called nitrum flammaris ; etc. J. R. Glauber prepared nitric acid 
by distilling a mixture of nitre and alum or sulphuric acid ; and hydrochloric acid 
by distilling common salt with sulphuric acid. The term muriatic acid for this 
acid was also coined by him. The residue in the last-named operation is known 
to this day as Glauber's salt, or sodium sulphate, which J. R. Glauber regarded as a 
most wonderful salt — sal mirahile — for he ascribed to it extraordinary curative 
properties when used as a medicine. He said : 

This salt is the beginning and end of all things, and it increases and exalts their powers 
and virtues ; it is the true universal medicine ; not that I would have any man persuade 
himself, that in these words I would assert immortality, for my purpose tendeth not thither, 
seeing that I am not ignorant there is no medicine against death. 

J. R. Glauber 22 also studied the products of the distillation of bones, and of wood. 
He described the preparation of pyroligneous spirit or wood vinegar — acetum 
lignorum — by the destructive distillation of wood, and stated that it could be made 
as virtuous as wine vinegar — acetum vini — by re-distillation. He also noted the 
preservative action of wood tar. J. R. Glauber recognized the law of chemical 
exchange — double decomposition — in the action of sulphuric acid on common salt, 
and of potassium silicate on gold chloride. He said that the potash of the silicate 
neutralizes the acid of the gold salt, so that the silica and gold are both deprived 
of their solvents, and are precipitated. 

F. Sylvius de la Boe (1614-1672), C. Glaser (1615-1673), 0. Tachen (1620-1690), 
Robert Boyle (1627-1691), J. Kunckel (1630-1715), N. Lemery (1645-1715), J. K. 
Dippel (1673-1734), and many other interesting chemists flourished during this 
period. Their work will be discussed more specifically later on. Most of these 
men believed in the alkahest, the philosopher's stone, and in the transmutation of 
the metals. Their faith may have been largely founded upon J. B. van Helmont's 
assurance that he had verily witnessed the transformation of mercury into gold.23 
Fortunately, these men were indefatigable workers, and did not fritter away much 
time on fantastic fictions. Said S. Brown (1851) : It is never the originators of a 
great but useful scientific error, nor yet its true believers, but it is the indolent 
perpetuators, who will not move to the music of a new fact and the new time, that 
are ridiculous, shifty, ambiguous, and not respectable. 

Some important works, written under the worn de plume Basil Valentine, 
probably in the sixteenth or seventeenth century, were for a long time wrongly 


supposed to have been the work of a fifteenth-century Benedictine monk, before 
Paracelsus. On account of the many parallel statements in the writings of 
Basil Valentine and Paracelsus, J. B. van Helmont and others assumed that the 
latter was indebted to the former for many of his ideas and facts. The truth is more 
probably the direct converse of this, and the imposition of Basil Valentine as a pre- 
Paracelsian writer has been called " a seventeenth-century hoax." Anachronisms 
in the supposed writings of Basil Valentine show that these could not have been 
written so early as the fifteenth century. In common with the later views of H. Kopp, 
J. Ferguson, K. Sudhoif, M. Berthelot, F. Strunz, C. W. G. Kastner, etc., W. Hommel 
says that all the evidence indicates that the name was a pseudonym for Johann 
Tholde who, in 1603-1604, first published the works of Basil Valentine, and pretended 
that he had translated them from an old Latin manuscript which he had discovered. 2* 
The writings are characterized by some clearness, particularly when describing the 
results of experiments. The masterpiece, Triumph-Wagen des Antimonii, published 
at Leipzig, in 1624, seems to include almost all that was known about antimony up 
to the seventeenth century. Basil Valentine precipitated gold from its solution by 
the addition of mercury ; copper from its solution by means of iron ; and iron from 
its solution by potash ; he obtained metallic mercury by the distillation of corrosive 
sublimate with chalk ; and he is sometimes regarded as one of the founders of 
analytical chemistry. A. number of other works are attributed to the same writer. 
Special attention should be directed to Robert Boyle, who, more than any 
previous worker, emphasized the importance of the science or, as he called it, the 
philosophy of chemistry. 25 He has accordingly been called "the father of chemistry," 
although the same cognomen has been applied to several others. R. Boyle claimed 
that those who had previously studied chemistry regarded it as a means of pre- 
paring medicines or improving the metals, while he considered the art neither as 
a physician nor as an alchemist, but rather as a philosopher. Chemistry, he claimed, 
had been too often practised by illiterate arrogant impostors who wrote in a language 
which could scarcely be understood by a philosopher. 

Without seeking the grand elixir, chemistry may greatly promote om: knowledge of the 
works of nature. It is certain that some meliorations of metalline and mineral bodies may 
be made, useful medicines prepared, and various productions serviceable in particular 
trades may be obtained by means of chemistry, and therefore this subject may be studied 
to advantage. 

R. Boyle further claimed that he had a larger view in cultivating the science— no 
less a purpose, indeed, than the general advancement of natural philosophy. 

Chemistry is eminently conducive to extend the empire of mankind by enlarging our 
views, and giving us a command of nature. Just as the Bologna stone would never become 
luminous unless it were chemically prepared, so many natural bodies would never afford 
light to philosophy xmless it be struck to them by chemical operations. 

In his remarkable Sceptical Chymist (Oxford, 1661), Robert Boyle introduced the 
modern conception of an element, and dropped the four principles or elements of 
the peripatetic school, and the prima tria of the alchemists. In 1 660, Boyle designed 
a new air pump based upon that of 0. von Guericke. Between 1660 and 1672, 
R. Boyle tried the effect of a reduced pressure upon the properties of many substances, 
and he made many experiments on the elasticity of gases. He demonstrated what 
is now known as Boyle's law ; he showed that air expanded by heat (1662) ; he 
studied the action of alkalies on vegetable tinctures (1663) ; and attempted a classi- 
fication of substances into acids, bases, and salts (1680). He also studied the cal- 
cination of metals in sealed vessels (1673), and assumed that during the calcmation 
"a subtle fluid is able to pierce into the compact and solid bodies of metals 
imparting to them " no despicable weight." Robert Boyle had a clear conception 
of the ponderable character of air, for he several times attempted to determine its 
weight, and showed that the weight of a bladder of air appears to be greater in 
vacuo than in air. 



^ C. Krumbacher, Oeschichte der byzantinischen Literatur, Miinchen, 1897. 

* A. D. White, A History of the Warfare of Science and Theology, London, 1896 ; P. Lacroix, 
Les arts au moyen dge et a Vepoque de la renaissance, Paris, 1869. 

» V. Weech, ZeU. Geschichte Oherrheins, 25. 468, 1873. 

* F. Pouchet, Histoire des sciences naturdles au moyen dge. Alberius Magnus et son ^poqu£, 
Paris, 1853 ; W. J. Townsend, The Great Schoolmen of the Middle Ages, London, 1881 ; Albertua 
Magnus, Opera Omnia, Lugduni Batavorum, 1653 ; Theatrum chemicum, Argentorati, 2. 23, 
423, 1615; 4. 809, 825, 841, 1617; H. Fronober, Die Lehre von Materie und Form nach Albert 
dem Grassen, Breslau, 1909. 

^ R. B. Vaughan, St, Thomas of Aquin : His Life and Labours, London, 1871-2 ; Theatrum 
chemicum, Argentorati, 3. 267, 278, 1617 ; 4. 960, 1619 ; 5. 806, 1661 ; Secretu alchemice. Colon, 
1679 ; Thesaurus alchemice, Lugduni Batavorum, 1602 ; De esse et essentia mineralium, Venetice, 

^ S. Vogl, Die Physik Roger Bacon, Erlangen, 1906 ; Theatrum chemicum, Argentorati, 2. 
377, 1615 ; 5. 834, 1619 ; Fr. Rogeri Bacon, Opera quoedam hactenus inedita (J. S. Brewer), 
London, 1859 ; H. G. Bridges, The Opus majus of Roger Bacon, Oxford, 1897 ; H. F. Wiistenfeld, 
Oeschichte der arabischen Aerzte, Gottingen, 1840 ; E. Charles, Roger Bacon, sa vie, ses ouvrages, 
ses doctrines, d'apres des textes inedits, Bordeaux, 1861 ; R. Bacon, Thesaurus chemicum, Franco- 
furti, 1620; Fr. Rogeri Bacon, Ordinis minorum, Opus majus (S. Jebb), London, 1733; 
R. Adamson, Roger Bacon: the Philosophy of Science in the Middle Ages, London, 1876; 
J. E. Sandys, Roger Bacon, London, 1914. 

' Arnoldus de Villanova, Opera Omnia, Lugduni Batavorum, 1520 ; Theatrum chemicum, 
Argentorati, 1. 128, 1613 ; 2. 108, 1615 ; 3. 128, 137, 1617. 

* P. 0. Keicher, Raymundus Lullus und die Grundzuge seines philosophischen Systems aufgezeigt 
als ein Reaktionsversuch gegen die arabische Philosophic, Miinster, 1 908 ; Theatrum chemicum, 
Argentorati, 3. 165, 295, 1617; 4. 1, 135, 171, 515, 1619; Opera alchemia, London, 1673; 
Opera omnia, Argentorati, 1677, 

* J. Kepler, Nova astrcmomia seu physica codestis tradita commentares de motibus stellce martis, 
Prague, 1609; Harmonices mundi, Linz, 1019; G. Galilei, Discorsi e dimostrazioni matematicJie, 
Ley den, 1638. 

^° A. Comte, Cours de philosophic positive, Paris, 3. 7, 1864. 

^1 M. M. P. Muir, The Chemical Essence and the Chemical Elements, London, 1 894 ; J. C. 
Draper, Amer. Chemist, 5. 1, 1874 ; H. Kopp, Quart. Journ. Science, 5. 21, 1868. 

^2 E. 0. Lippmann, Abhandlungen und Vortrdge zur Geschichte der Naturmssenschaften, Leipzig, 
2. 185, 1913. 

^3 M. Berthelot, Les origines de Valchimie, Paris, 1885 ; Introduction a V etude de la chimie des 
ancient et du moyen dge, Paris, 1889 ; La chimie au moyen dge, Paris, 1893 ; M. Berthelot and 
P. E. Ruelle, Collection des anciens alchimistes grecs, Paris, 1887-8; H. Kopp, Die Alchemic in 
dlterer und neuer Zeit, Heidelberg, 1886 ; Beitrdge zur Geschichte der Chemie, Braunschweig, 
1869; Veber der V erf all der Alchemic und die hermetische Gesellschaft, Giessen, 1847; G. P. 
Nenter, Berichte von der Alchemic, Niirnberg, 1727 ; E. A. Hitchcock, Remarks upon Alchemy 
and the Alchemists, Boston, 1857 ; H. S. Redgrove, Alchemy ; Ancient and Modem, London, 
1911 ; G. Letz, Die Alchemic, Bonn, 1869; E. 0. von Lippmann, Entstehung uvd Ausbereitung 
der Alchemic, Berlin, 1919. 

1* J. Gildenemeister, Zeit. deut. morg. Qes., 30. 534, 1876 ; S. Brown, Essays, Edinburgh, 1858. 

15 C. H, Bolton, Monthly Journ. Science, (3\ 9. 715, 1879. 

1* A. M. Stoddart, Life of Paracelsus, London, 1911 ; A. E. Waite, The Hermetic and Alchemical 
Writings of Paracelsus the Great, London, 1894 ; F. Hartmann, The Life of Philippus Theophrastus 
Bombast of Hohenheim known hy the name of Paracelsus, London, 1896 ; J. M. Stillman, Monist, 
27. 390, 526, 1917 ; F. Mook^ Theophrast^is Paracelsus— eine kritische Studie, WUrsburg, 1876 ; 
R. Netzhamraer, Theophrastus Paracelsus, Einsiedeln, 1901 ; J. Ferguson, Encyc. Brit., 18. 236, 
1885 ; H. Magnus, Paracelsus der Ueberartz, Breslau, 1906 ; M. Neuburger and J. Pagel, Handhuch 
der Geschicht der Medizin, Jena, 3. 403, 1905 ; S. Brown, Essays, Edinburgh, 131, 1858 ; W. Luzi, 
Das Ende des ZeiUdters der Alchemic und der Beqinn der iafrochemischen Periode, Berlin, 1892; 
R. Browning, Paracelsus, London, 1835. 

" A. N. Whitehead, B. A. Rep., 355, 1916 ; S. Brown, Essays, Edinburgh, 1. 131, 1868. 

1* G. Agricola, De re metallica, London, 1912. 

*• A. Libavius, Alchemia, Francofurti, 1595 ; Opera chymica, Francofurti, 1604. 

^ A. Sala, Opera medico-chymica omnia, Rothomagi, 1650. 

21 M. Meslens, Note historique sur J. B. Hehnont, Paris, 1874 ; J. B. Tan Helmont, Works, 
London, 1664. 

2 2 J. R. Glauber, Works, London, 1689. 

2' G. de Mengel, Journ. Alchem. Soc., 1. 49, 1913 ; K. C. Schmieder, Oeschichte der Alchemic, 
Halle, 1832. 

2* M. Berthelot, Introduction a V etude de la chimie des anciens etdu moyen dge, Paris, 279, 1889 ; 
K. Sudhoff, Beitrdge aus der Geschichte der Chemie dem Geddchtniss von G. W. W. Kahlbaum, 254, 
1909 ; F. Strunz, Theophrastus Paracelsus seine Leben und seine Persohnlichkeit, Leipzig, 30, 



1903 ; K. C. Schmeider, Geschichte der Alchetnie, Halle, 1832 ; C. W. Kestner, MedicinMches 
GeUhrten- Lexicon, Jena, 1740; C. W. G. Kastner, Beytrdge zur Begrnndung eines wissen- 
schaftlichen Chemie, Heidelberg, 1807; J. M. StiUmann, Pop. Science Monthly, 81. 591, 1912- 
W. Hommel, Zeit. angew. Chem., 32. 73, 1919 ; H. Kopp, Geschichte der Chemie, Braunschweig*, 
1. 74, 1843; Ansichten uher der Aufgabe der Chemie, Braunschweig, 110, 1875; Beitrdge zur 
Geschichte des Chemie, Braunschweig, 3. 112, 1875 ; Die Alchemie, Heidelberg, 1. 29, 1886 ; J. Fer- 
guson, Bibliotheca Chemica, Glasgow, 1906; B. Valentine, The Triumphal Chariot of Antimonv 
I^ndon, 1893 ; P. S. Wellby, Journ. Alchem. Soc, 2, 91, 1914. 

25 Robert Boyle, Works, London, 1744; The Philosophical Works, London, 1725; The 
Sceptical Chymist, Oxford, 1661 ; H. B. Dixon, B. A. Rep., 594, 1894; T. E. Thorpe, Essays in 
Historical Chemistry, London, 1, 1894. 

§ 13. The Evolution of Ideas regarding the Nature of Calcination 

Let all the greatest minds in the world be fused into one mind and let this great mind 
strain nerve beyond its power ; let it seek diligently on the earth and in the heavens ; let 
it search every nook and cranny of nature ; it will only find the cause of the increcised 
weight of the calcined metal in the air.^ — Jean Rey (1630). 

The principle operations of the earlier chemists were performed by fire, and 
one of the many names applied to chemistry in its early days was Pyrotechnia — 
TTv/o, fire ; rcxi^^y, art. Calcination has always been one of the most important 
operations in the chemical laboratory. Paul de Canotanto,i about the middle of 
the fifteenth century, defined this operation as involving " the incineration of the 
metals, or the destruction of the igneous principle." 

The term calx (calcis) is the Latin word for lime, but the meaning was extended by the 
alchemists to anything produced in the same way as quicklime — namely, by roasting to a 
powder or friable substance. The operation of heating or roasting was called calcination. 
Consequently, as the Latin Geber expressed it in his De alchemia, calcination is the pulveri- 
zation of a thing by fire by the deprivation of the humidity consolidating its parts— in 
illustration, the ash of wood, the oxide of a metal, and the ignited residue of a substance 
dissolved in acid Were all calces. The alchemists regarded the calx as the purest and most 
refined residuum of a substance which remained after the coarser parts had been dispelled 
by heat. 

It was probably known very early that limestone loses weight during its con- 
version into a calx, and it came as an incredible surprise to find that an 
increase in weight occurs when the metals are converted into calces. Near the end 
of the fifteenth century— November, 1489— P. Eck de Sultzbach 2 was probably 
the first to demonstrate experimentally that when a metal is calcined in air, the 
resulting calx — or cineris fixi, as he called it — is heavier than the original metal. 
He also showed that an amalgam of silver and mercury increased in weight 50 per 
cent, when heated for eight hours in air. The increase in weight, which many later 
observers also noticed, seems to have puzzled the earlier chemists. P. Eck de Sultz- 
bach attributed the increase to the union of a spirit (gas) with the metal ; and, 
as will soon appear, he was nearly right. No notice seems to have been taken of 
P. Eck de Sultzbach's surmise, and many probable and improbable explanations of 
the increase in weight, and of the change in the appearance of the metal, were made 
during the sixteenth and seventeenth centuries. Two sets of hypotheses now 
struggled for existence. j j r 

One set of hypotheses assumed that the metals are naturally compounded of 
a substance lighter than air which buoys up the metals, so to speak, against gravi- 
tation ; during calcination this component is driven from the metal and the caJx 
remains. Thus, H. Cardan, in his book De suUilitate (Basil, 1553), stated : 

The metal during calcination dies, for the celestial l^eat^aior ccKi.^t.^--w^ich gav^^^^^ 
life and rendered it light, is dissipated, and the metal consequently becomes heavier during 

Paracelsus expressed a similar idea a short time previously : ^^^''^^'''t^u'lhU 
separates watery moisture, fat, natural heat, odour, and whatever else is combus ible 
Accordingly, terms like terra damnata and caput mortuum were applied to the rebidues 


left after the spirit had been driven from the metals by calcination, and the residua 
were often symbolized pictorially by a skull and cross-bones. 

In another set of hypotheses, it was assumed that something ponderable is 
absorbed by the metal. R. Boyle attributed the increase in weight to " the 
arresting of igneous corpuscles/' and N. Lemery,3 to the assimilation of corpuscles 
de feu by the metal. In R. Boyle's -essay. Fire and flame weighed in the balance 
(London, 1672), a number of experiments are described showing the actual gain in 
weight which occurs when metals are calcined in air ; thus, an ounce of copper 
filings gained 49 grains in two hours, and an ounce of lead gained 28 grains in the 
same time. R. Boyle inferred that *' glass is pervious to the ponderous parts of 
flame " because tin or lead are partially calcined when heated in hermetically sealed 
vessels ; and he stated that the increase in weight arises from the assimilation of 
the " extinguished flame " by the calx. It is rather remarkable that R. Boyle did 
not attribute the increase in weight to the action of the air on the heated body, 
because, shortly afterwards, in an essay entitled Suspicions about some hidden 
qualities of air (London, 1674), apparently following R. Hooke's experiments, q,v.j 
R. Boyle suggested that " air contains some odd substance, either of a solar, astral, 
or other foreign nature ; on account whereof the air is so necessary to the sub- 
sistence of flame ; " and he further added that " this substance is not improbably 
a volatile nitre akin to that which seems so necessary for the maintenance of other 
flames." In opposition to H. Cardan, Boyle also says that the calx of a metal must 
be the metal plus, not minus, something acquired during calcination, and not its 
terra damnata. J. Kunckel (1677), J. J. Becher (1690), J. Romberg (1700),* and 
others also attributed the increase in weight of a metal during calcination to the 
absorption of what J. Kunckel called particulce ignicB. In an analogous manner, 
0. Tachen (1666) & assumed that the increase is due to the absorption of an acid 
existing in the flame, and he found that when lead burns to red lead, it increases 
its weight one-tenth, and returns to its former weight when reduced to the metallic 
state. H. Boerhaave (1732) ^ must have suspected that something was wrong, 
since he kept mercury at a slightly elevated temperature for fifteen years in order 
to find if there was any increase in weight due to the absorption of the alleged fire 
particles ; and, in opposition to Boyle's hypothesis, no increase due to this cause 
could be detected. He also demonstrated that the weight of certain metals — 
e.g. silver — was the same whether at ordinary temperatures or at a red heat. 

A. Cfesalpin, in his De metallicis (Romse, 1596), summarily dismissed the subject 
by assuming that the increase in weight is due to the deposition of soot in the interior 
of the metal during calcination, and others supposed the increase was due to the 
retention of the vapours of charcoal, or the volatile salt of charcoal or the matter 
removed from the calcining vessel. J. Hartmann, in his Praxis chymiatrica (Lipsise, 
1625), showed that the increase could not be due to the assimilation of soot, or the 
vapours of charcoal, because antimony increased in weight when heated in the focus 
of a burning lens with sunlight ; and N. le Febvre ^ supposed that when the metal 
is calcined by means of a burning glass, the increase in weight is due to the absorption 
of the matter of light, which J. Mayow called particulce niti-o-aercB, and which were 
supposed to be derived, not from the air but from the sun, which he regarded as a 
chaos of these particles. 

Jean Rey appears to have been the first to critically examine the different 
hypotheses which had been proposed to explain the increase in weight which occurred 
when the metals are calcined. J. Rey's work was published in an obscure pamphlet 
entitled Essays de Jean Rey, docteur en medicine, sur la recherche de la cause pour 
laquelle Vestain et le plomb augmentent de poids quand on les calcine (Bazas, 1630),^ 
which at that time does not seem to have attracted much attention from those 
interested in the subject, since the discovery of the pressure of air, shortly after- 
wards, diverted the minds of investigators away from a study of the chemistry of 

1. The facts. — In order to clarify the mind, the facts must be reviewed. 


Investigators of nature, said D. Sennert us in hmEpitotne naturalis scienli(B{Oxioid, 
1664), are warned not to look for the causes of phenomena before there is a complete 
agreement as to the facts. Four things are present during the calcination of the 
metal in air : (1) The containing vessel or crucible ; (2) The metal being calcined ; 
(3) The air ; and (4) The source of heat. Again the metal weighs more after the 
calcination than it did before. 

2. The hypotheses. — In applying the inductive method of investigation to these 
facts, it is necessary to review every rational explanation consistent with the facts, and 
to examine each hypothesis rigorously and impartially, since, as emphasized above, it 
is necessary to show that the explanation finally selected is alon£ consistent with 
the facts. This extension of the inductive process might be called the method of 
exhaustion ; its importance was recognized by Epicurus (c. 300 b.c.).^ It is a 
mistake to confine the attention to one hypothesis, because that might seriously 
limit the range of the inquiry. The mind unconsciously assimilates evidence in 
favour of a pet hypothesis ; and a pet hypothesis is apt to grow from a favoured 
child to a tyrannical master. Four plausible hypotheses may be suggested to 
explain the cause of the increase in weight : (1) the gases, etc., from the source 
of heat unite with the containing vessel ; (2) the air unites with the containing 
vessel ; (3) the gases from the flame penetrate the crucible, and unite with the 
metal ; and (4) the air unites with the metal. 

3. Testing the hypotheses by experiment.— By heating the crucible alone, 
without the metal no change in weight occurs. This blank, dummy, or control 
experiment shows that neither the first nor the second hypothesis will account for 
the increase in weight of the metal. The third hypothesis can be tested by heating 
the crucible and the metal out of contact with the air. There is then no change 
in the weight of the metal. The third hypothesis is therefore untenable. This 
method was not practicable for the early chemists, and hence J. Key employed 
a less decisive test. It might be expected that if the results depend upon the 
absorption of the flame gases, different residts must be obtained by using different 
sources of heat — sun-glass, etc. — but the same results are obtained. in every case, 
and accordingly, the third hypothesis is probably wrong. 

4. The conclusion.— Key thus examined all the previously suggested explana- 
tions, and rejected them one by one ; the remaining unchallenged factor was air. 
The sole invariable antecedent of a phenomenon is probably its cause. Hence, 
unless something has been overlooked, it is concluded that when metals are calcined 
in air the increase in weight is due to the fixation of air by the metal, and not to the 
absorption of furnace gases, nor to variations in the weight of the vessel in which 
the calcination is made. The idea was not far from F. M. A. de Voltaire's mind lO 
a century later, for in 1737 he said : 

II est tr^s possible que 1' augmentation du poids soit venue de la mati^re r^pMidue 
dans I'atmosphere, done dans toutes les autres operations par lesquelles les matieres 
calcinees acquierent du poids cette augmentation pourrait aussi leur Hre venue ae la 
meme cause, et non de la matiere ignee. 

Similar remarks apply to R. A. Vogel's Experimerda chemicorum de in^emento 
ponderis corporum quorundam igne calcinatorum examinat (Gottingen, 1753) made m 
ignorance of J. Key's work. . . , 

J. Key attempted to explain how air alone could produce an increase m tne 
weight of a metal during calcination. J. Key imagined that when air is heated, it 
separates into a heavier and a lighter part, and that when a metal is calcined m air, 
the lighter part of the air is distilled off, and the denser portion-/ air epats- alone 
attaches itself to the metal and forms an ash or calx. J. Key did not prove th^ 
subsidiary hypothesis, viz. that only a part of the air attaches itself to the meta^ 
to form a calx. The increase in weight which occurs during calcination was com- 
pared to the wetting of sand with water-most of the water can be drained away, 
but a little remains adherent to the sand : 


The condensed air becomes attached to the calx, and adheres, little by little, even to 
the smallest of its particles. Thus the weight increases from the beginning to the end. 
When all of it is saturated, it cannot take up more. 

J. Rey's explanation proved to be fallacious. The great merit of J. Rey's work 
lies in his demonstration that air is a ponderable fluid ; and the analogy between 
air and a liquid regarded as ponderable fluids enabled him to grapple with an 
intangible body, and to reason on that which from its subtlety had hitherto eluded 
the grasp of the philosophers of all previous ages.ii 

5. Confirmatory experiments. — S. Hales i^ and J. Juncker also explained the 
increase in weight by assuming that particles of air were absorbed by the metal, 
and S. Hales showed that when " 1922 grains of red lead is heated there arises 
34 cubic inches of air." He did not consider it necessary to test the gas expelled 
from the red lead since he assumed that it was elemental air. J. Rey's idea that the 
increase in weight which occurs when a metal is calcined in air is due to the fixation 
of air by the metal, was confirmed by the work of P. Bay en is early in 1774. Bay en 
showed that mercurial calx owes its " calcined state " to its intimate combination 
with an elastic fluid, the weight of which, in adding itself to that of mercury, " con- 
stitutes the cause of the observed increase in weight " of the mercury during cal- 
cination. The experiment was made by reversing J. Rey's procedure and heating 
the calcined mercury until it decomposed into the original mercury and an elastic 
fluid. The mercurial calx and the revived mercury were weighed before and after 
the calcination : 

Mercurial calx . .^. . . . . .576 grains 

Revived mercury . . . . . . . . 518 „ 

Difference . . . . . . . . 58 „ 

P. Bayen added : "I cannot state positively that the 58 grains represent the true 
weight of the elastic fluid, liberated from the 576 grains of mercurial calx, but 
clearly everything leads to that conclusion." 

J. Rey also made the interesting unforeseen observation that " nature, in her 
inscrutable wisdom, has set limits which she does not overstep " ; in other words, 
however long a metal may be heated in air, a definite weight o£ each metal can 
combine with only a definite maximium amount of air. Students to-day regularly 
repeat J. Rey's experiments on the metals, under various guises, as class exercises — 
Table I. for example. 

Table I. — -Action of Air on the Calcination of the Metals. 


Weight of metal 

Weight of calx 

Increase in weight 

Ratio weight air 

absorbed : metal 



Zinc .... 

Aluminium . 


Tin .... 





Hence, one gram of the 

(Absorbed air). Magnesh 
1 1-52 

absorbed air is 

im. Zinc. 

respectively eq 


uivalent to 




6. Anticipation of new phenomena. — A good hypothesis ought to predict 
phenomena which have not been observed, and to foretell the results of new 
experiments ; because, if the hypothesis be true, it ought to include all other cases. 
A hypothesis which is not illogical and which does not contradict known facts 
is to be judged by its usefulness. The end justifies the means. G. J. Stoney has 
expressed the idea neatly : "A theory is a supposition which we hope to be true ; 


a hypothesis is a supposition which we expect to be useful. Fictions belong to the 
realm of art ; when allowed to intrude elsewhere, they become either make-believes 
or mistakes." When the consequences of a hypothesis are logically deduced, a 
good hypothesis should not only explain, but it should anticipate new facts. Key's 
hypothesis can be used to predict new results. In his Memoire sur la calcination 
de retain dans les vaisseaux fermes, et sur la cause de Vaugrmntalion du poids 
qu'acquiert ce metal pendant cette operation (1774), A. L. Lavoisier i* wrote : 

Thus did I at the beginning reason with myself. ... If the increase in weight of a metal 
calx (calcined in a closed vessel) be not due to the addition of fire matter, nor of any other 
extraneous matter, but to the fixation of a portion of the air contained in the vessel, the 
whole vessel after calcination must be heavier than before, and must merely be partly 
void of air, and the increase in the weight of the vessel will not occur until after the air 
required has entered. 

A. L. Lavoisier confirmed this inference experimentally on November 12, 1774, 
although the gifted Russian chemist, M. W. Lomanossofi,!^ had come to the same 
conclusion in 1756, eighteen years before A. L. Lavoisier. 


^ Paul de Canotanto, Theoria ultra estimationem peroptima ad coqnitionem totiua alhimia 
veritatis. Manuscript No. 7159 at the Bibliotheque royalo, Paris — vide F. Hoefer, Histoire de la 
chimie, Paris, 1. 444, 1842 

2 P. Eck de Sultzbach, Theatrum chemicum, Argentorati, 4. 1007, 1622 ; G. F. Rod well, 
Chem. News, 8. 113, 186, 246, 1863; 9. 14, 26, 50, 242, 1864; 10. 74, 195, 208, 1864; 11. 38, 
74, 160, 291, 1865 ; 12. 62, 74, 293, 1865 ; 14. 51, 1866 ; 16. 29, 43, 1869. 

' N. Lemery, Cours de chimie, Paris, 1675. 

* J. Kunckel, Chymische Anmerkungen de principiis chymicis salihus, acidis,alcalibu8, Wittem- 
berg, 1677 ; J. J. Becker, Physica subterranea, Franckfurt, 1690 ; J. Homberg, Mem. Acad., 
64, 1700. 

5 O. Tachen, Hippocrates chemiciis, Venice, 210, 1666. 

^ H. Boerhaave, Elementa chemice, Lugduni Batavorum, 1732. 

' N. le Febvre, Traicte de la chymie, Paris, 1660 : J. Mayow, De sal-nitre et spiritu nitro-aereo, 
Oxford, 1669. 

8 Alembic Club Reprints, 11, 1895 ; R. P. Beraud, Dissertation sur la cause de VaugmerUation de 
poids que certaines matieres acquierent dans leur calcination, Haye, 1748. 

* E. Zeller, The Stoics, Epicureans, and Sceptics, London, 424, 1870. 

10 F. M. A. de Voltaire, Mem. Acad., 169, 1737. 

11 G. F. Rodwell, Chem. News, 10. 208, 1864. 

12 S. Hales, Vegetable Staticks, London, 1. 288, 1727 ; J. Juncker, Conapectus chemtca themettco- 
practicce, Halle, 1749. 

i» P. Bayen, Journ. Phys., 3. 135, 281, 1774. 

" A. L. Lavoisier, (Euvres, Paris, 2. 103, 1862. 

i« A. Smith, Journ. Amer. Chem. Soc, 34. 109, 1912 ; Ostwald's Klassiker, 178, 1910. 

§ 14. The Evolution of Ideas regarding the Nature of Burning 

step by step we cross great eras in the development of thought ; there is no sudden 
gigantic stride ; a theory proceeds by slow evolution until it dominates or is destroyed. 
— G. F. RoDWELL (1869). ,.^ ^ ., , . 

Slowly, gradually and laboriously one thought is transformed into a different tnougni, 
as in all likelihood one animal species is gradually transformed into a new species. J"^^ 
ideas arise simultaneously. They fight a battle for existence not otherwise than diet tne 
Ichthyosaurus, the Brahmin and the horse. Thoughts need their own time to ripen, grow, 
and develop. — E. Mach. 

The beautiful fiction of Greek mythology, as related by ^Eschylus, teUs how 
Prometheus stole fire from heaven, and gave the sacred gift to man as the most 
useful of all his necessaries. To many ancient worshippers, fire was a thmg divme, 
the supreme manifestation of God himself, and it soon became the one visible 
symbol of God. Even to-day the sacred fire exists among the races of the iiaikans. 
Accordingly, the Zoroastrian fire worshippers called their god the one fire, or the pure 
fire ; i and the sun was worshipped first as an emblem of the deity— Hre— ana 
afterwards as itself a god.2 Fire thus came to be the first and most potent oi an 


the elements, and it is easy to understand how Heracleitos regarded subtle fire as 
the sole primal element from which all things were created ; and how fire was 
canonized by Pythagoras and Empedocles as one of the four indispensable and all- 
sufficient components of the universe. 

Some of the early philosophers promulgated a dynamical theory of heat and fire. 
Epicurus (c. 300 B.C.) regarded heat as a result of the rapid motion of minute spherical 
particles which insinuated themselves in the pores of the densest substances ; cold 
was likewise produced by angular particles moving more slowly. Lucretius (c. 80 
B.C.) similarly referred heat to the motion of primary particles which penetrated 
every material thing. H. Cardan ^ (1557) spoke of a ynotus ignis and a motus 
caloris. R. Fludd (1617), F. Bacon (1620), A. Kircher (1644), and others have 
propounded views which amount to a denial of the elemental nature of fire, since 
they virtually assumed that heat is a violent motion of the particles of bodies, or 
that fire is air which has been made to glow by the vehement collision of its particles, 
and that the heat so generated changes combustible matter into flame. 

Rene Descartes, in his Principia fhiloso'phice (Amsterdam, 1644), assumed that 
originally all matter consisted of square particles endowed with two kinds of 
motion : a rotation of each particle about its own centre ; and a rotation of groups 
of particles about a common centre. The angles of the particles were abraded by 
collisions producing three kinds of particles which he called elements : (1) Materia 
primi elementi, or fine dust, which he also called materia subtilis, or materia coslestis, 
because the sun, stars, and fire were supposed to be composed of this material. 
(2) Globuli secundi elementi, or rounded particles which were supposed to make up 
the atmosphere and everything between the stars and the earth. (3) Particulce 
tertii elementij or particles which retain some of their angles and are partially 
rounded ; these were assumed to make up the earth and all terrestrial bodies. The 
particles of the materia coelestis were supposed to be in far more rapid motion than 
the other particles. The different forms of matter were supposed to be determined 
by the relative proportions and motions of these three elements ; and every natural 
phenomenon, the result of the conduction of motion from one body to another. 
Fire, according to R. Descartes, consisted of the third element rapidly agitated by 
the tnateria coelestis ; and the particles of combustible bodies were supposed to be 
peculiarly adapted to receive the motions of the materia coelestis. It was all a 
transmission of motion, not substance. N. Lemery adopted the main tenets of th^ 
Cartesian theory in his famous Cours de chimie (Paris, 1675) : 

I understand by igneous corpuscles — corpuscles ignees — a subtle form of matter which 
having been thrown into rapid motion, still retains the capacity of impetuous motion 
when it is enclosed in grosser matters ; and when it finds bodies which by their texture or 
figure are easily put in motion, it draws them about so strongly that their parts develop 
heat by being rubbed violently against one another. . . . The particles of sulphur, for 
instance, are very susceptible to motion . . . and it seems probable that fire is only violent 
motion of minute bodies about their common centre. 

Flame, said R. Descartes, is directed upwards because it contains much materia 
coelestis which is lighter than air, and the cause of lightness in bodies generally. 
Descartes' materia coelestis approximates to the modern conception of an aether 
more subtle than air, and filling the interstices between the molecules of air with a 
continuous series of globules which pervade the pores of glass, and of the densest 
substances without interruption ; and propagating light by communicating impulses 
from one molecule to another so as to produce a kind of pressure without locomotion. 
Isaac Newton * postulated a similar aether " pervading and lurking in dense 
bodies, but not yet sufficiently manifested by experiments." R. Hooke introduced 
the notion of vibratory impulses in this medium, and the idea was elaborated by 
C. Huygens and T. Young into the undulatory theory of light which is now generally 
accepted. The communication of the vortex motion of the materia coelestis to the 
atoms is thus described by R. Boyle : ^ 

The restless agitation of the materia coelestis wherein the particles of air swim, so whirls 


them round that each corpuscle endeavours to beat off all others from coming within the 
little sphere requisite to its motion about its own centre . . . their elastic power is made 
to depend upon the vehement agitation which they receive from the fluid sether Imateria 
coelestis) which swiftly flows between them. 

Several early observers noticed that fire cannot subsist without air. Theo- 
phrastus,6 for instance, in the fourth century B.C., in his treatise On Fire, noticed 
that air plays an important part in the maintenance of flame ; Hero of Alexandria 
(c. 117 B.C.) demonstrated this by placing a lighted lamp in a closed vessel, and 
showing that under these conditions the flame was extinguished— Hero said that the 
fire consumed and rarefied the air ; and from a similar experiment in the thirteenth 
century, Roger Bacon inferred that aer est cihus ignis—aii is food of flame— in agree- 
ment with Theophrastus— 315 B.C.— who said, " It is not at all irrational to believe 
that flame is maintained or supported by an aeriform bo4y." Near the beginning 
of the sixteenth century (c. 1500), Leonardo da Vinci 7 clearly recognized that air 
is necessary for the sustenance of the flame of a burning candle, for he said : " There 
is smoke in the centre of the flame of a wax candle because the air which enters 
into the composition of the flame cannot penetrate to the middle. It stops at the 
surface of the flame and condenses there." Leonardo da Vinci also showed that 
air is necessary for respiration ; and that air is not an element because one part 
of it alone is concerned in combustion. R. Fludd 8 noticed in 1617 that when a 
candle is burnt in a glass vessel over water, the water rises in the vessel as the air 
is consumed, for " air nourishes fire, and in nourishing consumes it." H. Cardan 
also, in his De rerum varietate (Basil, 1557), classified different substances as corti- 
hustihle or incomhustiUe. Flame, said he, is nourished by a ga,s— flatus — which wiU 
ignite a glowing splint, and which exists in saltpetre. H. Cardan was here ver}' 
near to the discovery of facts which in the hands of A. L. Lavoisier produced une 
revolution iintnense dans la science. 

After his discovery of the air-pump in 1650, one of the first experiments tried 
by 0. von Guericke ^ was to ascertain if a candle would continue burning in an 
exhausted receiver, and it was found that owing to the want of air the flame of a 
lighted candle expired more quickly under the exhausted receiver of an air-pump 
than when the receiver was not exhausted ; fire, said Guericke, consumes air. In 
his first treatise on pneumatics, New experiments, fhysico-mechanical, touching the 
spring of air (London, 1660), R. Boyle mentions several proofs that combustion 
cannot proceed in a space void of air ; and in 1672, R. Boyle, in an essay On the 
difficulty of pr^eserving flatne without air (London, 1672), showed that when placed 
under the receiver of an air-pump, the flame of burning gas, derived from the action 
of an acid on iron, is suddenly enlarged on exhausting the air, and finally is ex- 
tinguished ; and he showed that sulphur does not burn if heated in vacuo. These, 
and other experiments on similar lines, clearly showed that air is necessary for 

Robert Hooke outlined a theory of combustion in his Micrographia (London, 
1665). He noticed the similarity in the actions produced by air and by saltpetre,io 
and hence suggested that air is mixed with a substance which is like, if not identical 
with, that which is fixed in saltpetre, and that only this portion of air is required 
to support combustion and respiration. A similar conclusion had been hinted at 
by R. Fludd,ii who said : " The substance of saltpetre is nothing but air congealed 
by cold." Again, in his Lectiones cutleriance (London, 1674-9), Robert Hooke 
assumed that burning is produced by the solvent action of the surrounding air 
which is dissolved by the burning body much as water dissolves salt. He said : 

Air is a menstruum that dissolves all sulphurous bodies by burning, and without air, 
no such dissolution will follow, though the heat applied be never so great which was 
particularlv tried by charcoal enclosed in an iron case with a screw stopper, which though 
violently heated yet the coke was not burned nor wasted when taken out. . . . Ihat 
shining transient body we call flame is but a mixture of air and volatile sulphurous parts 
of combustible bodies which are acting upon each other as they ascend. . . . The action 
is performed with so great violence and does so minutely act, and rapidly agitate the 


smallest parts of the combustible matter, that it produces in the diaphanous medium of 
air the action or pulse of light. 

J. Mayow (1669)12 subjected the guess or hypothesis of Hooke to the test of 
observation. The following experiment is a more refined form of one made by 
J. B. van Helmont, circa 1640 : — 

J. Mayow arranged a candle in water so that the wick was between 9 and 10 cm. above 
the surface of the water. A glass cylinder, A, Fig. 6, was lowered over the burning candle 
so that the level of the water inside and outside the cylinder was 
the same. A small syphon, B, was used for the purpose. Im- 
mediately the cylinder was in position, the syphon was removed. 
The flame of the candle soon expired, and water rose in the 
jar. Some gas still remained in the jar, but it could not be air 
because one of the characteristic properties of air is to support 
the burning of the candle, and the flame of the candle is ex- 
tinguished in the residual gas. Mayow obtained analogous 
results by confining a mouse under the jar. The mouse died, 
and the water rose in the jar. 

Hence, Mayow inferred that air contains two kinds of 
F 6— Ma ow's Experi Pa^*ticles, One of which — the nitro-aerial particles — 
ment on Combustion " ^ withdrawn and destroyed by the burning candle. 
J. Mayow also stated : 

Though the particles of air are very minute, and are vulgarly taken for an element of 
the greatest simplicity, it appears to me necessary to judge them to be a compound. . . . 
It is manifest that the air is deprived of its force by the respiration of animals much in the 
same manner as by the deflagration of flame. 

Mayow does not seem to have quite grasped the idea that the nitro-aerial particles 
which support combustion actually combine with the burning body, although he 
correctly inferred that air was a mixture containing nitro-aerial particles as one 
constituent. The nitro-aerial particles were indiscriminately called fire-air, nitre-air, 
and nitro-aerial spirit. Mayow' s observations appear to show that air is a mixture 
of two gases one of which is withdrawn during combustion, and the remaining gas 
does not support combustion. Stephen Hales i^ also noticed that in the combustion 
of phosphorus under a bell- jar, white fumes are produced and air is absorbed. When 
the experimenters of the seventeenth century spoke of the destruction of the 
elasticity of a portion of the air, they meant that some of the air was lost — 
presumably by absorption by the confining liquid, etc. 

Some modern commentators consider that J. Mayow's nitro-aerial spirit repre- 
sented oxygen, and his aerial spirit, nitrogen. It has been said that J. Mayow's 
nitro-aerial particles were made to explain too much, for he applied them to all 
sorts of phenomena — e.g. the formation of acids, fermentation, the production of 
nitre, calcination, combustion, and respiration— rather is this a tribute to J. Mayow's 
genius. J. Mayow considered the nitro-aerial particles to be fixed as the acid 
component of nitre because the effects produced by nitric acid and by the burning 
glass on antimony were the same. He extended his views to other substances — 
particularly the acidification of sulphurous and fermenting substances by exposure 
to the atmosphere — and thus inferred that his nitro-aerial particles are the active 
agents in combustion and acidification. When J. Mayow regarded these same 
particles as the principle by which metals increase in weight when calcined in air ; 
the principle by which vegetables germinate and. grow ; and by which the blood 
changes its colour in the lungs during respiration, he seems to have generalized 
with far greater precision from a few facts than the greater part of the next 
generation did from many.i^ 

J. Mayow, however, did mix some fantastic hypotheses with his eminently logical 
interpretations of ingenious experiments, and in some cases the relevant matter is 
mixed with so many irrelevancies, that it is difficult to tell which is which unless 
his statements are interpreted in the light of what is now known to be true. To-day, 
J. Mayow's brilliant reasoning would be accepted as a logically conclusive proof of 


the existence of oxygen as a distinct substance ; but his demonstration was a 
century ahead of its time. Instead of his unique experimental talents 
being encouraged by his contemporaries, they were damped by the coldest of 
receptions. His work was evidently above the heads of his contemporaries The 
historian of science, said G. F. Rodwell,i5 should endeavour to grasp the precise 
mode of thought of the man of whom he writes, to think as he thought, to view 
the phenomena in the light of the age in which he lived, and then to reason on them 
as he reasoned. Evidently, then, E. Hooke and J. Mayow got very near to the 
present-day theory of combustion, but unfortunately, the latter's ingenious experi- 
ments had very little, if any, influence on the subsequent development of chemistry, 
because the lowering clouds of the phlogiston hypothesis appeared as a grey after 
dawn and gradually darkened the sky of chemistry until the chemical world appeared 
to be enveloped in an impenetrable fog. For another century more trust was placed 
in phantasms of the imagination than in facts obtained by precise observations. 

It must be added that in the Far East, the Chinese philosopher Mao-Khoa, who flourished 
about the eighth century, is said to have had a fairly clear idea of the composition of air, 
and of the part played by oxygen— which he called yin—m. combustion and respiration! 
This historical information, however, played no part in European discoveries since it is 
but a comparatively short time ago that Mao-Khoa's views were reported, and im- 
familiarity with the language and literature has prevented many examining the claims of 
_^ the Chinese scholar to a proud place in the history of chemistry. 

* References. 

1 T. Stanley, History of ChaUaick Philosophy, London, 1662 ; V. Titelbach, Open CouH, 15. 
143, 1901. 

2 Malachi, 4. 2 ; / Chronicles, 21. 26 ; // Chronicles, 7. I ; / Kings, 18. 38 ; Exodus, 3. 38 ; 
19. 18 ; Deuteronomy, 4. 12. 

' H. Cardan, De rerum varietate, Basil, 1557 ; R. Mudd, Utriusque cosmi majoris scilicet et 
minoris metaphysica, physica atque technica historia, Oppenheim, 1617 ; F. Bacon, Novum organum, 
London, 1620 ; A. Kircher, Ars m/igna lucis et umbrcs, Rome, 1644 ; G. F. Rodwell, PhU. Mag., 
(4), 35. 1, 1868. 

* Registry Book of the Roy. Soc., 5. 67, 1675-9 ; Letter from Newton to Halley, 1686 ; Letter 
from Newton to Boyle, 1678 ; Isaac Newton, Opticks, London, 1717. 

5 R. Boyle, New Experiments, Physico-mechanical, touching the Spring of Air, London, 1660. 

* Theophrastus, Uepl irvpSs, Paris, 1567. 

' J, B. Venturi, Notice de quelques articles appartenant a Vhisloire naiurellede la chimie, iiris de 
Vessai stir les ouvrages de Leonard de Vinci, Paris, 1797 ; M. Libri, Histoire des sciences mathe- 

^matiques en Italic, Paris, 3. 27, 1838-41 ; E. O. Lippmann, Leonardi da Vinci als Gelehrter und 
Techniker, Stuttgart, 1900 ; E. Muntz, Leonardo di Vinci, London, 1898. 
^ R. Fludd, Utriusque cosmi majoris scilicet et minoris metaphysica, physica atque technica 
historia, Oppenheim, 1617. 
' 0. von Guericke, Experimenta Magdehurgica, Amsterdam, 1672; G. Berthold, Wied Ann., 
54, 724, 1895. 
i» R. Bathurst and N. Henshaw, Aerochalinos, or a Register for the Air, London, 1677. 
^^ R. Fludd, Utriusque cosmi majoris scilicet et minoris metaphysica, physica atque technica 
historia, Oppenheim, 1617. 
12 J Mayow, De sal-nitro et spiritu nitro-cereo, Oxford, 1669 ; Tractatus quinque medico-physici, 
Oxford, 1674 ; Alembic Club Reprints, 16, 1907 ; J. B. van Helmont, Orius medicince, Lugduni 
Batavorum, 84, 1656. 

13 S. Hales, Vegetable Staticks, London, 1727. 

14 W. V. Harcourt, Phil. Mag., (3), 28. 478, 1846 ; J. B. Cohen, CAcw. WorU, 3. 247, 1914 ; 
A. Smith, Journ. Amer. Chem. Soc., 34. 109, 1912 ; G. D. Yeates, Observations on the Claims of 
Moderns to some Discoveries in Chemistry and Physiology, London, 1798. 

15 G. E. Rodwell, Chem. News, 14, 25, 1866. 

§ 15. The Phlogiston Theory 

During the greater part of the eighteenth century, the doctrine of phlogiston was not 
only the lamp and guide of chemists but it remained the time-honoured and highest 
generalization of physical chemistry for over half a century. — S. P. Langley. 

Phlogiston died as an old king,— once infinitely dominant, somewhat tyrannical, not 
always just ; now deposed, decrepit, utterly senile, forsaken by all.— W. Odling. 

Up to about the middle of the fourteenth century, combustion was explained by the 


aid of the assumption that all combustible bodies contained a common element, 
the essence of fire, that is, an inflammable principle which enabled them to burn. 
This obviously means little more than saying that substances burn because they 
are combustible. The idea of a subtle fire innate in matter has pervaded philosophy 
from the earliest times. Zeno (c. 450 B.C.) called it drcKveKov -n-vp — barren fire ; 
Heracleitos (c. 450 B.C.), dvaOvfiiai^ ; Lucretius (c. 80 ac), suUilis ignis, coelestis 
ignis, or tenuis ignis ; Paracelsus (c. 1500), sideric sulphur ; H. Cardan (c. 1553), 
color coelestis ; and R. Descartes (c. 1664), materia coelestis. The alchemists of the 
Middle Ages variously styled it elemental fire, astral fire, sulphurous principle, or 
materia ignis. 

The empyrean i element of the ancient Greeks was consecrated under the 
classical name phlogiston by the hierophants of a newer chemistry. The word 
phlogiston is derived from the Greek <f>\oyL^oi, to inflame, and is related to ^Aeyw, 
to burn, and <^Ao^, flame. In some cases phlogiston was believed to resemble 
that subtle fiction we now call cether, and J. Juncker,^ in 1744, called it materia 
igtiea OBtherce. J. Kunckel (1676) thought that the inflammable principle must be 
sulphur, and wrote ubi ignis el color, ihi sulphur — where there is fire and heat there 
is sulphur. Virtually all chemists of this period attributed the combustibility of 
a substance to the presence of sulphur. There were many sulphurs — e.g. the sulphur 
of wood (carbon), the sulphur of wine (alcohol), etc., and Robert Boyle in his essay 
On the difficulty of preserving fiame without air (London, 1672), called the fume or 
gas which is evolved when an acid acts upon iron the volatile sulphur of Mars ; and 
in his essay On the producihleness of chemical principles (London, 1680), he speaks 
of the sulphur of the chemist as being a combustible and inflammable principle. 

Twenty-five years after the appearance of R. Descartes' Principia, and about 
the time of J. Mayow, J. J. Becher began to publish the chemical side of a theory 
analogous in many respects with the physical theory of Rene Descartes. The most 
important work of J. J. Becher is his Physica suhterraneo (Lipsise, 1669), and the 
three supplements dated 1671, 1675, and 1680 respectively — J. J. Becher's term 
suhterraneo is probably equivalent to the modern inorganic. J. J. Becher advocated 
the importance of experiment in chemical science. He rejected the four-elements 
and the quintessence of the ancients, but he did so only to promulgate four elements 
of his own devising — fire ; the earthy principle ; the combustible element ; and 
the metallic one. This enabled him to classify material substances into fiery or 
imponderable bodies, earth, combustibles, and metals. The combustibles and metals 
were later grouped together, and his system was simplified into fire, the first kind 
of substance ; earths, calces, and acids, the second ; and combustibles and the 
metals, the third ; otherwise expressed, J. J. Becher's triad included fire, the 
products of combustion, and combustibles. It was not the custom, in J. J. Becher's 
time, to keep one specific technical term for one specific thing. He seems to have 
used the terms vitrifiable earth — terra lapida or terra vitrescihilis — inflammable 
earth — terra pinguis — and mercurial earth — terra fluida or terra mercurialis — almost 
in the same sense that the alchemists spoke respectively of salt, sulphur, and 
mercury. He regarded his three elements as three varieties of sulphur ; vitrifiable 
earth was called fixed sulphur, tnercuriol earth, or volatile sulphur ; and infiam- 
mahle earth was indiscriminately called combustible sulphur, sulphur adustible, 
sulphur ardens, or phlogistic sulphur. J. J. Becher said : 

Combustible sulphur is the innate heat of the metals. . . . The base metals contain an 
inflammable principle which by the action of fire goes into the air, leaving behind a metal 
- calx. 

This recalls the hypothesis promulgated by H. Cardan a century earlier. J. J. 
Becher supposed the different forms of matter to be compounds of one or more of 
these elements differently arranged with or without water. G. F. Rodwell,^ in a 
valuable article On the theory of phlogiston, says that J. J. Becher never used the 
word ^Xoyia-rov as a noun to designate the matter or principle of fire. It was 


reserved for J. J. Becher's disciple G. E. Stahl, near the beginning of the eighteenth 
century, to employ the term phlogiston for the materia ignis of the early writers. 

Towards the end of the seventeenth century, G. E. Stahl sketched an outline 
of the theory of phlogiston in his Zymotechnia fundamentalis (Franckfurth, 1697) ; 
and in his Specimene Becheriano (Franckfurth, 1702), he elaborated J. J. Becher's 
Physica subterranea, a work which he rated very highly, and vaunted it to be opus 
sine pari — a work without a peer ; primum ac princeps — first and foremost ; liber 
undique et undique primus — a book everywhere supreme ; etc. G. E. Stahl also 
wrote his Fundamenta chymice dogfnaticce et experimentalis (Norimberga?, 1723) as 
a text-book of phlogistic chemistry, and he described in his Experimenta, observa- 
tiones, animadversiones, CCG numera, chymicce et physicw (Berlin, 1731), a nimiber 
of experiments in support of the theory, and answers to some questions. Like the 
ancients, G. E. Stahl believed in the existence of two kinds of fire : (i) ordinary 
visible fire, or mundane fire, or gross earthy fire which he called ignis or flame ; 
and (ii) pure, subtle, invisible fire, materia ignis, or phlogiston, which became ignis 
only when associated with material particles which assimilated its motion. It 
therefore follows that J. J. Becher's terra infiammibilis, terra pinguis, combustible 
sulphur, sulphur ardens, or phlogistic sulphur ; and G. E. Stahl's phlogiston are 
new names for an old time-honoured principle. The dominant functions of Des- 
cartes' materia coelestis were conferred on phlogiston, and some new properties were 
added. Phlogiston, said G. E. Stahl, is the materia aut principium ignis, non ipse 
ignis. Although the real nature and properties of phlogiston were unknown, its 
existence was pure conjecture, yet G. E. Stahl did not hesitate to speak very 
definitely about this creature of the imagination. He said : 

Phlogiston is a very subtle matter capable of penetrating the densest substances ; it 
neither bums, nor glows, nor is visible ; it is agitated by a rapid motion- — igneo motu — and 
it is capable of communicating its motion to material particles adapted to receive it. The 
particles when endowed with this rapid motion constitute visible fire. . . . Fire is an 
aggregate of a great number of particles in vehement motion. The maieria of fire is phlogis- 
ton — a thin all-pervading medium composed of movable particles — the forina is the motion 
itself ; the materia is passive, the forma is active. The motion of phlogiston is gyratoriua 
seu verticillaris and not progressive. . . . Heat is an intestine motion of the particles of 

G. E. Stahl taught that in the act of combustion, phlogiston, an intrinsic con- 
stituent of every combustible body, was set at liberty. Oxidation was said to be 
due to the escape of phlogiston ; deoxidation or reduction to the absorption of 
phlogiston. When a metallic oxide was heated with a substance rich in phlogiston 
— e.g. charcoal or reducing agents generally — the charcoal supplied the calx 
or metallic oxide with phlogiston, and reproduced a compound of phlogiston with 
the metallic oxide which was the metal itself. Metals were thus supposed to be 
compounds of phlogiston with their calces or oxides. The noble metals were sup- 
posed to have their phlogiston so firmly fixed that nothing can take it from them. 
While the base metals are turned to calces when roasted in air, the royal metals 
remain intact during the fiercest trial. If phlogiston escaped, the metalUc oxide or 
calx remained. The idea is symbolized 

Metal ^ I-hlogiston 4- Metal calx or oxide 

The body from which phlogiston escapes, when no longer capable of supporting 
combustion, was said to be dejMoqisticated, and conversely, the body- solid, liquid, or 
gas -with which the phlogiston' was combined, or by which it was absorbed, was 
said to be phlogisticafed. Apparently overlooking the theories of R. Hooke (1664) 
and J. Mayow (1674), which were developed while Stahl was in the nursery, 
M. E. Chevreul * claimed that on doit a Stahl la premiere explication de la combustion. 

The phlogistians are said to have been most assiduous in collectmg instanttw 
convenientes, but very reluctant in accepting instanti(B inconstanti(E. G. E. Stahl, by 
denying that the calx of mercury weighed more than the mercury from which it was 

VOL. I. *" 


derived, sacrificed fact to theory. Phlogistic chemistry was thus established in 
opposition to facts which at first sight appeared to carry its own refutation, for if 
the calcination of a metal be attended by the expulsion of phlogiston, the calx 
should weigh less than the metal. When the facts that the loss of phlogiston is 
always associated with a gain in weight, and vice versa, became too insistent, and 
could no longer be denied, G. E. Stahl, in his Fundamenta chymice (Norimbergse, 
1723), frankly evaded the difficulty by introducing another perplexity. He said : 

The fact that metals when transformed into their calces increase in weight, does not 
disprove the phlogiston theory, but, on the contrary, confirms it, because phlogiston is 
lighter than air, and, in combining with substances, strives to lift them, and so decreases 
their weight ; consequently, a substance which has lost phlogiston must be heavier than 

Thus, the phlogistians said that phlogiston also embodied the principle of levity, 
and conferred a negative weight upon bodies. Consequently, when phlogiston is 
associated with matter, the weight is lessened, just as inflated bladders lessen the 
water-weight of a swimmer. 

It may not seem rational to postulate the existence of a substance weighing 
less than nothing. It will be observed, however, that the assertion, all 7natter 
is heavy and possesses weight, is one way of saying that the attraction of gravitation 
exists between all masses of matter. This is by no means a self-evident principle, 
because it is just as easy to conceive of two masses of matter repelling one another, 
and easier still to imagine two masses of matter neither attracting nor repelling one 
another. Thus, G. B. Airy ^ said : "I can easily conceive that there are plenty of 
bodies about us not subject to this mutual action, and therefore not subject to the 
law of gravitation." Hence, the assumption of a phlogiston weighing less than 
nothing is not so silly as is sometimes supposed. If phlogiston be a principle of 
levity, however, with a negative gravity, it would not be attracted but rather repelled 
by other substances. Consequently, in order to explain Ifow phlogiston becomes 
fixed in combustible bodies, it would be necessary to invent another force stronger 
than gravitation. It is quite true that no form of matter with a negative gravity 
has been detected, and accordingly, it is assumed that a form of matter weighing 
less than nothing does not exist, and that, other things being equal, an increment 
in weight is necessarily an effect of an increment of matter. 

The era of phlogiston presents serious claims to be regarded as the period when 
chemistry began to take shape as a definite science. It represented a definite attempt 
to group diverse chemical phenomena about a rational principle which seemed 
adequate to embrace the then known facts. The doctrine of phlogiston was 
invented to render chemical phenomena intelligible to the mind ; it was founded 
on fact ; and it owed its value in the minds of a race of eminently practical chemists, 
to the facts which it represented. New facts soon began to accumulate which 
could not be explained in terms of the original simple hypothesis, and auxiliary 
hypotheses were framed in quick succession ; these made the theory contradictory 
and unmanageable. In his Reflexions sur le phlogistique (Paris, 1783), A. L. 
Lavoisier ^ said : 

Chemists have turned phlogiston into a vague principle, one not rigorously defined, 
and which consequently adapts itself to all the explanations for which it might be required. 
Sometimes this principle has weight, sometimes not ; sometimes it is free fire, sometimes 
it is fire combined with the earthy element ; sometimes it passes through the pores of 
vessels, sometimes the vessels are impervious to it ; it explains both causticity and non- 
causticity, transparency and opacity ; colours and their absence ; it is a veritable Protean, 
changing in form each instant. 

A. L. Lavoisier's explanation of the increase in weight which occurs when lead 
is calcined, seems so obvious that it is now difficult to appreciate the difficulty as 
set forth by P. J. Macquer (1769) 7 : 

The phenomenon is un vrai paradoxe chimique. While it is easy to prove the fact, it 


is difficult to find a satisfactory explanation. The phenomenon is outside all physical ideaa 
which we have formed, and it is only in the future that a solution of the difficulty can be 

The chief difficulties encountered in the application of the theory of phlogiston 
were : (i) The increase in weight which occurs when metals are calcined ; (ii) The 
necessity for the presence of air during combustion ; (iii) The change of mercury 
calx into a metal without the addition of phlogiston. In the latter case, P. j. 
Macquer, indeed, used the fact that mercury calx can be converted into a metal 
by merely heating it jper se in the absence of a body containing phlogiston, to argue 
that the mercury calx is not a real calx, but merely a substance which has acquired 
par r action dufeu Fapparence d'une chaux metallique. P. J. Macquer endeavoured 
to remove the objection by assuming that phlogiston is light and that during com- 
bustion, light and air mutually precipitate one another ; during the calcination 
of a metal, the air unites with the metal and disengages phlogiston ; and during 
the reduction of a metal calx, light unites with the metal and liberates air. C. W. 
Scheele ® supposed that heat, light, and inflammable air were compounds of air 
and phlogiston which are convertible into one another by the addition or subtraction 
of phlogiston — -inflammable air was assumed to contain most, and heat least 
phlogiston. During calcination, the metal either attracted air by means of its 
phlogiston and thus formed heat, or else communicated phlogiston to the air, and 
attracted heat from the fire ; in either case the air remained in the calx and im- 
parted an overplus of weight. When a calx is reduced by inflammable air, heat, 
or light, the latter is decomposed and the phlogiston remains united to the reduced 
metal. The fact that oxygen supported combustion better than air led to the 
hypothesis that air contains more phlogiston than oxygen, which was hence called 
dephlogisticated air. At one time H. Cavendish (1766) ^ assumed that inflammable 
air is itself the phlogiston of the ancient chemists, and that a certain amount is fixed 
in all combustible bodies. Inflammable air, i.e. hydrogen gas, was accordingly 
called phlogisticated air. This hypothesis substituted a definite tangible material 
for a vague principle, but many of the properties of G. E. StahFs phlogiston were 
utterly at variance with those of hydrogen, and the hydrogen hypothesis completely 

About 1770, it had been definitely proved that there is an increase in weight 
during the conversion of a metal into a calx by calcination of the metal in air. The 
fact was qualitatively explained, somewhat clumsily, by the phlogiston hypothesis 
which was based upon the subtilis ignis of the ancients, or the materia coekstis of 
R. Descartes. R. Hooke, J. Rey, and J. Mayow had recognized that air somehow 
plays an important part in the process of calcination and combustion, but while 
their ideas on the general principle were clear, the details were somewhat hazy and 


1 Empyrean — the highest heaven where the ancients supposed pure fire subsisted. 

2 J. Juncker, Conspectus chemice theoretico-practicce, Magdeburg, 1744 ; J. von Lowenstern 
Kunckcl. Nfdzliche Ohservationes, Hamburg, 1676. 

3 G. F. Rodwell, Phil. Mag., (4), 35. I, 1868. 

* M. E. Chevreul, Campt. Rend., 59. 977, 1864. 

5 G. B. Airy, Gravitation, London, 1885; R. Hare, Jmer. Journ. Sctence, (1), 42, 200, 184-i; 
W. Whewell, Cambridge Phil. Soc, 7. 197, 1842. 

« J. B. Dumas, Lecons sur la philosophic chimique, Paris, 161, 1837. 

' P. J. Macquer, Mem. Acad., 153, 1769 ; Mments de chimie pratique. Pans, 1 /5I. 

8 C. W. Scheele, Chemische Abhandlungen von der Luft umf. dem Feuer, Ixsipzig, 1782. 

« H. Cavendish, Phil. Trans., 56. 141, 1766; R. Kirwan, An Essay on Phl^tston and the 
Constitution of Acids, London, 1789 ; Phil. Tram., 72. 236, 1782 ; W.^Nicholson, The Controversy 
between Kirwan and the French Academicians on Phlogiston, London, 1 /87. 



§ 16. Lavoisier's Experiments on Combustion and Calcination 

Nature is ever making signs to us, she is ever whispering to us the beginnings of her 
secrets ; the scientific man must be ever on the watch, ready at once to lay hold of nature's 
hint, however small ; to listen to her whisper, however low.— M. Foster. 

The beginning and end of every exact chemical process is weighing. — W. Nicholson (1808) . 

In 1772, Antoine Laurent Lavoisier began to publish accounts of a brilliant 
series of investigations which in a few short years banished phlogiston completely 
from chemical science. Chemistry had grown too great to be governed by the 
mystic phantom — phlogiston. In his Opuscules physiques et chimiques (Paris, 
1774), A. L. Lavoisier first showed that phosphorus and sulphur increase in weight 
and absorb large volumes of air when they are burnt, and he obtained similar results 
with lead and mercury in closed vessels. A. L. Lavoisier pursued the subject 
further in a Memoire sur la calcination de F Stain dans les vaisseaux fermcs, et sur la 
cause de F augmentation de poids qu'acquiert ce metal pendant cette operation (1774). 
He found that the vessel containing the air and tin did not increase in weight, 
although part of the air was absorbed. When the flask was opened, air rushed in, 
and the increase in the weight of the vessel was found to be equal to the increase in 
weight which the tin alone had suffered. Hence, A. L. Lavoisier concluded, with 
J. Rey, that the increase in the weight of the tin was solely due to an absorption 
of the air in which the calcination had occurred. There was not sufficient air in 
the flask to saturate all the tin, and yet some air always remained as a residue. 
Hence, A. L. Lavoisier concluded further that only part of the air can combine 
with the metal during the calcination ; he also found that the increase in the 
weight of the tin during calcination is equal to the decrease in the weight of the air. 
Hence, it seems as if air contains at least two constituents, only one of which is 
absorbed by the heated metal. This inference was tested by an important ex- 
periment described in his Traite elementaire de chimie (Paris, 1789). 

The mercury was confined in a glass retort with an S-shaped neck which dipped under 
a bell-jar in a trough of mercury, as illustrated in Fig. 7. The air in the retort was in 

commimication with the air in the bell- jar. The level of 
the mercury in the bell-jar was adjusted at a convenient 
level, and its position " very carefully marked with a strip 
of gummed paper." By means of a charcoal furnace, the 
mercury in the retort was heated- — not quite to its boiling 
point (357°). A. L. Lavoisier said : " Nothing of note 
occurred during the first day. The second day I saw little 
red particles swimming over the surface of the mercury, and 
these increased in number and volume during four or five 
days ; they then stopped increasing and remained in the 
same condition. At the expiration of twelve days, seeing 
that the calcination of the mercury made no further pro- 
gress, I put the fire out." The red particles were identified 
with the calx of mercury now called red oxide of mercury, 
or mercuric oxide, and then called mercurius calcinatus 
per se. 

After making allowance for variations of tempera- 
ture and pressure, A. L. Lavoisier found that when 
mercury was calcined with a given volume of air in a 
closed vessel, 50 cubic inches of air were reduced to 
, . , between 42 and 43 cubic inches ; the difference, 7 to 8 
Fig. 7— a. L. Lavoisier s ^^^-^ inches, that is, one-fifth or one-sixth of the total 
Experiment on the Com bus- , f ,i • i i i i ,i , 

tion of Air. volume 01 the air, was absorbed by the mercury, formmg 

the red calx of mercury. The air which' remained 
in the retort was not absorbed by the excess of hot mercury ; it was rather 
less dense than ordinary air ; it extinguished the flame of a burning candle 
immersed in the gas ; and a mouse was quickly suffocated when placed 
in the gas. Hence, A. L. Lavoisier first called the gas moufette atmospherique, and 


later azote, " from the a privative of the Greeks, and ^a>^, life." In France, the 
gas is still called azote, though in Britain it is called nitrogen. 

By collecting the red mercury calx, and re-heating it in a suitable retort 
(probably to 400°), Lavoisier obtained between seven and eight cubic inches of a gas 
which had been previously removed from the air by the hot mercury. The gas was 
exactly analogous in properties with the dephlogisticated air, discovered on August 
1st, 1774, by Joseph Priestley, by heating mercurius calcinatus per se by means of a 
burning lens. When a burning candle was immersed in the gas, the candle burnt 
with eclat eblouissant—hlmdmg brilliancy — as Lavoisier expressed it ; a smoulder- 
ing splinter of wood burst into flame when plunged in the gas ; and the gas did not 
suffocate a mouse like azote. A. L. Lavoisier first called this gas Vair eminemment 
respirable, pur, ou vital, and afterwards oxygen. The latter term is its present-day 
designation. In this manner, A. L. Lavoisier proved that atmospheric air is 
made up of two gases— oxygen and nitrogen — of different and even opposite 
natures, the oxygen alone combines with the metal during calcination, and is the 
cause of the increase in weight. 

A. L. Lavoisier further showed that the sum of the weights of the mercury and 
oxygen obtained by heating mercury calx is exactly equal to the weight of the 
calx ; and that the increase in the weight of the mercury in the formation of the 
calx is equal to the weight of the oxygen taken from the air. In his Reflexions 
sur le phlogistique (Paris, 1783) A. L. Lavoisier said that during the combustion of 
phosphorus in oxygen gas (vital air) : 

There is a total absorption of vital air, or rather of oxygen, in the combustion of phos- 
phorus, and the weight of the phosphoric acid obtained is found to be rigoroasly equal 
to the weight of the phosphorus, added to that of the vital air employed in the combustion. 
The same agreement of weights is observed in the combustion of inflammable air, in the 
combustion of charcoal, etc. 

Hence, the mechanism of combustion according to A. L. Lavoisier is : Metal -|- Oxygen 
= Metal calx, and not, as G. E. Stahl supposed to be the case : Metal - Phlogiston 
= Metal calx. The phenomenon which occurs when oxygen unites with a metal 
to form a calx is called oxidation, and the resulting calx is called an oxide. 
A. L. Lavoisier thus showed that it is not the calces that are simple and the metals 
compound, but just the reverse; so that the phlogistians have therefore been 
said to have perceived the relations between these two classes of bodies upside 
down. In all reductions with charcoal, said A. L. Lavoisier, fixed air is obtained 
owing to the union of the charcoal with the pure portion of the air — oxygen 
which was fixed in the calx during the oxidation of the metal. 

If I take a metallic calx and heat it with carbon in a closed vessel, at the moment the 
calx is reduced to the metallic state— at the moment, for example, when litharge, the calx 
of lead, is changed into metallic lead, there reappears the air, which had become fixed 
when the metallic lead had been made into a calx, and an aerial product — fixed air— -can 
be collected at least a thousand times more bulky than the solid litharge employed, inia 
experiment appears to be one of the most interesting which has been made smce the time 
of Stahl. 

Assuming that this interpretation of the experiments is correct, A. L. If vo^^ier 
inferred that by mixing azote and oxygen in the right proportions, it ought to be 
possible to reproduce atmospheric air. This A. L. Lavoisier did, and the mixture 
was found to behave with respect to '' combustion, respiration, and the calcination 
of metals similar in every respect to atmospheric air." Lavoisier similarly showed 
that if the calcination of the metal is attended by the union of the vital air--oxygen 
—of the atmosphere with the metal, then, when the calcination is ettected in an 
inverted glass vessel containing vital air, the whole should be absorbed, inis 
deduction was " proved by weight and measure." . . , , 

According to the phlogistians, a lighted candle burns because it is a compound oi 
candle-matter and phlogiston. The compound is decomposed little by iittie, as 


the candle burns from tip to base, and the phlogiston passes into the surrounding 
atmosphere. A. L. Lavoisier inverted Vancienne hypothese. He supposed the 
hydrogen and carbon of the candle, during the burning, to unite with the oxygen 
of the air to form the oxides of carbon and of hydrogen ; and generally, when a 
substance is burned, it does not give out an imaginary levitative phlogiston, but 
rather takes in real gravitative oxygen. 

In his Mhnoire sur la combustion en general (Paris, 1777), A. L. Lavoisier alto- 
gether rejected the principle of combustion advocated by G. E. Stahl, and argued 
that his own hypothesis " seemed to be more probable, more conformable with the 
laws of nature, and to involve less strained explanations and fewer contradictions " 
than the doctrine of G. E. Stahl. About ten years later, A. L. Lavoisier collected 
and organized such an array of facts in defence of his proposition that he was able 
to write with much greater confidence in his Reflexions sur le fhlogistique (Paris, 
1783), and he claimed the phlogistic doctrine to be an error fatal to the progress of 
chemistry : 

If in chemistry everything can be satisfactorily explained without the aid of phlogiston, 
it thereby becomes eminently probable that phlogiston does not exist, that it is a hypo- 
thetical being, a gratuitoiis assumption. 

It is easier to make new discoveries than to eliminate old prejudices. Chemists 
were painfully slow to recognize the part played by air in combustion and calcina- 
tion. In his Reflexions sur le pJilogistique (Paris, 1783), A. L. Lavoisier said : 

Chemists have turned phlogiston into a vague principle, one not rigorously defined, 
and which consequently adapts itself to all the explanations for which it might be required. 
Sometimes this principle has weight, sometimes not ; sometimes it is free fire, sometimes it 
is fire combined with the earthy element ; sometimes it passes through the pores of vessels ; 
sometimes the vessels are impervious to it ; it explains both causticity and non-causticity, 
transparency and opacity, colours and their absence ; it is a veritable Protean, changing 
in form each instant. 

A. F. de Fourcroy began to teach A. L. Lavoisier's theory in 1787 ; C. L. Ber- 
thollet joined the new cause about the same time. Then followed L. B. Guy ton de 
Morveau, and nearly all the French and British chemists. The Berlin Academy 
abandoned phlogiston in 1792, and the controversy which had waged for some 
years between the phlogistians was virtually at an end.i The downfall of phlogiston, 
a relic of Egyptian and Chaldean lore, was celebrated by Madame Lavoisier, 
habited as a Greek priestess, burning the writings of G. E. Stahl upon an altar 
dedicated to the new positive science. At the beginning of the new century a few- 
petrified spirits, unable to march to the music of the new chemistry, still lingered 
behind. Robert Boyle's admonition in his Considerations touching experimental 
essays in general (1661), may have been forgotten : 

It ought to be esteemed much less disgraceful to quit an error for a truth than to be 
guilty of the vanity and perverseness of believing a thing still because we once believed 
it. . . . Until a man is sure he is infallible it is not fit for him to be unalterable. 

The observed facts were sterile and barren before they were vivified by the 
fire of Lavoisier's genius. Indeed, enthusiasts have said that chemistry as a science 
was not born until A. L. Lavoisier's theory of burning had been demonstrated. 
Many writers — e.g. S. Brown (1858) 2— have emphasized that tradition and 
prejudice were all against A. L. Lavoisier, and however much he owed to his pre- 
decessors and contemporaries — J. Rey, P. Bayen, J. Priestley, H. Cavendish, and 
C. W. Scheele — he scarcely owed them one glimmering ray of thought — rather 
the reverse. The legacies of fact inherited by A. L. Lavoisier were beclouded and 
distorted by the false hypotheses through which their discoverers saw them, and 
it required a master mind to co-ordinate the facts accumulated by many workers 
into one system. We can feel with A. Wurtz when — following A. F. de Fourcroy 
(1797) — he opened his Histoire des doctrines chimiques (Paris, 1869) thus : La chimie 


est la science francaise, ellefut constituee par Lavoisier d' immortelle tnemoire; other- 
wise expressed, chemistry is a French science, it was founded by Lavoisier of 
immortal memory. This statement seems to have needlessly irritated some of 
our own historical writers. Can we wonder that Frenchmen are proud of their 
Lavoisier ^ Surely '' we can amiably pass without protest this ardent hero- 

At first sight, it does seem curious that such a long period of time should have 
been required to work from P. E. de Sultzbach's note in 1489 to the effect that 
metals increase in weight when calcined in air, to A. L. Lavoisier's proof in 1774 
that the increase in weight is due to the absorption of oxygen from the air. This 
will occasion no surprise when we remember the difference between the properties 
of air which cannot be seen, and the properties of solids and liquids which can be 
readily seen and handled. As G. F. Rodwell has emphasized, the most obvious 
property of matter is its visibility, and the conception of matter divested of this 
quality is no small effort to a mind untutored in invisible bodies, which exercise no 
apparent effect on surrounding objects, and it belongs to an advanced order of experi- 
mental philosophy. There were no means of recognizing even the more salient 
properties of air at the disposal of the chemists until a comparatively late period, 
and the earlier chemists, accordingly, believed air to be intrinsically different in its 
essence from more familiar visible substances. To illustrate the ideas about air 
which prevailed at the end of the eighteenth century, the opening words of A. L. 
Lavoisier's Memoire sur la nature du principe qui se combine avec les metaux pendant 
leur calcination et qui en augmente le poids (Paris, 1775) may be quoted : 

Do different kinds of air exist ? Is it enough that a body should be permanently ex- 
panded for it to be considered a particular kind of air ? Are the different airs found in 
nature or formed by us specific substances, or are they modifications of atmospheric air ? 

Again, altogether apart from the skill required in the manipulation of gases, 
it is not at all surprising that writers on chemistry in the Middle Ages failed to 
interpret the mechanism of the burning of a candle in air when the knowledge 
required to explain the chemical side of the phenomenon is recalled : 

(i) Air is composed of two gases both sparingly soluble in water ; (ii) During combiistion 
one of the gases unites and the other does not imite with the burning body ; (iii) Air 
contains four volumes of the inert gas, and one volume of the gas which unites with the 
burning body ; (iv) A gas soluble in water is produced during the combustion ; and (v) 
The increase in weight of the combustible body during the burning is equal to the decrease 
in the weight of the air. 

The phlogiston hypothesis is sometimes held up to ridicule. It must be borne 
in mind that the hypothesis was adopted by nearly all the leading chemists in the 
earlier part of the eighteenth century when it appeared to be as firmly fixed among 
the root principles of chemistry as the kinetic theory does to-day. Thus, the ardent 
and devoted J. Priestley could say : 

If any opinion in all the modern doctrine concerning air be weU-founded, it is certamly 
this, that nitrous air is highly charged with phlogiston. If I have completely ascertamea 
anything at all relating to air, it is this ; 

and the diplomatic P. J. Macquer, in his Mernents de chymie pratique (Paris, 1751), that 

We cannot say how phlogiston is fixed by substances ; but without pretending to gueM 
the cause of the phenomenon, let us rest contented with the certamty of the tact. 

The phlogiston theory represented the most perfect generalization known to the 
best intellects of its day, and J. J. Becher and G. E. Stahl were the prophets of a 
new mode of viewing chemical mutations. The doctrine served to give coherence 
to the thoughts and work of a race of chemists extendmg from J.J. l^echer ana 
G. E. Stahl down to H. Cavendish, J. Priestley, and C. W. Scheele. 

The phlogistic hypothesis enabled chemistry to escape m part from mystic and 


mediaeval empiricism,^ for it introduced a certain amount of order among a chaotic 
mass of facts. Like phenomena were grouped together, and chemistry thrived and 
multiplied its proportions while under its sway. Phlogiston prepared the way 
for A. L. Lavoisier's balance, just as the balance heralded J. Dalton's arithmetic. 
There is what A. Comte * called la hi de succession running through history. The 
early struggles of man in quest of knowledge and truth were not in vain. The 
sun-worshipper and the phlogistian, each in his own way, had been working to a 
common end. All generations — past and future — thus seem to be linked in one 
common service. 

It is inconceivable that men like T. Bergmann, H. Cavendish, J. Priestley, and 
C. W. Scheele would counsel what they considered to be an inconsistent doctrine. 
Phlogiston was regarded by them, not as a temporary hypothesis, but as a permanent 
acquisition, an enduring conquest of truth. To-day the word is but an empty 

It mxist be added that H. St. C. Deville (1860), C. Brown (1866), and W. Odling (1871) ^ 
have pointed out that phlogiston occupied a similar position in the chemistry of the eighteenth 
century that potential energy does to-day. Said Deville : On arrive a admeUre que Vajjinite 
(en intensite) n'est pas autre chose que la quantite de chaleur latente ou phlogistique enfermee 
dans les corps. Even A. L. Lavoisier, in his Traite eUmentaire de chimie (Paris, 1. 60, 1793), 
considered oxygen to be made up of caloric and the matter of oxygen. Lavoisier's caloric 
• — a veritable ghost of phlogiston- — was supposed to be the matter of heat possessing no 
weight whatever. Ordinary oxygen thus contained the principle of oxygen plus caloric. 
The latter has also been identified with potential energy. Here then the old revives in the 
new. The chemistry of to-day is not materialistic, for it is concerned with both energy 
and matter. 

Theories perish, facts remain. — Much of what we think best in the theories of 
to-day may to-morrow be rejected, with phlogiston, worthless. There is no reason 
to suppose that fewer errors are believed to-day than in the days when phlogiston 
reigned supreme ; and it is not at all improbable that posterity will smile at our in- 
explicable ignorance in some departments of science. This need cause the student 
no embarrassment. A fallacious theory may be a valuable guide to experiment. 
Experiment and labour applied to the explication of the most extravagant hypo- 
thesis are not always lost. Guided by wrong hypotheses, men haves ought one thing 
and found another ; Columbus sought the Indies and found America. W. Whewell 
has pointed out that when a theory, which has been received on good evidence, 
appears to fail, the really essential and vital part of it survives the fall, that which 
has been discovered continues to be true. It is necessary, however, to follow 
Rene Descartes advice : Give unqualified assent to no proposition which is not 
presented to the mind so clearly that there is no room for doubt. As Aristotle 
would have said, we do not need to cultivate the art of doubting, but rather the art 
•of doubting well ; for the art of doubting well is the necessary antecedent of 
progress. Doubt is the parent of inquiry. 

It is not always expedient to follow the history of each hypothesis and each 
conquest of truth, step by step, as in the case of air. That of course would be an 
ideal plan of work ; but it is not always a waste of time to study the exploded fallacies 
once cherished by the potentates of old. The right attitude of mind towards an 
hypothesis or law can be developed only when history has taught how man has 
had to climb with slow faltering steps until he obtained a clear view of each new 
principle of chemical science. J. W. Goethe was quite right : The history of a science 
is the science itself : The past is key to the present ; although, as A. Comte (1839) 
expressed it : On ne connait pas complctement une science tant qu'on n'en sail que 
Vhistoire. Unfortunately, time cannot always be spared to wander with the original 
investigators into the byways of knowledge, and a more economical plan must 
usually be followed. If every one had to pass through all the stages traversed by 
all who have gone before, it would be impossible to reach the vantage ground gained 
by the labours of his predecessors. 



1 J. Priestley, Considerations on the doctrine of phlogiston and the decomposition of loater, 
London, 1796 ; R. Kirwan, An essay on phlogiston and the constitution of acids, London, 1789. 

2 S. Brown, Essays, Scientific and Literary, Edinburgh, 1. 186, 1858 ; P. Duhem, La chimie est- 
elk une science fran^aise ? Paris, 1916 ; R. Jagnaux, Histoire de la chimie, Paris, 1891. 

* R. Lote, Les origines mystiques de la science allemande, Paris, 91, 1913 ; F. le Dantec, Rev. 
Scient., 51. 740. 1913. 

* A. Comte, Cotirs de philosophic positive, Paris, 1839 ; J. H. Bridges, Essays, Ix)ndon, 1907. 

6 W. Odling, Proc. Boy. Inst., 6. 323, 1871 ; A. C. Brown, Proc. Roy. Soc. Edin., 5. 328, 1806 ; 
H. St. C. Deville, Conipt. Rend., 50. 534, 584, 1860 ; Lemons sur la dissociation, Paris, 1860. 


§ 1. What is an Element ? 

The elements count as simple substances not because we know that they are so, but 
because we do not know that they are not. — J. von Liebig (1857). 

A. L. Lavoisier showed that atmospheric air is no more an elementary principle 
than the water of the ocean, for it can be resolved into two simpler gases — oxygen 
and nitrogen. It is further possible to resolve all known substances — air, water, 
etc. — into about eighty distinct elemental or primitive forms of matter. The 
present-day concept of an element is one of those ideas which has gradually grown 
into chemistry. Epicurus, about 300 B.C., held that corporeal things are either 
composite, or else they are the constituent parts of which the composite things are 
compounded ; and that the continued division of the composite must at last 
furnish ultimate, indivisible, unchangeable particles of the elements. Aristotle, in 
his De coelo (3. 3), also defined an element. He said : 

Everything is either an element or composed of elements. . . . An element is that into 
which other bodies can be resolved, and which exists in them either potentially or actually, 
but which cannot itself be resolved into anything simpler, or different in kind. 

This precise and accurate concept was soon beclouded with the idea that all the 
different varieties of matter observed in nature are composed of a primitive element 
with varying proportions of wetness or dryness, or of coldness or hotness. This 
quaternary of attributes gradually materialized into earth, water, air, and fire. 
All the different forms of matter were vaguely supposed to have been compounded 
in some inscrutable manner from varying proportions of this quartet. 

In 1661, Robert Boyle's attention was arrested by the loose way in which the 
term element was employed, and in his The Sceptical Chymist (Oxford, 1661), 
he gave a clear concept for an element. He said : 

I mean by elements, as those chy mists that speak plainest do by their principles, certain 
primitive and simple, or perfectly unmingled bodies ; which not being made of any other 
bodies, or of one another, are the ingredients of which all those called perfectly mixt bodies 
are immediately compounded, and into which they are ultimately resolved. ... I must 
not look upon any body as a true principle or element, which is not perfectly homogeneous, 
but is further resolvable into any niimber of distinct substances. 

N. le Febvre,! whom J. B. Dumas called riiomme d' imagination, flourished in 
the seventeenth century about the time of Robert Boyle. N. le Febvre showed 
that Empedocles' analysis of wood into four elements — flame or fire, smoke or air, 
moisture or water, and ashes or earth— does not include all the principles of which 
this form of matter is compounded. By the destructive distillation of wood, 
he found that water charged with acetic acid and an oily inflammable liquid 
condensed in the receiver ; a gas escaped ; and charcoal remained. The charcoal 
burnt in air, giving fire and ashes ; and the ashes were resolved by water into a 
soluble salt, and an insoluble earth. N. le Febvre thus resolved wood into six 
ingredients, and he got very near to recognizing that the only proof of an elementary 
principle is the fact of its yielding nothing else to analysis. He maintained that 
chemistry is not the doctrine of the four elements, an art of transmutation, or a 

• 74 


science of mixts ; but is rather the art of analysis with a view to discover la con- 
nmssance de toutes les choses que Dieu a tirees du chaos par la creation—a knowledge 
of all the ingredients of all the various kinds of matter which God has created out 
of chaos. 

Even as late as the latter part of the eighteenth century, fire, air, water, and 
earth were regarded as elemental. Thus, P. J. Macquer, in his Dictionnaire de 
chymie (Pans, 2. 4, 1778), gave a juste definition of an element, and added : 

Although fire, air, water, and earth are reputed to be simple, it is possible that they 
are not so ; they may be very complex, and may result from the union of several other more 
simple substances . . . but as experience teaches us absolutely nothing on this subject, we 
may consider without inconvenience, and indeed in chemistry we ought to consider fire, 
air, water, and earth as le8 corps simples, because they really act as such in all chemicaJ 

We are also indebted to A. L. Lavoisier (1789) for further clarifying the concept 
of an element. A. L. Lavoisier, quite logically, considered lime, magnesia, baryta, 
and alumina to be elements. We now know that these elements of A. L. Lavoisier 
are compounds of oxygen with calcium, magnesium, barium, and aluminium 
respectively. This was not known to Lavoisier, and he rightly said : '* We are 
certainly authorized to consider them simple bodies until, by new discoveries, their 
constituent elements have been ascertained." Again, in 1811, the question whether 
chlorine^ — ^then called oxymuriatic gas — was really an element or a compound of 
oxygen with some other element was raised by Humphry Davy. H. Davy claimed 
that chlorine is an element because, although oxygen was believed to be present, 
none could be found. " Hence," added H. Davy, " we have no more right to say 
that oxymuriatic gas (i.e. chlorine) contains oxygen than to say that tin contains 
hydrogen. . . . Until a body is decomposed, it should be considered simple." 

It is not possible to improve upon Lavoisier's conception of a chemical element, 
and I feel compelled to quote his words, although written before 1789 : 2 

When we apply the term elements or principles to bodies to express our idea of the leist 
point which analysis is capable of reaching, we must admit, as elements, all substances 
into which we are able to reduce bodies by decomposition. Not that we are entitled to 
affirm that these substances which we consider as simple, may not themselves be compoimded 
of two, or even of a greater number of more simple principles ; but since these principles 
cannot be separated, or rather, since we have not hitherto discovered the means of separat- 
ing them, they are, with regard to us, as simple substances, and we ought never to suppose 
them compo\uided until experiment and observation have proved them to be so. 

The definition of an element is not founded upon any intrinsic property of the 
elements, but rather upon the limited resources of the chemist. To find if a given 
substance is an element or a compound, it is usual to assume that it is a compound 
and then to apply all known methods for resolving compounds into simple sub- 
stances. If the methods fail to effect a decomposition, the substance is said to be 
an element. Hence, the statement that any given substance is an element has 
been said to be a confession of the impotence of human powers. In fine, element 
is a conventional term employed to represent the limit of present-day methods of 
analysis or decomposition. ^ We may, therefore, summarize these ideas in the 
definition : An element is a substance which, so far as we know, contains only 
one kind o£ matter. To say the substances we call elements caw/<of be decomposed 
may be regarded as an unwarranted reflection on the powers of our successors. 
The moment Auer von Welsbach (1885) proved that didymium was a mixture of 
praseodymium and neodymium, one element ceased to exist, and two elements 
were born. If it were found to-morrow that the element chlorine is really a com- 
pound of two new elements previously unknown, the fact would be important 
and it would change the face of chemistry, but it would not render useless any facts 
we know about chlorine. . . , 

The old alchemists sought to transform some of the common metals mto go d. 
Whenever the attempt has been made with materials known to be free from gold, 


no transmutation has been observed. There is nothing intrinsically absurd in the 
notion, but at present, no authentic transmutation has been deliberately, or rather 
intentionally, accomplished. Works like P. J. von Lewinheim Sachs' Aurum 
chymicum (Genevae, 1702) and K. C. Schmeider's Geschichte der Alchemie (Halle, 
1832) professed to examine critically the authenticity of the legendary reports by 
the alchemists of the reality of the transmutation of the metals, and concluded that 
in some cases the legends are above suspicion, and this in spite of the fact that 
H. von Osten, in his Eine grosse Herzstdrkung fiir die Chymisten (Berlin, 1771), had 
exposed some forty-five tricks and deceptions practised by alchemical knavery. 
All the reports now stand discredited. K. C. Schmeider criticized the legends 
imperfectly, and failed to recognize that fictions may be plausible as well as 
extravagant. When the evidence has permitted a critical examination, every re- 
corded instance has been traced to a mal-observation ; and evidence which cannot 
be tested is outside the range of scientific methods. In the words of J. M. Wilson 
(1917), in science, there is no statement of fundamental importance that depends 
on history or on any testimony which cannot be verified. 

The next inquiry arises from the question : What relations subsist between 
the weights and volumes of the different elements which make up the different 
kinds of matter known to man ? 


1 N. le Febvre, Traicte de la chymie, Paris, 1660 ; London, 1664 ; J, B. A. Dumas, LeQons sur la 
philosophie chimique, Paris, 51, 1837 ; S. Brown, Lectures on the Atomic Theory, Edinburgh, 9, 

2 A. L. Lavoisier, Traite elementaire de chimie, Paris, 1789 ; H. Davy, Phil. Trans., 98. 39, 
1808 ; 99. 450, 1809 ; 100. 231, 1810 ; 101. 1, 1811. 

* H. Spencer, Essays, London, 3. 234, 1891 ; Justus Liebicfs und Friedrich Wohler's Brief- 
wechsel in dem Jahren 1829-1873, Braunschweig, 2. 43, 1888. 

§ 2. The Law of Constant Composition— Proust's Law 

Nature in her inscrutable wisdom has set limits which she never oversteps. — Jean Rey. 

The proportion in which one element can unite with another is fixed by nature, and the 
power of augmenting or diminishing this pondua naturce is not given to man.' — J. L. Proust 

Attention must now be directed to the singular observation made by Jean Eey 
(1630) that during the calcination of a metal in air, " the weight of the metal 
increased from the beginning to the end, but when the metal is saturated, it can 
take up no more air. Do not continue the calcination in this hope : you' would 
lose your labour." The examples previously quoted — Cap. I, Table I — have shown 
that one gram, and only one gram, of air is absorbed by definite amounts of the given 
metals under the conditions of the experiment, and Lavoisier's work proves that 
the oxygen of the air is alone absorbed. Accordingly, one part by weight of oxygen 
is equivalent to 1-52 parts magnesium ; 4-06 of zinc ; 1*12 of aluminium ; 3-97 of 
copper ; and 3" 72 of tin. Instead of taking the weight of oxygen unity, it will be 
more in accord with general usage to make oxygen 8. Hence, multiply the 
preceding numbers by 8 : 

Oxygen. Magnesium. Zinc. Aluminium. Copper. Tin. 
8 1216 32-48 8-96 3196 29*76 

When magnesium is calcined in the presence of oxygen, or air, the metal always 
unites with the oxygen in the proportion of one part of oxygen per 1*52 parts of 
magnesium, or 8 parts by weight of oxygen per 12-16 parts by weight of magnesium. 
The same principle obtains when magnesium oxide is made in several dilierent ways ; 
and likewise with the other metallic oxides. The proportions in which two elements 
unite together do not vary in a fortuitous manner, but in fixed and definite 


proportions. Hence, as P. J. Hartog 1 puts it : two like portions of matter 
have the same composition. The converse of this statement is not necessarily 
true, for two portions of matter compounded from the same proportions of the 
same elements are not necessarily alike. 

The exact work of J. S. Stas 2 and T. W. Richards and many others has firmly 
established the constancy of the composition of the regular type of chemical com- 
pounds. J. S. Stas, in his famous Recherches sur les lots des proportions chimiqites 
(1860-65), for example, studied among other things, the composition of silver 
chloride prepared by four different processes at different temperatures. He found 
that 100 parts of silver furnished 132-8425, 132'8475, 132-8480 parts of silver 
chloride ; and that neither the temperature nor the method of preparation had any 
influence on the composition of the chloride. The difference between the two 
extremes is less than 0-006 part per 100 parts of silver. This shows that the errors, 
incidental to all experimental work, are here remarkably small. J. S. Stas likewise 
proved that ammonium chloride prepared from quite different sources, and purified 
in different ways, always contains exactly the same proportion of chlorine. Still 
further, he proved that the combining weight of an element is not affected in the slightest 
degree hy the various elements with which it might combine. For example, silver com- 
bines with iodine to form the iodide, and with iodine and oxygen to form the iodate. 
The ratio of silver to iodine in both compounds is the same, and is not modified 
by the large quantity of oxygen present in the iodate. Hence, J. S. Stas stated : 
" If the recognized constancy of stable chemical compounds needed further de- 
monstration, I consider the almost absolute identity of my results has now com- 
pletely proved it." 

The law o£ constant proportions, however, can never be proved with mathe- 
matical exactness. However skilful a chemist may be, it is impossible to make an 
exact measurement without committing an error of observation or an error of 
experiment. It is assumed that the small difference O'OOS per cent, between the 
two extreme results of J. S. Stas (1) is wholly due to the unavoidable errors of 
experiment, for we cannot expect an exact solution of the problem ; and (2) is 
not due to a very slight inexactitude in the law of constant proportions. In 1860, 
J. C. G. de Marignac considered that the experiment did not prove definitely 
that the composition of compounds might not vary within very minute 
limits : 

I do not consider that it has been absolutely demonstrated that chemical compounds 
do not normally have an excess of one of the constituents. It is true that this excess is 
very minute, but it is still appreciable in very delicate measurements. 

The composition of a definite compound appears to be independent of its mode 
of formation, and therefore it is inferred that substances always combine in definite 
proportions. If an excess of one substance be present, the amount in excess remains 
uncombined as extraneous matter. This deduction from the observed facts is 
called the law of definite proportions, or the law of constant composition. The law 
is sometimes enunciated : a particular chemical compound always contains the same 
elements united together in the same proportions — hy mass. This statement, if inter- 
preted literally, holds good for a particular mixture, as well as for a particular com- 
pound ; and it has nothing to say as to the distinction between a mixture and a 
compound. Probably no generalization in chemistry is more firmly established than 
that like compounds possess the same quantitative composition. So great is the faith 
of chemists in the truth of this generalization that a few accurate and careful 
experiments are considered sufficient to settle, once for all, the composition of a sub 
stance. For instance, if a substance possessing all the properties of magnesium 
oxide be given to a chemist, without taking any more trouble, he knows that it 
will contain 12-16 parts of magnesium for every eight parts of oxygen. The law 
of constant composition furnishes a kind of a priori control over quantitative 
analysis. Constancy in composition is regarded as a proof of purity, and purity 


is attended by constancy in composition. Hence arose the concept of a chemical 
compound as distinct from a mixture. ^ 


1 P. J. Hartog, Nature'bO. 149, 1894 ; B. A. Rep., 618, 1894. 

* J. S. Stas, (Euvres completes^ Bruxellos, 1894 ; T. W. Richards, Experimenlelh Unler- 
auchungen uher Atomgewichte, Hamburg, 1909. 
» E. J. Mills, Phil. Mag., (4), 40. 259, 1870. 

§ 3. History o! the Law of Constant Composition 

Ce n*est que du conflit des opinions contraires que jaillit la veritc'-. — ¥. Hoefer (1843). 

The law of constant composition was not discovered by any particular man, 
but it gradually grew among the doctrines of chemistry. The law was tacitly 
accepted by many before it was overtly enunciated. This is shown by J. Rey's 
views (1630), previously stated. In 1699, G. Homberg,^ in his Observations sur la 
quantite d'acides absorbes par les alcalis terreux, described measurements of the 
amounts of different acids (vinegar, spirits of salt, aqua fortis, and vitriolic acid) 
required to saturate a given amount of potassium carbonate (salt of tartar) ; he 
evaporated the saturated liquid to dryness and weighed the resulting solid. His 
results were compiled in the form of a table which has been regarded as embodying 
the first hint of the law of definite proportions. G. Homberg considered that the 
quantity of ^n acid which unites with an alkali is la mesure de la force passive de eel 
alcali, and further added that by la force des acides he means the solvent action of 
the acid, and similarly for the alkalies. F. Hoefer (1843) suggests translating 
G. Homberg's " solvent action " by " neutralizing," and " solubility " by " neu- 
tralizable." Isaac Newton referred to the saturation capacity of acids for different 
metals in the thirty-first query of his Opticks (London, 1704). G. E. Stahl also in 
his Fundamenta chymim (Norimbergse, 1723) spoke of the pondus naturcp. as the 
proportions which ought to exist between the masses of the different ingredients 
in order that a determinate compound be produced. In 1717, E. F. Geoffrey 
analyzed saltpetre and stated its quantitative composition. A. S. Marggraf 
(1749) ; H. T. Scheffer (1750) ; T. Bergmann (1775-84) ; R. Kirwan (1790-1800) ; 
J. Black (1794) ; M. H. Klaproth (1795) ; V. Rose (1803-5) ; C. F. Bucholz 
(1799-1802) ; and L. N. Vauquelin (1812) all based analyses of chemical com- 
pounds on the tacit assumption that this law is valid ; and W. Higgins' theory of 
atoms (1789) implies that chemical compounds must have a constant composition. 
A. L. Lavoisier appears to have had no doubts on the subject. In every oxide, said 
he, the relation of oxygen to the metal is constant. 

In 1767, H. Cavendish said that those quantities of bases — e.g. potash and 
lime — are equivalent which neutralize the same amount of acid ; and, in 1788, 
he showed that this equivalency is independent of the nature of the acid. C. F. 
Wenzel (1777) had a fairly clear idea that a definite weight of a base neutralized 
a definite amount of a given acid, and in his Lehre von der Venvandschafi der Korper 
(Dresden, 1777), he gave measurements of the weights of over twenty metals and 
bases which were required to saturate about a dozen acids ; and he also examined 
quantitatively the products of some reactions — e.g. copper sulphate and lead acetate ; 
mercuric sulphide and silver chloride ; etc. Shortly after C. F. Wenzel's book had 
appeared, J. B. Richter, in an important study of this subject, published evidence 
in his Ueber die neueren Gegenstdnde der Chemie (Breslau, 1791-1802), and in his 
Anfangsgrunde der Stocky ometrie oder Messkunst chymischer Elemente (Breslau, 1792- 
4), which demonstrated conclusively that the weights of the various acids which 
neutralize certain fixed weights of the bases are the same ; and the same 
numbers hold good for the neutralization of all acids by the bases ; otherwise 
expressed : Acids and alkalies unite in constant proportions to form salts- this 


is Richter's law of proportionality, or Richter's law of equivalent ratios. Conse- 
quently, it is possible to assign equivalent numbers to the acids and bases. For 
instance, using modern data and terms : 





. 3505 
. 37-06 
. 40-01 
. 56-00 

Hydrofluoric acid 
Hydrochloric acid 
Sulphuric acid 
Nitric acid . 

. 2001 
. 36-47 
. 49-04 
. 63-02 

Ammonium hydroxide . 
Calcium hydroxide 
Sodium hydroxide. 
Potassium hydroxide 

J. B. Kichter gave separate tables for the neutralization equivalents of each acid 
and each base ; but Gr. E. Fischer, in an appendix to his German translation of 
C. L. Berthollet's Recherches sur les his de Vaffinite, showed that J. B. Richter's 
data could be reduced to a single table containing twenty-one numbers divided into 
two columns as just indicated. These tables can be regarded as the first tables of 
equivalent weights published. The weights of the acids in one column represent 
the amounts required to neutralize the quantity of any of the bases indicated in 
the other column ; and conversely, the weights of the bases in the second colunm 
represent the amounts required to neutralize the quantity of any one of the acids 
indicated in the first column. Thus 56 grams of potassium hydroxide will neutralize 
20"01 grams of hydrofluoric acid, 36'47 grams of hydrochloric acid, 49*04 grams of 
sulphuric acid, 63*02 grams of nitric acid, etc., and 63"02 grams of nitric acid will 
neutralize 35'05 grams of ammonium hydroxide, 37*06 grams of calcium hydroxide, 
etc. Richter claimed that the rule he gave is a true touchstone — Probierstein — for 
the proportions wherewith the acids and bases neutralize one another, because if 
the observed numbers do not agree with those demanded by the rule, they may 
be regarded as erroneous. 

J. B. Richter mixed up much valuable work with several fantastic hypotheses ; 
he supposed that the weights of the bases required to neutralize a constant weight 
of acid are in arithmetical progression ; and the weights of the acids required to 
neutralize a constant weight of any base are in geometrical progression. Richter 
appears to have cooked some of his results to make them fit his erroneous hypo- 
thesis so that the numbers represent what he thinks he ought to have obtained 
rather than what he actually observed. Such a procedure is quite antagonistic to 
the spirit of science, and made chemists reasonably sceptical about the accuracy 
of the whole of Richter's work. It was thought, wrongly as it happens, falsus in 
uno, falsus in omnibus (false in one, false in all). Consequently, the above 
generalization did not attract the attention it deserved. On reading J. B. Richter's 
work on chemical ratios, said J. J. Berzelius (1827), we are astonished that the 
further study of the subject could ever have been neglected. 

It must be added that the validity of the law of definite composition was the 
subject of an interesting controversy during the years between 1800 and 1808. 
J. L. Proust 2 maintained that constant composition is the invariable rule ; C. L. 
Berthollet did not assert that cases of constant composition are non-existent, but 
he argued that these instances were due to special circumstances, and maintained 
that constant composition is the exception, variable composition the rule. J. L. 
Proust's words are worth quoting : 

According to my view, a compound is a privileged product to which nature has assigned 
a fixed composition. Nature never produces a compound even through the agency ot 
man, other than balance in hand, pondere et messura. Between pole and pole compounds 
are identical in composition. Their appearance may vary owing to their manner ot aggre- 
gation, but their properties never. No differences have yet been observed between tne 
oxides of iron from the South, and those from the North ; the cmnabar of Japan has tne 
same composition as the cinnabar of Spain ; silver chloride is identically tlie same whetner 
obtained from Peru or from Siberia ; in all the world there is but one sodnini chloride ; 
one saltpetre ; one calcium sulphate ; and one barium sulphate. Analysis confirms tnese 
facts at every step, 

It might be thought that positive assertions of this kind, backed by accurate 


experimental work, would leave no subject for disputation ; but, surveying the 
battlefield in the light of the present-day knowledge, it seems that another quite 
different phenomena was confused with the law of constant composition ; and the 
methods of analysis were not very precise. Some, probably from the unfounded 
belief that " Proust deservedly annihilated Berthollet," call the generalization 
discussed in this chapter, Proust's law. The arguments against the law of constant 
composition was silenced not by J. L. Proust, but by the work which developed 
from J. Dalton's atomic theory. 3 J. L. Proust did not satisfactorily answer all 
C. L. Berthollet's objections. 

According to C. Daubeny (1850) it has been stated that something hke the theory of 
constant composition can be found among the dogmas of the old sage Pythagoras (c. 520). 
This philosopher is sometimes supposed to have derived what is the most valuable part of 
his philosophy from the Egyptian priests during his sojourn in Egypt. Pythagoras taught 
that number — whatever was meant by that term — is a bond sustaining by its power the 
permanent existence of everything on the earth. The influence of Pythagoras has been 
traced in the doctrine laid down by Philo the Jew— or who ever wrote the apocryphal 
book of wisdom— God ordained all things by measure, number, and weight. It is, however, 
certain that European chemistry is in no way indebted to the Egyptian priesthood or to 
the Pythagorean philosophy for the concept of the law of constant composition. It would 
indeed require the exercise of a good deal of ingenuity to disentangle the law of chemical 
combination from the conflicting statements which have been made as to the meaning to 
be attached to Pythagoras' doctrine of numbers. 

F. Wald (1895-9) ^ argues that the composition of chemical compounds is 
variable, and that the observed constancy in the composition of chemical com- 
pounds must be attributed to the selection by chemists of special preparations. 
Hence, says F. Wald, the statement of the law of constant composition is quite 
empirical, and the assumption that these selected preparations are alone true com- 
poimds is quite arbitrary. 


1 G. Homberg, Mim. Acad., 64, 1700. 

2 J. L. Proust, Ann. Chim. Phys., { 1), 32. 26, 1799 ; Journ. Phys., 53. 89, 1801 ; 55. 325, 1802 ; 
59. 260. 265, 321, 350, 1 804 ; 60. 347, 1805 ; 63. 421, 1806 ; C. L. Berthollet, Pecherches sur les lois de 
Vaffinite., Paris, 1801 ; Essai de statique chimique, Paris, 1803 ; Journ. Phys., 60. 284, 347, 1805 ; 
61. 352, 1805. 

3 A. N. Meldrum, Mem. Proc. Manchester, Lit. Phil. Soc.,5^. 7, 1910 ; C. Daubeny, An Intro- 
duction to the Atomic Theory, London, 1850. 

4 F. Wald, Zeit. phys. Chem., 17. 337, 1895 ; 19. 607, 1896 ; 22. 253, 1897 ; 23. 78, 1897 ; 24. 
315, 634, 1897 ; 25. 525, 1898 ; 26. 77, 1898; 28. 13, 1899 ; Chem. Ztg., 30. 963, 978, 1906 ; 31. 756, 
769, 1907 ; 0. Kuhn, ib., 31, 688, 1907 ; 32. 55, 1908 ; E. Bauer, Zeil. anorg. Chem., 50. 199, 
1906 ; C. Benedicks, ih., 49. 284, 1906 ; L. Henry, Bvll. Acad. Belgique, 975, 1904 ; S. Cannizzaro, 
Rend. Soc. Chim. Roma, 2. 128, 1904; R. Nasini, Rend. Accad. Lincei, (5), 5. 119, 1905; 
L. Duhem, Le mixte et la comhinaison chimique, Paris, 1902 ; W. Ostwald, The Fundamental 
Principles of Chemistry, London, 1909 ; Journ. Chem Soc., 85. 506, 1904. 

§ 4. Pure Substances 

Pure water is never found in nature. One may oven say that no man has over seen or 
handled absolutely pure water. It is an ideal substance, to which some specimens of 
highly purified water have nearly approached. — M. M. P. Mum. 

It is only in " tall talk " or in advertisements that any human preparation, elementary 
or not, can be spoken of as chemisch rein. — P. G. Tait (1881). 

The substance we call water has its own properties, but sea-water, spring-water, 
rain-water, and distilled water show certain differences in their properties. The 
differences, however, are not due to the water, but to the substances — impurities — 
which the water has dissolved from its surroundings. If sea-water be distilled, the 
'* impurities " — sodium chloride, magnesium chloride, etc. — remain behind. Sea- 
water is therefore a homogeneous substance, but, rightly or wrongly, it is often 
stated to be a mixture, because water and various salts can be separated by simple 


evaporation or by freezing. Table salt is more or less impure sodium chloride 
The presence of a little magnesium chloride in table salt makes the salt more 
hygroscopic, so that the contaminated table salt deliquesces more readily than if 
magnesium chloride were absent. 

The term deliquescence— from deliqnescere, to melt or dissolve —refers to the process 
of absorbmg moisture from the air so that a salt becomes moist, or even dissolves in the 
moisture it has absorbed from the air ; e.g. when potassium carbonate is exposed to the 
atmosphere it rapidly gains in weight. The term hygroscopic— from Sypos, wel^refers 
to the absorption or adsorption of moisture from the atmosphere. Most substances 
particularly when powdered, are hygroscopic, even if they do not deliquesce. The term 

efflorescence from efflorescere, to blossom, refers to the formation of a crust generally 

white — on the surface of a body. The phenomenon is very often due to the loss of water 
from the surface of certain crystalline salts ; e.g. when crj^stals of washing soda are exposed 
to a dry atmosphere, they gradually lose weight. 

Air is a mixture of oxygen and nitrogen, with a little carbon dioxide, and it is 
habitually moist owing to the presence of a varying proportion of water vapour. 
In a chapter contained in J. B. Porta's Magice naturaUs (Naples, 1589), on the 
extraction of water from air, it is shown that if a large glass flask be filled with a 
mixture of ice and nitre, water condenses from the air to the outer walls of the 
vessel, and trickles down into a basin below as receiver. Isaac Newton i said that 
potassium carbonate deliquesces in air because of an attraction between the salt 
and the particles of moisture in the atmosphere, and asked : Why does not common 
salt or nitre deliquesce in the same way except for want of such an attraction ? 
In H. B. de Saussure's Essais sur Vhydrometrie (Neuchatel, 1783) there is an 
excellent study of the moisture which is normally present in atmospheric air. He 
exposed " equal quantities of salt of tartar, quicklime, wood, lime, etc., all dried 
as perfectly as possible," to the same air, and found that they " imbibed water 
and increased in weight in unequal quantities." The salt of tartar took more than 
the lime, and the lime more than the wood. H. B. de Saussure said that " these 
differences can only proceed from the different degrees of the affinity of these bodies 
for water," and he called this affinity, the hygroscopic affinity of the bodies for the 
vapour, so that the amount of vapour imbibed by different substances from the air 
" is proportional to their affinity for water vapour." H. B. de Saussure also showed 
that the thirst or the attractive force of the body for aqueous vapour diminishes 
from moment to moment " in proportion as it drinks the vapour," otherwise 
expressed, the hygroscopic activity of the body diminishes in proportion as it 
approaches the point of saturation. 

Lavoisier's experiments on the transformation of water into earth.— A com- 
pound may be contaminated with impurities in many ways — from the raw materials 
used in preparing the compound ; from the vessels in which it was prepared or 
stored ; by exposure to the atmosphere ; by the partial decomposition of the 
substance when exposed to light, etc. It was once believed that air can be condensed 
to water, as was thought to be proved by the falling dew ; and that water can be 
changed into an earth, as is evidenced by the residue obtained when rain-water or 
distilled water is evaporated to dryness in glass vessels. Thus, 0. Borrichius in 
his Hernietis, Mgyjptorum et cheynicorum sapientia (Hafnia?, 1674), said that " when 
100 pounds of snow, hail, or of rain-water, are evaporated, the water is transformed 
into a dusty earth which contains some common salt ; " R. Boyle ^ found on 
distilling and re-distilling pure rain-water, time and again, in glass vessels, a white 
powdery substance was obtained each time the water was evaporated ; and the 
more the water distilled from a given glass vessel, the larger the amount of whit€ 
powder. He added that a friend, of unsuspected credit, had distilled water two 
hundred times " without finding the liquor grow weary of affording the white earth." 
It seemed to him as if water " might be very nigh totally brought into earth, since 
out of an ounce of distilled rain-water he had already obtained nearly three-quarters 
of an ounce, if not more, of the often-mentioned earth." A. L. Lavoisier 8 first 

VOL. 1. ^ 


traced the true source of this earth. In his paper Sur la nature de Veau et sur les 
experiences par lesquelles on a pretendu prouver la possihilite de son changement en 
terre (1770), A. L. Lavoisier described experiments with the object of " settling by 
decisive experiments whether water can be changed into earth as was thought by 
the old philosophers, and still is thought by some chemists of the day." By heating 
water in hermetically sealed glass vessels, after some days, the water became turbid 
and little white specks separated from the water and floated about. The hermeti- 
cally sealed glass vessels were weighed before and after the experiment ; it was 
proved : (1) The earth does not come from outside the vessel, because the weight 
of the vessel and its contents does not alter. This is against Boyle's hypothesis 
that fine igneous particles are able to pass through the glass, and are precipitated 
in the form of a white powder when they come in contact with water. 

Consequently, I conclude that nothing can pass through the pores of the glass, and these 
little white particles, be they caused by what they may, are not caused by igneous particles 
passing through the glass. 

Still further, it was shown (2) The earth does not come from the water, because 
the weight of the water remains the same before and after the experiment ; (3) The 
earth comes from the vessel, because the vessel loses in weight ; and (4) The earth 
comes wholly from the vessel, because the loss in weight of the vessel is virtually 
equal to the weight of the earth formed. Hence, adds Lavoisier, " it follows from 
these experiments that the greater part, possibly the whole of the earth separated 
from rain-water by evaporation, is due to the solution of the vessels in which the 
water has been collected and evaporated." C. W. Scheele (1777) ^ deduced a similar 
conclusion from other experiments. He analyzed tlie earth produced during the 
evaporation of water in glass vessels and showed that it has a similar composition 
to the stuff of which the vessel was made. 

K. F. von Walther (1915) has an interesting experiment to demonstrate the solubility 
of glass in water, 500 c.c. of water are placed in a common litre flask with sufficient alizarine 
to produce a pale yellow colour, the colour changes to a reddish -violet owing to the dis- 
solution of alkali from the glass. By adding dilute sulphuric acid from a burette, the colour 
changes back to pale yellow when the alkali is neutralized. He found that after an hour's 
boiling, alkali equivalent to 18*3 c.c. of centinormal sulphuric acid had been dissolved from 
a glass vessel. 

The purity of commercial compounds. — The term pure or cJiemically pure, is 
unfortunately used when it is desired to emphasize the fact that the substance has 
not sufficient impurity to influence appreciably the most exact work for which it 
is to be employed. There cannot be degrees of purity. A thing is either pure or 
impure. It may be convenient to use terms like highly pure, all but pure, very 
impure, etc., but the term, chemically pure, in the sense of nearly pure, is objection- 
able. This recalls Basil Valentine's statement that water exists in three degrees 
of excellence — ^the pure, the purer, and the purest ! The labels on commercial 
reagents with their pare, purissimum, and chemically pure, are almost equivalent. 
F. Mylius * proposed distinguishing degrees of purity as of the first, second, and 
sixth grades according as they contain one part of total impurity in 10, 10^, . . . 
10* parts. 

The terms reagents and chemicals are applied to the substances used in chemistry 
for producing special reactions with other substances. The former term is more 
particularly used in analytical work. Chemically pure substances, paradoxical as 
it may seem, are sold with a statement on the labels indicating what impurities are 
present as well as how much of each. Commercial reagents, on the other hand, 
have not been specially purified, and hence are sold at a cheaper rate than the 
chemically pure substances. Purification is an expensive operation, and the cheaper 
commercial reagents are used whenever specially purified materials are not required. 
Some hold that " perfectly pure substances are unknown." This is possible, but to 
establish the proposition, we should be involved in a metaphysical discussion, and 
\^e might be led to say with A. Laurent : " Chemistry is the science of substances 


which do not exist," or perhaps with G. W. F. Hegel : " Pure being is pure 

Positive and negative evidence. — One positive proof may demonstrate an 
indefinite number of negatives. Thus, if a test proves that a given substance is 
silver chloride, it at the same time proves that the substance is not metallic copper, 
arsenic oxide, etc. On the other hand, inability to prove a direct negative is not 
to be regarded as equivalent to a positive proof. Thus, let it be asserted that a 
third substance, say moisture, must be present when two substances interact chemi- 
cally. Against this, it can be shown that substances like mercury and chlorine do 
react when most carefully purified and dried ; but it could be, and has been argued 
that this circumstance is due to the presence of an unrecognized impurity. Similarly, 
some argue that if the elements could be obtained absolutely free from unknown 
impurities their atomic weights would be whole numbers. Negative arguments of 
this type are invulnerable in controversies because they cannot be controverted by 
proofs to the contrary. True, the most skilful workers with the most refined 
instruments cannot find an impurity, but still, it can be asserted that better equipped 
searchers might be more successful. This might, however, does not prove the thesis 
in question. Nevertheless, the argument is often used. For instance, H. Davy's 
quest for oxygen in chlorine ; T. Bergmann's proof of the individuality of nickel ; 
W. Ostwald's statement that catalytic agents can change only the velocity of 
existing reactions ; etc. 

The effect of traces of impurity on the properties of a compound. — It may be 
well to emphasize, just here, that sometimes a minute trace of impurity is of vital 
importance. Some reactions proceed quite differently in the presence, and in the 
absence of traces of moisture or maybe other impurities. The properties of many 
substances, too, are modified in a remarkable manner by small traces of impurity. 
H. Vivian says that f^th part of antimony will convert the best selected copper 
into the worst conceivable ; Lord Kelvin, that the presence of ^^^th part of 
bismuth in copper would reduce its electrical conductivity so as to be fatal to the 
success of the submarine cable ; H. le Chatelier, that the absorption of a quite 
imperceptible weight of gas changes the melting-point of highly purified silver nearly 
30° ; G. le Bon (1900) that the presence of ,-Woo*^ part of mercury in magnesium 
makes the metal decompose water at ordinary temperatures ; and W. C. Roberts 
Austin, that ^t]i part of bismuth in gold will render gold useless from the point 
of view of coinage, because the metal would crumble under pressure in the die. 
J. F. W. Herschel (1851) considered the fact that such minute proportions of 
extraneous matter should be found capable of communicating sensible properties 
of a definite character to the bodies with which they are mixed, to be perhaps 
one of the most extraordinary facts that has appeared in chemistry. 


^ I. Newton, Opticks, London, 1704. . . , 

2 R. Boyle, The origin of forms and qualities, Oxford, 1666 ; A. L. Lavoisier, Mem. Acad^ 
73, 107, 1770. ^^ , ___ 

3 C. W. Scheele, Chemische Abhandlungen von der Luft uvd dem Feuer, Upsala, 1777. 

* F. Mylius, Zeit. Elektrochem., 23. 152, 1917; T. S. Hunt, Amer. Journ Science (i), Jb. i n>, 
226, 1853 ; (2), 16. 203, 1854; R. F. von Walther, Journ. prakt. Ckem., (2), 91, 33A l^i^- 

§ 5. Physical and Chemical Changes. 

Most of the substances belonging to our globe are constantly undergoing aJtor«^»^" jj^ 
sensible quantities, and one variety becomes as it were transmuted mto another &uc 
changes, whether natural or artificial, whether slowly or r^P^^i^y Pf ^°"^*';*; ,*rj„Xa 
chemical. Thus, the gradual and almost imperceptible decay of the leaves and branches 
of a fallen tree exposed to the atmosphere, and the rapid combustion of v^ood in our hres, 
are both chemical operations.— H. Davy. , 

The early chemists did not clearly distinguish between uniform mixtures and 
homogeneous compounds so that many substances now known to be mechanical 


mixtures were classed with substances known to be homogeneous compounds ; 
again, owing to the fact that they were seldom able to prepare compounds of a high 
degree of purity, the properties of compounds seemed more or less variable. Even 
so late as the end of the seventeenth century, chemists were not all clear that 
substances could be obtained with fixed and invariable properties. The properties 
of a substance are those qualities, or attributes, by which its nature is manifested. 
About 1730, H. Boerhaave distilled mercury five hundred times with the idea of 
finding if its properties thereby suffered any change. About this time, it was 
recognized that a homogeneous pure substance always has the same properties and 
behaves in the same way, when the conditions are the same ; and generally, that 
one element or compound is distinguished from all other elements or compounds 
in possessing certain specific and characteristic properties ; or, in the words of an 
old alchemist : " God hath sealed each substance with a particular idea." 

First and foremost, a chemical compound has a fixed and definite composition ; 
then again, a compound or element usually melts and boils at definite temperatures ; 
its specific gravity, specific heat, crystalline form, colour, odour, behaviour when 
in contact with other substances, etc., are characteristic of one particular chemical 
compound. When the melting-point of, say, pure silver chloride has been once 
accurately determined, it follows that all other samples of pure silver chloride will 
melt at the same temperature under the same conditions. Many changes in the 
properties of matter are not immediately perceptible to- the senses ; and in the 
majority of cases, the processes for the identification and difierentiation of the 
different forms of matter are based upon their behaviour towards certain reagents. 
The more salient characteristic properties of an element or compound are employed 
for its identification — that is, for distinguishing it from other known elements or 
compounds. Thus, a student would be probably correct in stating that a solution 
contained a silver compound if it gave a white precipitate when acidified with 
hydrochloric acid, and the precipitate was insoluble in hot water, and soluble in 
aqueous ammonia ; and if the spectrum of a burning body has a yellow line in a 
particular part, the presence of sodium would be inferred. About 1661, Robert 
Boyle 1 noticed many examples of the use of chemical reagents for the detection 
or identification of certain substances, and in 1780, T. Bergmann collected together 
a number of reagents useful for detecting the commoner elements or acids, and 
described the effects produced. M. H. Klaproth, L. N. Vauquelin, J. J. Berzelius, 
F. Wohler, H. Rose, C. R. Fresenius, and others built upon this foundation the 
present system of qualitative analysis. ^ 

Physical changes. — -When liquid water becomes ice or steam there is no change 
in the chemical nature of the substance, for the matter which makes steam and ice 
is the same in kind as that of liquid water. A substance can generally change its 
state, as when liquid water becomes steam or ice. The idea is further emphasized 
by the fact that in most cases a substance is called by the same name, whether it 
be in a solid, liquid, or gaseous state of aggregation. For instance, we speak of 
liquid oxygen, liquid air, tnolten silver chloride, etc. Again, matter may change 
its volwne by expansion or contraction ; it may change its texture, as when a porous 
solid is fused to a vitreous mass ; it may change its magnetic qualities, as when a 
piece of soft iron in contact with a magnet attracts other pieces of iron, etc. It is 
conventionally agreed to say that in none of these cases of physical change is there 
any evidence of the formation of a new substance ; and that the matter does not 
lose or change those properties which distinguish it from other forms of matter. 
A physical change involves an alteration in the properties of a substance without 
the formation of a new substance. 

Chemical changes.— When magnesium metal is heated in air, a white powder 
is formed, and when mercuric oxide is similarly treated, mercury and oxygen are 
obtained. The action of heat in both cases furnishes forms of matter with very 
different specific properties from those forms of matter employed at the start. A 
chemical change involves the formation of a fresh substance with different 


specific properties from the original substance or substances. In both chemical 
and physical changes the total weight of matter before and after the change remains 
constant, but in chemical changes alone the kind of matter alters. 

It is not always easy to distinguish between physical and chemical changes, 
because the only real distinction between the two turns on the question : is there 
any evidence of the formation of a new substance during the change ? The evidence, 
as we shall soon see, is not always conclusive. When red mercuric iodide is heated 
above 126° it turns yellow, and the red colour is resumed on cooling. Two chemical 
changes are involved, because the new substance produced on heating the iodide 
re-forms the original compound on cooling. So, when water is heated, complex 
aggregates of simple particles are riven asunder to again coalesce or associate to- 
gether on cooling. To the physicist, with his attention fixed on the temperature, 
or volume, the heating of water is a physical process ; to the chemist, with his 
attention on the nature of the constituent particles, it is a chemical process, because 
when heated the particles of water become less and less complex as the temperature 
rises. What we call a body, said E. Mach, is a complex of properties which affects 
the senses in different ways. . . . One or more properties of the complex are altered 
in a physical change, while in a chemical change, the whole complex is affected.3 

The distinction between chemical and physical changes is a subject for the end, 
not the beginning of chemistry. It is remarkable that the first principles of a 
science are really the most difficult to grasp, because, said J. F. Ferrier, that 
which is first in the order of nature, is last in the order of knowledge : 

The apotheosis and final triumph of the human reason will be, when, having traversed 
the whole cycle of thought, she returns- — ^enriched only with a deeper insight and clearer 
consciousness — to be merged in the glorious innocence of her primitive and inspired 


1 Robert Boyle, The Sceptical Chymist, Oxford, 1661 ; Experiments and Observations on 
Colours, London, 1663 ; T. Bergmann, De minerarum docimasia humida, Holmise, 1780. 

2 M. H. Klaproth, Beitrdge zur chemischen Kenntniss der Mineralkorper, Freiberg, 1795 ; L. N. 
Vauquelin, Scherer's Journ., 3. 410, 1799 ; J. J. Berzelius, De Vanalyse des corps inorganiques, 
Paris, 1827 ; H, Rose, Handbuch der analytischen Chemie, Berlin, 1829 ; F. Wohler, Praktische 
Uebungen in der chemischen Analyse, Gottingen, 1862 ; C. R. Fresenius, A nleitung zur qualitativen 
chemischen Analyse, Bonn, 1841. 

^ P. V. Wells, Journ. Washington Acad., 9. 361, 1919; I. La,ngmmr, Journ. Anier Chem.Soc., 
39, 1848, 1917 ; L. Gurwitsch, Zeit. phys. Chem., 87. 323, 1914; E. J. Mills, Phil. Mag., (5), 1. 
1, 1876; J. F. Ferrier, Institutes of Metaphysics, London, 12, 1854. 

§ 6. Compounds and Mixtures 

The common operations of chemistry give rise in almost every instance to producta 
which bear no resemblance to the material employed. Nothing can be so false as to expect 
that the qualities of the elements shall be discoverable, in an altered form, in the com- 
pound.' — W. Whewell (1840). 

In his De generatione et corruptione, Aristotle regarded the difference between 
what we call to-day physical and chemical mixtures, as dependent on the distinction 
between what is potential and what is actual. Aristotle recognized a form of com- 
bination—now called physical mixture— in which the elements were supposed to 
exist actually ; and another— chemical combination— in which the elements were 
supposed to exist potentially— e.^. the elements oxygen and hydrogen exist actually 
as such in a free state, but "'in water they exist potentially, for they can be educed 
and become actual onlv by the destruction of the water or of that special form 
which in water they actually possessed.i Consequently it may be said what is 
actually one substance may be potentially another. In a mere mixture said Aris- 
totle, you have only mixture, juxtaposition or o-uVl^co-i? ; but m chemical combina- 
tion you have a mingling or fxC^a where the elements disappear as such, but they 


still remain potentially. This kind of combination — chemical combination — is 
defined very well by Aristotle as " the unification of mingled elements that have 
changed their nature as elements." 

1. The constituents of a compound are combined in definite proportions. — 
The law of constant proportions is of fundamental importance in forming a con- 
ception of the meaning of the term " chemical compound." If a substance produced 
in different ways be not constant in composition, it is not considered to be a chemical 
compound, but rather a mixture. R. Bunsen (1846), for example, showed that the 
proportion of oxygen to nitrogen in atmospheric air is not constant, because the 
ox}^gen varies from 20'97 to 20'84 per cent, by volume, by methods of measurement 
with an error not exceeding 003 per cent. Hence, the oxygen and nitrogen in 
atmospheric air are said to be simply mixed together, and not combined chemically. 
The so-called eutectic mixtures and cryohydrates show that substances with a definite 
composition are not always chemical compounds. 

2. Compounds are homogeneous, mixtures are usually heterogeneous. — It is 
comparatively easy to detect particles of sugar and sand in a mixture of the two ; 
and a simple inspection of a piece of Cornish granite will show it is a mixture of at 
least four constituents — silvery flakes of mica ; black patches of schorl ; whitish 
crystals of felspar ; and clear glassy crystals of quartz. Although the particles of 
felspar, mica, schorl, and quartz differ from one another in size and shape, no 
essential difference can be detected in the composition and properties of different 
samples of pure quartz, felspar, mica, and schorl. Hence, it is inferred that the 
sample of granite is a mixture of schorl, felspar, quartz, and mica ; and that each 
of these minerals is a true chemical compound. Very frequently the constituents 
of a mixture are too small to be distinguished by simple inspection, and the body 
appears homogeneous. A microscopic examination may reveal the heterogeneous 
character of the substance. Blood and milk, for instance, appear to be homogeneous 
fluids, but under the microscope the former appears as a colourless fluid with red 
corpuscles in suspension ; and milk appears as a transparent liquid containing 
innumerable white globules (fat). Naturally, too, the stronger the magnification, 
the greater the probability of detecting whether the body is homogeneous or not. 
Sometimes the microscope fails to detect non-homogeneity under conditions where 
other tests indicate heterogeneity. 

Before constant composition can be accepted as a proof of chemical combination, 
it must also be shown that the substance is homogeneous. Chemical individuals 
are homogeneous. A homogeneous substance is one in which every part has 
exactly the same composition and properties as every other part. A substance 
may have a fixed and constant composition and yet not be homogeneous — e.g. 
cryohydrates and eutectic mixtures to be described later. A substance may be 
homogeneous, for all we can tell to the contrary, and yet not have a constant 
composition — e.g. atmospheric air ; a solution of sugar in water, etc. This simply 
means that all chemical compounds are homogeneous, but all homogeneous sub- 
stances are not chemical compounds. Indeed, it is sometimes quite impossible 
to tell by any single test whether a given substance is a mixture or a true chemical 
compound. It is therefore not satisfactory to classify matter into (i) homogeneous 
bodies (meaning elements and chemical compounds), and (ii) mixtures, because some 
mixtures would have to be included with homogeneous bodies. It might also be 
added that the term substance is used in chemistry in two ways : It is employed as 
a synonym for body or matter, and also for a specific form of matter which is 
chemically homogeneous. 2 

3. The constituents of a mixture can usually be separated by mechanical 
processes. — The properties of a mixture of finely powdered iron and sulphur have 
been used in chemical text-books from the beginning of the nineteenth century in 
order to illustrate the difference between mixtures and compounds. It would be 
difficult to find a better example. If a mixture containing, say, 6 grams of iron 
and 4 grams of sulphur be rubbed in a mortar, (1) the colour of the mixture is 


intermediate between the colour of the iron and of the sulphur ; (2) the particles of 
iron and sulphur can be readily distinguished under the microscope ; (3) most of 
the iron can be removed without dijSiculty by means of a magnet ; and (4) the two 
constituents can be separated quite readily by washing the mixture on a dry filter 
paper by means of carbon disulphide. The sulphur dissolves in the carbon disulphide ; 
and the former can be recovered by evaporating the carbon disulphide from the 
filtered solution. Sulphur remains behind as a crystalline residue. The metallic 
iron remains on the filter paper. Here then the constituents of the mixture have 
been separated by the mechanical processes— (1) magnetting, and (2) the action of 

In 1826, J. J. Berzelius published analyses of the precipitate obtained when hydrogen 
sulphide is passed into a slightly acid solution of a salt of tellurous acid, and these showed 
that the proportions of sulphur and telluriiun satisfied the law of constant composition, 
and hence J. J. Berzelius inferred that a true chemical compound — tellurium sulphide — 
was formed. Accordingly, tellurium sulphide — with its method of formation and a de- 
scription of its chemical and physical properties — was regularly described in chemical 
literature. This sulphide is now considered to be a myth, because half a century lat«r, 
F. Becker (1876) discovered that when the material was digested with carbon disulphide, 
the sulphur dissolved and tellurium remained imdissolved. Hence it was inferred that 
Berzelius' sulphide is not a chemical individual, but a mixture of siilphur and tellurium 
in constant proportions. The assumption is of course made that the carbon disulphide 
does not decompose the precipitate. 

It is generally stated that " a solution of sugar or of salt in water is a mechanical 
mixture because, though homogeneous, the salt or sugar can be recovered unchanged 
from the water by the mechanical process of evaporation." This is an unwarranted 
assumption. The salt and water may have combined, and the product of the 
chemical combination may be decomposed into salt and water during the process 
of evaporation. The intervention of a solvent sometimes decomposes a compound 
into its constituents, or conversely, causes the constituents of a mixture to 
combine in such a manner as to produce compounds which previously did not 

The so-called mechanical processes of separation usually include: (1) Magnetting, hand- 
picking, sieving, etc. (2) Elutriation, or treatment with water flowing at different speeds 
such that the lighter particles are carried off by the slower streams, and the heavier particles 
by the faster streams. Settling and lixiviation are modifications of this type of separation. 

(3) Flotation, or fractional levigation. If some mixtures be placed in liquids of the right 
specific gravity, the lighter constituents will float and the heavier constituents will sink ; 
and if some mixtures be treated with oils, etc., the oil so affects the particles of some 
substances that they are buoyed up in liquids where otherwise they would sink — such 
substances can be separated in this way from other substances not so affected by the oil. 

(4) Fractional solution, or crystallization, depend on differences in the solubility of the 
constituents in suitable solvents. (5) Distillation, evaporation, freezing, liquation, melting, 
diffusion, cupellation, etc. 

4. A mixture usually possesses the common specific properties of its consti- 
tuents ; the properties of a compound are usually characteristic of itself alone. — 

The properties of a mixture are nearly always additive, i.e. the resultant of the 
properties of the constituents of the mixture. For instance, a mixture of equal 
parts of a white and black powder will be grey, whereas sodium metal and greenish- 
yellow chlorine gas give a white pulverulent compound — common salt. 

Specific gravity is a number which expresses how much heavier a given substance 
is than an equal volume of a standard substance (say water at 4°) taken at a standard 
temperature and pressure. In the case of gases, either air=unity, oxygen = 16, hydrogen 
= 1, or hydrogen = 2 is taken as standard ; and in the case of liquids and solids, water at 
+ 4°, or at 0°, is taken as unity. The great value of specific gravity data lies in the fact 
that specific gravity is a number which enables volume meastirements to be convertea into 
weights, and weight measurements to be converted into volumes, for weight = specific gravity 
X volume. Specific gravity may thus be regarded as the weight of unit volume if water 
=unity be taken as a standard, and the weights are reckoned in grams, and volumes in 
cubic centimetres. There is no need here to elaborate distinctions between density ana 


specific gravity. The density is the mass of unit volume, so that if D, m, andt; respectively 
denote the density, mass, and volxime of a substance, D =mjv. 

The specific gravity of a mixture of equal volumes of two substances of specific 
gravity 3 and 5 will be 4, because if one c.c. of water weighs one gram, there will 
be a mixture of 05 c.c. weighing I'S gram of one substance ; 0*5 c.c. of the other 
substance weighing 2*5 grams ; and l-54-2'5=4 grams per c.c. It must be added 
that the specific gravities of compounds are not necessarily a mean of the specific 
gravities of their components ; indeed, if elements mix without change in volume 
that fact alone is strong presumptive evidence that a compound has not been formed. 
It must be added, too, that a small contraction would not be considered a sufficient 
proof of chemical action because liquid chlorine and bromine contract a little when 
mixed together, and this reaches a maximum — 2 per cent. — when the mixture 
corresponds approximately with the atomic proportions Br -f- CI. The specific 
gravity of compounds may be greater or less than the average specific gravity of 
their constituents. This shows that the force which causes compounds to 
unite chemically is not an attractive force independent of the nature of the 
combining sub.stances. Hence, although this force is sometimes called chemical 
attraction, the term is used metaphorically. Some properties of compounds 
— like weight — are additive, for they are the sum of the properties of their 

Examples. — (1) What is the specific gravity of air containing a mixture of one volume 
of nitrogen when the specific gravity of oxygen is 16, and the specific gravity of nitrogen 
14-01 ? One-fifth volume of oxygen weighs 3*2 units, and four-fifths volume of nitrogen 
weighs 11*2 luiits. Hence, one volume of the mixture will weigh 14*4 units. 

(2) Ozonized air- — ^a mixture of air and ozone — has a specific gravity 1-3698, and it 
contains 13-84 per cent, by weight of air, specific gravity unity, and 86*16 per cent, of 
ozone. What is the specific gravity of ozone ? Here 13-84 grams of air occupy 13-84 -^ 1 
volumes ; and 86-16 grams of ozone occupy 86-16-i-ic volumes, where x denotes the specific 
gravity of ozone. Hence, 100 grams of ozonized air occupy 100-^1*3698 = 73 volumes; 
and 73-00 = (86-16-i-a;)+ 13-84; ora;=l-46. 

The law of mixtures may be stated in symbolic form. If a mixture of two 
substances contains x fractional parts of a substance of specific gravity ^j, it will 
contain 1 — x fractional parts of the other substance of specific gravity Sg- Then 
if S be the specific gravity of the mixture, xsi-[-{\—x)s2=8. 

Example. — Lord Rayleigh and W. Ramsay (1895) found that a mixture of argon and 
nitrogen had a specific gravity 2-3102 (air unity), and the specific gravity of nitrogen alone 
is 2-2990 ; what is the specific gravity of argon if the mixture contained 1-04 per cent, of 
argon? Here a; = 0-0 104; 1— a;=0-9896; «2=2-2990; >S' = 2-3102. By substituting these data 
in the above expression, 2-2990 + (2-3102— 2-2990) -hO-0104=Si, or the specific gravity of 
argon (air unity), is Si= 3-376. 

If a portion of the mixture of finely divided sulphur and iron be placed in a 
hard glass test-tube and warmed over Bunsen's flame, the contents of the tube 
begin to glow and a kind of combustion spreads throughout the whole mass. When 
cold, break the test-tube, and note that (1) the porous black mass formed during 
the action is quite different from the original mixture ; (2) the microscope shows 
that the powdered mass is homogeneous ; (3) it is not magnetic like iron (provided 
the iron was not in excess); and (4) it gives up no sulphur when digested with carbon 
disulphide (provided the sulphur was not in excess). These facts lead to the assump • 
tion that there has been a chemical reaction between the sulphur and the iron. 
When chemical combination occurs, the reacting constituents appear to lose their 
individuality or identity more or less completely, and each neiv substance which is 
formed has its own distinctive j)roperlies. 

5. Thermal, actinic (light), or electrical phenomena usually occur during 
chemical changes. Attention must be directed to the fact that a great deal of heat 
was developed during the combustion of the iron and sulphur. The heat required 
to start the reaction does not account for the amount of heat developed during the 


reaction. This point is perhaps better emphasized by placing an intimate mixture 
of powdered sulphur and zinc on a stone slab. After the flame of a Bunsen's burner 
has been allowed to play on a portion of the mixture for a short time to start the 
reaction, the zinc and sulphur combine with almost explosive violence. Large 
amounts of heat and light are developed during the reaction. 

If a plate of commercial zinc be placed in dilute sulphuric acid, bubbles of gas 
are copiously evolved, and if a thermometer be placed in the vessel, the rise of 
temperature shows that heat is generated during the chemical action. If the zinc 
be pure, very little, if any, gas is developed. It makes no difference if a plate of 
platinum be dipped in the same vessel as the zinc, provided the plates are not 
allowed to come into contact with one another. If the two plates are connected 
by a piece of copper wire, a rapid stream of gas bubbles arise from the surface of 
the platinum plate, and some gas also comes from the zinc plate. The platinum 
is not attacked by the acid in any way, but the zinc is rapidly dissolved. If a 
voltmeter and shunt or an electric bell be interposed in the circuit between the two 
plates, the deflection of the needle or the ringing of the bell will show that an electric 
current passes from the platinum to the zinc. The electric current is generated by 
the chemical reaction between the zinc and the acid, which results in the formation 
of zinc sulphate and a gas. The action will continue until all the acid or the zinc 
is used up. 

For convenience, the zinc plate of the cell B is conventionally called the positive plate 
and is often represented by a short thick line, and the platinum plate is likewise called the 
negative plate and is represented by a longer thinner line as illustrated by the plan, Fig. 1. 
Here G represents the voltmeter or galvanometer and shunt. The vessel of acid with ita 
two plates is called a voltaic cell, and this particular combination can be symbolized : 

. Platinum | Dilute sulphuric acid | Zinc 
The voltaic cell originally used by A. Volta (1800) had copper in place of platinum. 

The chemical reaction just indicated is far from being the most economical mode 
of generating electricity, but all the different forms of voltaic cell on the market 
agree in this : Electricity is generated during chemical action. 

The development of heat, light, or electrification are the usual concomitants of 
chemical action. The absence of such phenomena when substances are simply 
mixed together is usually taken as one sign that chemical action has not taken 
place. When nitrogen and oxygen are mixed together in suitable proportions to 
make atmospheric air, there is no sign of chemical action, and this fact is sometimes 
cited among the proofs that air is a mixture. The argument is not conclusive 
because the condensation of steam and the freezing of water are usually cited as 
physical changes although heat is evolved during both transformations. 

The tests for distinguishing chemical compounds from mixtures involve answers 
to the following questions : (1) Is the substance homogeneous ? (2) Are the 
different constituents united in definite and constant proportions ? (3) Are the 
properties of the substance additive 1 (4) Were thermal, actinic, or electrical 
phenomena developed when the substance was compounded 1 (5) Can the con- 
stituents be separated by mechanical processes 1 The list does not necessarily 
exhaust the available tests, but in spite of what we know, there is sometimes a 
fingering doubt whether a particular substance is a mixture or a true chemical 
compound. This arises from the fact that some of the tests are impracticable, 
others are indecisive. Owing to our ignorance, it is not always easy to state " the 
truth and nothing but the truth." As P. J. Hartog 3 has emphasized, C. L. Berthol- 
let repeatedly asked J. L. Proust to furnish an experimental distinction between 
chemical compounds and mixtures, but without success. Even to-day, there is 
no experimental method of generally distinguishing the two. The usual definition 
is a theoretical distinction based on molecules, but one can also be adapted from 
the phase rule (q.v.). 



» F. W. Bain, On the Realization of the Possible, London, 171, 1899. 

* W. Ostwald, The Fundamental Principles of Chemistry, London, 1909 ; Natural Philosophy, 
London, 1911. 

» P. J. Hartog, Nature, 50. 149, 1894 ; B. A. Rep., 618, 1894. 

§ 7. Circumstantial and Cumulative Evidence 

To find the truth is a matter of luck, the full value of which is only realized when we 
can prove that what we have found is true. Unfortunately, the certainty of our knowledge 
is at so low a level that all we can do is to follow al(^ng the lines of greatest probability. — 
J. J. Berzelius. 

Suppose a substance is suspected to be a chemical compound because it appears 
to be homogeneous ; on investigation, we find that it has a fixed definite com- 
position. This verifies our first suspicion, and the joint testimony gives a very 
much more probable conclusion than either alone. By piling up the evidence in 
this manner, for or against our suspicion, we can make a chain of circumstantial 
evidence which enables a highly probable conclusion to be drawn. Each bit of 
evidence by itself is not of much value, but all the evidence taken collectively has 
tremendous weight. A successful hypothesis is strengthened by the testimony 
furnished by diverse facts, and the more numerous and significant the particular 
instances embraced by the hypothesis the more nearly will their joint testimony 
mount to the altitude of proof, and plausible hypotheses neatly dovetailed may fit 
together so well as to apparently strengthen rather than weaken one another. How- 
ever, it is easy to see that the probability of an hypothesis being valid becomes less as 
the number of unproved assumptions on which it is based becomes greater. We can 
even get a numerical illustration. // the definite- compound test be right nine 
times out of ten, the probability that a given substance of definite composition 
is a true compound is ~ ; similarly, if the homogeneity test be right three times out 
of four, the probability that the given homogeneous substance is a chemical com- 
pound is I ; and the probability that the given homogeneous substance of definite 
composition is a true compound is ||. Every bit of additional evidence in favour 
of a conclusion multiplies the probability of its being correct in an emphatic 
manner; and evidence against a conclusion acts similarly in the converse 
way. Thomas Huxley has stated that one of the tragedies in science is 
the slaughter of a beautiful hypothesis by one incongruent fact : a conclusion 
based solely upon circumstantial evidence is always in danger of this Damoclean 

A writer has said : " When two facts seem to be in conflict, we may be driven 
to decide which is the more credible of the two." This statement may give rise to 
a misunderstanding. We cannot admit the possibility of two contradictory facts. 
Facts can, and often do, contradict hypotheses. Again, a fact is a fact and cannot 
be disputed ; all facts are equally true. Scientific knowledge cannot be arranged 
in two compartments, one for truth and one for error. The degree of confidence to 
be placed in a statement can be made onlv after the evidence has been sifted and 
weighed. If there be any doubt about the truth of an alleged fact, something is 
wrong. The laboratory, not the study, is the place to decide if the alleged fact is 
the result of an incomplete or of a mal-observation. Facts qua facts cannot be 
graded in degrees of probability or credibility, since the difference between 
probability and certainty does not represent any quality of the objective fact, 
it merely describes a state or attitude of the mind which ranges from ignorance to 



§ 8. Analysis and Synthesis 

The earliest chemists were familiar with changes due to the union of distinct 
forms of matter to produce a different substance with new properties of its own ; 
and also with the separation of two or more definite substances from another quite 
different substance. The term spagyric art {(nrdv, to separate ; dyeipetv, to 
assemble), applied to chemistry about the sixteenth century, emphasized the fact 
that chemical changes were regarded as involving either combinations or decom- 
positions ; and as the balance came into more and more extended use, it was 
gradually recognized that when elements or compounds have suffered a chemical 
change, the original substances can be recovered, qualitatively and quantitatively 
the same, by reversing the chemical operation. 

The term synthesis — from avv, with ; nOio), I place — is employed for the 
operations involved in the formation of a particular compound from its constituents. 
The term analysis — from dvd, back ; Avw, I loosen — is employed for the process 
of separating the constituents of a compound or mixture. Thus mercuric oxide is 
broken down into its constituents when heated. The object of the analysis may be 
to answer the question : What are the constituents of the mixture or compound ? 
The analysis is then said to be qualitative. If the relative quantities of the different 
constituents are to be determined, the analysis is said to be quantitative. 

There is one period in the history of chemistry when the discovery or synthesis 
of new substances was considered to be the main aim of the chemist ; new sub- 
stances were made unmeasured and unclothed with properties, which now re- 
quire to be critically scrutinized all over again. The style of some old text-books 
on chemistry was not far removed from that of cookery recipe books, for they gave 
a long dreary list of modes of preparing different substances which led E. J. Mills 
(1876) to say : Chemistry has become an art of breeding (new compounds). The 
pioneer workhasbeenuseful, for it has furnished modern chemistry with raw empirical 
material to be worked up into science ; indeed a great many more empirical data are 
now available than chemists have been able to co-ordinate and assimilate into their 
science. Consequently, we are beginning to recognize the truth of the inspired 
words of M. W. Lomonossoff, cited above, though written in 1751 ; and the growing 
use of tables of measurements and of squared paper in chemical text-books is a 
sign of the times. In the words of K. Fittig : 

We are now forced to increase the number of compounds, not merely in order to prepare 
new substances, but in order to discover natural laws. 

The solution which remains when the dilute sulphuric acid can dissolve no more 
zinc, may be filtered and evaporated over a hot plate until a drop of the hot solution 
crystallizes when placed on a cold glass plate. Crystals of zinc sulphate will separate 
as the solution cools. By evaporating a large volume of the solution very slowly, 
crystals over a foot long have been obtained. This experiment illustrates the 
synthesis of zinc sulphate from metallic zinc and dilute sulphuric acid. The earlier 
alchemists assumed that when a metal dissolves in acid, the metal is destroyed, 
J. B. van Helmont 1 showed that this assumption is ill-founded because just as 
when a certain amount of common salt is dissolved in water, the same amount of 
salt can be recovered from the solvent, so, when silver is dissolved in aqua fortis, 
the metal passes into solution, but is not essentially altered. In the present case, 
the zinc dissolved by the acid can be recovered as zinc sulphate, and if need be as 
metallic zinc. 

The analysis of aqueous solutions of zinc sulphate by the electric current.-- 
An electric current is developed during the reaction between dilute sulphuric acid 



and metallic zinc which results in the formation of zinc sulphate and the evolution 
of a gas. 

Place two platinum plates, E, Fig. 1, and pure distilled water in the clean glass jar, 
which will now be called the " electrolytic cell." Connect the two platinum plates with 
an accumulator or secondary battery, and a voltmeter and shunt as indicated in Fig. 1. 
The object of the accumulator is to generate an electric current. If the water is pure the 
needle of the voltmeter moves very little, if at all. Add a concentrated solution of zinc 
sulphate to the water in the glass jar. The jump of the needle of the voltmeter shows 
that a current of electricity is flowing through the circuit and hence also through the 
solution of zinc sulphate. If chloroform, benzene, or an aqueous solution of cane sugar 
had been used in place of the solution of zinc sulphate in the electrolytic cell, no current 
would p€tss through the circuit. Hence, liquids may be either conductors or non-conductors 
of electricity. 

An electric current passing through an aqueous solution of zinc sulphate produces 
some remarkable changes : (1) a spongy mass of metallic zinc accumulates about one 
of the platinum plates ; (2) if the solution be tested, particularly in the neighbour- 
hood of the other platinum plate, sulphuric acid will be found to be accumulating 
in the solution during the process of electrolysis ; and (3) bubbles of oxygen gas, 
easily tested by collecting some in a test-tube, rise from the same platinum plate 


Cathode Anode 
r>j 7 ^ Platinum 

Platinum & ^/P/^fp 

^^EleclroW fie Cell 
Accumulator ^v_x_v w i/v; iv vyiyufjc^ uNi>-^^ ^ 

Fig. 1. — Chemical Action induced by Electric Current — Electrolysis. 

about which the acid accumulates. If the experiment be continued long enough, 
metallic zinc and sulphuric acid will be produced in appreciable quantities. If 
the accumulator be disconnected, and the connecting wires be joined together, the 
zinc will redissolve in the acid, re-producing zinc sulphate ; and an electric current 
will be generated during the dissolution of the zinc. 

The process of decomposition or analysis by the aid of the electric current is 
called electrolysis. The liquid which is decomposed is called the electrolsrte. The 
passing of the electric current through the conducting copper wires, and through 
the conducting platinum plates, produces no change in these metals. Hence, we 
recognize two kinds of conductivity — in one the conducting medium is decomposed 
by the current — electrolyte ; and in the other the conducting medium is not 
decomposed by the current — non-electrolyte. The plate at which the zinc collects 
is called the cathode— from Kara, down ; 0805, a path— and the other plate, about 
which the acid collects, is called the anode— from dm, up ; 080s a path. The 
anode and cathode together are called the electrodes. These terms were suggested 
to M. Faraday by W. Whewell.2 With the conventions already indicated as to 
direction, the electric current is said to enter the electrolytic cell via the anode, 
and to leave the cell ma the cathode. The two electrodes are thus " the doors 
or ways by which the current passes into or out of the decomposing body." It seems 
as if the electric current first splits the decomposing liquid into two parts which 
pass to the electrodes. The term anion — from avidv^ that which goes up — is applied 


to those parts of the decomposing fluid which go to the anode ; those passing to 
the cathode are called cations — from KartoV, that which goes down ; and when 
reference is made to both the anions and the cations, the term ions- from, Ton', 
traveller— is employed. Ion is thus a general term for those bodies which pass 
to the electrodes during electrolysis ; or for the two parts, no matter how 
complex, into which the electrolyte is primarily divided during electrolysis. This 
notation was proposed by M. Faraday in 1834. 

The experiments indicated above illustrate an important principle — the principle 
of reversibility : If an antecedent event A produces an effect B, then an antecedent 
event B will reproduce the effect A. Thus, chemical action can produce an electric 
current, and conversely, an electric current can produce chemical action, Fig. 1. 
The one can undo the work of the other. Many other examples of the principle will 
be recalled — for example, heat causes gases to expand ; conversely, if a gas expands 
by its own elastic force, the gas will be cooled ; a crystal of tourmaline is electrified 
by uniformly raising its temperature, and Lord Kelvin (1877) showed that the reverse 
effect can be induced, for a change of temperature occurs when the electrical state 
of the crystal is changed ; etc. 


1 J. B. van Helmont, Ortua medicince, Lugduni Batavorum, 1656. 

2 I. Todhunter, William Wheivell,D.D.,London, 2, 178, 1876; M. Faraday, Phil. Trans., 124. 
77, 1834. 

§ 9. Dalton's Law of Multiple Proportions 

If Dalton's hypothesis of multiple proportions be found correct, we shall have to regard 
it as the greatest advance chemistry has yet made towards its development into a science. 
— J. J. Bebzelius (1811). 

The formation of chemical compounds is not a capricious and fortuitous process, 
but it proceeds in an orderly fashion. Chemical combination is restricted to certain 
fixed proportions of matter. These limitations appear to have been prescribed by 
nature as part of her scheme in building the material universe. This fact arrested 
the attention of J. Rey in 1630. J. Key's conclusion that in the calcination of the 
metals " nature has set limits which she does not overstep," agrees with many 
facts ; but there are certain limitations. If one gram of lead be calcined for a long 
time at 500°, never more than 1'103 gram of a red powder — red lead — is obtained. 
Here, 64 grams of oxygen correspond with 621 grams of lead. If the lead be 
calcined at about 750°, one gram of lead will not take up more than 0-078 gram of 
oxygen to form a yellow powder — litharge; otherwise expressed, 64 grams of oxygen 
correspond with 828 grams of lead. Here then nature has set two limits; lead 
forms at least two definite oxides— a red oxide stable at a dull red heat, and a 
yellow oxide stable at a bright red heat. A puce oxide can also be obtained by 
treating the red oxide with nitric acid, and the puce oxide contains 414 grams of 
lead for 64 grams of oxygen. The relative proportions of lead and oxygen in the 
three oxides are as follows : 

Oxygen. Lead. 

Puce oxide (lead peroxide) . . 64 414 = 207x2 

Red oxide (red lead) ... 64 621=207x3 

Yellow oxide (litharge) ... 64 828 = 207x4 

This means that for a given weight of oxygen, the yellow oxide has four-thirds as 
much lead as the red oxide, and twice as much as the puce oxide. Smularly, carbon 
forms two well-defined oxides, called respectively carbon monoxide, and carbon 
dioxide. In these we have : ^ , 

Oxygen. Carbon. 

Carbon dioxide 8 l-t^l 

Carbon monoxide . • • • ^ H-JXZ 


Perhaps the oxides of nitrogen furnish the most convenient illustration of the 
principles ; at least six have been reported (the real existence of the hexoxide has 
not been established satisfactorily). In these, the relative proportions of nitrogen 
and oxygen are as follows : 



Nitrogen monoxide 


8 = 8X1 

Nitrogen dioxide 


16 = 8X2 

Nitrogen trioxide 


24 = 8X3 

Nitrogen tetroxide 



Nitrogen pentoxide 


40 = 8X5 

(Nitrogen hexoxide 


48 = 8X6) 

These six compounds of the same elements united in different proportions form 
a series of substances so well marked and contra-distinguished that it is questionable 
if the most acute human intellect would ever have guessed a priori that they contained 
the same constituents. Starting from the compound with the least oxygen, we see 
that for every 14 grams of nitrogen, the amount of oxygen increases by steps of 8 
grams. Accordingly, in all six compounds of nitrogen and oxygen the masses of 
nitrogen and oxygen are to one another as mxl4 : wx8, where m and n are whole 

If an aqueous solution of sodium hydroxide be mixed with successive small 
quantities of hydrochloric acid, the relative proportions of the two substances can 
be varied at pleasure, but there is not an infinite variety of compounds of soda and 
acid. The one sole product of the reaction is sodium chloride, and this has always 
one fixed and definite composition. If an excess of either acid or soda be present, 
it is assumed that the excess remains uncombined, because, when the solution is 
concentrated by evaporation, crystals of sodium chloride are obtained along with 
the excess of soda or of acid if such be present. If sulphuric acid be substituted 
for hydrochloric acid, crystals of two distinct and definite products can be separated 
— the one is called sodium hisulphate, and the other normal sodium sulphate — 
according as the acid or alkali is in excess. Here then is an apparent exception 
to the old saw, natura non facit saltum, for nature does make jumps. The leaps 
are shown in the relations by weights between the soda and acid in the two products : 





Sodiiun bisulphate 

. 52 , 





Normal sodium sulphate 

. 52 





Hundreds of cases equally simple might be cited. Similar facts helped to 
establish an idea deduced by J. Dalton (1802-4) from the atomic theory, and now 
called the law of multiple proportions : when one substance unites with another 
in more than one proportion, these different proportions bear a simple ratio to one 

There is no difficulty in tracing the simple ratio m: n in the cases which 
precede, but it is not always easy to detect the simplicity of this ratio in perhaps 
the larger number of cases. Eor instance, the ratio w : ?? for compounds of carbon 
and hydrogen passes from 1 : 4 in methane, up to 60 : 122 in dimyrcyl, and still 
more complex cases are not uncommon ; the methods of analysis are scarcely 
sensitive enough to distinguish the comparatively simple triacontane where carbon : 
hydrogen is as 30 : 62, from hentriacontane where this ratio is 31 : 64. Again, the 
masses of carbon which unite with one of hydrogen, in methane, ethylene, and acety- 
lene are 3, 6, and 12 respectively, but in methane, ethane, propane, hexane, eico- 
sane, and anthracene, j;hey are 3, 4, 4-5, 5143, 5-714, and 168 respectively. Several 
attempts have been made to get around the difficulty, by rewording the statement 
of the law. Thus, B. D. Balaref[ i recommends : " The masses of the different 
elements in a compound are directly proportional to their equivalent weights or 
to simple multiples of their equivalents," but E. Puxeddu has discussed these 
various forms and shown that they are intrinsically different in meaning from the 
original Daltonian law. 


Still the Daltonian law is considered to be so well founded that it can be applied 
to predict the composition of compounds which have never been prepared. Thus, 
if an oxide of nitrogen containing rather more oxygen than nitrogen hexoxide be 
made, it may be predicted that it wilJ contain 7x8=56 parts of oxygen for 
every 14 parts of nitrogen by weight. Again, if a substance be found to contain 
oxygen and nitrogen, not in the proportion 14 : 8 or a multiple of 8, it is in all 
probability a mixture, not a true compound. Again, air contains oxygen and 
nitrogen, but the proportions of nitrogen to oxygen is as 14 : 4-29. This is 
usually given along with other circumstantial evidence to show the probability that 
air is a mixture and not a chemical compound. 

Are solutions chemical compounds or mixtures ?— Our definitions say mixtures, 
because the composition of solutions follows neither the constant nor the multiple 
proportion law. We might easily be led to reason in a vicious circle — in circulo 
prohando — by a rigid application of the so-called constant and multiple proportion 
laws. Salts dissolve in water in all proportions up to a certain limiting value. 
The process of solution, in some cases, seems to be otherwise indistinguishable 
from chemical combination, and C. L. Berthollet (1803) 2 considered that " solution 
is a true combination " produced by " a feeble combination which does not cause 
the characteristic properties of the dissolved body to disappear." It is sometimes 
said that the process of solution cannot be a case of chemical combination because 
there are no signs of abrupt per saltum changes characteristic of combination in 
multiple proportions. The composition of homogeneous solutions can vary con- 
tinuously within certain limits while a chemical compound has one fixed and 
definite composition ; accordingly, we refuse to call substances compounds which 
do not conform with this definition. Hence, in virtue of arbitrarily compiled 
definitions, solutions are said to be mixtures, not chemical compounds, and this in 
spite of the fact that the dissolution of salts may be accompanied by those very 
phenomena which are usually recognized as characterizing chemical combination 
— changes in volume, specific heat, temperature, etc. — so that the product of the 
reaction (solution) has different properties from the average of its components. 

One writer has said : " Efforts have been made to find compounds which do 
not conform to the laws of chemical combination, but all attempts have resulted in 
failure ; " another writer says, " The law of multiple proportions has been tested by 
the analysis of thousands of compounds, and, like the law of constant proportions, 
it is one of the perfect laws from which no deviation has been discovered." From 
what has been said, if exceptions to the laws of chemical combination were 
discovered, chemists would refuse to call them compounds, and the quest for ex- 
ceptions must therefore end in failure. For the same reason, the .appeal to ex- 
perience is useless, it can neither establish nor refute the laws of constant and 
multiple proportions. More bluntly expressed : a prejudice in favour of the defini- 
tions in question may warp the judgment to such an extent as to lead to a denial 
of the possibility of contradictory phenomena. Such a perversion of the judgment 
must be detrimental to the progress of science. Hence the danger of cherishing 
a blind faith in our so-called laws of nature, which, at the present day, are little 
more than conventional definitions. With such definitions one can easily be deluded 
with the belief that he worships in the temple of certainty as indicated in the above 
two quotations. 


1 D. B. Balareff, Journ. prakt. Chem., (2), 95. 397, 1911 ; E. Puxeddu, f/azz. Chim. ItaL, 49. 
i, 203, 1919 ; P. Duhem, Le mixte et la comhinaison chimique, Paris, 1902. 
* C. L. Berthollet, E-^sai de statique chimique^ Paris, 1803. 


§ 10. The History of the Law of Multiple Proportions 

Communities of atoms are called clieraical combinations, and they possess every degree 
of stability. The existence of some is so precarious that the chemist in his laboratory can 
barely retain them for a moment ; others are so stubborn that he can barely break them up. 
The more persistent or stable combinations succeed in the struggle for life and are found in 
vast quantities as in the cases of common salt and of the combinations of silicon. Stability 
is a property of relationship to siu-rounding conditions ; it denotes adaptation to environ- 
ment. Thus, salt is adapted to the struggle for existence on earth, but it cannot withstand 
the severer conditions which exist on the sun.- — G. H. Darwin (1905). 

William Higgius, in his book A comparative view of the phlogistic and 
antiphlogistic theories with inductions (London, 1789), stated that one particle of 
sulphur and one of oxygen constitute sulphurous acid, while a* particle of sulphur 
and two particles of oxygen constitute sulphuric acid ; he also stated that in the 
compounds of nitrogen and oxygen, the particles of the two ingredients are to each 
other respectively in the ratio 1 : 1 or 2, 3, 4, or 5. According to C. Daubeny (1850), 
owing to imperfections in the available chemical analyses, W. Higgins could not 
have estabHshed the proposition as a general rule ; and judging from the cursory 
manner in which Higgins refers to the relation between the proportions in which 
the constituents of these compounds unite to form compounds, he did not attach 
much importance to the principle. W. Higgins here appears to have followed 
Isaac Newton, who, in his Opticks (London, 1704), said : 

The smallest particles of matter may cohere by the strongest attractions and compose 
bigger particles of weaker virtue ; and many of these may cohere and compose bigger 
particles whose virtue is still weaker, and so on for divers successions, until the progression 
ends in the biggest particles on which the operations in chymistry depend. 

It has been suggested that Newton's idea of chemical affinity, dependent on the 
successive addition of atoms, may have given W. Higgins and J. Dalton the hint 
which they needed for producing the law of multiple proportions. 

Even before John Dalton enunciated the law of multiple proportions, many 
observations had shown that compounds unite together in more than one proportion. 
Indeed, it now seems strange that chemists should have failed to notice the law of 
multiple proportions when numerous analyses were available. E. von Meyer ^ 
attributes this to the results being calculated in such a way as to hide the law, 
but A. N. Meldrum has shown that the data were frequently stated in precisely 
the way required. J. B. Richter (1792) noticed that certain metals have the 
power of combining with oxygen to form oxides with two different proportions of 
oxygen ; J. L. Proust (1799) obtained a similar result in connection with copper, 
but partly owing to inaccurate analyses, and partly owing to the fact that he 
had no guiding principle, he failed to recognize the law of multiple proportions. 
A. L. Lavoisier (1789) knew that certain substances united with oxygen in several 
different proportions each of which corresponded with a fixed and constant relation 
between the weights of the combining elements. F. Clement and J. B. Desormes 
(1801) also analyzed carbon monoxide and found that it contained just half the 
amount of oxygen contained in carbon dioxide, and it afterwards struck J. Dalton as 
curious that the two French chemists did not take more notice of this remarkable 
result. J. Bostock's analyses of the lead acetates in 1805 were shown by J. Dalton 
to be in good agreement with the law. Between 1802 and 1807, J. Dalton gave a 
number of examples of the law of multiple proportions from his own analyses and 
those of others. 

In 1808, in a memoir On oxalic acid, T. Thomson ^ showed that, in the formation 
of the two potassium salts of oxalic acid, the quantity of potash which reacts with 
a given amount of oxalic acid is in one case j ust double the proportion in the other ; 
similar results were obtained with the two strontium oxalates — one of which is 
obtained by saturating oxalic acid with strontia water, and the other by mixing 
solutions of ammonium oxalate and strontium chloride. It is remarkable, said 


T. Thomson, that thefirst contains just double the proportion of base contained in 
the second. In a paper On swper-acid and sub-acid salts (1808), W. H. Wollaston 
^also found that the amounts of carbonic acid relative to a given amount of potash 
in the two potassium carbonates are related as 1 : 1 and 1:2. These two papers 
are of historical interest, and they attracted some attention because, at that time, 
so few facts were known which could be employed to test the law of multiple pro- 
portions. In 1810, J. J. Berzelius began to pubHsh a series of investigations designed 
" to find the fixed and simple ratios in which the constituents of inorganic nature 
are combined ; " he gave a number of accurate analyses which enabled him to say 
that if two substances A and B unite in more thap one ratio, the various masses 
of A which unite with a fixed mass of B bear a simple ratio to one another. These 
experiments played so important a part in establishing the law of multiple proportions 
that the law itself has been called Berzelius' law. Some years later, in reviewing 
J. Dalton's hypothesis, J. J. Berzelius said : 

It may be doubted if J. Dalton was sufl&ciently cautious in applying the new hypothesis 
to the system of chemistry. It appeared to me that the paucity of analyses given in support 
of the generalization indicated a desire on the part of the experimenter to obtain a certain 
result ; but this is just the attitude which must be avoided when proofs for or against a 
preconceived theory are sought. Notwithstanding all this, to Dalton belongs the honour 
of discovery that part of the doctrine of chemical composition termed the law of multiple 
ratios, which no one had previously observed. 

In the celebrated Proust v. Berthollet controversy, C. L. Berthollet showed that 
some elements unite in more than one proportion, and therefore he argued that 
compounds do not necessarily have a fixed and definite composition ; but J. L. 
Proust demonstrated that when a metal unites with, say, oxygen in more than one 
proportion, the proportion in which the two elements combine do not vary in a 
continuous manner, but they proceed in jumps, per saltum, and each of the compounds 
has then a fixed and definite composition. J. L. Proust, however, failed to recognize 
the law of multiple proportions subsequently developed by J. Dalton. 


^ E. von Meyer, History of Chemistry, London, 195, 1906 ; J. L. Proust, Ann. Chim. Phys., 
(1), 28. 214, 1798 ; Journ. Phys., 54. 92, 1802 ; 55. 330, 1802 ; 59. 324, 352, 1804 ; 62. 138, 
1806 ; 63. 431, 1806 ; A. N. Meldrum, Mem. Manchester Lit. Phil. Soc., 55. 6, 1911 ; F. Clement 
and J. B. Desormes, Gilbert's Ann., 9. 409, 1801 ; J. Bostock, Nicholson's Journ., 11. 75, 1805; 
29. 150, 1811 ; A. L. Lavoisier, Traite elementaire de c^imie, Paris, 1789; J. B. Richter, Ueber 
die neueren Gegenstdnde der Chymie, Breslau, 1791-1802. 

2 T. Thomson, Phil. Trans., 98. 63, 1808 ; W. H. Wollaston, t&., 98. 96, 1808 ; J. J. Berzelius, 
GiWerfs Ann., 40. 320, 1812 ; 42. 274, 1812. 

§ 11. Richter's Law of Reciprocal Proportions 

After long centuries of painful and continuous effort, chemistry has discovered that the 
elements combine with one another in definite and unchanging ratios of quantity ; and 
that, when their compounds are decomposed, they yield up those identical ratios.— S. 
Brown (1843). 

Between 1810 to 1812, J. J. BerzeUus i pubHshed the results of a careful study 
of the quantitative relations of some of the elements— T'erswc^ die hestimmten wid 
einfachen Verhdltnisse aufzufinden nach welcken die Bestandtheile der unorganischen 
Natur mit einander verhunden sind. He found that 100 parts of iron, 230 parts of 
copper, and 381 parts of lead are equivalent, for they unite with 296 parts of ox>'gen 
forming oxides, and with 58-73 parts of sulphur, forming sulphides. Hence, smce 
58-73 parts of sulphur and 29-6 parts of oxygen unite respectively with 138 parts 
of lead, then, if sulphur and oxygen unite chemically, 58-73 parts of sulphur will 
unite with 29-6 parts of oxygen, or, taking the law of multiple proportions into 
consideration, with some simple multiple or submultiple of 29' 6 parts of oxygen. 
In confirmation, J. J. Berzelius found that in sulphur dioxide, 5873 parts of sulphur 

VOL. I. " 


are united with 57*45 parts of oxygen. The difierence between 2 X 296 = 59*2 and 
57 '45 is rather great, but some of the methods of analysis were crude in the time of 
J. J. Berzelius, and very much closer approximations — very nearly 1 in 50,000—" 
have been obtained in recent years, 

J. B. Richter, some twenty years before J. J. Berzelius' work, proved that a similar 
relation held good for the combination of acids and alkalies. J. J. Berzelius extended 
J. B. Richter's law to combinations between the elements. The above relations are 
included in the generalization sometimes called the law of reciprocal proportions, 
or the law of equivalent weights. The weights — multiple or submultiple — of the 
various elements which react with certain fixed weight of some other element 
taken abitrarily as a standard, also react with one another. If each of two sub- 
stances, A and B, combines with a third substance C, then A and B can combine 
with each other only in those proportions in which they combine with C, or in some 
multiple of those proportions. This law does not mean thatif each of the elements 
A and B combines with C, then the elements A and B will combine with one another. 
A. L. Lavoisier, in his Traite elementaire de chimie (Paris, 1. 116, 1789), argued that 
if two elements have une grande appetence for a third element, they should have an 
affinity for one another : qucB sunt eadem uni tertio sunt eadem inter se ; and he 
added : c'est ce qu'on observe en effet. Further knowledge has shown that the direct 
converse is more nearly in accord with facts. 

The law of reciprocal proportions may be regarded as a corollary of the law of 
multiple proportions on the further assumption that A, B, and C can form binary 
compounds — AB, BC, CA — with one another. Consequently it follows that if a 
compound be formed by the union of two elements A and B, it is only necessary 
to find the proportions in which a third element C unites with one of the two elements, 
say A, to be able to predict the proportions in which C will unite with B ; if the law 
of reciprocal proportions did not hold, this prediction would be impossible. These 
numerical relations come out very clearly by comparing the proportions ii;i which 
the difierent members of a series of elements, selected at random, combine with 
a constant weight of several other elements. Suppose the analysis of a substance 
shows that its ingredients are not in those proportions which we should expect 
from the known combinations of each of its components with another substance, 
we might safely infer that the substance analyzed is a mixture, and not a single 
compound. At ordinary temperatures, alcohol mixes in all proportions with ether 
and with water, but ether and water cannot be mixed in all proportions. 

Example. — If one gram of hydrogen unites with eight grams of oxygen to form water, 
and if one gram of hydrogen iinites with 35"5 grams of chlorine to form hydrogen chloride, 
in what proportion will oxygen and chlorine be likely to combine ? Ansr.' — If oxygen 
and chlorine unite at all, they will be likely to do so in the proportion of 8 grms. of oxygen 
to 35*5 grms. of chlorine, or some multiple or submultiple of this ratio. As a matter of 
fact, 8 grms. of oxygen do unite with 35' 5 grms. of chlorine to produce chlorine monoxide. 

The laws of constant, multiple, and reciprocal proportions are wonderful examples 
of the beauty and harmony of nature ; and yet, we have glimmering hints that these 
are but symbols of a sublimer generalization which, when discovered, 

Will make one music as before 
But vaster. 


1 J. J. Berzelius, Gilbert's Ann., 37. 249, 415, 1811 ; 38. 161, 227, 1811 ; 40. 162, 235, 1812 ; 
42. 276, 1812 ; Essai sur la theorie des proportions chimiques et sur Vinfluence chimique de V electricity, 
Paris, 1819 ; J. B. Richter, Ueber die n^ueren Oegenstdnde der Chymie, Breslau, 1791-1802. 


§ 12. Combining, Reacting, or Equivalent Weights 

Since it is already settled for us by custom that quantities of different substances are 
to be called equal when or because they are equivalent gravimetrically, we have no choice 
but also, from the chemical point of view, to call those quantities of substance equal which 
mteract in single chemical changes. — E. Divers (1902). 

The following numbers represent the results obtained by the chemical analysis 
of a number of substances selected at random : 

Per cent. Per cent. 

Silicon dioxide . . . Silicon 46-93 ; Oxygen 63-07 

. Hydrogen 2-76 ; Chlorine 97-23 

. Magnesium 25-53 ; Chlorine 74*47 

. Hydrogen IMS; Oxygen 88-81 

. Silver 75-26; Chlorine 24*74 

. Silver 70*05; Fluorine 29-95 

Hydrogen chloride 

Magnesium chloride 


Silver chloride 

Silver fluoride 

Analyses are generally calculated so that the sum of all the constituents is 100 
(per cent.) within the limits of experimental error. This is simply a convention of 
the analyst, for the results could be just as intelligibly summed to any other number. 
Taking any one of the elements as a standard, let us calculate what amount of each 
of the other elements will combine with a given quantity of the selected element. 
To save time, take oxygen = 8 as the standard. Starting with silicon, 53"07 parts 
of oxygen are combined with 46"93 parts of siHcon. Consequently, we have the 
proportion 53'07 : 8 = 46*93 : a? ; or, a; = 7'07, for siUcon when the unit oxygen ia 8. 
Similarly, for water, hydrogen is 1-008 when oxygen is 8. Again, in hydrogen chloride 
when hydrogen is r008, chlorine is 35*4:5 ; in silver chloride, silver is 107*88 when 
chlorine is 35-45 ; when silver is 107*88, fluorine is 19*0 ; and when chlorine is 35*45, 
magnesium is 12*16. Collecting together the results of these calculations, we get 















We have previously obtained a number of results for some metals for the standard 
oxygen 8 by a different process, and the number for magnesium obtained by an 
indirect process : Oxygen -> Hydrogen (water) -> chlorine (hydrogen chloride) -» 
magnesium (magnesium chloride) gives the same results within the Hmits of experi- 
mental error as was obtained by a totally different process. Similar results are 
obtained in all cases, subject, of course, to the greater risk of experimental error 
when a long chain of compounds is involved. As a rule, there is no need to follow 
such an extended series as we have done, for fluorine and for magnesium. Most 
of the elements unite directly with oxygen ; and with the other elements, one 
intermediate step usually suffices. 

We are therefore able to deduce an important generalization : The combining 
weights of the elements are specific constants, i.e. they change from element to 
element, but for each element, the combining weight is fixed and invariable. Other- 
wise expressed: A number can be assigned to each element ; this number — called 
the combining, reacting, or equivalent weight— represents the number of parts by 
weight of the given element which can enter into combination with 8 parts by 
weight of oxygen, or one part by weight of hydrogen. All combining weights are 
relative numbers, and they are conventionally referred to oxygen 8, or hydrogen 
unity. When an element unites with another element in more than one proportion, 
the higher proportions will always be simple multiples of the combining weights— one 
for each element. This is the so-called law of combining or reacting weights : 
when a substance enters into chemical combination it always does so in quanti- 
ties which are proportional to its combining weight ; and the law of multiple 
proportions becomes : i The quantities of the different elements in a compound 
are simple multiples of their equivalent weights. The term equivalent weight 
is generally attributed to W. H. Wollaston (1814), and combining weight to 
T. Young (1813).2 


If the combining weights of the elemeijts are fixed, as they undoubtedly are, 
and since the elements can combine to form compounds which, in turn, can form 
compounds with other elements and with one another, jt follows that the com- 
pounds themselves also have combining weights if they also can enter into chemical 
combination. Hence the so-called law of compound proportion — the combining 
weight of a compound body is the sum of the combining weights of its components. 
This deduction from the Jaw of combining weights is as firmly established experi- 
mentally as the law of combining weights itself. The neutralization of acids by 
bases, and numerous other chemical reactions, can be cited in illustration. 

The experimental results, indicated in § 2, raise the suspicion that there is a 
difference between chemical and gravimetric equahty. E. Divers (1902) has 
pointed out that in the latter, equal quantities of the different forms of matter are 
represented by equal weights ; whereas, in a chemical sense, equal quantities of 
matter are the weights or masses of different forms of matter which unite with one 
another chemically. Consequently, chemical union may be regarded as a measure 
of the amounts of the different forms of matter which are chemically equivalent. 
Chemical equality is thus as clearly defined as gravimetric equaUty. The former is 
a measure of chemical and the latter a measure of physical phenomena ; the latter 
is wholly independent of, and the former mainly dej)endent upon the nature of the 
substances compared. 


1 D. B. Balareff, Journ. prakL Ghem., (2), 95, 397, 1911. 

* W. H. WoUaston, Phil. Trans., 104. 1, 1814 ; T. Young, Introdiiction to Medical Literature, 
London, 1813 ; E. Divers, B. A. Rep., 557, 1902. 

§ 13. The Perdurability of Matter 

The annihilation of matter is unthinkable for the same reason that the creation of matter 
is unthinkable, the reason namely that nothing cannot be an object of thought. — H. 
Spenckr (1851). 

I cannot see what warrant there is for assuming that when a weight A of one substance 
combines with another whose weight is B, the weight of the resulting compound is uni- 
versally and necessarily A-\-B. — A. D. Risteen (1895). 

In 1774, A. L. Lavoisier heated tin with air in a closed vessel and found that 
the weight of the whole system, before and after the calcination of the tin, was the 
same, thus showing that the whole system neither gained nor lost in weight during 
the oxidation of the metal. H. Follinus also noticed, in 1613, that mercur}^ could 
be transformed into the sulphide and the product transformed back to the metal 
without a change in the weight of the mercury, and Jean Rey was very emphatic, 
for he said in 1630 : 

I now give a flat denial to the erroneous maxim which has been current since the birth 
of philosophy — that the elements mutually undergoing change, one into the other, lose 
or gain weight according as in changing they become rarefied or condensed. With the arms 
of reason I boldly enter the lists to combat this error, and to sustain that weight is so closely 
united to the primary matter of the element that they can never be deprived of it. The 
weight with which each portion of matter was endowed at the cradle will be carried by it 
to the grave. 


J. R. Glauber, in his Furni novi philosophici (Amsterdam, 1648), described ti 
reaction between a solution of gold in aqua regia and a solution of siUca in potai 
lye, by stating : 

The potash paralyses the action of the acid with the result that the gold and silica are 
respectively deprived of their solvents, and are accordingly precipitated. The weight of 
the precipitate so obtained is the sum of the weights of the silica and gold originally taken. 

These experiments are here mentioned because they emphasize very well the 
fact that, in spite of the most painstaking care, every time all the substances taking 


part in a chemical reaction are weighed before and after the change, there is no 
sign of any alteration in the quantity of matter. The need for assuming the per- 
durability or constancy of matter emphasized in the so-called Imv of the indestructi- 
bility of matter has been recognized from the very beginning of the Ionian physics ; 
for example, Democritus said twenty-four centuries ago : Nothing can ever become 
something, nor can something become nothing— eic niUlo nihil fit, et in nihilum nihil 
potest reverti. J. B. van Helmont's experiment on the transformation of water into 
vegetable substances, and the analytical work indicated in connection with the law 
of constant composition, all tacitly assume the principle of the indestructibility of 
matter. A. L. Lavoisier is generally supposed to have first demonstrated the law 
in 1774 by experiments like that cited above, but the law is very much older ; it was 
definitely enunciated in 1756 by M. W. LomonossofE; and the law must have been 
at the back of J. Black's mind when he worked on the alkaline earths in 1755. 
The alleged demonstrations that " in all changes of a corporeal nature, the total 
quantity of matter remains the same, being neither created nor destroyed," illustrate 
but do not prove the proposition, and they assume that no new substance can 
possibly come into or go out of existence. 

The chemist's law of " indestructibility of matter " really means that, in all 
cases which have been examined, the total iveight of the elements in any reacting 
system remains constant through all the physical and chemical changes it is made 
to undergo ; although the observed facts are better generalized as the law of 
persistence of weight : no measurable change in the total weight of all the 
substances taking part in any chemical process has ever been observed. If A and 
B represent respectively the weights of two compounds which form two other 
compounds M and N ; and if the symbol = be employed in place of " produces," 
and + for " together with," the law of persistence of weights can be symbolized 
algebraically A + B = M+N. If the weight of one of these four compounds be 
unknown, it can be computed by solving the equation. Chemists constantly use 
this principle in their work, for, as A. L. Lavoisier said in 1774 : 

Experiments can be rectified by calculations, and calculations by experiments. I 
have often taken advantage of this method in order to correct the first results of my ex- 
periments, and to direct me in repeating them with all proper precautions. 

When faith in magic was more prevalent than it is to-day, many believed 
that by some potent incantation or charm, matter could be called out of nothingness, 
or could be made non-existent. i^ Superficial observation might lead to the belief 
that a growing tree, the evaporation of water, and the burning of a candle prove 
the creation and the destruction of matter, but a careful study of these and in- 
numerable other phenomena, has shown that the apparent destruction of matter 
is an illusion. Matter may change its state as when liquid water is vaporized, and 
when a candle is burnt. In the case of a growing tree, the nutrition the tree receives 
from the soil and from the air (carbon dioxide) is overlooked. There is an old 
demonstration experiment commonly used to illustrate the fact that the apparent 
destruction of matter in the burning of a candle is illusory : 

A candle is fixed on one pan of a balance below a cylinder fitted with wire gauze, quick- 
lime, soda lime, and glass wool. Weights are added to the right scale-pan until the beam 
of the balance is horizontal. The candle is lighted. The gases rising from the flame pass 
through the cylinder where the products of combustion are absorbed by the soda lime. 
In 3 or 4 minutes the pan carrying the candle is depressed. The increase in weight is due 
to the fixation of the products of combustion by the soda lime. The products of com- 
bustion are formed by the combination of the carbon and hydrogen of the candle with 
oxygen from the air ; this oxygen was not included in the first weighing. The fact illus- 
trated by this experiment is undoubtedly true, but the experiment, though popular, is 
inconclusive because quicklime and soda lime both absorb moisture and carbon dioxide 
from the air. Hence, to make the experiment conclusive, it would be necessary to remove 
these compounds from the air used in the burning of the candle, or else to make due allow- 
ance for them. This would involve complicated operations ; the test has been made, and 
the result is qualitatively the same as with the simpler experiment. 


Every time a chemical reaction takes place in a closed vessel, which permits 
neither the egress nor the ingress of matter, the total weight remains unchanged 
within the limits of experimental error. The more carefully the experiments are 
made, the more nearly do the values approach identity. Both A, Heydweiller 
(1901) and J. J. Manley (1912) have tried to find if a loss in weight occurs during 
chemical action, taking the most extreme precautions known to man in order to 
secure the utmost accuracy. 

The experiment may be illustrated by introducing a solution of silver nitrate into one 
limb of the ^-shaped tube by means of a suitable funnel and a solution of potassium 
chromate in the other limb. The opening of the tube is then sealed, the tube is weighed 
and tilted so as to mix the solutions and start the reaction. The tube is again weighed. 
When the reaction is over and the conditions of temperature, etc., are the same as when 
the first weighing was made (for illustrative work on the lecture table, the opening of the 
tube may be corked and the solutions mixed). Other pairs of solutions are : a solution 
of potassiima iodate, slightly acidulated with hydrochloric acid, and potassium iodide ; 
lead acetate and sodium sulphide ; acidulated potassium chromate and sodium sulphite ; 

No diiference has been detailed in the weights of the initial and final products 
of the reaction within the limits of experimental error — 0*000006 grm. After an 
examination of fifteen different reactions, H. Landolt (1909) ^ again failed to detect 
a variation in weight ; and added, " since there seems no prospect of pushing the 
precision of the experiments further than the degree of exactness attained, the 
experimental proof of the law may be regarded as established." 

The law of the persistence of weight or the so-called law of the indestructibility 
of matter means that a variation in the total weight of the substance taking part in 
chemical reactions, greater than the limits of experimental error, has never been 
detected. Hence it is inferred that in chemical reactions, substance persists while 
matter changes its form. It might also be added that the many and varied deter- 
minations of the atomic weights of the elements furnish valuable illustrations of 
the law in question. The law of persistence of weight is quite empirical like the law 
of excluded perpetual motion. It is shown later, that if a real difference of weight 
in the substances taking part in a reaction could be detected, perpetual motion 
would be possible. 

If immeasurably small and trifling differences be taken into consideration, as 
is sometimes done in theoretical speculations, objection might be made to the state- 
ment that the weight of a compound must be equal to the weight of the separate 
constituent elements, for, as I. Todhunter ^ pointed out in 1876, the converse is the 
strict truth. The weight of a body depends upon the positions of the component 
particles, and, in general, by altering the positions of the particles, the resultant 
effect which we call weight is altered, though it may be to but an inappreciable extent. 
Moreover, even the time at which the weighing is performed is theoretically important, 
for the weight must change to a trifling extent with the changing position of the sun 
and moon in the sky. It is quite conceivable, too, that the weight of the iron in, 
say, magnetic oxide of iron might appear to be greater than the same amount of 
iron in, say, potassium ferrocyanide because of the effect of the earth's magnetic 
field upon the former. But if such an effect were observed, it would not interfere 
with our faith in the law as soon as the disturbing effect was recognized. 

H. Spencer considers that all the so-called experimental proofs by weighing 
tacitly assume the object being proved, since weighing impUes that the matter forming 
the weights remains unchanged in quantity ; or as H. S. Redgrove puts it, " weight 
measures matter because matter is indestructible, and matter is indestructible 
because weight measures matter." 


^ H. Spencer, First Principles, London, 1884. 

2 H. Landolt, Zeit. phys. Chem., 55. 589, 1906 ; Ueher die Erhaltung der Masse hei chemischen 
Umsetzungen, Halle a. «., 1909 ; A. Heydweiller, Ann. Physik, (4), 5. 394, 1901 ; Lord Rayleigh, 


Nature, 64. 181, 1901 ; P. Joly, Trans. Roy. Soc. DvJblin, 8. 23, 1903 ; A. W. Surdo, N\U)vo Cimenio, 
(5), 8. 45, 1904 ; (5), 12. 299, 1906. 

' I. Todhunter, William Whewell, D.D., London, 1876 ; H. S. Redgrove, AkJtemy Ancient and 
Modern, London, 1910 ; H. Spencer, First Principles, London, 1884. 

§ 14. The Atomic Theory of John Dalton 

It seems probable to me, that God in the beginning formed matter in solid, massy, hard, 
impenetrable, movable particles, of such sizes and figures, and with such other properties, 
and in such proportion to space, as must conduce to the end for which He formed them ; 
and that these primitive particles, being solids, are incomparably harder than any porous 
body compounded of them, even so hard as never to wear or break in pieces ; no ordinary 
power being able to divide what God Himself made one in the first creation. . . . The 
changes of corporeal things are to be placed only in the various separations and new associa- 
tions and motions of these permanent particles. . . . These principles I consider not as 
occult qualities, but as general laws of nature by which the things themselves are formed ; 
their truth appearing to us by phenomena, though their causes be not yet discovered. — 
Isaac Newton. 

The three laws of chemical combinatioD : (1) the law of constant composition ; 
(2) the law of multiple proportions ; (3) the law of reciprocal proportions ; and 
the law of the persistence of weight, summarize observed facts. They exist quite 
independently of any hypothesis we might devise about their inner meaning ; but 
we have an intuitive feeling that there must be some peculiarity in the constitution 
of matter which will account for the facts. 

An atom is the unit of chemical exchange. — Chemists in imagination have 
invested matter with a granular structure. Matter is supposed to be discrete, 
and built up of corporeal atoms. The imagination can subdivide matter inde- 
finitely ; the chemist says that however true this may be, nothing less than an 
atom ever takes part in a chemical reaction. The atom is the limiting size so 
far as chemical combination is concerned. An atom cannot be subdivided by 
any known chemical process. What A. Kekule wrote in 1867 appUes equally 
well to-day, in spite of some interesting though abortive attempts to eliminate 
atoms from chemistry. Should the progress of chemistry lead to a different view 
of the constitution of matter, it will make little alteration to the chemist's atom. 
The chemical atom will always remain the chemist's unit. As a chemist, con- 
tinued A. Kekule,! the assumption of atoms appears to be not only advisable but 
absolutely necessary provided that the term be understood to denote those particles 
of matter which undergo no further division in chemical transformations. 

Compare this hypothesis with observation. Fix the attention on the facts: 
Elements combine with one another either in amounts which correspond with their 
combining weights (law of constant composition), or with multiples of their combining 
weights (law of multiple proportions). Otherwise expressed, definite amounts of 
matter — the atoms — corresponding with the combining weights, act as chemical 
units. Keactions between different elements are reactions between these uoits. 
Atoms of the same element all have the same constant weight, and atoms of different 
elements have different weights. All this is in agreement with the law of constant 
combining weights. It is not the mass per se but the constituent particles of the 
elements which combine each to each. 

Fractions of an atom do not take part in chemical changes.— The proportions 
in which one element combines with another can alter only by steps one atom at 
a time ; 1, 2, 3, , . . atoms of one element can combine with 1, 2, 3, . . . atoms of 
another element. This is but one way of stating the laws of multiple and reciprocal 
proportions. The weight of an atom of each element is a constant quantity, and 
therefore elements can only combine with each other in certain constant proportions 
or in multiples thereof. The atoms of the elements are the units from which nature 
has fashioned all the different varieties of matter in the universe. One atom of 
mercury unites with one atom of oxygen to form mercuric oxide. If two atoms ot 


mercury united with one atom of oxygen, the result would not be mercuric oxide, 
but some other oxide of mercury — if otherwise, the law of constant composition 
would be false. As a matter of fact, such a compound is known, but it is mercurous 
oxide. Mercurous oxide has its own specific properties which are different from those 
of mercuric oxide. 

The analyses of C. F. Wenzel (1777), J. B. Richter (1791) J. L. Proust (1800), 
J. Dalton (1801), J. J. Berzelius (1810), and a host of followers are summarized 
in the laws of chemical change, and these laws, in turn, are rendered luminous 
and coherent by the hypothesis which assumes that all the different forms of matter 
in the universe are aggregates of insensibly small homoeomeric particles which all 
the powers of chemistry cannot further subdivide. We thus adopt the view of 
J. B. Dumas and of M. Faraday that whether matter be atomic or not, this much 
is certain, granting it be atomic, it would behave in chemical transformations as 
it does now ; A. Kekule expressed similar views in 1867 : 

The question whether atoms exist or not has but little significance from a chemical point 
of view ; its discussion belongs rather to philosophy. In chemistry we have only to decide 
whether the assumption of atoms is a hypothesis adapted to the explanation of chemical 
phenomena, . . . and to advance our knowledge of the mechanism of chemical phenomena. 

It remains to find the canons by which chemists have been able to fix the relations 
between the weights of the atoms of different elements. 

Atomic weights are relative. — The combining weights of the atoms can be 
expressed in any desired units ; it is quite immaterial whether a grain or a ton be 
imagined. In dealing with combining or atomic weights, the conception of abso- 
lute quantity is irrelevant. Given sufficient oxygen, 100 tons, kilograms, pounds, 
grams, or grains of mercury will give respectively 108 tons, kilograms, pounds, 
grams, or grains of mercuric oxide — no more, no less. Several different lines of argu- 
ment, given by 0. E. Meyer in his The Kinetic Theory of Gases (London, 1899), 
indicate that there are about 1280,000000,000000,000000 or 12*8 X lO^o hydrogen 
atoms in a milligram, so that the weight of an atom of hydrogen is not far from 
i28o,oooooo.oooooo.oooooo th or 12-8 X 10-20 of a milligram. This estimate may not be 
exact, and it is not here emphasized as a fact, although it is probably not far 
out. Suppose for the sake of illustration it is true, then, with the evidence so 
far adduced, an atom of mercury will weigh 100 X 12"8 X 10" -^tb milligram, and 
an atom of oxygen 8 X 12*8 X lO-^o mgrm. We do not know the absolute weights 
with any degree of precision, but the relative weights are known with a fair degree 
of accuracy. Given the relative weights, and the weight of an atom of one of the 
elements, the absolute weights of the atoms of all the other elements can be com- 
puted, for the masses of the other elements bear the same ratios to one another 
that are assigned to them in the table of atomic weights. The ratio of the weights 
of the different kinds of elements in a compound represents the relation between 
the weights of the several different kinds of atoms (or aggregates of atoms) which 
make up the compound. 

J. Dalton's atomic hypothesis. — It is impossible to say who invented the 
atomic theory, because it has grown up with chemistry itself. It certainly did not 
arise by one effort of modern science, as W. Nernst supposes, " like a phoenix from 
the ashes of the old Greek philosophy." In the work of William Higgins and his 
predecessors, the hypothesis was little more than an inanimate doctrine. It 
remained for Dalton to quicken the dead dogma into a living hypothesis. John 
Dalton's atomic hypothesis explains the structure of matter and of chemical com- 
bination upon the following postulates, which may be regarded as a very brief 
statement of what is called Dalton's atomic theory : 

1. Atoms are real discrete particles of matter which cannot be subdivided 
by any known chemical process. 2. Atoms of the same element are similar 
to one another, and equal in weight. 3. Atoms of different elements have 
different properties— weight, afi&nity, etc. 4. Compounds are formed by the 


union of atoms of different elements in simple numerical proportions — 
1:1; 1:2; 2:1; 2:3; etc. This led Dalton to deduce the law of 
multiple proportions which was later confirmed by experiments. 5. The com- 
bining weights of the elements represent the combining weights of the atoms. 
J. Dalton seems to have assumed that the atoms are in perfect repose, unless 
disturbed by mechanical or chemical forces.^ 

Some defects in Dalton's atomic theory.— The hypothesis of Dalton's respecting 
atoms, and more particularly atomic weights, is not quite that which prevails in 
modern chemistry. According to the atomic theory : an atom is the smallest 
particle of an element which can enter into or be expelled from chemical com- 
bination. The assumption that the combining weights of the elements represent 
the combining weights of the atoms has caused some difficulty. How is the smallest 
combining weight of an atom to be fixed 1 In carbon monoxide, for example, we 
have oxygen and carbon in the following proportions by weight : Oxygen : carbon 
8 : 6, and in carbon dioxide : Oxygen : carbon 8:3 or as 16 : 6. What is the 
atomic weight of carbon if the atomic weight of oxygen is 8 ^ Obviously, the 
evidence now before us would be consistent with many, different views. Carbon 
monoxide may be a compound of one oxygen atom with two carbon atoms each 
with a combining weight of 3 ; or a compound of one oxygen atom with one 
carbon atom with a combining weight of 6. In the latter case, carbon dioxide 
is a compound of one carbon atom of combining weight 6 with two oxygen atoms, 
and the same combining weights would have been obtained if any number n of 
carbon atoms were combined with 2n oxygen atoms. Again in, order to ascertain 
the complexity of a combination of atoms, J. Dalton ^ stated that 

If only one combination of two elements exist, it must be presumed to be binary ; if 
two combinations exist, one will be a binary compound and the other a ternary compound. 

This hypothesis was also adopted by J. J. Berzelius,^ but in the case of the 
so-called carbon dioxide or carbon monoxide, there is at present nothing to show 
which is the binary and which the ternary compound. Similar difficulties arise 
when the idea of atoms so far developed is applied to other combinations of the 
elements. There is therefore some confusion. The concept of the atom becomes 
more or less indistinct and vague when the attempt is made to develop a 
consistent system on the basis of the atomic hypothesis as propounded by 
Dalton. Dalton's theory is defective because it lacks a standard for fixing the 
atomic weights of the different elements. The difficulty was removed only when 
chemists had learned the value of Avogadro's hypothesis in fixing a definite 
standard" for evaluating atomic w'eights. Chemists then conventionally came 
to an understanding as to the relation between the composition and specific gravity 
of a vapour or gas. 


1 J. B. A. Dumas, Lt>^on.s sur la philosophie chimique, Paris, 1836 ; A. W. Williamson, 
Journ. Chem. Soc, 22. 328, 1869; A. Kekule, Zeit. Chem., (2), 3. 216, 1867; M. M. P. Muir, A 
History of Chemical Theories and Laws, New York, 1907 ; 1. Freund, The Study of Chemical 
Composition, Cambridge, 1904; M. Faraday, Phil. Mag., (3), 24. 136, 1844. 

2 J. Dalton, A Neiv System of Chemical Philosophy, London, 1. 135, 136, 147, 189, 190, 180». 

3 J. Dalton, A New System of Chemical Philosophy, London, 1. 214, 1808. 

* J. J. Berzelius, Essai sur la tMorie des proportions chimiques et aur Vtnjluencc chimtque de 
Velectricite, Paris, 117, 1819. 

§ 16. The Evolution of the Atomic Theory up to the time of Dalton 

II est regrettable que les traitees modemes negligent I'histoire et pr^sentent comme des 
monuments acheves des sciences en perpetuelle Evolution. — F. OsMONr> (190b). 

The atomic theory seems to have been born in the twiUght of liistory. The 
earhest philosophers of the Eastern fore-world made many quamt guesses at the 


constitution of matter. Among these guesses, there is one which appears to have 
been promulgated by Kanada as a doctrine among the ancient Hindus i long prior 
to the rise of Grecian philosophy. This doctrine assumed that the world of sensible 
matter is produced or constituted by the concourse of substantial or concrete 
monads or atoms moving more or less freely about one another. A similar guess 
was propounded by Leucippus about 450 B.C., and advocated as a doctrine about 
thirty years later — 420 B.C. — by his disciple Democritus.2 About 300 B.C. the 
same guess was elaborated by Epicurus into a definite system, and the same guess 
still lives, more or less modified, in modern chemistry. 

From the imperfect fragments which have been transmitted to us, it is scarcely 
possible to dissociate the ideas of Leucippus from those of Democritus. Epicurus 
taught Democritus' views, which thence passed to Lucretius, and were summarized 
in an immortal poem De rerum natura (written about 80 B.C.). According to C. Dau- 
beny,3 a Phoenician named Mochus promulgated similar views before Leucippus 
time ; and it has also been stated that the ideas of Pythagoras (c. 500 B.C.) about 
corpuscular monads, mentioned by Aristotle, in his Metaphysics (12. 6), were derived 
from the Egyptian priests. E. Zeller * has argued that the available evidence does 
not justify the assumption that Leucippus derived his hypothesis from Mochus, 
and he further considers that Democritus adopted nothing but mathematics from 
Pythagorean sources, since there is no affinity between the two philosophies. De- 
mocritus, however, travelled extensively on his own account ; and it is probable 
that he visited the Egyptian priests, the Chaldeans, and the Persians. There are 
traces of atomistic views in the writings of Empedocles (c. 500 B.C.), Anaxagoras 
(c. 450 B.C.), and Heracleitus (c. 450 B.C.). P. Gomperz ^ has emphasized his belief 
that the atomic theory of Leucippus and Democritus was a resultant of the labours 
of their predecessors, and that it " was the ripe fruit on the tree of the old doctrine 
of matter which has been tended by the Ionian philosophers." 

In the fifth century before Christ, Anaxagoras' attempt to compress an inflated 
bladder led him to recognize the impenetrability of matter. If matter be con- 
tinuous it was not so easy to see how movement without appreciable hindrance 
could be possible in air, and yet be impassably resisted by a rock. The atomic 
theory of Leucippus provided a satisfactory explanation. Motion in a medium 
is easy or difficult according to the disposition of the constituent atoms which makes 
it easy or difficult for the atoms to be displaced. The Hellenic theory of atoms 
seems also to have been opposed as a counter-proposition to the idea of Zeno (c. 460 
B.C.) that matter is infinitely divisible. Zeno argued that whatever be the dimen- 
sions of matter, it must be geometrically divisible, for however small a particle may 
be, it can be supposed to be halved, quartered, or split into a thousand parts. The 
atomicians, however, postulated that the monads or atoms could not be cut, bruised, 
broken, or frayed ; otherwise they would wax old, crumble, and lose their shape. 
Consequently, substances formed by the aggregation of wearable atoms would 
gradually change their characteristics. Water and earth, said Isaac Newton, 
composed of old worn particles would not be the same in nature and texture as water 
and earth originally composed of unworn particles. There is no reason to suppose 
that there has been any change in the character of water and earth in past ages, 
and hence, in order that nature may be enduring and permanent, it was inferred 
that the atoms must be adamantine and perdurable. Zeno's concept is quite 
different from that of the atomicians'. % The latter could have readily admitted with 
Zeno that atoms are capable of geometrical subdivision, but reserved the right to 
hypothecate that further subdivision does not occur. Consequently, with those 
apparently opposing tenets, said S. Brown, the disputants did not argue in answer 
to one another at all. They crossed swords without touching one another. Each 
fought his own shadow. 

Among other names for atoms, Democritus employed &to/xo, but Lucretius does not use 
this term. Lucretius' favourite expression is primordia or rerum primordia, which is trans- 
lated " the first elements " or " the first beginnings of things." Lucretius also uaes figures, 


semina, or aetnina rerum — the seeds of things ; materia corpora genitalia or prima ; corpora 
or corpora rerum or corpora materia ; elementa ; and corpuacula — but never atom. Cicero 
used Democritus' term atomi for these primitive corpuscles. The derivation of the term 
atom— a, not ; Te>i/a>, 1 cut — means something which cannot be subdivided. The present- 
day definition of an atom says nothing about its ultimate nature. John Dalton certainly 
considered the atom to be indivisible, and this is illustrated by his favom-ite aphorism : 
*' Thou knowest no man can split an atom." Thomas Graham (1842) defined the atom, 
not as a thing which cannot be divided, but as one which has not been divided. The term 
atom was once used for a small interval of time — according to Ducange, the ^|th part of 
a second— a moment. Thus, in the Greek text of Paul's First Epistle to the Corinthians 
15. 52), there is an expression : iv arSfi^, iv piirr) otpdaXnov — in an atom or moment, in the 
twinkling of an eye. 

Concrete indivisible atoms.— The more characteristic features of the Hellenic 

theory of the atomic constitution of matter, as expounded by Lucretius,^ are best 
illustrated by quotations from II. A. J. Munro's translation of Lucretius' poem. 

1. Matter is discrete, not a continuum. 

However long you may hold out by iirging many objections, you must needs in the end 
admit that there is a void in things. . . . Wherever there is empty space which we call 
void, there body is not. ... If there were no empty void, the universe would be solid . . . 
for without void, nothing seems to admit of being crushed in, broken up, or split in two. 

2. All substances are formed of soUd atoms which are separated from one 
another by void space. Each atom is a distinct individual. 

First beginnings are of solid singleness, massed together and cohering closely by means 
of least parts, not compounded out of a union of those parts, but, rather, strong in ever- 
lasting singleness. . . . First beginnings are strong in solid singleness, and by a denser 
combination of these all things can be closely packed and exhibit enduring strength. 

3. The atoms are impenetrable, indivisible, and indestructible. They are as 
perfect and fresh to-day as when the world was new. 

There are therefore certain bodies which can neither be broken in pieces by the stroke 
of blows from without, nor have their texture undone by aught piercing to their core, nor 
give way to any other kind of assault. . . . Since by the laws of nature it stands decreed 
what these things can do and what they cannot do, and since nothing is changed, but all 
things are constant . . . they must sure enough have a body of unchangeable matter also. 
Therefore, if first bodies are as I have shown solid and without void, they must be everlasting. 
. . . For if the first begumings or things could in any way be vanquished and changed it 
would then be uncertain too what could and what could not rise into being, in short on what 
each thing has its powers defined, its deepest boundary mark. . . . From these parts nature 
allows nothing to be torn, nothing further to be worn away, reserving them as the seeds of 

4. The atoms differ from one another in shape, size, and weight. 

Next in order apprehended of what kind and how widely differing in form are the b^in- 
ning of things, how varied by manifold diversities of shape. . . . The things which are able 
to affect the senses pleasantly consist of smooth round elements ; while all those, on the 
other hand, which are found to be bitter and harsh, are held in connexion by particles that 
are more hooked and for this reason are wont to tear open passages in our senses, and on 
entering in to break through the body. . . . And quickly as we see wines flow through a 
strainer, sluggish oil on the other hand is slow to do so, because sure enough it consists of 
elements either larger in size or more hooked and tangled in one another. . . . Again things 
which look hard and dense must consist of particles more hooked together, and be held in 
imion because compacted throughout with branch-like elements. . . . Those things which 
are liquid and of fluid body ought to consist more of smooth and round elements. 

5. There is a finite number of different kinds of atoms, but an infinite number 
of homoeomeric atoms of each kind. 

The first beginnings of things have different shapes, but the number of shapes is finite. 
If this were not so, then once more it would f oUow that some seeds must be of mfinite bulk 
of body. . . . Wherefore you cannot possibly believe that seeds have an infinite variety of 
forms, lest you force some to be of monstrous hugeness. . . . The first begmnings of things 
which have a like shape one with another, are infinite in number. For smce the difference 
of forms is finite, those which are like must be infinite or the sum of matter will be finite 
which I proved not to be the case. ... It is clear then that in any class you like the tirst 
beginnings of things are infinite, out of which all supphes are furnished. 


6. The properties of all substances depend upon the nature of the constituent 
atoms, and the way the atoms are arranged. In his Metaphysics, Aristotle illustrated 
the effect of shape, arrangement, and position by examples borrowed from the Greek 
alphabet, and his illustration may be interpreted : The difference of sha^e is illus- 
trated by the opposition of A and N ; the difference of arrangement or contact, by 
AN and NA ; and that of 'position^ by the conversion of N to Z by turning the former 
on its side. 

It often makes a great difference with what things and in what position the same first 
beginnings are held in union and what motions they mutually impart and receive ; for the 
same make up heaven, sea, lands, rivers, sun ; the same make up com, trees, living beings ; 
but they are mixed up with different things and in different ways as they move. Nay, you 
see throughout even in these verses of ours many elements conunon to many words, though 
you must needs admit that the lines and words differ one from the other both in meaning 
and in the sound wherewith they sound. So much can elements effect by a mere change of 
order, but those elements are the first beginnings of things can bring with them more 
combinations out of which different things can severally be produced. 

7. The atoms are in constant motion ; motion is an inherent property of atoms. 

Solid bodies of matter fly for ever unvanquished through all time. . , . The first 
beginnings of things move of themselves, . . . No rest is given the first bodies through the 
imfathomable void, but driven on rather in ceaseless and varied motion they partly, after 
they have been pressed together, rebound leaving great spaces between, while in part they 
are so dashed away after the stroke as to leave but small spaces between. . . . Herein you 
need not wonder at this, that though the first beginnings of things are all in motion, yet 
the sun is seen to rest in supreme repose, unless where a thing exhibits motions with its 
individual body. For all the nature of first things lies far away from our senses beneath our 
ken ; and therefore since they are themselves beyond what you can see, they must with- 
draw their motions from sight also ; and the more so that the things which you can see, do 
yet often conceal their motions when a great distance off. For often the woolly flocks as 
they crop the glad pastures on a hill, creep on whither the grass jewelled with fresh dew 
sununons and invites each, and the lambs, fed to the full, gambol and playfully butt ; all 
which objects appear to us from a distance to be together and to rest like a white spot on 
a green hill. 

8. Combination or aggregation is due to the coalescence of moving particles. 
Democritus supposed the particles to move in straight lines, and the collisions to 
be accidental. In order to better the account for the coalescence, Epicurus supposed 
that the atoms moved in paths which deviated sUghtly from the rectilineal. 

When bodies are borne downwards sheer through void, at quite uncertain times and 
uncertain points of space they swerve a little from their equal poise ; you just and only 
just can call it a change of inclination. If they did not swerve, they all would fall 
down, like drops of rain, through the deep void, and no clashing would have been begotten, 
nor blow produced among the first beginnings ; thus nature never would have produced 

A. A. Cournot ^ believes that none of the ideas bequeathed to us by the ancients 
has had a greater or even a similar success to the atomic doctrine of Leucippus and 
Democritus. So far as the experimental evidence available to the Grecian philo- 
sophers in support of this particular doctrine is concerned, its long life, in the form 
of the chemist's atomic theory, can be attributed to chance, for if a sufficient 
number of thinkers speculate about the structure of matter, without checking their 
conclusions with facts, it is but in accord with the laws of probability that some of 
them will approximate to the truth. As C. Daubeny has said : 

The earliest philosophers appear to have often lighted upon the most sublime truths, 
astonishing us with an intermixture of the noblest views of nature with the most crude and 
vulgar conceits, and often leaving to their successors little more than the task of selecting 
from the mass of error, the grains of truth which are disguised by and confounded with it. 

The modern theory, unlike the older speculation, is based upon the observed laws 
of chemical change, and can scarcely stand apart from them. 

There is almost an historical continuity in the treatment of the doctrine from 
Leucippus to John Dalton (1801) — with a break during the dark ages. The atomism 


of Deinocritus and Epicurus grew into the corpuscular mechanics of the seventeenth 
century, and into the atomic theory of the nineteenth century. Francis Bacon ® 
was one of the first of the Renaissance philosophers of the seventeenth century to 
recognize the importance of Democritus' doctrine of atoms ; but he later regarded 
the study as unprofitable : 

Men do not cease from dissecting nature until they arrive at the atom ; a thing which 
if true, can do but little for the welfare of mankind. 

The atomic hypothesis was accepted with minor modifications by Robert 
Boyle,® who said in his Sceptical Chymist (Oxford, 1661) : 

There are clusters wherein the particles do not stick so close together, but they may 
meet with corpuscles of another denomination, disposed to be more closely united with them 
than they were among themselves ; and in such case, two corpuscles thus combining, losing 
that shape, size, or motion upon whose account they exhibited such a determinate quality, 
each of them really ceases to be a corpuscle of the same denomination as it was before ; 
and from the coalition of these, there may result a new body, as really one as either of the 
corpuscles before they were confounded. 

If this were paraphrased into the language of to-day it would be taken to embody 
the idea of a chemical affinity uniting atoms into compounds. Robert Hooke (1665), 
John Mayow (1669), Nicolas Lemery (1675), and most of the philosophers of 
the Renaissance — R. Descartes (1644), Pierre Gassend (1647), C. Huygens (1690), 
G. Amontons (1702), N. de Malebranche (1712), and M. N. LomonossofE (1741) 
— were atomicians.^® Rene Descartes seems to have beheved in the existence of 
atoms, but he substituted in place of an interatomic void, a subtle imponderable 
atomic fluid, the materia coelestis, which occupied the space between the atoms of 
matter. Therefore, while a given space could be freed from ponderable matter, 
the materia coelestis still remained. This is equivalent to the more modern state- 
ment that an aether- vacuum is impossible. N. de Malebranche (1712) dogmatically 
asserted that 

The matiere subtile or ether ee is necessarily composed of petUs tourhillona- — small vortices 
— which are the natural cause of all material changes, and of the most general phenomena 
— e.g. hardness, fluidity, weight, buoyancy, the refraction and reflection of light, etc. 

Isaac Newton (1675) ^ assumed that the atoms of a compound were held together 
by attractive forces so long as they did not approach within a certain limiting 
distance ; within this limit repulsive forces were supposed to come into play which 
prevented absolute contact and gave rise to the resilience of the particles during 
impact. Newton also tried to explain Boyle's law on the assumption that gases 
were made up of mutually repulsive particles, which recede from one another as far 
as the pressure of the superincumbent atmosphere will let them ; and he referred 
chemical changes to different associations of atoms. 

R. Kirwan (1783), Bryan Higgins (1776), and William Higgins (1789),i2 with 
more or less confidence, explained the constant composition of salts in terms of 
atoms. Bryan Higgins recognized seven elements composed of " atoms homo- 
geneal, impenetrable, immutable, in figure inconvertible, and globular ; " and he 
appears to have held the view that two different atoms combine in the proportions 
of 1 : 1, and in that proportion only. William Higgins imagined a combination in 
multiple proportions, but believed that the binary combination 1 : 1 was the most 
stable. Thus, he said : 

In volatile vitriolic acid, a smgle ultimate particle of sulphur is united only to a single 
particle of dephlogisticated air ; and in perfect vitriohc acid, eveiy smgle particle of sulphur 
is imited to two of dephlogisticated air, being the quantity necessary to saturation. 

This idea appears to have arisen in Higgins' mind because it was assumed that 
atoms of the same kind are mutually repulsive and that a combmation contaimng 
two similar atoms would have a greater tendency to disruption on account of the 


assumed mutual tendency of similar atoms to break apart. About this time, 
W. Nicholson i^ defined chemistry as a science of atoms, for he said : 

Chemistry, as a science, teaches the methods of accounting for the changes produced 
in bodies by the motions of their parts amongst each other which are too minute to affect 
the senses individually ; and, as an art, it consists in the application of bodies to each other 
in such situations as are best calculated to produce those changes. 

Then followed John Dalton's announcement of the atomic theory and the law 
of multiple proportions at a lecture delivered at the Royal Institution, London, in 
1803-4 ; the theory was described in T. Thomson's System of Chemistry (Edinburgh, 
1807), and by Dalton himself in the following year, in the first part of his remarkable 
book, A New System of Chemical Philosophy (Manchester, 1808-10), where he sa} b : 

It is one great object of this work to show the importance and advantage of ascertaining 
the relative weights of the ultimate particles, both of simple and compound bodies, the 
number of simple elementary particles which constitute one compound particle, and the 
number of less compound particles which enter into the formation of one or more compoimd 

Quite a number of different suggestions have been made to explain how Dalton 
came to give to the atomic hypothesis he had no doubt imbibed from Isaac Newton, 
the distinguishing features which led to its being called Dalton's atomic theory. 
Dalton's own accounts of the genesis of the hypothesis are not always consistent, 
so much so, that H. Debus i* has breathed an improbable suspicion that J. Dalton 
dehberately made a mystery of the evolution of the theory. In his System of 
Chemistry^ 1807, T. Thomson stated that the theory was suggested to J. Dalton by 
a comparison of the analyses of marsh gas and olefiant gas ; but J. Dalton's note- 
books show that the experiments on these gases were made in the summer of 1804, 
nearly a year after the first table of atomic weights had been compiled. H. E. 
Roscoe and A. Harden,i^ in opposition to H. Debus,i6 attempted to prove that Dalton 
was influenced in the development of the theory by experiments on the diffusion 
and solubiHty of gases, which led him to try to find the relative sizes of the particles 
of different gases. This involved a determination of the relative weights of the 
particles of each gas, which, in turn, necessitated a determination of the chemical 
composition of the gas. The results so obtained led J. Dalton to deduce the atomic 
theory. In a series of important papers on The Development of the Atomic Theory 
(1909-11), A. N. Meldrum i7 showed that the facts admit of a somewhat different 

At the beginning of the nineteenth century, the diffusion of gases was supposed 
to be the work of chemical affinity, and the oxygen and nitrogen in the atmosphere 
were supposed to be chemically combined. In 1801, J. Dalton ^^ argued that the 
phenomenon is physical and that the mixture of oxygen and nitrogen gases in 
atmospheric air is mechanical because the " nitric acid gas " formed by the union of 
these two elements is "an elastic fluid totally distinct in its properties from either 
of the ingredients." Dalton frequently quoted Newton's views on the attraction 
and repulsion of atoms, and, in a lecture in 1810, Dalton explained that he did not 
at first consider a possible difference in the sizes of the particles of the two elastic 
fluids, but he said that in 1805, he considered that the sizes must be different because, 
no equilibrium can be established by particles of different sizes pressing against 
each other. In Dalton's notebooks, these views are dated Sept. 14th, 1804. 
According to H. E. Roscoe and A. Harden, these dates are wrong, for they assume 
that, having established a difference in the sizes of the particles of the elastic fluids, 

Dalt-on thence proceeded to determine the relative sizes and tveights, together with the 
relative numbers of atoms in a given volume. This led the way to the combination of gases. 
. . . Thus a train of investigation was laid for determining the number and weight of all 
chemical elementary principles which enter into any sort of combination with one another. 

Otherwise expressed, it is assumed that Dalton first satisfied himself that the 
atoms of different gases have different sizes, and then devised the chemical theory. 



A. N. Meldrum (1911), however, has shown that J. Dalton did not conclude that the 
atoms of different gases were different in size until after the chemical theory had 
been formed. In J. Dalton's notebook, dated Sept. 6th, 1803, the first table of 
atomic weights appears in the annexed form : 

Ult. at. hydrogen 

,, azote 

„ carbon 

„ water 

„ ammonia. 

„ nitrous gas 








Ult. at. 

nitrous oxide . 

. 13-66 

nitric acid 

. 15-32 


. 17 

sulphuroTis acid 

. 22-66 

sulphuric acid . 


carbonic acid . 


oxide of carbon 

. 10-2 

A. N. Meldrum has also indicated that John Dalton probably arrived at the 
law of multiple proportions as a result of experiments on the combination of nitric 
oxide and oxygen whereby he was able to write in his notebook, Aug. 4th, 1803, 
that 100 measures of air could take 36 or 72 of nitric oxide. J. Dalton then probably 
framed the rule that atoms combine in the proportion 1:1, and on considering the 
more complex cases, he tested the possibility of combination in other proportions 
by the available analytical data, so that, in the following month, Sept. 6th, he was 
able to draw up the table of atomic weights. 

Punctual atoms or centres of force. — The Lucretian school has never receded 
from the primary assumption that matter is composed of ultimate, solid particles — 
potentially divisible, but physically incapable of further subdivision ; but another 
school of atomicians has assumed that there is no limit to the divisibiUty of the 
particles of matter, and that the smallest conceivable particle still consists of an 
infinitude of smaller particles. Bene Descartes has said : i^ 

It is very easy to recognize that there can be in substance no atoms, that is to say parts 
of bodies or matter which are by nature indivisible, as some philosophers have imagined ; 
in as much as however small we may suppose these parts to be, yet, since they must be 
extended, we see there is not one of them that cannot be further divided into two or more 
others, of smaller size, and is accordingly divisible ; 

and I. Kant (1781), in his Observations on the Second Antinomy ^^^ argued that those 
who object to the infinite sub-divisibility of matter do not recognize the clearest 
mathematical proofs as propositions relating to the constitution of space. Zeno 
(460 B.C.) previously argued that matter must be made up of indivisible and un- 
extended points. Some such particles as these — foints de substance — were imagined 
by G. W. von Leibniz (1695), and called monads — /xova?, a unit— a term which 
was employed by Pythagoras, and which is said to have been suggested to Leibniz 
by G. Bruno's De ynonade (Frankfurt, 1591), or during his intercourse with F. M. 
van Helmont. Leibniz's ideas were described in his posthumous La monadohgie 
(Berlin, 1840) : 21 

Material atoms, still composed of parts, are contrary to reason, for the inviolable attach- 
ment of one part to another — if we could reasonably conceive or suppose such a thing — 
would not destroy their diversity. 

Newton himself seems to have had some misgivings about the indivisibility of atoms, 
for he said in his Philosophice natiiralis principia mathematica (Londoni, 1687) : 

Whether these parts, distinct, and as yet imdivided by material forces, are able to be 
divided and sundered in their turn is uncertain. 

The main difficulty with Leibniz's animated points is to understand how a body 
can possess extension in space if it be made up of components which have no spatial 
dimensions, for, as J. C. Maxwell (1877) observes, that which has neither figure nor 
extent can have no existence. The Democritians— Newton, etc.— assumed that 
it is necessary to suppose that the ultimate particles must possess some bulk, other- 
wise they could not produce bulk by aggregation ; on the contrary, Zeno, \V olf , 
Schelling, etc., do not consider this assumption necessary, for a number of self- 
repulsive points in limited space can also communicate bulk to the body they 


compose. For instance, if a point were endowed with the irresistible power of 
repelling the hand from a radius of one inch, the result would be the same as if 
the hand were to grasp a 2-inch ball of adamant. 

R. J. Boscovich,22 in 1763, attempted to improve Leibniz's ideas by assuming 
that matter is made up of unextended points which mutually attract one another, 
but which never come into contact because, as soon as they approach within a certain 
limiting distance, they mutually re-pel one another ; the repulsive forces increase 
more and more in intensity as the points approach closer and closer together, so that 
they never come into absolute contact. Extension in space is an effect of this 
repulsion, and the aggregation of matter is an effect of the attractive forces. He 

Matter is not mutually penetrable, but each atom-centre extends, so to say, throughout 
the whole of the solar system, yet always retaining its own centre of force. 

R. J. Boscovich assumed that when attraction predominates, the body is a solid, 
and a gas when repulsion predominates, while if the two forces are more equally 
balanced, a liquid results. 

A great deal has been written in favour of both hypotheses — Newton's that 
an atom is a solid nucleus surrounded by spheres of repulsive and attractive forces ; 
and Boscovich' s that an atom is a mathematical point with a sphere of a repulsive 
force surrounded by a sphere of an attractive force. In 1844, in A speculation 
touching electrical conduction and the nature of matter, M. Faraday 23 points out that 
in the ordinary atomic theory it is assumed that solids, liquids, and gases are com- 
posed of material atoms occupying a definite space, and are held together by cohesive 
forces ; and further, in order to explain the contraction in volume which occurs 
on cooling or compressing solids, liquids, or gases, it is assumed that atoms cannot 
be in actual contact, but must be separated by an intervening space. These 
assumptions involve the following dilemma : If space is a non-conductor of elec- 
tricity in non-conducting bodies, and a conductor in conducting bodies, we are 
compelled to assume that space possesses opposite and contradictory qualities, 
for if space be an insulator, it cannot exist in conducting bodies, and if it be a 
conductor, it cannot exist in insulating bodies. Hence, M. Faraday wrote : 

I feel a great difficulty in the conception of atoms of matter with intervening spaces not 
occupied by the atoms. . . . The atoms of Boscovich appear to me to have a great advantage 
over the more usual notion. His atoms are mere centres of forces or powers, not particles 
of matter in which the powers themselves reside. 

There is a similar dilemma involved in connection with the transmission of light, 
and the physicists, A. M. Ampere (1835), A. L. Cauchy (1836), and M. Seguin (1853), 
have accordingly regarded atoms as centres of force infinitely small, without 
extension in space. Cauchy's punctual atoms were supposed to vibrate differently 
in different directions so that the elasticity varied accordingly. J. F. Redten- 
bacher (1857) 24 regarded this as an impossible assumption. The atoms, said Ampere, 
regarded as les centres d' actions moleciilaires, ne doivent pas etre considerees seulement 
comme tres petites relativement aux distances qui les separent, mais conime rigoureuse- 
ment nulles. 

The difference in the two sets of hypotheses turns on whether cohesive or other 
forces emanate from imniaterial points of zero volume, or from material particles 
each occupying a definite volume. Which hypothesis is to be accepted ? It must 
be remembered that we can persuade ourselves that matter itself can be spirited 
away by trying to conceive the residuum which remains when each property known 
to be a manifestation of energy is subtracted from matter. An extended nothing, 
said G. W. von Leibniz, is meaningless, an extended something must have quality, 
and to call that quality extension is to cover up the difficulty with a name. J. 
Locke (1690), and G. Berkeley (1713), M. Faraday (1844), W. Ostwald (1892), as well 
as earlier and later philosophers, have emphasized how impossible it is to conceive 
or imagine the existence of matter independent of energy ; we have evidence of 


the existence of energy, and therefore, the supposition that a material world reallv 
exists apart from energy is undemonstrable and false. The chemist, however, 
progresses with his work on the assumption that he lives in a material world which 
it is his business to investigate. 

The atomic theory is the only satisfactory hypothesis which has correlated the 
numerous facts relating to the transformations of matter. It may be perfectly 
true. Lord Kelvin (1874) has pointed out, that the assumption of atoms can explain 
no property of a body which has not previously been attributed to the atoms 
themselves. This, added H. von Helmholtz, is not evidence against the existence 
of atoms, but is rather against efforts to derive the foundations of theoretical physics 
from purely hypothetical assumptions as to the atomic structure of natural bodies. 
The assumption of atoms has none the less proved an invaluable aid in forming 
vivid mental pictures of the different phases of a chemical reaction ; it has served 
as a wonderful stimulus to the chemical explorer, for it has enabled chemists to 
anticipate successfully the results of experimental research. The vitality of this 
time-honoured theory is remarkable ; it is ever assimilating new facts, and ever 
enticing the chemist to fresh fields and pastures new. Innumerable prophecies 
based on the atomic hypothesis have been completely verified so that the atomic 
theory is now regarded as a pyramid of truth. Consequently, although no one has 
ever seen an atom, A. R. A. Smith (1884) could say : We believe in atoms because, 
so far as we can see, nature uses them. The greater the number of facts con- 
sistently explained by one and the same theory, the greater the probability of its 
being true. The overwhelming mass of circumstantial evidence, direct and in- 
direct, which modern chemistry and physics offer, has justified the faith of Dalton ; 
and almost, but not quite, demonstrated the real existence of tangible atoms. 


^ H. T. Colebrooke, Asiatic Researches of Calcutta, 5. 1, 1799. 

2 J. Ferguson, Proc. Phil. Soc. Glasgow, 16. 36, 287, 1884. 

^ C. Daubeny, An Introduction to the Atomic Theory, Oxford, 1831. 

* E. Zeller, Die Philosophic der Griechen, Leipzig, 1876-82 ; The Pre-Socratic Philosophy, 
London, 2. 207, 1881 ; F. A. Lange, Geschichte der Materialismus, Leipzig, 1. 3, 1908; 2. 181, 

5 H. C. Bolton, Amer. Chemist, 3. 326, 1873 ; P. Gomperz, Greek Thinkers, London, 1. 323. 
1901 ; S. Brown, Critical Lectures on. the Atomic Theory, Edinburgh, 1843 ; T. Graham, Elements 
of Chemistry, London, 1842. 

« H. A. J. Munro, T. Lucreti Cari de natura rerum, Cambridge, 1873 — ^the translations in the 
text are mainly Munro's ; J. Masson, The Atomic Theory of Lucretius, London, 1884 ; A. Brieger, 
Die Urbewegung der Atome und die Weltentstehung bei Leukipp und Demokrit, Halle, 1884 ; H. C. 
Liepmann, Die Mechanik der leucipp-democrif schen Atome, Berlin, 1885 ; P. Gomperz, Greek 
Thinkers, London, 1. 316, 1901 ; J. Burnet, Early Greek Philosophy, London, 380, 1908; J. C. 
Maxwell, Encyc. Brit, 3. 36, 1877 ; 1. Freund, The Study of Chemical Composition, Cambridge, 1904 ; 
M. Giua, Gazz. Chim. Ital., 49. ii, 1, 1919 ; J. Gregory, Science Progress, (2), 14. 479, 1920. 

' A. A. Cournot, Traite de V enchainement des idees fondamentules datis les sciences et dans 
Vhistoire, Paris, 1. 245, 1861 ; C. Daubeny, An Introduction to the Atomic Theory, Oxford, 1831. 

8 F. Bacon, De principiis atque originibus, London, 1612 ; Novum Organum, London, 1620. 

» R. Boyle, The Usefulness of Experimental Philosophy, Oxford, 1663 ; The Sceptical Chymist, 
Oxford, 1661 ; R. Hooke, Micrographia, London, 1665 ; J. Mayow, De sal nitre et spiritu nitro- 
aereo, Oxford 1, 669 ; N. Lemery, Cours de chimie, Paris, 1675. 

*» P. Gassend, Opera omnia, Florentiae, 1727 ; F. Bemier, Abrege de la philosophic de Gassendi, 
Lyons, 1684 ; C. Huygens, Discours de la cause de la pesanteur, Leiden, 1690 ; N. de Malebranchc, 
Recherche de la verite, Paris, 1712 ; M. W. Lomonossoff, Elementa chymice mathematica, St. Peters- 
burg, 1741 ; Ostwald's Klassiker, 178, 1910 ; R. Descartes, Principia philosophic, Amsterdam, 

11 L Newton, Opticks, Jjon^on, 1704 ; Philosophicenaturalis principia mathematica, C&mhndge, 
1687. .^ , J . 

12 W. mggins. Comparative View of the Phlogistic and Antiphlogistic Theories with Inductions, 
London, 1789 ; B. Higgins, Philosophical Essay concerning Light, London, 1776 ; A. N. Meldrum. 
Mem. Proc. Manchester Lit. Phil. Soc., 55. 4, 1910. 

i\W. Nicholson, A Dictionary of Chemistry, London, 1795. 

1*; H. Debus, Zeit. phys. Chem., 29. 266, 1899. 

VOL. T. ^ 


^^ H. E. Roscoe and A. Hai:den, A New View of the Origin of Dalton'a Atomic Theory, London, 

** H. Debus, Ueber einige Fundamentcdsdtze der Chemie inshesondere das Dalton-Avogadrosche 
Oesetz, Cassel, 1894; Phil. Mag., (5), 42. 350, 1896; Zeit.phys. Ghent., 20. 359, 1896; 24. 325, 
1897 ; 29. 266, 1899 ; 30. 556, 1899 ; G. W. A. Kahlbaum, ih., 29. 700, 1899 ; H. E. Roscoe and 
A. Harden, ih., 22. 241, 1897 ; Phil. Mag., (5), 43. 153, 1897. 

1' A. N. Meldrum, Mem. Proc. Manchester Lit. Phil. Soc., 55. 5, 6, 1911. 

18 J. Dalton, Mem. Proc. Manchester Lit. Phil. Soc., 5. 538, 1802. 

i» R. Descartes, (Euvres, Paris, 3. 137, 1824. 

20 I. Kant, Kritik der reinen Vernunft, Riga, 1781 ; London, 274, 1860. 

21 G. W. von Leibniz, The Monadohgy, Oxford, 1898. 

22 R. J. Boscovich, Theoria philosophice nnturalis reducta ad unicam legem virium in natura 
existentium, Venetiis, 1763. 

2» M. Faraday, Phil. Mag., (3). 24. 136, 1844 ; (3), 27. 345, 1845 ; E. J. Mills, ih., (4), 42. 112, 

1871 ; R. Laming, ib., (3), 27. 420, 1845 ; H. Sloggett, ih., (3), 28. 443, 1846 ; W. H. Walenn, ih., 
(4), 39. 123, 1870 ; C. R. A. Wright, ih., (4), 43. 241, 503, 1872 ; R. W. Atkinson, ih., (4), 43. 428, 

1872 ; (4), 44. 118, 1872 ; A. Tribe, ih., (4), 44. 121, 1872 ; A. W. Williamson, Jaarn. Chem. Soc, 22. 
328, 1869. 

2* A. L. Cauchy, Memoire sur la dispersion de la lumiere, Prag, 1836 ; J. F. Redtenbacher, 
Das Dynamiden-system, Mannheim, 1857. 

§ 16. The Language o£ Chemistry 

However certain the facts of any science, however just the ideas derived from these 
facts, we can only communicate false or imperfect impressions to others, if we want words 
by which these may be properly expressed. — A. L. Lavoisier. 

Words are the footsteps of reason. — Francis Bacon. 

The nomenclature of a science, that is, the group of technical terms pecuUar 
to that science, is of vital importance. It is virtually impossible to separate the 
nomenclature from the science itself. Lavoisier emphasized the importance of this 
in his classical Traite elementaire de cJiimie (Paris, 1789). Every science consists 
of three things : (1) the facts which form the subject-matter ; (2) the ideas repre- 
sented by those facts ; and (3) the words in which those ideas are expressed. Like 
three impressions of the same seal, said Lavoisier, the word ought to produce the 
idea ; and the idea ought to be a picture of the fact. 

Special technical words have been invented to fix and describe the ideas and 
principles of chemistry — as of all other sciences. Technical terms should be precise 
and clear, and not tainted with ambiguity and vagueness. Such technical terms 
form part of the current language of chemistry, and they are of international value. 

Technical terms are obtained in two ways : (1) Owing to the poverty of language, 
words in colloquial every-day use are commandeered, and are given, by a special 
definition, a specific meaning. Such words are a proHfic source of error and con- 
fusion, and they ofttimes lead to needless controversies because they have a variety 
of difierent meanings — energy, force, atom, etc., are examples. (2) Terms are 
specially invented for a specific purpose — electron, and telegraph, are examples. 
These terms are much less liable to misapprehension than adaptations of every-day 
words which possess several meanings. However strange the special terms may 
appear at first, they soon grow familiar to the ear, and they can be used without 
effort. W. Whewell has pointed out, very aptly, that " technical terms carry the 
results of deep and laborious research. They convey the mental treasures of one 
period to the generations that follow ; and laden with this, their precious freight, 
they sail safely across the gulfs of time in which empires have suffered shipwreck, 
and the language of common fife has sunk into oblivion." Witness : some of the 
terms used in the chemistry of to-day ware coined by the early Arabian chemists 
— e.g. alcohol, alkali, borax, elixir, lac, etc. 

Naming the elements. — A great number of the elements have been endowed 
with names which refer to some salient property or characteristic, e.g. iodine — 
from its violet vapour ; chlorine — from its green colour ; chromium — from the 
colour of its compounds ; rhodium — from the rose colour of its salts ; osmium — 


from its smell ; argorir-hom its indifference to chemical reagents ; similarly with 
platinum which refers to the silvery appearance of the metal— from the Spanish 
plata, silver. Likewise with the names phosphorus, radium, quicksilver, bromine, 
nitrogen, oxygen, hydrogen, argon, glucinum, iridium, praseodymium,' thallium,' 
mdium, caesium, and rubidium. Other elements have been named more or less 
capriciously ; thus some elements are named after particular localities— sfrow^iwrn, 
from Strontian (in Scotland) ; ruthenium, from Ruthenia (Russia) ; yttrium, ytter- 
hium, erbium, and terbium are all derived from Ytterby (in Sweden). Some elements 
have been named in honour of some country or from association with some other 
event at the time of their discovery — e.g. helium, from its occurrence in the sun ; 
gallium, from Gallia (Gaul) ; germanium, from Germany ; lutecium, from Leutece, an 
old name for Paris ; 'palladium was named after the planetoid Pallas discovered 
about the same time : uranium was likewise named in honour of the discovery of 
the planet Uranus ; etc. Some names refer to the minerals in which they occur ; 
beryllium is derived from the name of the mineral beryl ; zirconium, from the 
mineral zircon : similarly with molybdenum, and many others. Some names 
refer to renowned personages — e.g. victorium, from Queen Victoria ; similarly 
with gadolinium, from J. Gadolin; and mosandrum, after G. Mosander. 
Other names refer to mythological personages — e.g. thorium, from Thor, 
the son of Odin, a god in Scandinavian mythology ; vanadium, from a Scan- 
dinavian goddess, Vanadus ; tantalum, from Tantalus in Grecian mythology ; 
niobium, from Niobe, daughter of Tantalus ; i and similarly with cerium, titanium, 
palladium, and uranium. Some names are emblematic — e.g. selenium, cobalt, 
and nickel. 

Unfortunately some elements have not yet been christened with a name recog- 
nized by all. Niobium — symbol Nb — and columbium — symbol Cb — are two different 
names for one element : glucinum— symbol Gl — and beryllium — symbol Be — are 
two different names for another element. There is at present a struggle for exist- 
ence between these terms, no doubt the fittest will survive. The first terms here 
employed were recommended by the International Association of Chemical 
Societies, September, 1913 ; and F. W. Clarke wrote a strong protest, and claimed 
columbium in place of niobium for historical reasons. 

Symbols. — The old alchemists used to represent different substances by quaint 
sometimes fantastic symbols — an example is given in Fig. 1, Cap. I. The hieroglyphs 
of the Hermetic priests in Egypt, and the fantastic symbols of the alchemists of the 
Middle Ages, were attempts to hide knowledge from the vulgar, and to surround 
the study of nature with difficulties and mysteries. The symbols of the modern 
chemist, on the contrary, are intended to facilitate the study of chemistr}^ by 
abbreviating complicated expressions so that their meaning can be seen at a glance. 
Some of the older symbols did come under this category ; for example, gold has 
been represented by the picture of a king on his throne ; by the symbol O or Q, 
for the sun, etc. ; silver, by (I, the moon ; etc. Fantastic symbols, like that 
indicated in Fig. 1, Cap. I, could lead only to confusion. Symbols were employed 
by Raymond fully somewhat frequently in the thirteenth century. Possibly the 
alchemists intended the symbols to convey some idea of the peculiarities of the metals 
they represented ; indeed, it has been suggested that the circle which appears in 
certain of the symbols was intended to illustrate the perfection of the metalhc 
state, and the half circle, an approximation thereto. In any case the alchemists 
were very fond of symbols, and of obscuring their meaning by using mystic triangles 
and special hieroglyphs so as to make their writings like cryptograms which required 
a key before the meaning could be deciphered. 2 Thus, Raymond Lully in his 
Testamentum, duobus libris universam artem chimicam complectens (Colon, 1568), used 
the symbol yl to represent God the Creator, 5 stood for mercury,^, for saltpetre. . . . 
These symbols were not in general use, and each writer devised his own. The 
alchemists of the thirteenth century also represented Aristotle's four elements by 
triangles : A, fire ; A, air ; V, water, and V, earth. Other symbols gradually 


came into more or less general use ; thus, about the fourteenth century the symbol 
A for sulphur was fairly common in the writings of the alchemists. 
■^ At the beginning of the eighteenth century, symbols for chemical compounds 
began to be used more frequently, not with the idea of making the literature 
obscure and unintelUgible to the uninitiated, but rather for conciseness, brevity, 
and clearness. St. F. Geoffroy, in his Table de differents rapports observes en chimie 
entre differerUes substances (1718), used the ordinary alchemical symbols for the 
metals and introduced a number of others, e.g. for salt ; >0 for hydrochloric acid ; 
>CD for nitric acid ; >0-< for sulphuric acid, etc. In his De attractionibus electiviis 
(Upsala, 1775), T. Bergmann represented chemical reactions by symbols and signs. 
The two subjoined diagrams illustrate T. Bergmann's method. The symbols to 
the right and left, outside the brackets, represent the substances which react together ; 
and those above and below, the products of the reaction, if any, which separate from 
the system. The symbols within the brackets represent the reacting components ; 
and the disposition of the brackets is intended to indicate whether the products of 
the reaction are solid, or solution, or volatile. Thus, 

Represents the action which occurs Represents the action which occurs 

when an aqueous solution (V) of calcium when an alloy of gold and copper (0 $ ) is 

sulphide (^4^) is treated with sulphuric a^id fused (A) with antimony sulphide (^); 

((^). The lime {^) and sulphuric acid The copper ($) and gold ( ) are separated ; 

((^) unite together to form calcium sul- ,, , _ . , , , \x 

r: /u^rTL X t,- I, • • -^ * J / X *li® copper ($ and sulphur (^) unite 

phate (TUh,) which is precipitated (^-v— ) , ^^ ^ ^' ,. - v + 

^ v+ I./ 11 V ; together to form a sohd (-v-), and the 

and the sulphur ( A ) also remains as a gold ( ) and antimony ( 5 ) also unite to 
soUd. ( -^ ). form a solid ( —^ ). 

A. F. de Fourcroy ^ employed a similar method in 1784. It must be added that, 
about 1756, W. CuUen is said to have been the first to employ diagrams to illustrate 
chemical reactions. A. L. Lavoisier used the symbol v for water ; i^ for oxygen ; 
etc., and, like T. Bergmann (1775), he represented chemical reactions by combining 
these symbols in various ways. 

John Dalton, in his New System of Chemical Philosophy (Manchester, 1808), 
made a step in advance by representing the atoms of the elements by symbols, 
and combining these symbols so as to show the elements present in a compound. 
Thu3, represented hydrogen ; O oxygen ; # carbon, etc. Water was repre- 
sented by O0 ; carbon monoxide by 0# ; carbon dioxide by 0#O ; etc. These 
symbols have all been abandoned. They are too cumbrous. To-day, we follow 
J. J. Berzelius' method, suggested in various editions of his Larbok i Kemien 
(Upsala, 1811), and use one or two leading letters from the recognized name of 
the element to represent any particular element. The first letter is always a 
capital ; the second, if present, is always a small letter. 

Thus, O representib oxygen ; H, hydrogen ; C, carbon ; N, nitrogen ; CI, chlorine ; etc. 
The names of ten elements start with C, and to prevent the possibility of confusion, a second 
leading letter is selected either from the name, or from the alternative Latin name of the 
element. Thus C (carbon), Ca (calcium), Cb (columbium), Cd (cadmium), Ce (cerium), CI 
(chlorine), Co (cobalt), Cr (chromium), Cs (caesium), and Cu {cuprum)^ copper). The ele- 
ments with alternative I^atin names are symbolized : Sb for antimony {stibium) ; Cu for 
copper {cuprum) ; Au for gold {aurum) : Fe for iron (ferrum) ; Ag for silver {argentum) ; 
Pb for lead {plumbum) ; Hg for mercury {hydrargyrum,) ; K for potassium {kalium) ; Na 
for sodium {natrium) ; and Sn for tin (stannum). 

Naming the compounds. — Each element forms with other elements a group of 



compounds which are said to contain the respective elements, because the elements 
in question can be obtained unchanged from the compounds. Consequently, every 
compound has an elementary or ultimate composition. Compounds are symbolized 
by joining together the letters corresponding to the different elements in the 
compound. Thus, HgO represents mercury oxide, a compound of mercur}^ and 
oxygen. When only two elements are united to form a compound, the name of 
the second element is modified so that it ends in ide. 

The symbol for the element also represents one of its atoms. If more than 
one atom is present in a compound, a small figure is appended to the bottom— in 
France, generally at the top right-hand — corner of the symbol of the element, to 
indicate the number of atoms present. Thus, HgO represents a molecule of water, 
i.e. a compound containing two atoms of hydrogen and one of oxygen ; CO repre- 
sents a molecule of carbon monoxide — a compound containing one atom of carbon 
and one atom of oxygen ; NagCOs represents a molecule of sodium carbonate — a 
compound containing two atoms of sodium, one atom of carbon, and three atoms 
of oxygen. A letter affixed in front of a group of symbols represents the number of 
times that group is contained in the given compound. Thus, crystalUzed sodium 
carbonate is symbolized : NagCOs.lOHgO, meaning that this compound contains 
one equivalent of NagCOs, and ten equivalents of the group HgO. 

J. J. BerzeHus (1814) * represented two atoms of an element in a compound by drawing 
a bar through the symbol of the element ; for instance, HO represented HgO ; ¥^0^ ; FegOg ; 
OttO represented CugO ; etc. J. J. Berzelius also represented an atom of oxygen united with 
an element by placing a dot over the symbol of the element, and an atom of sulphur by a 
dash in a simUar position ; thus, Cu represented CuO ; Pb, PbOg ; Ca(5, CaOCOg ; CuS + ofi 
represented CuO, SOg + SHgO ; and Fe represented FeSg. This system did not last long 
in chemical literature, although the mineralogists used it for a longer time. 

Compounds of an element with oxygen are called oxides, and the process of 
combination is called oxidation. When an element forms more than one oxide, 
a Greek numerical suffix is often prefixed to the word oxide. Thus SOg is sulphur 
dioxide ; SO3, sulphur trioxide ; CO, carbon monoxide ; COg, carbon dioxide ; 
PbO, lead monoxide ; PbOg, lead dioxide or lead peroxide. The AngUcized Latin 
and Greek numerical prefixes are indicated in Table I. 

Table I. — Latin and Greek Numerical Prefixes. 










































Septa- : 



























































Half Whole Equal Many One and a half One third Four thirds 

Semi- Omni- Equi- Multa- Sesqui- Tertia- Quadritertia- 

Hemi- Hole- Homo- Poly- Hemitri- Trita- Tetratrita- 

It is considered bad style to mix Latin and Greek root words and pre6xes. Conse- 
quently we usually try to keep Greek with Greek, and Latin with Latin. Thus, we say 



" diatomic," not " biatomic " ; " bimolecular,'* not " dimolecular " ; " bivalent," not 
" divalent " ; and " bivariant," not " divariant " ; because " atomic " is derived from the 
Greek word, while " molecular," '* variant," and " valent," are derived from Latin words.^B 
There are, however, many hybrids universally recognized. E.g. millimetre, centimetre, etc.^H 
Monovalent, divalent, etc., are also used in spite of their hybrid character. In the appli- 
cation of the Greek numerals in organic chemistry, some hybrids are \ised — -e.g. in the 
methane series of hydrocarbons, Greek numerals are generally employed excepting for 
C9H20, C19H40, C29H60, • • • and for C11H24, C21H44, . . . where Latin numerals are used. 
The series thus runs pentane, C5H12 ; hexane, C6H14 ; heptane, CjHig ; octane, CJin; 
nonane, C9H20 ; decane, C10H22 ; undecano, C11H24; etc. For consistency nonane should 
be enneadecane, and undecane, hendecane, etc. The custom is so general, and so deeply 
rooted in the literature of organic chemistry, that, as F. Beilstein ^ says, the rectification 
gegenwdrlig nicht mehr empfehlenswert erscheint. This state of crystallization has not yet 
been attained in the naming of inorganic compounds, and the Greek n\imerical prefixes 
can be consistently used if thought desirable ; but " sesqui " is generally used whether Greek 
or I-.atin aflfixes are employed. However, we cannot always be purists without defying 
custom, which, as Horace has said, decides the language we must use. 

Sometimes the termination -ic is affixed to the name of the metal for that oxide 
which contains the greater proportion of oxygen, and -OUS for the oxide containing 
the lesser proportion of oxygen. For instance, SnO is either stannous oxide, or tin 
monoxide, and Sn02 is either stannic oxide or tin dioxide ; FeO is ferrous oxide ; 
and Fe203 ferric oxide. For historical reasons, the names of some compounds 
do not conform to this system because the affix "ic" was assigned to the compound 
first discovered, and the compounds subsequently discovered were named accord- 
ingly. Consequently, when only one series of compounds is known, the use of 
either termination is now avoided — thus, potassium, sodium, and magnesium are 
preferred to potassic, sodic, and magnesic respectively. The method of naming 
the compounds now under discussion is not always satisfactory when the elements 
form more than two compounds. To get over the difficulty, a prefix hypo- (under, 
or lesser) is sometimes added to a compound less rich in oxygen than the -OUS com- 
pound, and per-, hyper-, or super- (beyond, above) is added to the one with more 
oxygen. Thus, 

Persulphuric acid 

. H2S2O8 

Perchloric acid . 

. HCIO4 

Sulph\iric acid . 

. H2SO4 

Chloric acid 

. HCIO3 

Sulphurous acid 

. H2SO3 

Chlorous acid 

. HCIO2 

Hyposulphurous acid 

. H2S2O4 

Hypochlorous acid 


The six nitrogen oxides — nitrogen monoxide, dioxide, trioxide, tetroxide, pentoxide, 
and hexoxide — would be awkwardly named by this system. 

It will be observed that ous from the Latin osus means " richness," so that stannous 
means rich in tin, and etymologically stannous oxide means an oxide richer in tin than 
stannic oxide, and by implication poorer in oxygen. In actual use, therefore, the etymological 
meaning is inverted, and the implied signification has been universally adopted. Etymolo- 
gically the term hypo means less rich, so that hypochlorous means less rich in chlorine than 
chlorous — in practice the very opposite is the case, for hypochlorous acid has less oxygen 
than chlorous acid, and it contains a higher proportion of chlorine. Similar remarks apply 
to the prefixes per^ super, and hyper. 

Oxides Hke alumina — ^Al203 ; ferric oxide — Fe203, etc., are sometimes called 
sesquioxides {sesqui, one-half more). Compounds which have less oxygen than 
the normal are sometimes called suboxides [suh, below) instead of hypo-oxides, 
e.g. while CuO represents cupric oxide, CU2O represents cuprous oxide, and also 
copper suboxide ; similarly, while AgCl represents the normal silver chloride, 
Ag2Cl represents silver sw6chloride. Custom has restricted the use of hypo- to the 
acids or acidic oxides, and sub- to the basic or indifferent oxides. The oxides can 
be roughly divided into two classes. Some oxides, with water, form acids, and 
others act as bases. It is not very easy to draw a sharp line of demarcation between 
the two. The acidic oxides have a sour taste, and turn a solution of blue litmus 
red ; the basic oxides usually turn a solution of red litmus blue, and have a soapy 

The nomenclature of inorganic chemistry is thus based upon the principle that 


the different compounds of an element with other elements can be named by a simple 
change in the beginning or termination of the word — witness ferric and ferrous 
oxides ; and also by the addition of a numerical suffix showing the relative number 
of atoms of the corresponding element in its compounds. The systematic name of 
a compound thus indicates its composition.^ These little artifices, apparently trivial, 
are really important advances in the language of chemistry. The method has 
some defects, but when the necessity for a modification becomes acute, it will 
probably not be difficult to change. Language generally lags in the wake of 


1 P. Diergart, Journ. prakt. Chem., (2), 61. 497, 1900 ; J. Berendes, Chem}ztg., 28. 103, 663 
1899 ; H. Diels, EUmentum, Leipzig, 1899. 

2 G. B. Plowright, Pharm. Journ., 20. 289, 726, 1905 ; 22. 583, 1906; A. L. Lavoisier, Mem. 

Acad., 492, 1782. 

^ A. F. de Fourcroy, Memoires et observations de chimie, Paris, 308, 1784. 

*• J. J. Berzelius, Lehrbuch der Chemie, Dresden, 1827 ; Ann. Phil., 3. 51, 363, 1814. 

^ F. Beilstein, Handbuch der organischen Chemie, Berlin, 1. 49, 1918. 

* W. Whewell, The Philosophy of the Inductive Sciences, London, 1840. 

§ 17. The Evolution o! the Chemist's Nomenclature 

For a language to be perfect, it is not sufficient that each substance, each idea, each 
modification of form, time, place, etc., should be represented by one word, or by one invari- 
able symbol, it is necessary in addition, both to aid the memory and to facilitate the opera- 
tions of the mind, that analogous words sho\ild designate analogous substances, analogous 
ideas, and modifications of ideas. It is thus that the words of our language represent to 
us by similar terminations or augments, similar modifications of ideas represented as 
when we say : je vois, j'aperQois, je reQois ; nous voyons, nous apercevons, nous recevons. 
In like manner do chemists make use of the expressions sulphate, nitrate, chloride, etc. — 
A. Laurent (1854). 

In the British Association's Report on Chemical Nomenclature,^ it is shown 
that the evolution of the chemist's nomenclature is largely conditioned by the history 
of chemistry itself. No attempt to name substances systematically appears to 
have been made before the time of Geber — about the thirteenth century. The names 
in vogue for chemical substances up to the middle of the eighteenth century were 
more or less arbitrary, for they were (i) relics of alchemists' terms — for instance, 
aquafortis (nitric acid), aqua regia, etc. ; or derived (ii) from the name of their 
discoverer — for instance, Cadefs fuming liquid (alkarsine) ; or (iii) from one who had 
made a special study of the substance— for example, Glauber's salt (sodium sulphate) ; 
or (iv) from the name of the locality where they occurred — for example, Epsom 
salts (magnesium sulphate) ; or (v) from some prominent property or quaUty they 
possessed— for instance, tartar emetic (potassium antimony tartrate) ; or (vi) the 
names were based upon some superficial resemblance, and thus what J. B. A. Dumas 
called le langue des cuisinieres — the language of the kitchen — was applied; for 
instance, antimonious chloride was called butter of antimony because of its buttery 
appearance ; zinc chloride for the same reason was called butter of zinc ; and arsenic 
chloride, hatter of arsenic. On account of this superficial resemblance, these sub- 
stances for a time were classed along with butter from milk! Similarly, oi/ of 
vitriol (sulphuric acid), oil of tartar (deliquesced potassium carbonate), oUve oil, 
and the fatty oils generally were classed together ; so also were such unlike sub- 
stances as spirit of wine (alcohol), spirit of salt (hydrochloric acid), Libavius' furmng 
spirit (stannic chloride), Boijle's fuming spirit (ammonium sulphide), Glaubers 
fuming spirit of nitre (nitric acid), and spirits of hartshorn (ammonia) were mcluded 
in one class. ' This virtually means that the names of the compounds were the 
basis of the classification. The names were arbitrarily assigned, and hence the 
classification was almost as arbitrary and confusing as if the compounds had been 


classified according to the number of the letters in their names. Liquids were 
once called mercurys — mercury itself was mercurius communis, alcohol, mercurius 
vegetahilis, etc. Salts were distinguished by their taste — satis acida, salis alcalina 
— ^and by their volatility — salis alcalina fixa, salis alcalina volatila, etc. There are 
here, however, signs of a feeble attempt at a truer classification. 

Towards the end of the seventeenth century, chemists began to assign similar 
names to salts having the same origin — more particularly in reference to the acidic 
component of the salts. Thus, salts derived from sulphuric acid were called vitriols ; 
and those from nitric acid were called saltpetres. A century later, P. J. Macquer 
and A. Baume, in their Plan d'un cours de chimie experimentale et raisonnee (Paris, 
1757), emphasized the need for designating substances similar in composition by 
similar names so as to enable chemists to cope with a rapidly growing list of new 
compounds. The confused state of chemical nomenclature, even at the beginning 
'of the nineteenth century, is shown by an illustration from Joseph Black's Lectures 
on the Elements of Chemistry (Edinburgh, 1803), where sometimes a dozen synonyms 
for a salt are listed. 

About 1770, T. Bergmann advocated a new system of nomenclature which was 
described in his Meditationes de systemate fossilium naturali ; the system was based 
as far as possible on the terms then in use, and founded on the phlogiston theory. 
T. Bergmann also proposed to represent substances of analogous composition by 
similar symbols, and so compounded the symbols that each substance had its own 
special symbol. For instance, he called potassium sulphate, alkali vegetahile 
vitriolatum ; sodium chloride, alkali fossile salitum ; ammonium nitrate, alkali 
volatile nitratum ; and similarly for sodium nitrate, sulphate, etc. His 
system was excellent for its time, and shortly afterwards (1782), Guy ton de 
Morveau 2 gave a consistent nomenclature for the salts which he described as 
compounds of acids and bases, and he illustrated the advantages of his system 
by applying it to 474 substances — e.g. vitriol de harote (barium sulphate) ; nitre de 
mercure (mercury nitrate) ; muriate de cake (calcium chloride) ; fluor de calce (calcium 
fluoride) ; etc. In the choice of names for chemical compounds, said G. de Morveau, 
the following five principles should be observed : 

(1) A phrase like liqueur alkaline aaturie de la matiere colorante de bleu de Prusse is not 
a name and it should be replaced by V alkali prussien. Both terms were then in vogue. 
(2) The name should correspond as nearly as possible with the object. When a name 
is made up of a noun and an adjective, the former should be applied to the least changeable 
and more essential constituent. The names of discoverers should be excluded from the 
system. (3) If the constitution of a body is not known, a term with no meaning is better 
than one which may ultimately prove to be a wrong one. Hence Valkali prussien is 
preferable to Valkali phlogistique. (4) New names are best derived from roots of the best 
known dead languages — Greek and Latin. (5) Names should be adapted to the peculiari- 
ties of the particular language in which they are to be used. 

G. de Morveau's system, like Bergmann's, was founded on the phlogiston theory. 
These two schemes were probably the first attempts to devise a complete system 
of naming inorganic compounds so that each name indicates the qualitative com- 
position of the substance for which it stands. These two systems are not very 
different, and are not much unlike the one in use to-day. 

In 1787, A. L. Lavoisier and G. de Morveau, with the assistance of C. L. Ber- 
tholet and A. F. de Fourcroy, presented details of a new Methode de nomenclature 
chimique to V Academic des Sciences 3 in Paris. The proposed method was really an 
elaboration of T. Bergmann's and G. de Morveau's systems adapted to the duaHstic 
hypothesis. Most chemists felt the need for a precise nomenclature independent 
of the phlogiston which the French chemists were rapidly driving out of chemical 
science. In the proposed system the names assigned to the various compounds 
were intended (i) to indicate the compound ; (ii) to define the compound ; (iii) to 
recall its constituent parts ; (iv) to classify it according to its composition ; and 
(v) to indicate the relative proportions of its constituents. 




The French report laid the foundations of the chemical language of to-day— of 
course, after making due allowance for the development of the science which has 
necessitated many modifications. The terms ic (ique) and ate, ous (eux) and ite, 
for respectively distinguishing the higher and lower acidic oxides and their salts, 
are employed for the first time. In 1804, T. Thomson * introduced the plan of dis- 
tinguishing the different oxides of an element by prefixing the Greek sufl&xes 
proto, first ; deuto, second, ... for the first, second, . . . compound of a series — 
e.g. CuCl would be the ^/-o^o-chloride of copper ; and CUCI2 the (^to-chloride. In 
1808, J. Dalton explained his notation in his New System of Chemical Philosophy 
(Manchester, 1808). J. J. Berzelius' modifications ^ followed in 1811 as indicated 
above. Berzelius introduced the term ide, or French ure, as a termination for 
simple compounds. 

Various other systems of nomenclature have been proposed from time to time in which 
artificial words replace the arbitrary names applied to well-known substance* — each vowel 
or consonant of the artificial word representing either a substance or a number. ^ These 
systems have been found to be unworkable. There are also systems based on M. Dewey's 
Decimal Classification and Relativ Index (Boston, 1885) ; for example, A. L. Voge, in his 
The Inorganic Compounds (Zurich, 1911), arranges 14,000 inorganic compounds on Dewey's 
system. He gives 

NgO NO N2O3 NO2 N2O4 N2O5 

Symbols . . 133211 133311 133411 13361181 13361182 133611 

These systems have possible uses in libraries and for card indexes. 

The Methode de nomenclature contained as appendices two Memoir es sur de nouveaux 
caracteres a employer en chimie devised by J. H. Hassenfratz and P. A. Adet. In these, 
54 straight and curved lines representing the combining units, were arranged in various 
v/ays to represent possible compounds. The appearance of the combined symbols, in many 
cases, recalls some of the modem systems of shorthand writing. The idea of using " short- 
hand systems " is revived every now and again, but has never come into general use. 


1 Report on Chemical Nomenclature, B. A. Rep., 39, 1884 ; 262, 1885; H. G. Madan, Joum. 
Chem. Soc., 23. 22, 1870. 

2 G. de Morveau, Journ. Phys., 19. 310, 382, 1782 ; Ann. Chim. Phys., (1), 25. 205, 1798. 

3 Methode de nomenclature chimique proposee par MM. de Morveau, Lavoisier, BerthoUet et 
de Faurcroy, Paris, 1787 ; London, 1799. 

* T. Thomson, A System of Chemistry, Edinburgh, 1804. 

5 J. J. Berzelius, Journ. Phys., 72. 266, 1811 ; 83. 253, 1816 ; L. Gmelin, Handbuch der 
anorganischen Chemie, Heidelberg, 1. 149, 1870; A. Laurent, Methode de chimie, Paris, 1854; 
J. AIr. Newlands, Chein. News, 4. 281, 332, 1861. 


§ 1. The History of Pneumatic Chemistry 

The history of human knowledge is a history of false inferences and erroneous inter- 
pretations of facts. — ^Max Nordau. 

The attention of the early workers in chemistry was mainly directed to visible and 
tangible liquids and solids, while the gases — spirits, fumes, vapours, and airs, as 
they were variously called — which escaped when different substances reacted 
together, were usually considered to be unwholesome effluvia, best avoided. Indeed, 
about the middle of the eighteenth century J. Black i could say : 

In their distillations, chemists have often observed that part of a body has vanished 
from their senses, notwithstanding the utmost care to retain it ; and upon further inquiry, 
they have always found that subtle part to be air, which, having been imprisoned in the 
body under a sohd form, was set free and rendered fluid and elastic by the fire. 

In the third century, Clement of Alexandria beheved that the suffocating 
properties of some gases were manifestations of a diabolical nature, and J. B. van 
Helmont, who was the most advanced student of gases at the beginning of the 
seventeenth century, appears to have had a hazy belief that the gases he had 
discovered were in some senses living spirits — diabolic or divine. Even as late as 
the middle of the seventeenth century, G. Agricola 2 hinted that the gases in mines 
were manifestations of malignant imps ; and the idea had not been altogether 
exorcised at the beginning of the eighteenth century. 

The old chemists used the term spirit or air where we use the term gas generically for 
aeriform elastic fluid. Thus, in the first century of our era, Pliny, in his Historia natura 
(2. 4), spoke of that spiritus which both the Greeks and the Romans called aero. The 
terms sjnritus, flatus, halitus, aura, and emanatio nubila were also applied to aeriform fluids 
disengaged by heating other substances, and they are common in the writings of the 
alchemists of the Middle Ages. J. B. van Helmont, in speaking of the spiritum sylvestrem 
which he had obtained by the combustion of carbon, etc., said, " This spirit, unknown up 
to the present, I call by a new name groa," and he says elsewhere ^ that in order to distinguish 
the vapour given off by water at ordinary temperatures from the vapour which is derived 
from boiling water, " by the Hcence of a paradox, for want of a name, I call the vapour 
rising from water at ordinary temperatures, a gas, being not far severed from the chaos of 
the auntients (ancients)." Just as the " chaos of the auntients "■ — Hesiod's xaos — was a 
confused mixture of elements from which the Creator produced the universe ; so, to van 
Helmont, the vapoiu' of water was a confused mass of elements from which all material 
substances could be produced. The word chaos was very frequently used by Paracelsus 
with a similar meaning. " Chaos," said he, " is an air like the wind. Air is nothing more 
than a chaos. What air is, that is chaos. The element air is named chaos." Stephen 
Hales (1727 also said that atmospheric air is a veritable Proteus and a chaos. It is an 
easy transition from chaos to chas, which has the so\ind of gas. According to M. Speter, 
the ch and ao of chaos when converted into Netherland speech become respectively g and a, 
so that van Helmont transformed Paracelsus' term to suit the language of his country. 
Some derive the word from the geest — spirit, volatile liquid, or refined fluid — of the Dutch ; 
or from the gdscht— yeast — of the Germans.* 

Near the beginning of the seventeenth century, J. B. van Helmont, in his essay 
De flatihus, distinguished gas sylvestre — ^given off by fermenting liquids — from 
the inflammable gases which he named gas pingue, gas sicum, or gas fuliginosum. 
J. B. van Helmont seems to have adopted the common opinion that gases are 



different combinations of elastic air with various exhalations or impurities, for at 
that time chemists regarded the different gases as chaotic mixtures of various 
substances with atmospheric air. The term sylvesire was intended to imply that 
the artificial gases which he had prepared were untameable and uncondensable. 
In a letter to R. Boyle & in 1678, Isaac Newton stated that he considered that the 
ferrous gas (hydrogen) which R. Boyle had obtained by the action of acids on iron, 
and the cuprous gas (nitrogen oxide), which C. Huygens 6 had obtained by the action 
of nitric acid on copper, contained ultimate particles respectively of iron and copper 
brought to a state of aerial elasticity ; but the idea of a, ferrous gas from iron, and a 
cuprous gas from copper was disproved when H. Cavendish ^ demonstrated the 
identity of the gases obtained by the action of acids on iron and on zinc. According 
to J. Priestley, " Boyle ^ was the first who discovered that what we call fixed air, 
and also inflammable air, are really elastic fluids capable of being exhibited in a state 
unmixed with common air." R. Boyle extended his experiments on factitious 
(artificial) airs separable from fixed bodies to a variety of substances, and he noticed 
the condensability of hydrogen chloride (1676) ; the orange colour of nitrogen 
peroxide (1672) ; and the evolution of an air by heating red lead in the focus of 
a burning glass (1678). He also obtained an air from oyster shells and red coral 
(1661), and noted the inflammability of hydrogen obtained by the action of acids 
upon iron (1671). R. Boyle employed the term air generally (1676) in the same 
sense that the word gas is used to-day. Tout corps invisible et impalpable, said 
R.Descartes (1664), se nomme air. J. Mayow ^ examined the relative elasticities 
of the two gases obtained by R. Boyle by the action of nitric and sulphuric acids on 
iron, and decided that there exist various elastic fluids other than air. J. Mayow's 
conclusion was opposed by the elder Bernoulli,!^ ^Jio claimed that there are no 
other elastic fluids besides air ; and, overlooking the constant diminution of volume 
which Mayow found to occur when air is breathed or burnt, J. Bernoulli further 
claimed that animals are suffocated and flames are extinguished in certain airs 
because the airs are charged with miasmata inimical to life and combustion. 

It is sometimes said that S. Hales, in his Vegetable Staticks (London, 1727), 
confused the different gases which he prepared with atmospheric air. This erroneous 
idea has appeared because Hales focussed his attention on the generic physical 
properties of gases rather than on their specific chemical characteristics. Thus, 
W. V. Harcourt " has pointed out that when Hales states that " the airs generated 
by effervescences . . . resemble true permanent air " he really means that they 
are true elastic fluids with the same permanence of constitution, and the same 
elastic force as common air. Hales heated a number of substances in vessels 
arranged so that the gases evolved could be collected over water, and he measured 
the proportion of gas furnished by definite weights- of different substances. He also 
collected airs furnished by fermentation processes, and airs generated by the action 
of acids on metals. S. Hales did not make any special experiments on the chemical 
properties of different gases — hydrocarbons, carbon dioxide, nitrogen oxides, 
oxygen, nitrogen, hydrogen, cyanogen, and chlorine— which he probably collected, 
nor on the aqueous solutions of the more soluble gases^hydrogen chloride, sulphur 
dioxide, and ammonia — which he must have prepared. In spite of the experimental 
facts which S. Hales thus accumulated, his attention was so preoccupied with their 
generic physical properties that he did not observe their specific chemical differences 
— oculos habuit et non videbat — and he was thus prevented from making many 
capital discoveries. 

J. B. van Helmont seems to have believed that while gases could be prepared 
artificially in many ways, they could not be caught and held in vessels— r^a^, vasts 
incoercible, foras in aerem prorumpit. S. Hales is generally credited with the 
invention of the gas-collecting or pneumatic trough. J. B. van Helmont did not 
know how to isolate and preserve the gas sylvestre which he discovered near the 
beginning of the seventeenth century, and he distinctly stated that the gas cannot be 
confined in any vessel, since it overcomes all obstacles and mixes with atmospheric air. 



R. Boyle (1661) and J. Mayow (1669) used a glass globe, Fig. 6, Cap. I, inverted in a 
basin of water for confining air ; they filled the globe with water and inverted it in the 
basin of water so that the gas generated by the action of an acid on some scraps of 
iron in the basin displaced the water and collected in the globe. M. d'Element, 
in a brochure 12 pubUshed at Paris in 1719, had abready shown that air could Ih 
manipulated and measured like other bodies by confining it in vessels over water ; 
and in 1621, J. C. Drebbel had noticed the bubbling of gas from a retort heated 
with its beak dipping in water. S. Hales devised the apparatus indicated in Fig. 1 , 
for collecting the gases evolved when different substances are heated in a retort — 
a glass vessel was used for generating the gases at low temperatures, and a bent 
gim barrel for high temperatures. The vessels used for collecting the gas were hung 
by strings mouth downwards below the surface of the water. H. Cavendish (1766) 
used a similar device. W. Brownrigg 13 used a shelf with two holes larger than 
the gas jar and above the level of the liquid in the trough ; the latter were prevented 
sinking too deeply by means of wedges. J. Priestley introduced the use of a per- 
forated shelf below the level of the liquid in the trough for supporting the vessel to 

be filled with gas. Modifications of S. Hales' 
and J. Priestley's pneumatic troughs were 
employed very effectively in chemical re- 
searches on gases by C. W. Scheele (1770) and 
A. L. Lavoisier (1772). Joseph Priestley also 
substituted mercury for water ; and, by means 
of the mercury pneumatic trough, he collected 
and isolated gases — ammonia, hydrogen chlo- 
ride, sulphur dioxide, silicon fluoride — which 
are so soluble in water that their existence had 
been overlooked when water was the confining 

The study of gases began to occupy serious 
attention towards the end of the eighteenth 
century, so that in 1779, although " only eight 
gases were certainly known with respect to 
their composition," yet chemists were so proud 
of their knowledge that T. Bergmann was 
able to write : " During the last ten years 
chemistry has not only soared into regions of invisible aerial substances, but it has 
dared to explore the nature of these substances, and to search into their constituent 
principles." The nineteenth-century chemists devoted a great deal of time and 
attention to the imperceptible, intangible gases ignored by the earlier workers. 
Indeed, chemistry could never have progressed very far if the gases and vapours 
had been ignored. The work of Joseph Priestley, between 1770 and 1780, gave 
such a stimulus to the study of gases that G. Cuvier, in his Eloge historique de 
Priestley (Paris, 1806), called him un des feres de la chimie moderne. 

Fig. 1.— S. Hales' Pneumatic Trough. 


^ J. Black, Experiments upon Magnesia alba, Quicklimey and other Alcaline Substances, 
Edinburgh, 1755 ; Alembic Club Reprints, 1, 1893. 

2 G. Agricola, De animantibus svbterraneis. Bale, 1657 ; J. B. van Helmont, Opera omnia, 
Franckfurti, 1707. 

^ J. B. van Helmont, Oriairike, or Physick Refined, London, 1662 ; Orius medicinal, Amsterdam; 

* G. F. Rodwell, Chem. News, 10. 196, 1864 ; M. Speter, Chem. Ztg., 34. 193, 1910 ; E. von Lipp- 
mann, ib., 34. 1, 1910 ; 35. 41, 1911 ; Abhandlungen und Vortrdge, Leipzig, 2. 361, 365, 1913. 

^ R. Boyle, Works edited by Thomas Birch, London, 1744. 

« C. Huygens, Phil. Trans., 10. 443, 1675. 

' H. Cavendish, Phil. Trans., 55. 141, 1766. 

' R. Boyle, Physico-mechanical experiments to show the spring and effects o/air, London, 1661 ; 


New experiments touching the rdationbetween flame and air, London, 1671 ; Phil. Trans., iO. 1675 ; 
Second continuation of new experiments, physico-mechanical, touching the spring and weight of air, 
London, -1676. 

^ J. Mayow, Tractatus de parte aerea igneaque spiritus nitri, Oxford, 1669. 

^» J.'BeTno\illi,Dissertatiodeeffervescentiaetfermentationen^vahypothesifundata,Base\ 20 1670 
" W. V. Harcourt, Phil. Mag. (3), 28. 106, 478, 1846. 

^2 M. d'Element, La maniere de rendre Vair visible, Paris, 1719 ; J. C. Drebbel, Een kort tractaei 
van de natuere der elementen, Rotterdam, 1621 — German edition, 1624. 
13 W. Brownrigg, Phil. Trans., 55. 235, 1765. 

§ 2. Hydrogen— Preparation and Properties 

It can scarcely be said that pneumatic chemistry was properly begun till Mr. 
Cavendish published his valuable paper on carbonic acid and hydrogen gas, in the year 
1766.— T. Thomson (1813). 

The discovery of hydrogen.— It is inconceivable that the alchemists knew 
nothing about this gas, for they were perpetually operating with various metals 
in contact with acids. It must therefore have been known for a very long time 
that an inflammable air or gas is produced when iron is dissolved in dilute sulphuric 
acid. Paracelsus, in the sixteenth century, described the action somewhat 
quaintly. He said that when the acid acts on iron " an air arises which bursts 
forth like the wind." Near the beginning of the next century, J. B. van Helmont 
described this gas as a peculiar variety of air which was combustible and a non- 
supporter of combustion, but his ideas were somewhat hazy, for he confused it with 
other inflammable gases ; indeed, up to about 1766, writers generally used 
inflammable air as a general term to include this gas, as well as the hydrocarbons, 
hydrogen sulphide, carbon monoxide, and other combustible gases. Hydrogen 
was sometimes specifically distinguished as the inflammable air from the metals. 
In 1650, T. Turquet de May erne i reported that the fumes evolved when dilute 
oil of vitriol acts on iron are inflammable, and in 1671 Boyle 2 observed that the 
flame was extinguished when placed under the receiver of an air-pump, but K. 
Boyle's chief concern was to show that the gas, which he called the volatile sulphur 
of Mars, was dilatable and compressible, and that it was really an air. Nearly a 
century later, J. Priestley also experimented with the gas, and S. Hales ^ found that 
iron filings and oil of vitriol gave scarcely any air, but on adding water, there was a 
copious evolution of the aeriform fluid. In 1766, H. Cavendish * showed that the 
combustible gas produced by the action of dilute sulphuric or hydrochloric acid on 
metals like iron, zinc, and tin is a distinct substance with definite properties pecuHar 
to itself ; hence, hydrogen was called inflammable air. Cavendish measured the 
amount of hydrogen obtained from a given weight of the different metals ; he also 
measured the specific gravity of the gas, and found it to be seven times lighter than 
atmospheric air ; he also showed that the specific gravity of the gas was the same 
whether zinc or iron were used in the preparation. F. de Lassone and C. W. 
Scheele discovered almost simultaneously that a solution of zinc in caustic lye 
furnishes the same gas. J. Watt (1783),' R. Kirwan (1781), H. Cavendish (1766), 
and J. Priestley (1784) identified the gas with the evanescent phlogiston, and 
they called it phlogiston, or phlogistic ated air; but neither this name nor 
inflammable air persisted very long, for both terms were ousted by the cognomen 
hydrogen which A. L. Lavoisier applied to the gas in 1783. In his Considerations 
generales sur la dissolution des metaux dans les acides (1784),^ A. L. Lavoisier, follow- 
ing a suggestion of P. S. de Laplace, traced the source of the hydrogen which is 
evolved when a metal dissolves in a dilute acid, to the decomposition of the water. 
He assumed that the oxygen of the water united with the metal to form a calx, 
and the hydrogen escaped in the free state. The calx united with the acid to form 
water and a salt. 

The preparation of hydrogen. — Hydrogen obtained by the action of dilute 


sulphuric or hydrochloric acid on metallic iron is not very pure, and it possesses 
a distinct smell owing to the presence of hydrocarbon gases, etc., formed by the 
action of the acid on the carbon compounds associated, as impurities, with com- 
mercial iron. The solution remaining after the action of sulphuric acid on the 
iron, when put aside in a cool place, soon forms beautiful pale green crystals of 
ferrous sulphate. Magnesium and aluminium furnish a fairly pure gas ; with 
aluminium the acid should be warmed to start the reaction. In these cases, not 
only is hydrogen gas evolved but crystals of magnesium sulphate and of aluminium 
sulphate can be obtained from the liquids in which the respective metals have been 
dissolved. The action of the acid on tin is rather slow ; granulated zinc is used 
for general laboratory work. 

Hydrogen gas is made in small quantities in the laboratory by placing granulated 
zinc in a bottle fitted with a stopper with two holes — one to take a funnel tube, the 
other to take an L-shaped tube for conducting away the gas. Instead, the 
granulated zinc may be placed in a two-necked Woulfe's bottle — so named because 
these bottles were first described by Peter Woulfe (1784). The one tubulure is fitted 
with a one-hole stopper carrying a tube funnel, and the other, with the gas exit tube. 
The zinc is covered with water, and sulphuric acid is added a little at a time through 
the tube funnel until the gas begins to come off vigorously. For many purposes 
there is no need to use the pneumatic trough for collecting hydrogen, since by 
bringing the gas- jar mouth downwards over a jet of hydrogen the gas will collect 
in the upper part of the jar, and displace the air downwards — hence the term collect- 
ing gases by the downward displacement of air — many writers call this collecting 
the gas hy wpward displacement. Hydrogen gas so prepared is always tested before 
iLse by collecting a test-tube of the gas, and while holding the tube upside down, 
applying a lighted taper. If the gas burns quietly at the mouth of the test-tube, 
all is well. 

Hundreds of different forms of apparatus ^ have been devised for supplying an 
intermittent stream of gas by the action of a liquid — e.g. hydrochloric or sulphuric 
acid— on a solid — e.g. zinc, ferrous sulphide, or marble. They are all based on the 
principle applied by J. W. Dobereiner in his hydrogen lamp. When the gas is no 
longer free to escape, the pressure generated by the gas drives the acid away from 
the solid ; this stops the further generation of gas. When the pressure is relieved 
by allowing the gas to escape, the acid again comes in contact with the solid. In 
the better types of apparatus (i) the freshest acid is brought in contact with the 
solid ; (ii) the emptying and recharging is simple ; and (iii) a great over-pressure is 

The properties of hydrogen. — Hydrogen gas is colourless and odourless — 
the impure gas may have a smell. The hydrogen gas streaming from the generating 
flask can be lighted, and a flame of burning hydrogen is obtained which was formerly 
called lumen philosophicum, or the philosopher's flame. To get the flame to burn 
steadily it is best to interpose between the exit tube and the jet, a wider tube loosely 
packed with granulated calcium chloride to arrest by absorption the water vapoui 
carried along with the gas. The hydrogen flame is very hot and melts ordinary 
glass ; a jet of hard glass, quartz glass, or platinum can be used. When a lighted 
taper is plunged into a jar of hydrogen held mouth downwards, the gas burns with 
a scarcely visible blue flame at the mouth of the jar, and the taper is extinguished 
showing that the gas is combustible and a non-supporter of combustion. When 
J. Black (1766) heard that H. Cavendish had found hydrogen to be much lighter 
than air, he thought that possibly a thin bag made from the allantois of a calf, when 
filled with hydrogen, would be buoyed up by air. Modifications of Black's idea 
are used as illustrative experiments on the lecture table, and not long afterwards 
the gas was used for filling balloons. The gas can be poured upwards from one jar 
to another, and it can be proved that the gas has actually been transferred from 
the one vessel to the other by testing the contents of each jar with a lighted taper 
before and after the pouring. 



The extreme lightness of hydrogen and its combustibility enable many ingenioiis 
cperiments to be performed with the gas. For instance, a cardboard box or a light glass 
ressel can be coimterpoised bottom upwards, on a balance ; the beam will ascend when 
lydrogen is poured upwards into the inverted vessel. Soap-bubbles blown with the gas, 
>r collodion balloons filled with the gas, rise to the ceiling very quickly. The gas may be 
^syphoned upwards from one vessel to another, or, the gas may be syphoned from, say a 
bell-jar and burnt at the long leg of the syphon. An explosive mixture with air is formed 
when the hydrogen has nearly all been syphoned away, and the flame at the top of the long 
leg of the syphon will then rush back and produce a loud but harmless explosion. 

The explosive character of a mixture of hydrogen with oxygen of air can be 
illustrated by mixing two volumes of hydrogen gas with either one volume of oxygen 
or five volumes of air in a soda-water bottle. A lighted taper applied to the mouth 
of the bottle causes the gas to detonate. The combustion of the whole mass is almost 
instantaneous. The explosion is so violent that we can understand why N.Lemery, 
in his Explication physique et chimique des eclairs et du tonnere (1700), tried to show 
that thunder and lightning are produced by the fulminations of hydrogen.'' The 
sound obtained when a long glass tube is placed about the flame of burning hydrogen 
led to W. Higgins (1777) calling the experiment the chemical harmonicon. The tones 
vary with the diameter, thickness, and length of the tube and on the nature of the 
jet. The sound appears to be the effect of an extremely rapid series of explosions. 
M. Faraday obtained a similar musical flame with inflammable gases and vapours 
other than hydrogen. M. Faraday's explanation is that a strong current of air is 
established ; this lengthens the flame, and small portions of air are mixed with the 
hydrogen in such a manner as to form small quantities of detonating gas, which, 
when set on fire, produces slight explosions succeeding each other quickly and 
regularly. C. Wheatstone found that while producing sound within a glass tube, 
regular intermissions in the intensity of the flame are observed, and these present 
a chain-like appearance on a revolving mirror, indicating alternate contractions and 
dilations of the flame corresponding with the sonorous vibrations of the column 
of air. 

Joseph Priestley ^ has told us that in 1776 his friend, J. Warltire, had noticed that 
when a flame of hydrogen is allowed to burn in air confined under a bell -jar, the 
whole of the receiver appears to be filled with " a fine powdery substance like a 
whitish cloud," when the flame was extinguished ; and the air left in the glass was 
found to be " perfectly noxious." In the same year P. J. Macquer ^ inquired 
whether the flame of hydrogen evolved smoke or soot. He thus described his 
experiment : 

By placing a saucer of white porcelain in a jet of inflammable gas (hydrogen) burning 
tranquilly at an orifice, I foimd that the part of the saucer which the flame licked was 
moistened by small drops of liquid as clear as water, and which, in fact, appeared to be 
nothing but pure water. 

It is probable that J. Warltire's white cloud was not produced by a finely powdered 
soHd, but by minute drops of water. In 1779, J. R. Sigaud de la Fond also mentioned 
the formation of water during the combustion of inflammable air. P. J. Macquer did 
not stop to inquire : Whence came the water ? He has been blamed because he felt 
no astonishment at that which is really astonishing, for he merely mentions, with- 
out comment, the appearance of the water. P.J. Macquer did not see before him a 
great discovery begging for recognition. Hence, asks F. J. Arago (1839), 1° is 
genius in the observational sciences to be reduced to the faculty of asking an 
appropriate Why 1 The inquiry can be made, (1) What happens to the surrounding 
air during the burning of a jet of hydrogen 1 and (2) Is the product of the action 
really water ? 

J. Warltire's 1776 experiment can be modified by making a jet of dried hydrogen 
burn under a bell- jar containing a measured volume of air standing over water. 
At first, there is a momentary expansion of the air due to the heating of the confined 
air by the flame ; immediately afterwards, the water rises in the jar, and the 
hydrogen flame gradually expires. Immediately this occurs the stream of gas is 


stopped to prevent it passing into the air in the bell-jar. The gas remaining in the 
jar has quite similar properties to the nitrogen gas remaining after mercury is 
calcined in air. It is the " perfectly noxious air " alluded to by J. Warltire. In 1777, 
C. W. Scheele described an analogous experiment in his Chemische Ahhandlungen von 
der Luftundvon dem Feuer (Upsala, 1777), but with other combustible agents. The 
experiment shows that when hydrogen bums in air, it unites with the oxygen 
and leaves nitrogen behind. If the experiment be carefully made, nearly four- 
fifths of the original volume of air remains. The burning hydrogen removes nearly 
one-fifth of the original volume of air. Hydrogen does not burn in the residual 
nitrogen — although about 7 or 8 per cent, of oxygen is still present. A certain 
amount of dew collects on the inner walls of the bell-jar, but that, of course, may 
come from the water in the dish below. In fine, the facts give reasons for supposing 
that hydrogen, in burning, combines with oxygen to form an oxide of hydrogen in 
the same sense that mercury, when calcined in air, combines with oxygen to form 
mercuric oxide. It remains to try and isolate a sufficient quantity of the hydrogen 
oxide whose existence has just been inferred, but not proved, in order that its 
properties may be examined more closely. 

The experiment of P. J. Macquer (1778) can be modified so that a jet of dried 
hydrogen is burned under a funnel, the stem of which is curved so that it passes into 
a two-necked globe ; the other neck of the globe is connected with an aspirator so 
that the products of combustion from the hydrogen flame can be aspirated through 
the system. The glass bulb is kept cold and a clear colourless liquid collects therein. 
This liquid has all the properties of water ; it 'is a clear, colourless, and tasteless 
liquid with no smell ; it freezes at 0°, and boils at 100°. The water does not come 
from the condensation of the moisture already present in the gas as it rises from 
the generating vessel, because the gas is dried by the " scrubbing " it receives as it 
passes along the tower of calcium chloride ; this statement can be tested by making 
a blank experiment with the un-ignited gas. It is therefore inferred that water 
is burnt hydrogen, or the calx of hydrogen ; otherwise expressed, water is hydrogen 
oxide formed when hydrogen bums in air. Hydrogen and oxygen are both 
gases, and it is therefore more difficult to find the combining ratio Hydrogen : 
Oxygen in the formation of hydrogen calx, by direct weighing, than is the case 
with the metallic calces. It remains therefore to show how chemists have solved 
the problem. 


1 T. Turquet de Mayerne, Pharmacopoea, London, L703. 

2 R. Boyle, New Experiments touching the relation between flame and air, London, 1671 ; 
N. Lemery, Mem. Acad., 101, 1700. 

3 S. Hales, Vegetable Staticks, London, 1727. 

4 H. Cavendish, Phil. Trans., 56. 141, 1766; 74. 119, 176, 1784; 75. 372, 1785; R. Kirwan, 
ib.y 72. 179, 1782; J. Watt, ib., 74. 329, 1784; J. Priestley, Experiments on Air, Birmingham, 
6, 1, 1786; F. de Lassone, Mem. Acad., 563, 1776; C. W. Scheele, Chemische Abhandlungen von 
der Luft und dem Feuer, Upsala, 1777. 

6 A. L. Lavoisier, Mem. Acad., 468, 1784 : (Euvres, Paris, 2. 509, 1862 ; J. B. A. Dumas, 
Lecons sur la philosophic chimique, Paris, 158, 1827. 

'« C. Cloez, Bull. Soc. Chim., (2), 43. 102, 1885 ; G. Tissandier, ib., (2), 43. 233, 1885 ; V. 
Wartha, Ber., 5. 151, 1872; J. Meister, Zeit. anal. Chem., 25. 373, 1886; R. Fresenius, ib., 12. 
73, 1873 ; W. Ostwald, ib., 31. 184, 1892 ; R. J. Friswell, Chem. News, 90. 154, 1904 ; 94. 106, 
1906; C. Thiele, Chem. Ztg., 25. 468, 1901 ; C. Arnold, ib., 26. 229, 1902; L. L. de Koninck, 
ib., 17. 1099, 1893 ; F. M. Perkin, Jovm. Soc. Chem. Ind., 20. 438, 1901 ; H. Hafelin, Pharm. 
Ztg., 50. 351, 1905 ; E. Egasse, Dinglers' Journ., 244. 54, 1882 ; H. Arzberger, Pharm. Post, 
37. 581, 1904; F. W. Kiister, Journ. prakt. Chem., (2), 48. 595, 1893; J. W. Dobereiner, 
Schweiggers' Journ., 38. 326, 1823 ; 39. 159, 1823 ; 63. 468, 1831 ; C. Aschmann, Chem. Ztg., 21, 
1049, 1897 ; U. Eebel, ib.,29. 141, 1905; J. D. Edwards, Journ. Ind. Eng. Chem., 11. 961, 1919. 

' N. Lemery, Mem. Acad., 101, 1700 ; W. Higgins, Nicholson's Journ., 1. 130, 1777 ; 
M. Faraday, Quart. Journ. Science, 5. 274, 1818 ; C. Wheatstone, Phil. Trans., 124. 586, 1834 ; 
F. Schaffgotsch, Pogg. Ann., 100. 352, 1857 : 101. 471, 1857 : 102. 627, 1857 ; J. Tyndall, Phil. 
Mag., (4), 13. 473, 1857 ; A. Schrotter, Sitzber. Akad. Wien, 24. 18, 1857 ; A. Terquem, Compt. 
Rend., 66. 1037, 1868. 


8 J. Priestley, Experiments and Observations on Different Kinds of Air, London, 3. 367, 1777. 
» r. J. Macquer, Dictionnaire de chimie, Paris, 2. 314, 1778; J. R. Sigaud de la Fond,' Kssai 
8ur differentes especes d'air, Paris, 1776. 

i» F. J. Arago, JiJloge historique de James Watt, Paris, 1834 ; (Euvres, Paris, 1. 454, 1854. 

§ 3. Dumas' Experiment on the Composition of Water by Weight 

After very careful examination of all the analytical researches made for the determina- 
tion of atomic weights, I emphatically declare that the researches of Dumas are the most 
important of all, marking as they do the beginning of the analysis of precision, and offering 
also the first instance of a true series of determinations, such as is required to furnish the 
absolute values of the atomic weights. — -G. D. Hinrichs. 

Several determinations of the combining weights of hydrogen and oxygen in 
the formation of water have been made. Prior to J. B. A. Dumas' work, there 
were the pioneer attempts to find the combining ratio of hydrogen and oxvgen by 
M. Monge, A. L. Lavoisier, and M. Meusnier i about 1786. They admitted 
measured volumes of hydrogen and oxygen into a globe, exploded the mixture, 
and after repeating the process 372 times, weighed the water produced, and 
calculated the weights of oxygen and hydrogen employed from the densities of the 
gases. It was found that in water the ratio of the weight of hydrogen to that of 
oxygen is as 1 : 6*61. In 1791, A. F. de Fourcroy, L. N. Vauquelin, and M. Seguin 
repeated M. Monge's work and found the ratio to be 1 : 6'17. In 1803, John Dalton 
estimated the ratio of hydrogen to that of oxygen to be 1 : 5' 66, a result further 
removed from the truth than the ratios found by the French savants. J. Dalton 
corrected his first result in 1808, and gave the ratio 1:7. In 1814, from J. L. Gay 
Lussac and A. Humboldt's observation that two volumes of hydrogen and one 
volume of oxygen unite to form water, and J. B. Biot and F. J. Arago's observation 
of the relative densities of these two gases, W. H. Wollaston calculated the ratio 
of the weights of hydrogen and oxygen in water to be 1 : 7 '545. This was followed 
by the work of P. L. Dulong and J. J. Berzelius in 1819, and of J. B. A. Dumas 
in 1842. 

Hydrogen does not combine readily with many of the elements, but it readily 
combines with oxygen, chlorine, fluorine, lithium, and a number of others. So 
great is the attraction of hydrogen for oxygen that it will very often remove oxygen 
from its combinations with the other elements. For instance, on March 6th, 1783, 
J. Priestley 2 reported that he had confined lead oxide (minium or red lead) in a 
tall cylinder containing inflammable air standing over water ; the red oxide of 
lead was heated in the focus of a burning glass. He observed : 

The minium became black, and then ran in the form of perfect lead ; at the same time 
the air diminished at a great rate, and the water ascended within the cylinder. . . . Seeing 
that metal to be actually revived, and that in a considerable quantity, at the same time that 
the air was diminished, I could not doubt that the calx was actually imbibing something 
from the air ; and from its effects in making the calx into a metal, it could be no other 
than that to which chemists had unanimously given the name phlogiston. . . . Con- 
sequently, phlogiston is the same thing as inflammable air. 

The experiment was varied by confining the gases over mercury in place of water, 
and using other calces— e.^r. the oxides of tin, bismuth, mercury, silver, iron, and 
copper. He further found that " 1 oz. of lead was revived by 108 oz. measures of 
inflammable air, and 1 oz. of tin by 377 oz. measures." The 108 oz. and 377 oz. 
measures of inflammable air would weigh nearly 4*4 and 15*4 grains respectively. 
Priestley's measurements are good, because these numbers are close to their ideal 
values, 4- 6 and 16'3 grains respectively. This remarkable experiment might have 
opened J. Priestley's eyes to the insufficiency of the phlogiston hypotheses. A. L. 
Lavoisier 3 has pointed out that J. Priestley did not notice that there was a decrease 
in the weight of the solid during the reduction, and that water was a product of 
the reaction. The true interpretation of the reduction observed by J. Priestley is 
due to A. L. Lavoisier. 

VOL. I. K 



In J. Priestley's experiment, the hydrogen is said to be oxidized ; and the 
metallic oxide, reduced or deoxidized. The hydrogen is called a reducing agent, that 
is, a reducer or deoxidizer ; and the copper oxide an oxidizing agent or 
oxidizer J because it oxidizes hydrogen to water. The reaction under consideration 
is both an oxidation and a reduction process. All depends upon whether the 
hydrogen or the copper be under consideration. In the fifteenth century, Paracelsus 
applied the term reduction to the preparation of the metals. During a reduction, 
the reducing agent is usually, not always, oxidized ; and during an oxidation, the 
oxidizing agent, reduced. If a known amount of copper oxide be reduced by 
hydrogen, and the water formed be collected and weighed, the weight of the reduced 
copper oxide will show how much oxygen has been used in forming a definite 
amount of water. This was done by P. L. Dulong and J. J. Berzelius ^ in 1820, and 
by J. B. A. Dumas in his celebrated Recherches sur la composition de Veau in 1843. 
J. B. A. Dumas' experiment is not the best of its kind, although it was the best 
of its time, and it has long and deservedly held an honoured place in chemical 
text-books. The experiment illustrates some important principles, and it is 

Hydrogen. Copper 

Water formed. Tube. 

Fig. 2. — Dumas' Experiment (abbreviated). 

therefore here described in outline. The first stage of the work involved the 
purification of hydrogen. 

The hydrogen was prepared by the action of zinc on sulphuric acid. It might be 
thought that pure zinc and pure sulphuric acid should be used. Experiment shows, 
curiously enough, that under these conditions the action is so very, very slow that some 
have jumped to the conchision that " absolutely pure sulphuric acid, even when diluted 
with pure water, has no action on perfectly pure zinc." Moreover, it is exceedingly difficult 
to prepare pure zinc and pure sulphuric acid. Hence, pure reagents were not used for the 
preparation of the hydrogen. Accordingly, the gas may contain nitrogen and oxygen 
derived from the air ; sulphur dioxide and hydrogen sulphide derived from the reduction 
of the sulphuric acid by the hydrogen ; carbon dioxide ; arsenic hydride (if the acid or the 
zinc contained arsenic) ; hydrogen phosphide (if the zinc or the acid contained phosphorus) ; 
nitrogen oxides (if the acid contained nitrogen oxides) ; and water vapour. Accordingly, 
J. B. A. Dumas (1842) used sulphuric acid, which had been well boiled, to get rid of dis- 
solved air, and then passed the hydrogen through a series of U -tubes- — ^Fig. 2- — containing: 
(1) pieces of glass moistened with lead nitrate to remove hydrogen sulphide ; (2) solution 
of silver sulphate to remove arsenic and phosphorus compounds ; (3) solid potassium 
hydroxide to remove sulphur dioxide, carbon dioxide, and nitrogen oxides ; and (4) phos- 
phorus pentoxide to remove moisture not absorbed by the solid potassium hydroxide. 
J. B. A. Dumas used three potassium hydroxide tubes, and two phosphorus pentoxide 
tubes — like (4) — only one of each is in the diagram. The phosphorus pentoxide tubes were 
placed in a freezing mixture. The tube marked (5) in the diagram contained phosphorus 
pentoxide, and it was assumed that the hydrogen passing through was quite dry — this 
tube is accordingly called a temoin tube {temoin, a witness) because it c€ua he employed as 


evidence that the hydrogen which passed through gave up no moisture to the desiccating 

J. B. A. Dumas passed the purified hydrogen over red-hot copper oxide, and 
determined the loss of weight (oxygen) which occurred. He then weighed the 
amount of water produced. 

The purified hydrogen was passed through a weighed bulb. A, containing copper 
oxide, and heated by the spirit lamp underneath. Most of the water condensed in the 
bulb B, and the remainder was absorbed in the U-tube G containing solid potassium 
hydroxide, and in D and E containing phosphorus pentoxide. The phosphorus pentoxide 
tube D was kept cool by a freezing mixture. The three tubes C, 2), E, and the bulb B, 
were weighed before and after the experiment. The last U-tube, F^ containing phosphorus 
pentoxide was followed by a cylinder, Q, of sulphuric acid through which hydrogen escaped. 
The vessels F and Q were not weighed ; they served to protect the other tubes from the 
external atmosphere. 

The average of nineteen experiments by J. B. A. Dumas (1842) gave : 

Copper oxide lost in weight . . . .44*22 grams 

Water produced . . . . . . 49*76 „ 

Hydrogen (by difference) . . 5*54 „ 

Hence, he inferred that 15*97 parts of oxygen united with two parts of hydrogen 
to form water, or 16 parts by weight of oxygen combined with 2 '004 parts by weight 
of hydrogen to form water. His nineteen values ranged between 15'892 and 
16'024, and his mean value for hydrogen is usually considered to be rather low. or 
the mean value for oxygen rather high. A later determination by E. W. Morley gave 
16 : 2'016. In approximate work, we may take it that 2 parts by weight of hydrogen 
combine with 16 parts by weight of oxygen to form 18 parts of water ; indeed, 
J. B. A. Dumas himself expressed his belief that the true value of the ratio Hydrogen : 
Oxygen is probably 2 : 16. 

It is common to append to the arithmetical mean of a series of observations the 
so-called probable error. For example, the mean of Dumas' nineteen determina- 
tions of the relative weights of hydrogen and oxygen in water is given as : Oxygen, 
15*96 ± 0*007, when hydrogen is 2 ; and 0. L. Erdmann and R. F. Marchand's eight 
determinations by a similar method are represented by the average 15*973 ± 0*011. 
The probable error in the one place is ±00*07 and in the other ±0*011. This does 
not mean that J. B. A. Dumas' results were nearer the true value than 0. L. Erdmann 
and R. F. Marchand's. The probable error does noo tell how nearly the average 
of a given number of similarly conducted experiments would approach the average 
actually found. In J. B. A. Dumas' result, the chances are even that the true average 
of the determination by his method lies between (15*96 + 0*007 =) 15*967 and 
(15*96 — 0*007 =) 15*953. If an unrecognized constant error affected all the 
results, the average actually found would still differ from the true value by this 
amount. As a matter of fact, when J. B. A. Dumas had nearly finished his work, 
he did find that his numbers were affected by a curious error, previously un- 
recognized, so that the concordance of his individual determinations did not ^ prove 
that his average was right. This error, if not corrected, makes the result appear 
a little low. The reduced copper retains some hydrogen very tenaciously ; 5 
similarly, when copper oxide is made, as is usually the case, by calcining the 
nitrate to redness in a current of air, it retains an appreciable amount of 
nitrogen. As a result, when the oxide is reduced in a current of hydrogen, the 
weight of the water formed is less than that which corresponds with the loss of 
weight which has occurred during the reduction of the copper oxide, assuming that 
water is really formed by the union of hydrogen and oxygen. 

The main objections to J. B. A. "^Dumas' work turn on the following facts : 
(1) There is a great difficulty in thoroughly removing all the air from a large com- 
pUcated apparatus ; (2) The absorption of air by sulphuric acid which is slowly 
evolved along with the hydrogen when the acid acts on zinc ; (3) M. Melsens showed 


that there is a retention or occlusion of hydrogen by the reduced copper ; (4) T. W. 
Richards and E. F. Rogers showed that the copper oxide was probably contaminated 
with occluded nitrogen and other gases ; (5) W. Dittmar and J. B. Henderson showed 
that there is a slight reduction of sulphuric acid by hydrogen to form gaseous sulphur 
dioxide (which is later absorbed by the potash) ; (6) The difficulty in drying the 
gas, etc., completely. The last is considered by T. W. Richards (1911) to be 
one of the most fertile sources of error in the determination of accurate equivalents. 
(7) Before the hydrogen reached the copper oxide, J. B. A. Dumas dried it with 
sulphuric acid and phosphorus pentoxide, and used calcium chloride to remove 
the aqueous vapour from the excess of hydrogen which left the copper oxide bulbs. 
Since phosphorus pentoxide removes more moisture from a gas than calcium 
chloride, it is possible that some aqueous vapour escaped. This would tend to 
give high results. (8) E. W. Morley has shown that hydrogen from sulphuric acid 
and zinc always contains carbon compounds which cannot be removed by absorption ; 
(9) J. J. Berzelius emphasized the fact that the displacement of hydrogen by air at 
the end of J. B. A. Dumas' experiment, saturated the liquid water with air and made 
its weight too large ; and (10) W. Dittmar has stated that J. B. A. Dumas did not 
correct his weighings for the buoyancy of air. This would make his weighing of 
the water produced appear too low. 

In 1892 G. D. Hinrichs^ argued that the combining ratio of oxygen in J. S. Stas' 
determinations is a function of the amount of potassium chlorate employed, such 
that with 30-35 grms. of chlorate the atomic weight is 16, and with 100 grms. of 
chlorate, 15'98. Similar results were obtained with the determinations of J. B. A. 
Dumas, J. S. Stas, and J. P. Cooke of the atomic weight of sulphur, chlorine, bromine, 
etc. Hence G. D. Hinrichs argues that the atomic weight should be calculated not 
from the mean, but from the limiting value corresponding with zero weight of the 
substance. P. A. Guye and E. Moles have shown that the relation observed by 
G. D. Hinrichs is confined to determinations in which the quantities of substances 
employed have been weighed in air and the reduction to vacuum effected by 
calculation ; and the results are satisfactorily explained by assuming that the 
anomaly is due to the surface condensation of air and moisture, and should there- 
fore disappear when the weighings are conducted in vacuo. The average deviation 
due to this cause is between 1 in 10,000 and 1 in 20,000. P. A. Guye and E. Moles 
found that with silver the error due to surface condensation is 2 X 10^ ^ gram per 
gram of metal. 


1 M. Monge, Mem. Acad., 78, 1786 ; A. L. Lavoisier and M. Meusnier, ib., 269, 1781 ; A. F. tie 
Fourcroy, L. N. Vauquelin, and M. Seguin, Ann. Chim. Phijs., (1), 8. 113, 183, 1791 ; (1), 9. 7, 
29. 1791; P. L. Dulong and J. J. Berzelius, ib., (2), 15. 386, 1820; J. B. A. Dumas, ib., (3), 8. 189, 
1843; W. H. WoUaston, Phil. Trans., 104. 20, 1814; J. Dalton, A New System of Chemical 
Philosophy, London, 1808 ; H. E. Roscoe and A. Harden, A New View of the Origin of Dalton' a 
Atomic Theory, London, 1896. 

2 J. Priestley, Experiments and Observations on Different Kinds of Air, Birmingham, 1786. 
' A. L. Lavoisier, Mem. Acad., 488, 1784. 

* P. L. Dulong and J. J. Berzelius, Ann. Chim. Phys., (1), 15. 386, 1820 ; J. B. A. Dumas, 
ib., (3), 8. 189, 1843. 

5 J. B. A. Dumas, Ann. Chim. Phys., (3), 8. 189, 1843 ; M. Melsens, ib., (3), 8. 205. 1843 ; 
W. Dittmar and J. B. Henderson, Proc. Phil. Soc. Glasgow, 22. 1, 1891 ; E. H. Reiser, Per., 20. 2323, 
1887; G. S. Johnson, Chem. News, 35. 232, 1879; 59. 272, 1889; W. A. Noyes, Amer. Chem. 
Journ., 12. 441, 1890 ; G. Neumann and F. Streintz, Monatsh., 12. 642, 189l'; J. J. Berzelius 
Lehrbuch der Chemie, Dresden, 3. 1183, 1848; W. Dittmar, Chem. News, 61. 76, 1890; T. W. 
Richards, Journ. Chem. Soc, 99. 1201, 1911. 

6 G. D. Hinrichs, Compt. Rend., 115. 1074, 1892; 116. 753, 1893; 118. 528, 1894; P. A. 
Guye and E. Moles, Journ. Chim. Phys., 15. 360, 405, 1917. 

§ 4. E. W. Morley's Experiment on the Composition of Water by Weight 

\n the determination of atomic weights, a small number of values are to be regarded 
as fundamental. They are the standards of reference ; and by comparison with them all 



the other atomic weights are established. The atomic weights of hydrogen and oxygen 
are primarj'^ ; that is, one or other of them is the basis of the entire system of 
atomic weights.^ — -F. W. Clarke. 

It will be observed that P. L. Dulong and J. J. Berzelius, and J. B. A. Dumas 
weighed the oxygen and the water, and estimated the hydrogen by difference. 
Then followed the work of J. Thomsen, J. P. Cooke and T. W. Richards, and E. H. 
Reiser, in which the hydrogen and water were weighed, and the oxygen estimated 
by difference. W. A. Noyes and Lord Rayleigh weighed the oxygen and hydrogen, 
and estimated the corresponding weight of water. In his memoir On the Density 
of Hydrogen and Oxygen, and the Ratio of their Atomic Weights (Washington, 1895), 
E. W. Morley first made what J. S. Stas called a synthese cotnplete by weighing all 
three quantities— oxygen, hydrogen, and water. He synthesized water by burning 
hydrogen in oxygen, and weighed both gases separately and afterwards in combina- 
tion. In this way he was able to determine the combining ratio of hydrogen and 
oxygen. Since the combining ratio of oxygen with a number of metals has already 
been determined, the combining ratios of the same metals with respect to hydrogen 
can be computed when once the ratio Hydrogen : Oxygen is accurately known. 

Known weights of pure dry hydrogen and pure dry oxygen were stored in two large 
glass globes. The vessels containing the hydrogen and oxygen were weighed separately. 
The hydrogen was prepared by heating palladium hydride, 
and the oxygen by heating potassium chlorate. The 
hydrogen was weighed as palladium hydride, and the 
oxygen was weighed in a compensated glass globe. The 
apparatus for storing and drying the hydrogen and 
oxygen is not shown in Fig. 3. The globe containing 
oxygen was connected with the apparatus, and the oxygen 
passed through a layer of phosphorus pentoxide, and thence 
into the glass chamber M, via one of the jets A ; the globe 
containing hydrogen was similarly connected with the other 
tube containing phosphorus j)entoxide, and the hy- 
drogen led into the chamber M rid one of the jets A. 
The phosphorus pentoxide was not intended to dry the 
entering gases — these had already been dried. The 
chamber M was previously evacuated and weighed. One 
of the gases, say oxygen, was allowed to enter M, and 
electric sparks were passed across the terminals F just 
over the jets A. Hydrogen was led into the apparatus 
and ignited by the sparks. The rates at which hydrogen 
and oxygen entered the chamber were regulated so that 
the formation of water was continuous. The water formed 
was condensed, and collected in the lower part of the 
chamber. To hasten the condensation the apparatus was 
placed in a vessel of cold water — dotted in the diagram. 
When a sufficient amount of water was formed, the 
apparatus was placed in a freezing mixture. The mixture 
of unconsumed oxygen and hydrogen remaining in the 
tube was pumped away, and analyzed. The weights of 
hydrogen and oxygen so obtained wore added to the 
weights of unconsumed hydrogen and oxygen remaining 
in the globes. The phosphorus pentoxide tubes prevented 
the escape of water vapour. The amount of water formed 
was determined from the difference in the weights of the 
system M before and after experiment. The amounts 

of hydrogen and oxygen were determined from the - 

weights of the corresponding globes before and after the experiment, ihe ^J^^^^^^\ 
water formed was determined from the increase in the weight of the above descriDea ve.sei 
before and after the combustion. 

Pjq, 3. — Morley 's Experiment 
—Synthesis of Water. 

E. W. Morley, as a mean of eleven experiments, found that : 

3-7198 grams 
. 29-5335 „ 
. 33-2630 „ 

Hydrogen used 
Oxygen used 
Water formed 

Hence, taking oxygen = 16 as the unit for combining weight, it follows that 16 
parts by weight of oxygen combine with 2-016 parts by weight of hydrogen 



to form 18*016 parts o! water — within the Hmits of the small experimental 
error, and, adds E. W. Morley : " Until further light is obtained concerning the 
sources of error which doubtless afEect all these experiments, this value is the most 
probable that can be derived from existing data." It might be added that the 
ratio Oxygen : Hydrogen = 16 : a; for twenty -five sets of determinations by other 
workers, made since 1821, using different methods, has values of x ranging 
between 2-003 and 2*018. 

§ 5. The Decomposition o! Water by Metals 

If, as I have tried to demonstrate, water is really a compound of hydrogen with oxygen 
... in order to obtain hydrogen, it is only necessary to bring water in contact with a 
substance for which the oxygen has more affinity than it has for hydrogen, in order to liberate 
the hydrogen as a gas. Iron is commonly used for this purpose, and it is necessary to raise 
the temperature to a red heat in order to effect the separation. . . . There is une veritable 
oxidation dufer par Veau. . . . The oxygen is fixed by unity with the iron, and the hydrogen 
is disengaged as an inflammable gas.' — A. L. Lavoisier (1789). 

Water remains permanent and stable so long as the balance of the forces between 
its constituent elements is maintained, but in the presence of a metal which can 
unite with one of these elements, the water may be decomposed. One element — say 
hydrogen — is set free, while the other element — oxygen — unites with the agent of 
destruction to form a new compound — oxide of the metal. The application of this 
principle was suggested to A. L. Lavoisier by the illustrious P. S. de Laplace ; and 
as a result, the first conscious analysis of water was made by A. L. Lavoisier, assisted 
by M. Meusnier, about 1784. This particular process has the disadvantage of 
isolating only one of the two elements of water. In their Memoire oil Von prouve 
par la decomposition de Veau, que ce jiuide n'est point une substance simphy et qu'il 
y a plusieurs moyens d'obtenir en grand Vair inflammable qui y entre comme princife 
constituant, A. L. Lavoisier and M. Meusnier (1771) ^ passed steam over hot iron, 
and found that the metaUic iron was converted into a *' black oxide precisely 
similar to that produced by the combustion of iron in oxygen gas " ; otherwise 
expressed, the iron is oxidized by the water, and the water is reduced by the iron, 
forming " a peculiar inflammable gas," which Lavoisier named hydrogen, because 
" no other term seemed more appropriate." The word signifies the generative 
principle of water, from the Greek vSwp, water, and ycwaw, I generate or produce. 
The German word for hydrogen is Wasserstqff — the stufi from which water is made. 

The following is a modernized form of M. Meusnier and A. L. Lavoisier's experiment :■ — 
Fill an iron, porcelain, or hard glass tube- — 60 cm. long and 1'5 cm. diameter- — with bright 
iron turnings or bright iron nails. In Fig. 4 a hard glass tube is used. This is drawn out 

at one end as shown in the 
diagram. This end is fitted 
with a delivery tube dipping in 
a gas trough. A roll of pre- 
viously ignited asbestos paper, 
6 cm. long, is inserted in the 
opposite end. This end is closed 
with a red rubber stopper and 
the exit tube of the flask so 
arranged that it passes a short 
distance into the core of the 
asbestos paper. The asbestos 
roll, later on, prevents the 
liquid water from coming into 
contact with the hot glass and 
breaking the tube. Water is 
boiled in the flask, and the 
When all the air has been driven 

Fig. 4. 

-Decomposition of Steam by Hot Iron — A. 
Lavoisier and M. Meusnier's Experiment. 

steam passing through the iron turnings is decomposed. 

out of the apparatus, hydrogen may be collected in the gas jar 

If zinc be used in place of iron, the temperature need not be much higher 


than the boiling point of water, since zinc reduces steam and forms zinc oxide at 
a comparatively low temperature. H. V. Regnault 2 found the zinc oxide is crystal- 
line if the reaction occurs at red heat. If a strip of magnesium ribbon be placed 
in a bulb of a hard glass tube and heated, in a current of steam, at a red heat, the 
metal appears to burst into flame, forming magnesium oxide. The resulting 
hydrogen can be ignited if the jet of steam be not too vigorous. According to 
A. Ditte (1871), magnesium decomposes water slowly at 70° ; and according to 
H. Fleck and H. Bassett (1895), magnesium amalgam decomposes cold water. In 
A. W. Knapp's experiment (1912) powdered magnesium is added to ten times its 
weight of water with a little palladious chloride in solution ; metallic palladium is 
formed and this metal acts catalytically or electrolytically on the water. The 
decomposition is then so vigorous that the water appears to boil, and the escaping 
hydrogen ignites spontaneously. Metallic calcium decomposes cold water and 
gives off hydrogen, but the action slows down very soon, probably because the 
calcium hydroxide is not all dissolved by the water, and in consequence a protective 
crust of this substance forms over the surface of the metal. The calcium can be 
advantageously warmed with water in a flask which is connected directly with a 
delivery tube leading to the gas trough. If the water is not free from carbonates, 
a crust of calcium carbonate also forms over the surface of the metal. Calcium 
hydroxide is formed as well as hydrogen. The reaction with strontium is rather 
more vigorous than with calcium ; and with barium more energetic than with 
strontium. The metal sodium decomposes cold water, giving off hydrogen, and 
forming sodium hydroxide. So much heat is generated during the reaction that 
the metal melts, showing that its temperature has risen over 95°. The experiment 
is liable to unpleasant explosions when the sodium is confined so as to enable the 
resulting hydrogen to be collected. The cause of the explosion has not been 
definitely established ; it has been attributed to the formation of a dioxide or a 
hydride. 3 It is more likely to be due to the formation of a film or bubble of water 
superheated above its boiling point. Potassium alone reacts so violently with 
water that the temperature rises high enough to set fire to the hydrogen. The 
hydrogen burns with a violet-tinged flame, owing to the presence of the vapour 
of potassium ; the hydrogen produced by the action of sodium on water burns 
with a yellow flame, owing to the contamination of the hydrogen with the vapour 
of sodium. According to J. J. Berzelius, a solution of metallic sodium in mercury — 
sodium amalgam — decomposes water much less turbulently than sodium alone ; 
the result is similar when a small piece of potassium amalgam — 3 or 4 mm. 
diameter — is placed on water. J. J. Berzelius says the gas obtained by the alkali 
amalgam is odourless, but if an acid or ammonium chloride is also present, the 
product smells like the gas derived from the dissolution of zinc in acids. 

This set of experiments gives a series of metals which appear to react with 
water with increasing violence ; the metals — iron, zinc, magnesium, calcium, sodium, 
potassium — seem to have an increasing avidity or affinity for oxygen so that they 
are able to tear the whole of the oxygen from the water, fix the oxygen, and thus 
liberate half or all the hydrogen as a gas. Under suitable conditions, by treatment 
with fluorine, chlorine, or bromine, the hydrogen is fixed and the oxygen liberated 
as a gas. Still further, by passing an electric current through water, both components 
are liberated in the gaseous state. 


1 M. Meusnier and A. L. Lavoisier, Mem. Acad., 269, 1784 ; A. L. Lavoisier, ib., 468, 1784. 

2 F. G. Benedict, Chemical Lecture Experiments, New York, 1901 ; M. Rosenfeld, Ber ,\b. 
161, 1882 ; 26. 59, 1893 ; Journ. praU. Chem., 12). 48. 599, 1893 ; G. T. Uoody, P roc. C hem. 
Soc., 7. 20, 1891; H. V. Regnault, Ann. Chim. Phys., (3), 43. 477, 1855; A. W. Hofmann, 
Introduction to Modern Chemistry, London, 1865; Ber., 15. 2663, 1882; J. B. Mevick, Amer. 
Chem., 7. 276, 1877 ; A. Senier, Chem. News, 91. 87, 1905 ; A. W. Knapp, ih., Ij^. 253, 1912 ; 
A. Ditte, Compt. Rend., 73. 108, 1871 ; H. Fleck and H. Bassett, Journ. Amer. Chem. 6oc., 17- 
789, 1895 ; J. J. Berzelius, Lehrbuch der Chemie, Dresden, 1. 769, 1825. 

3 R. Bottger, Journ. prakt. Chem., (1), 85. 397, 1862 ; M. Rosenfeld, ib., (2), 48. 699, 189.^. 



§ 6. The Decomposition of Water by Electricity 

Electricity is a key which will open a way into the innermost parts of nature.- 
RiTTER (1798). 


In 1758, G. B, Beccaria ^ exposed water to powerful electric sparks, and although 
he must have decomposed this substance, he does not seem to have been aware ot 
it ; in 1789, the Dutch chemists P. van Troostwijk and J. R. Deiman noticed that 

when an electric charge from a powerful electric 
machine was passed through water, bubbles of gas 
were obtained. They showed that the gases 
were not due to the expulsion of air dissolved 
by the water, since the same result was obtained 
by using distilled water, and water freed from 
dissolved air by a prolonged boiling. Hence, 
it can be inferred that water is decomposed 
into its constituent gases by the electric dis- 
charge. On May 2nd, 1800, W. Nicholson and 
A. Carlisle ^ happened to put a drop of water 
in contact with two wires from a voltaic battery, 
and noticed the formation of small bubbles of 
gas about the tips of the wires provided the wires 
were not in contact. They then immersed the 
two wires in a glass of water, and found that 

Fig. 5.-J. W. Ritter's Apparatus ^^f,^^ were formed about both wires; the gas 
(1800)for the Electrolysis ot Water collected about one wire was hydrogen, and 
— Gases separated. about the other wire, oxygen. Hence, hydrogen 

and oxygen are produced during the electrolysis 
of water. The gases were mixed and exploded. The result was water. This is 
very interesting — chemical combinations can produce an electric current ; here an 
electric current is used to produce chemical decomposition. H. Davy (1807) also 
showed that the hydrogen and oxygen liberated during the decomposition of water 
are in the proportions in which they combine to form water. 

The experiment of W. Nicholson and A. Carlisle appears to have excited a great 
deal of attention at the time, and many substances were treated 
in a similar manner. This culminated in the brilliant discovery of 
the alkali metals by H. Davy in 1807. J. W. Ritter's form of 
apparatus, shown in Fig. 5, is the prototype of the many ingenious 
forms of apparatus which have been devised for illustrating 
W. Nicholson and A. Carlisle's experiment. In place of J. W. 
Ritter's electrodes a and h, Fig. 5, adapted for the discharge from 
an electric machine, plates of gold or platinum, in communication 
with an accumulator or galvanic battery, are used. During the 
passing of the electric current, bubbles of gas accumulate on the 
metal plates and then rise into the test-tubes. More gas is given 
off at one plate than the other. In fact, the volume of the oxygen 
obtained approximates very closely to half the volume of the 
hydrogen. The gas in each tube can be identified by means of a 
lighted taper or otherwise. In the one tube, the taper burns with 
the " blinding brilliance " characteristic of oxygen ; and the gas in 
the other tube burns with the blue flame characteristic of hydrogen. 
The water to be decomposed or electrolyzed is usually acidified 
with a few drops of, say, hydrochloric or sulphuric acid. Some of 
the water disappears during the electrolysis, but no change can be 
detected in the amount of acid mixed with the water. Hence it is 
inferred that the water, not the acid, has been decomposed. The experiment 
succeeds equally well if a solution of sodium or potassium hydroxide be used with 

Fio. 6.— Elec- 
trolysis of 
Water — Gases 


nickel or iron electrodes. Here again the water, not the alkali, is decomposed. 
The acid or alkali is used because water alone does not conduct an electric current 
very well. In fact, pure water is said to be a non-conductor of electricity. 
Dilute solutions of acids or alkalies are good conductors. If iron electrodes are 
used in the acidulated liquid much of the oxygen formed during the decomposition 
of the water is used in oxidizing the metal. 

A mixture of one volume of oxygen and two volumes of hydrogen, called electro- 
lytic gas or detonating gas — A. Volta (1776) called it aura tonante — is often 
wanted in gas analysis, etc. This is easily provided by placing both electrodes 
under one receiver. The apparatus illustrated in Fig. 6 is often used for this 
work — it explains itself. The outer jacket keeps the electrolyte cool. Many 
forms of apparatus have been devised for the electrolytic preparation of small 
quantities of hydrogen and also of the mixed electrolytic gas. 3 

Are hydrogen and oxygen the sole products of the electrolysis of water ? 
—Electrolytic oxygen often contains a little ozone and the electrolyte some hydrogen 
peroxide ; both th^se compounds are formed by the electrolysis of acidulated 
water, but not if a solution of barium hydroxide be electrolysed. Besides oxygen 
and hydrogen, the early chemists noticed that an acid and an alkali are respectively 
formed about the positive and negative poles during the electrolysis of water. 
W. Cruickshank (1800) supposed the acid to be nitrous acid, and the alkah ammonia ; 
J. B. Desormes (1801) considered that hydrochloric acid and ammonia were the 
products ; while M. Brugnatelli (1802) explained the phenomenon by asserting 
that it is the nature of electricity to produce these substances, and he called the 
acid product electric acid. In 1807, Humphry Davy sought the origin of the acid 
and the alkali, and published an account of his experiments in a most important 
memoir entitled. On Some Chemical Agencies of Electricity . T. Thomson has styled 
this investigation " the finest and completest specimen of inductive reasoning 
which has appeared in the age in which Dav}^ lived." 

While accepting H. Cavendish's demonstration that water is a compound of 
oxygen and hydrogen, H. Davy considered the possibiHty that some product might 
result from the unexpected decomposition of oxygen and hydrogen, and he then 
divested the common experiment of every imaginable source of fallacy. It seemed 
to H. Davy that the acid and alkali are most likely produced : (1) from the water ; 
or (2) by the decomposition of the glass ; or (3) by the electrolysis of sodiimi 
chloride derived from the hands touching the instruments ; or (4) from substances 
derived from the ambient air which are decomposed by contact with the electrical 
apparatus. Instead of conducting the electrolysis in glass vessels, Davy tried 
vessels of gold, and by taking precautions to eliminate disturbances produced by 
the contact of the vessels with the hands, and by the presence of impurities in the 
water, H. Davy found that while an acid still continued to be formed, no alkali 
appeared, and he showed that the alkali is derived from the solution of the glass 
vessels during the electrolysis. H. Davy next conducted the electrolysis in an 
atmosphere of hydrogen, and he then found that neither an acid nor an alkali was 
developed, and hence he inferred that the acid which appears in the electrolyte is 
derived from the nitrogen in the atmosphere. Consequently, when precautions 
are taken to prevent the introduction of impurities from external sources, 
no acid or alkali is produced during the electrolysis of water. It has since 
been found that the volume of oxygen obtained during the electrolysis of a solution 
of lithium, potassium, sodium, barium, or calcium hydroxide is sensibly less than 
half that of the corresponding hydrogen. The hydrogen obtained is rather more 
than double the volume of the oxygen when an electric current of low density is 
used for the electrolysis. 

It must be emphasized that the decomposition of water by the electric current 
is not the same in kind as that produced by the disruptive discharge of an electric 
machine in P. van Troostwijk and J. R. Deiman's experiment. In the latter case, 
oxygen and hydrogen are evolved at both the poles dipping in the liquid, while in 


the former case, oxygen is evolved at the one pole, and hydrogen at the other. The 
electrolysis of water by the disruptive discharge is largely masked by the thermal 
decomposition of the water (A. L. Lavoisier and M. Meusnier's experiment). J. W. 
Ritter decomposed water the same year as W. Nicholson and A. Carlisle, but 
apparently in ignorance of their work. J. W. Ritter modified 
the experiment. He half-filled the two legs of a V-tube with 
concentrated sulphuric acid, aa, Fig. 7, so as not to wet the sides 
of the tube with acid ; he then carefully poured distilled water, 
WW, into each leg of the tube so as not to disturb the acid, when 
he found the water in the upper part of the legs of the tube did 
not affect litmus paper. When gold wires, gg, connected with a 
battery were dipped in the water, hydrogen collected at one pole, 
oxygen at the other. J. W. Ritter said that the water in one leg 
of the tube is not in communication with the water in the other 
Fig. 7.— J. W. Hit- leg. He therefore inferred that water is an elementary body, and 
ter's Experiment, that about the one pole water unites with negative electricity to 
form oxygen, and with positive electricity to form hydrogen, 
about the other pole. This conclusion conflicts with the evidence obtained when 
water is decomposed by agents other than electricity, and it was explained by 
Faraday's experiment on electrolysis. 

The formula for water used to be written HO when the atomic weight of 
hydrogen was taken unity, and oxygen 8. This agrees quite well with the deter- 
minations of E. W. Morley and of J. B. A. Dumas. But we naturally ask for an 
explanation of the result of the electrolysis of water. Does an atom of hydrogen 
occupy twice the volume of an atom of oxygen ? 


^ G. B. Beccaria, Lcttere delVElettricismo, Bologna, 1758 ; G. Pearson, Phil. Trans., 87. 142, 
1797 ; P. van Troostwijk and J. R. Deiman, Observations sur la physique, 35. 369, 1789. 

2 W. Nicholson and A. Carlisle, Nicholson's Journ., 4. 179, 1800 ; H. Davy, Phil. Trans., 
97. 1, 1807 ; J. W. Ritter, Voigt's Mag., 2. 356, 1800 ; Gilbert's Ann., 9. 284, 1801 ; 10. 282, 1802. 

' R. Bunsen, Gasometrische Methoden, Braunschweig, 72, 1857 ; 76, 80, 1877 ; A. W. Hofmann, 
Ber., 2. 244, 1869 ; J. N. von Fuchs, Schweigger's Journ., 15, 494, 1815 ; J. W. Dobereiner, 
Gilbert's Ann., 68. 55, 1821 ; A. Ehrenberg, Zeit. anal. Chem., 26. 226, 1887 ; E. W. Magruder, 
Amer. Chem. Journ., 19. 810, 1897 ; J. L. Beeson, Journ. Amer. Chem. Soc, 26. 324, 1904 ; S. S. 
Mereshkowsky, Centrb. Bakter., 11. ii. 786, 1904 ; M. Vezes and J. Labatut, Zeit. anorg. Chem., 
32. 464, 1902 ; J. J. Berzelius, Lehrbuch der Chemie, Dresden, 1. 185, 1825; M. BrugnateJli, Ann. 
chimica, 18. 136, 1800; J. B. Desormes, Ann. Chim. Phys., (1), 37. 284, 1801; H. Davy, Phil. 
Trans., 97. I, 1807; A. Volta, Letiera sulVaria inflammabile, M.ila,n, 1777; Strasbourg, 1778; 
F. Richarz and C. Lonnes, Zeit. phys. Chem., 20. 145, 1896; Lord Rayleigh, Journ. Chem. Soc, 
71. 181, 1897. 

§ 7. Cavendish's Experiments on the Synthesis of Water by Volume 

It is curious to note the changing fortunes of water in the history of chemistry. First 
the matrix of the whole universe ; then only one of the four elements, though the chief of 
the quaternion ; and at last discovered to be itself nothing but a liquid product of combustion, 
one oxide among many, the mere ash, rust, or calx of so much burnt hydrogen.' — S. Brown 

From the earliest dawn of scientific speculation, water has been regarded by- 
natural philosophers as one of the four primal elements, and they were quite right, 
so far as their knowledge went, because they did not know how to decompose it 
into simpler substances. The dogma had been reiterated so frequently that, down 
to the days of the first French revolution, no one appears to have entertained any 
doubts of the simple elementary nature of this liquid ; Basil Valentine called it 
'* the mother of the metals." At the beginning of the seventeenth century the 
sagacious J. B. van Helmont ^ planted a sprig of willow in a vessel suspended in 


air, and fed it on nothing but water ; he found the plant to grow apace — new 
branches, leaves, and roots sprouted forth. He said : 

I placed 200 livres of dried soil in an earthenware pot, and planted therein a sprig of 
willow weighing 5 livres. At the end of five years, the willow had increased to nearly 
69 livres, 3 onces. The vase had never been watered with anything but rain water or dis- 
tilled water. 

Hence it was inferred that the constituents of plants — wood, foliage, acids, salts, 
and earths — are embodied within elemental water in some mysterious inscrutable 
way. The experiment seemed to him a crucial one, but J. Woodward's researches 2 
showed the conclusion was fallacious because the parts played by the substances 
dissolved in the water, and by the atmospheric air surrounding the plant, were not 
recognized. The composite nature of water was not suspected until over a century 
after J. B. van Helmont's time. Even the shrewd Robert Boyle in his Sce/ptical 
Chymist (Oxford, 1661) lauded, beyond his predecessors, the importance of water : 

It seems evident that water may be transmitted into all the other elements . . . not 
only plants, but animals and minerals may be produced out of water. 

Near the end of the seventeenth century, Isaac Newton 3 noticed that while 
the refractive indices of various non-combustible substances increase proportionally 
with their densities, the increase with the refractive indices of combustibles — 
camphor, turpentine, oils — is greater than corresponds with their densities. " Water 
has a refractive index in a middle degree between those two sorts of substances, 
which consist as well of sulphureous, fat, and inflammable parts, as of earthy, 
lean and alcalizate ones." After the compound nature of water had been discovered, 
commentators read into Newton's statement a prediction that water would be found 
to contain an inflammable substance as one of its constituents, although it may be 
questioned if Newton intended to make any such assertion. 

In 1782, J. Priestley * thought that he had proved that water is converted into " air 
of the same purity as the atmosphere " by heating it in porous earthenware vessels so long 
as there is free access of air to the outside of the retort, but he found the following year, 
in agreement with a hint he had received from Josiah Wedgwood, that the supposed con- 
version was a mal-observation, because the air was transmitted from the outside to the 
interior through the pores of the retort. 

In 1776, p. J. Macquer ^ noticed the formation of a liquid resembling water when 
hydrogen burns in air, and the flame is allowed to impinge on a cold slab of porcelain. 
Soon after the discovery of oxygen, J. Priestley (1775) noticed that when hydrogen 
is mixed with certain proportions of oxygen, a violent detonation occurs when 
ignited by a flame. In the spring of 1781, J. Priestley ^ made what he called " a 
random experiment to entertain a few philosophical friends," in which a mixture 
of inflammable air with dephlogisticated air or oxygen was exploded in a closed 
vessel by means of an electric spark, as had been effected by A. Volta in 1777. The 
sides of the glass vessel were found to be bedewed with moisture after the explosion. 
Neither P. J. Macquer nor J. Priestley appears to have paid any particular attention, 
at the time, to the phenomena ; they both seem to have thought that the dew 
" was nothing else than the mechanical deposit of the moisture dispersed in common 
air." According to J. Priestley, John Warltire repeated this experiment with a 
copper vessel, and obtained a slight loss of weight which he thought might be due 
to the escape of ponderable matter in the form of heat, through the pores of the vessel. 
In the light of subsequent events, il est clair, said A. L. Lavoisier (1781), q^^e M. 
Priestley a forme de Veau sans s'en douter. Meanwhile, H. C. Cavendish ^ looked 
upon the deposition of the dew as a fact " well worth examining more closely " ; 
H. Cavendish also wished to find what became of " the air lost " during the com- 
bustion of hydrogen in common air. He tried (i) if the air had been changed into 
carbon dioxide ; (ii) if it had been changed into nitric acid ; and (iii) if it had been 
changed into sulphuric acid. He negatived these hypotheses one by one. In the 
summer of 1781, H. Cavendish followed up the subject by exploding mixtures of 


dephlogisticated air with inflammable air in closed vessels. A certain amount of 
the gaseous mixture lost its elastic form, and produced a certain amount of liquid 
water. In the fourth experiment on exploding gases — dated July 5th, 1781, in his 
laboratory notebook — H. Cavendish demonstrated the relations between the 
volumes of inflammable air and common air consumed in the formation of water, 
for he showed that by exploding a mixture of 7,344 volumes of inflammable air 
(from zinc and an acid) with 17,361 volumes of common air, there was a contraction 
of 10,630 volumes, and a gas -^-^^d of the specific gravity of common air remained. 
Before the end of the month, H. Cavendish had proved the liquid product to be 
pure water, for his notebook says : 

The liquid was not at all acid, nor gave the least red colour to paper tinged with red 
flowers, it yielded no pungent fumes on evaporation, and yielded scarce any sediment 
on evaporation to dryness. 

H. Cavendish stated his conclusion from these experiments when they were 
described in his paper Experiments on Air (London, 1784) : 

When inflammable air (hydrogen) and common air are exploded in proper proportion, 
almost all the inflammable air, and near one-fifth of the common air, lose their elasticity, 
and are condensed into dew. And by this experiment it appears that this dew is plain 
water, and consequently that almost all the inflammable air and about one-fifth of the 
common air, are turned into pure water. 

Cavendish repeated the experiment with a mixture of inflammable air (hydrogen) 
with nearly twice its volume of pure dephlogisticated air (oxygen), and found that 
almost the whole of the mixture in the globe formed pure water ; a quantity of 
water was collected by repeatedly introducing more gas into the globe and exploding 
the mixture. The vessel and its contents underwent no change in weight or parted 
with anything ponderable during the explosion, while a certain volume of gas was 
replaced by a certain weight of water. Hence, as A. L. Lavoisier ^ has expressed 
it : Veau n'est point une substance simple, et qu'elle est composee poids pour poids d'air 
inflammable et d'air vital — otherwise expressed, water consists, weight for weight, 
of the hydrogen and oxygen gases lost in its production. The results of H. Caven- 
dish's experiments, 1781-2, were communicated to J. Priestley not later than March, 
1783, and also to A. L. Lavoisier in June, 1783, and published in 1784 ; the delay 
in publication was occasioned by the need for investigating the puzzling appearance 
of nitric acid along with water when oxygen was substituted for atmospheric air. 
There appears to have been some extensive alterations in Lavoisier's paper before 
it was pubUshed, but there is no means of determining precisely the extent of the 
additions. J. Watt wrote a letter to J. Priestley, April 26th, 1783, containing an 
outline of a theory of the composition of water, and on June 19th, 1783, Joseph 
Priestley ^ read a paper before the Royal Society in which he stated in reference 
to the bedewed glass in his experiment : 

I carefully weighed a piece of filter paper, and then, having wiped with it all the inside 
of the glass, weighed it again, and always found, as nearly as I could judge, the weight of 
the decomposed air in the moisture acquired by the paper, 

H. Cavendish's public statement that he had previously communicated to 
J. Priestley every experiment which was needed to determine the composition of 
water was publicly acknowledged by J. Priestley. These facts have never been 
impugned, and they are supported by the entry in the Minute Book of the Royal 
Societ}^ 10 which was confirmed at the meeting on the 26th June, 1783. J. Priestley 
said : 

These arguments received no small confirmation from an experiment of Mr. Cavendish, 
tending to prove that the reconversion of air into water, in which pure dephlogisticated air 
and inflammable air were decomposed by an electric explosion, and yielded a deposit of 
water equal in weight to the decomposed air. 

The work of H. Cavendish was soon confirmed by M. Monge,ii in a memoir 
Sur le resultat de Vinflammation du gas inflammable et de lair dephlogistique dans des 


vaisseaux clos. M. Monge exploded measured volumes of hydrogen and oxygen 
in an exhausted glass globe, and by admitting fresh quantities of gas for explosion, 
he collected a relatively large amount of water. He calculated the weight of the 
original gases from their known densities, and weighed the liquid product. The 
results showed : 

L'air inflammable . 
L'air dephlogistique 

Total weights of components 
Total weight of product . 

Deficit . 

Onces. Gros. Grains. 

6 10-03 

3 58-53 

3 6 68-56 

3 5 101 

1 67-55 

Owing to the use of moist gas, M. Monge over-estimated the weight of the 
hydrogen which he had employed, and consequently there was a small deficiency 
between the observed weight of water and the estimated weights of gas required 
for the synthesis. The water produced was very slightly acid, and he assumed 
that the acidity is due to " the small quantity of vitriolic (sulphuric) acid which 
inflammable air carries when prepared by the dissolution of iron " in that acid ; H. 
Cavendish had already proved that nitric acid is a by-product of the reaction 
under certain conditions. M. Monge concluded (i) that the volume of hydrogen 
required for the formation of water is about twice as great as that of the oxygen ; 
and (ii) that " when inflammable air and dephlogisticated air, both pure, are 
exploded, there is no other product but pure water, heat, and light." Experiments 
similar in principle, but with highly purified materials, were made by A. Scott 
and E. W. Morley ^^ over a century later. 

J. Watt's claims to the first trae conception of water. — In 1783, James Watt,i3 
of engineering fame, expressed the opinion that " according to J. Priestley's experi- 
ments, dephlogisticated air unites completely with about twice its bulk of inflam- 
mable air . . . and therefore water is composed of dephlogisticated air and phlo- 
giston." It is possible, though doubtful, that J. Watt had in mind inflammable 
air or hydrogen when he used the term phlogiston, and by dephlogisticated air, 
what is now called oxygen. In his Thoughts on the constituent parts of water and 
of dephlogisticated air communicated to the Royal Society, November, 1783, 
J. Watt said that he was convinced by the arguments of R. Kirwan and J. Priestley 
that inflammable air is either wholly pure phlogiston or at least that it contains no 
admixture of any other matter ; but he added that in his opinion inflammable air 
contains a small quantity of water and much elementary heat. He regarded heat 
as a material substance, and invested it with the capacity of combining with other 
substances like other material elements, and of becoming the basis of those sensible 
qualities by which bodies are permanently distinguished from each other. According 
to his theory, dephlogisticated air is composed of water deprived of its phlogiston 
and united to elementary heat. He believed that dephlogisticated air and phlogiston 
can unite in certain degrees to form, not water, but fixed air, while under other 
circumstances they can unite to form neither water nor fixed air, but rather 
phlogisticated air. In spite of this, it has been claimed that J. Watt was the first 
to form the conception that water is a compound of dephlogisticated and inflam- 
mable air, and that H. Cavendish made the proposition good by unassailable ex- 
periments. Naturally, J. Watt's claims have been disputed, and the so-called waier 
controversy has been waged upon the rival claims of J. Watt, A. L. Lavoisier, and 
H. Cavendish. 

The controversy is exceedingly involved. The three rival claimants almost 
simultaneously arrived at analogous conclusions by different paths. H. 
Cavendish was at work on the products of the combustion of hydrogen ; 
J. Watt was speculating on the latent heat of steam ; and A. L. Lavoisier 
was studying the production of acids by the oxidization of inflammable substances. 
All these paths ultimately converged into the one line of inquiry which culminated 


in the discovery that water is a compound of hydrogen and oxygen. The issue is 
confused by the fact that, while the date of publication or receipt by a scientific 
society is usually taken to be decisive in questions of priority, this is not always 
satisfactory. In the present case, the observed results of the one were communicated 
to others before they were published, and alterations were made in some of the 
original papers, after they had been read and before they were published. Lord 
Jeffrey has shown that the case cannot be decided by those narrow and jealous 
canons of evidence derived from the rigid maxims of law or the precedents in cases 
of patent : 

Courts of law must proceed on inflexible rules, and can make no distinction of persons ; 
and are forced therefore peremptorily to reject all evidence proceeding from the parties 
concerned, or from those having any interest in the issue ; though it is certain by so doing 
they must occasionally decide against the truth, and against the conviction of all unpro- 
fessional observers. The question m a court of law, in short, is never really what the truth 
of a case is, according to the actual and conscientioiis belief of the judges or jury, after 
considering every atom of producible evidence that is in existence, but merely what is the 
import of the evidence that is legally admissible. ... In all questions before the public 
no evidence is inadmissible. 

J. Priestley tried to repeat the experiment on the formation of water which he 
said had been described to him by H. Cavendish. In order to ensure the absence of 
moisture, Priestley prepared his dephlogisticated air from nitre ; and his inflammable 
air, by heating what he called " perfectly made charcoal " in a retort. The gas from 
the charcoal would obviously be obtained by the diffusion of air and furnace gases 
through the walls of the retort and the reaction between these gases and the charcoal. 
J. Priestley failed because, through an extraordinary blunder, the wrong inflammable 
air was used. According to W. V. Harcourt (1846), " neither the phlogiston nor 
the inflammable air of Priestley and Watt were convertible terms for hydrogen, 
their notion of the change of air into water, and of water into air, had no reference 
to hydrogen, but first to nitrogen, and afterwards to a mixture of gases, the chief 
of which was carbon monoxide. J. Priestley's paper was communicated to the Royal 
Society on April 19th, 1783 ; H. Cavendish's communication to J. Priestley must 
therefore have been anterior to the speculation which J. Watt addressed to J. 
Priestley on the 26th of the same month, as well as to Lavoisier's experiments the 
following June." J. Priestley thus comes as an intermediate link, for through him 
an account of the experiments and conclusions of H. Cavendish were transferred to 

Lavoisier's claims to the discovery of the composition of water. — For a time, 
some claimed A. L. Lavoisier to have discovered the composition of water inde- 
pendently of H. Cavendish. According to M. Berthelot, the laboratory journal shows 
that as early as March, 1774, the attention of A. L. Lavoisier was directed to the 
product of the combustion of hydrogen since he believed that every inflammation 
ought to be attended by an increase in weight, and in 1777, he burnt hydrogen in 
air, and, in 1781, oxygen in hydrogen ; but A. L. Lavoisier's mind was preoccupied 
with the conviction that oxidation means acidification, and the production of water, 
which must have occurred, seems to have passed unheeded. Lavoisier said after- 
wards that he did not then know about Macquer's experiment. A. L. Lavoisier 
was always on the alert as to the nature of the products of the combustion of 
hydrogen, and in 1783 he was in such a position that the slightest hint would enable 
him to comprehend its true nature. This hint was furnished by the rumours of 
H. Cavendish's experiment which spread through the scientific world in the spring 
of 1783. C. Blagden communicated the result of H. Cavendish's experiment to 
A. L. Lavoisier, at Paris, on June 24th, 1783. A. L. Lavoisier confirmed the fact 
with a single hasty experiment made a few days after C. Blagden's communication, 
and described before the French Academy — partly in November and partly in 
December, 1783. Although the account of H. Cavendish's Experiments on Air was 
not read before the Royal Society until January 15th, 1784, Lavoisier i* said that on 



June 24th, 1783, " Mr. Blagden has informed us that Mr. Cavendish had burnt 
inflammable air in closed vessels, and that he had obtained a very sensible quantity 
of water." Consequently, H. Cavendish was undoubtedly first in the field, and 
he furnished his rivals with the grounds of their conclusions — J. Watt through 
J. Priestley, A. L. Lavoisier through C. Blagden. 

H. Cavendish was certainly not clear about the character of the reaction involved 
in his synthesis because his mind was unconsciously mystified by the phlogiston 
hypothesis. He seems to have rather incUned to the opinion that the indifference 
of hydrogen to oxygen at ordinary temperatures impUed the presence of some 
substance in the former which lessened the intensity of its affinity for oxygen, and 
he conceived that this substance could be water alone, because water is the sole 
residue of the combustion of hydrogen and oxygen. Thus, H. Cavendish remarked : 

From what has been said, there seems the utmost reason to think that dephlogisticated 
air is only water deprived of its phlogiston, and that inflammable air, as was before said, 
is either phlogisticated water, or else pure phlogiston, but in all probability the former. 

The indifference of free hydrogen and oxygen to one another at ordinary tempera- 
tures was a source of perplexity to others besides H. Cavendish. Thus, J. Watt is 
said : 

Priestley accounts for the facts by supposing that the two kinds of air, when formed 
at the same time and in the same vessel, can unite in their nascent state ; but that, when 
fully formed they are incapable of acting upon one another, unless they are first set in 
motion by external heat. 

It certainly required Lavoisier's system to give a significance to Cavendish's capital 
discovery, by showing that water is a definite oxide or calx of hydrogen formed 
whenever hydrogen is burnt in air or oxygen. 

The synthesis of liquid water. — The following is a modernized form of 
H. Cavendish's elegant experiment, although it is not any more demonstrative. In 
Cavendish's original experiment, the explosion vessel was weighed before and after 
the gases were exploded : 

A stout glass vessel, A, Fig. 8, is fitted with a stopcock, C, at one end, and with a piece 
of strong pressure tubing, D, con 
nected with a reservoir, at the other 
end. A pair of platinum wires, T, 
are sealed into the stout glass measur- 
ing vessel just below the stopcock. 
These wires are put in commimica- 
tion with an induction coil, which 
in turn is connected with an accumu- 
lator. The tube A is called the 
eudiometer^ or the explosion tube. 
This is filled with mercury by ad- 
justing the levelling tube B and the 
stopcock C. A mixture containing one 
volume of oxygen and two volumes 
of hydrogen is introduced into the 
explosion tube vid the stopcock G 
and by depressing the levelling tube. 
When the explosion tube is about half 
or three-foiu*ths filled, read the volume 
of its contents by bringing the mercury 
to the same level in both levelling tube 
and explosion tube. Then depress the 
levelling tube so that the mercury falls 
nearly to the bottom of the explosion 
tube. Pass a spark from the induction 
coil through the wire terminals of the 
explosion tube. The gases explode, 

and the level of the mercury is again • u . x 

adjusted after the apparatus has stood for a few minutes m order to regain the temperature 
The mercurv rises nearly to the level of the stopcock. The mixed gas probably 

Tube " 


8. —Modern Form of Synthesis of Liquid Water 
by Volume — Cavendish's Experiment. 

of the room. 



contained a trace of air, and probably also a slight excess of either oxygen or hydrogen. The 
advantage of this form of explosion vessel lies in the fact that the explosion takes place under 
diminished pressure, and is not so liable to fracture the apparatus because it is less 

The result shows that two volumes of hydrogen unite with one volume of oxygen 
to form water. Suppose the experiment he repeated a number of times with, say, 
one volume of oxygen and three volumes of hydrogen — one volume 
of hydrogen remains after the explosion ; again try the experiment with 
two volumes of oxygen and two volumes of hydrogen — one volume 
will remain uncombined after the explosion. It is inferred from this 
experiment, that two volumes of hydrogen and one volume of oxygen 
combine to form water, and if an excess of either oxygen or 
hydrogen be present, the excess will remain uncombined after the 

Gas analysis. — If a known volume of gas containing hydrogen be 
mixed with an excess of air or oxygen ; or if a known volume of a 
gas containing oxygen be mixed with an excess of hydrogen and 
exploded in a eudiometer, the contraction represents the volume of 
water formed, and the corresponding amount of the gas under 
investigation can be computed. A. Volta i^ utilized these .facts in 
devising a process to estimate the two gases. A metal cap, B, was 
fitted to the upper part of a graduated tube, A, Fig. 9, which con- 
stituted J. Priestley's eudiometer. The metal cap carries an insulated 
wire, C, which enabled a spark to be passed in the interior of the tube. 
A rubber ring, Z), was used in reading the level of the liquid in the 
tube. The funnel, F, connected with the stopcock. E, was used in 
fiUing the eudiometer with gas in the pneumatic trough. The Hmits of 
explosibility of mixtures of hydrogen and oxygen are approximately Hydrogen : 
Oxygen=5'4 : 94'6 ; and 94*7 : 5'3. No explosion occurs if the proportions of 
these two gases are outside these limits. 

Fig. 9— 
Vol t a's 

Example. — 20 c.c. of air were mixed with 20 c.c. of hydrogen and exploded. The 
mixed gases, after the explosion, occupied 28 c.c. Hence, the contraction shows that 12 c.c. 
of the mixture combined to form water. Of this two -thirds must have been hydrogen, and 
one-third oxygen. Hence, the original 20 c.c. of air contained 4 c.c. {i.e. one-third of 12 c.c. 
of oxygen). This illustrates an important principle used in gas analysis. 

J. Priestley was led astray by the presence of nitric acid in the water formed 
by the union of hydrogen with oxygen. According to H. Cavendish's notebooks,!^ 
he found in September, 1781, that the liquid formed by exploding oxygen 
with twice its volume of hydrogen contained some nitric acid. H. Cavendish also 
found that this acid was obtained whether the oxygen was prepared from mercury 
nitrate, from mercuric oxide, or from plants under the action of solar light ; and 
consequently he inferred that the nitric acid was not present as an impurity in the 
oxygen. In January, 1783, he showed that, if hydrogen is burnt in the presence 
of an excess of oxygen slightly contaminated with nitrogen, the excess of oxygen 
unites with the nitrogen forming nitric acid ; but if the hydrogen is burnt with 
oxygen mixed with a large proportion of nitrogen, " the heat of the explosion is 
80 much diminished that though the affinities of hydrogen and oxygen are sufficient 
to determine at that temperature the formation of water, the affinities of nitrogen 
and oxygen are not sufficient to determine the production of nitric acid." H. 
Cavendish thus demonstrated that the only product of the explosion of hydrogen 
and oxygen is water. 

E.. Bunsen (1857) ^^ noticed that when electrolytic gas is exploded with air, some 
nitric oxide is formed, and if an excess of oxygen be present, some nitrogen peroxide 
is also formed. According to K. Finckh (1905), the amount of nitric oxide so 



formed depends upon the temperature and pressure of the admixed gases. For 

Initial pressure of mercury 
Electrolytic gas per 100 vols, air 
Nitric oxide formed 











750 mm. 
210 vols. 
3*01 per cent. 

To reduce the proportion of nitrogen oxides formed during the explosion of hydrogen 

(or hydrocarbons) with air, R. Bunsen found it best to keep the amount of pure 

hydrogen between 3'81 and 1-55 per cent., for 

the resulting error is then negligibly small, If 

wider eudiometer tubes than those employed 

by R. Bunsen are used, these limits must 

be raised. According to A. SchuUer (1882), 

when hydrogen is exploded with an excess 

of oxygen, some hydrogen peroxide is formed 

at the same time. 

The volumetric synthesis of steam.— 
When hydrogen unites with oxygen to form 
water, is the product equal to the joint 
volume of the constituents when measured 
in the same state of aggregation, without 
allowing the gaseous water to condense to 
the liquid state ? Water is a gas — often 
called steam — when its temperature is a 
little above 100° at ordinary atmospheric 
pressures. In 1865 A. W. Hofmann modified 
an old experiment of J. L. Gay Lussac (1808) 
by placing a hot vapour jacket about the 
explosion tube so that the water remains Fig. 10. — Synthesis of Steam by Volume, 
in the gaseous condition and does not con- 
dense to a liquid after the explosion. A. W. Hofmann's experiment was described 
in his Introduction to Modern Chemistry (London, 1865), and a modification is 
illustrated in Fig. 10. 

The upper end of the glass jacket surrounding the explosion tube. Fig. 10, is connected 
with a flask, M, containing toluene, boiling at about 110°, or amyl alcohol, boiling at about 
130°. The lower end of the jacket is connected with a flask and condenser, N, so that the 
amyl alcohol can be recovered. When the amyl alcohol is steadily boiling, and the ex- 
plosion tube has been filled as described in the preceding experiment, the gases are sparked. 
In a few minutes, when the temperature has had time to adjust itself, bring the levelling 
tube in position for a reading. 

The result of this experiment is to demonstrate that two volumes of hydrogen 
unite with one volume of oxygen to form two volumes of steam, for the steam 
occupies just two-thirds the original volume of the mixed gases. Hence, A. W. 
Hofmann's form of J. L. Gay Lussac's experiment demonstrates that when water 
is synthesized at a temperature above its point of condensation — 100° — two 
volumes of hydrogen react with one volume of oxygen to form two volumes 
of steam. Several types of chemical problems are based on this fact. It is necessary 
to correlate the different results obtained when water is synthesized by volume 
and by weight. 


1 J. B. van Helmont, Orfus Medicince, Amsterdam, 1648; Lugduni Batavorum, 68, 1656. 

2 J. Woodward, Phil. Trans., 21. 193, 1699 ; H. Braconnot, Ann. Chim. Phys., (1), 61. 187, 

* Isaac Newton, Opticas, London, 75. 1704, 

4 J. Priestley, Phil. Trans., 23. 426, 1783. 

VOL. I. L 


* A. L. Lavoisier, CEuvres, Paris, 2. 335, 1862 ; P. J . Macquer, Dictionnaire de chimie, Paris, 

2. 314, 1778. 

* J. Priestley, Experiments and Observations on Different Kinds of Airy London, 2. 30, 1775 : 

3. 382, 1777 ; 5. 395, 1781. 

' H. C. Cavendish, Phil. Trans., 74. 119, 176, 1784 ; 75. 372, 1785 ; Alembic Club Reprints, 3 ; 
1893 ; R. Kirwan, Phil. Trans., 74. 154, 1784. 

« M. Lavoisier, Mim. Acad., 473, 1781 (printed 1784). 

» J. Priestley, Phil. Trans., 73. 414, 1783; Experiments on Air, Birmingham, 6. 29, 1780. 

i» W. V. Harcourt, B. A. Rep., 22, 1839. 

" M. Monge, Mem. Acad., 78, 1786. 

12 A.. Scott, Phil. Trans., 184. 643, 1893 ; E. W. Morley, Zeit. phys. Chem., 20. 68, 242, 417, 

" G. Wilson, The Life of the Honorable Henry Cavendish, London, 265-446, 1851 ; H. Kopp, 
Beiirage zur Geschichte der Chemie, Braunschweig, 1876 ; E. Grimaux, Lavoisier, 1743-1794, 
Paris, 1888 ; T. E. Thorpe, B. A. Rep., 761, 1890 ; M. Berthelot, La revolution chimique, Paris, 
1890 ; Notice historique sur Lavoisier, Paris, 1889 ; J. P. Muirhead, Correspondence of the late 
James Watt on his discovery of the theory of the composition of water, liondon, 1846 ; Lord Brougham, 
Lives of the Philosophers of the time of George III., London, 1855 ; W. V. Harcourt, B. A. Rep., 
22, 1839; Phil. Mag., (3), 28. 106, 478, 1846; J, W&tt, Phil. Trans., 74. 329, 1784; Anon., 
Quart. Rev., 77. 105, 1846; Lord Jeffrey, Edin. Rev., 57, 1848. 

1* A. L, Lavoisier, Mem. Acad., 468, 1784. 

15 .J. Watt, Phil. Trans., 74. 334, 1784. 

16 A. Volta, Ann. Chimica, 1. 171, 1790 ; 2. 26, 1791 ; 3. 36, 1791. 

17 W. V. Harcourt, B. A. Rep., 1, 1839 ; H. Cavendish, Phil. Trans., 74. 130, 1784. 

1^ R. Bunsen, Gasometrische Methoden, Braunschweig, 72, 1857 ; K. Finckh, Zeit. anorg. Chem., 
45. 116, 1905 ; A. Schuller, Wied. Ann., 15. 290, 1882. 


§ 1. The Atmosphere 

The atmosphere in which we live and breathe is really a part of the globe on which we 
stand. We are not surrounded by mere empty space. On the contrary, we live and move 
at the bottom of a vast ocean of air, which is just as material as the water which surrounds 
the flat-fish living at the bottom of the sea {1914)." 

Air was once considered to be a thin, pellucid, evanescent, inscrutable, and im- 
ponderable spirit — the spirit of life. Even to-day, air is still used as a symbol 
for what is spiritual and divine ; but to early man the analogy between the im- 
palpable breath of the physical heavens and the inscrutable spirit of God, was 
very real. It was quite a long time before air was recognized to be a gravic 
material essentially ponderable like earth and sea.^ 

The physical properties of air were studied long before its chemical properties 
were investigated. Anaxagoras, who lived about the sixth century B.C., cited two 
experiments to show that air is material : (i) A blown bladder resists compression, 
and (ii) the inside of an inverted drinking glass when plunged beneath the surface 
of water remained dry showing that the presence of air prevented the ingress of 
the water. These are among the earliest experiments on record. Aristotle (b.c. 
384), in spite of some confused ideas on the nature of gases, considered air to be a 
material substance which possessed weight, because he found that a blown bladder 
weighed less when empty than when inflated with air. Simphcius, a writer of the sixth 
century, commenting on Aristotle, said that Ptolemy showed that air has no weight 
when weighed in air, and that Aristotle's conclusion was vitiated by the condensa- 
tion of moisture in the bladder derived from the air blown from the lungs during 
the inflation of the bladder. About a century before Christ, Hero of Alexandria, 
in an important work on Pneumatics, described some experiments to prove that 
air is a material substance. For instance, he said : 

Let a vessel which seems to be empty be inverted, and, being carefully kept upright, 
pressed down into the water ; the water will not enter it even though it be entirely immersed ; 
so that it is manifest that the air, being matter, and having itself filled all the space in the 
vessel, does not allow the water to enter. Now if we bore the bottom of the vessel, the water 
will enter through the mouth, but the air will escape through the hole. Again, if before 
perforating the bottom, we raise the vessel vertically, and turn it up, we shall find the inner 
surface of the vessel entirely free from moistiu-e, exactly as it was before immersion. Hence, 
it must be assumed that the air is matter. 

A similar experiment was mentioned by Empedocles 2 (c%Vca 430 B.C.), and a 
correct explanation given. The same experiment was also described in the essay, 
De ingeniis spiritualibus, by Philo of Byzantium, about 300 B.C. 

The weight of air.— In his Book of the balance of wisdom, written in the fifteenth 
century, the Arabian Al-Khazoni recognized clearly that air has weight. He said : 

When a heavy body of whatever substance is transferred from a rarer to a denser air, 
it becomes lighter in weight ; and when transferred from a denser to a rarer air, it becomes 
heavier. . . . Although the weight of a substance in air does not appear to vary, there is 
an actual variation, owing to a difference of atmospheres at different times. 
However, GaHleo dei Galilei, in 1632, is usuaUy credited with having first demon- 
strated satisfactorily that air possesses weight ; and he made a rough determmation 



of the specific gravity of air by comparing the relative weights of equal volumes of 
air and water. G. Galilei found water to be 400 times heavier than air ; and twenty 
or thirty years later, R. Boyle (1661) found water to be 938 times heavier than air. 
Both measurements were very crude, and are quite unreUable; G. Galilei's result 
is too low, R, Boyle's too high. Refined experiments show that 1000 c.c. of dry 
air weigh 1*293 grms. under standard conditions — 760 mm. pressure, 0°, and at 
sea level in latitude 45°. Hence the specific gravity of air is 0001293 if water be 
unity. This means that a normal litre of dry air freed from carbon dioxide and at 
0° and 760 mm. weighs 1-2930 grams at sea-level and a latitude of 45°. The 
actual numbers are : 1 29276 (H. V. Regnault, 1847) ; 1-293085 (P. von JoUey, 
1879) ; 1-29284 (Lord Rayleigh, 1888-93) ; 1*29273 (A. Leduc, 1898) ; 1*2930 
(P. A. Guye, J. Kovacs, and E. Wourtsel, 1912) — vide Cap. on atmospheric air. 

The accidental or experimental errors affecting the number 1-2930 amount to 
less than one in ten thousand. The variations which have been observed show 
that the density of air is not constant but variable both with respect to place and 
time. This conclusion is in harmony with the variations which have been observed 
on the relative proportions of oxygen and nitrogen in air. Thus, P. A. Guye, J. 
Kovacs, and E. Wourtsel found the weight of a normal litre of air, collected during 
a rising barometric pressure, to be 1-2927 grm., and 1*2932 grm. when collected 
during a falUng barometric pressure. The former number is taken to mean that 
the air has a shght deficit in the proportion of oxygen, and the latter, a sHght deficit 
in the proportion of nitrogen — when the normal Utre is taken as 1-2930 grms. The 
specific gravity of air, referred to the standard hydrogen 2, is taken to be 28*75 ; 
or it oxygen 32 be the standard, 28*95. 

The terms atmosphere and air are sometimes taken to be synonymous and interchange- 
able, but the word air is often used when reference is made to a limited portion of the 
atmosphere. The word air was formerly used in the same general sense that the word gas 
is to-day. Later, the meaning of the word air was narrowed to connote the atmosphere. 
The word atmosphere is derived from the Greek aTfx6s, vapour ; acpaipa, the sphere. 
The term atmosphere is also applied to the gaseous envelope or medium surroiinding any 
body, whatever be the nature of the gas- — air, oxygen, carbon dioxide, etc. Hence the term 
atmospheric air is often used to emphasize the fact that air is the enveloping medium. 

Both Anaxagoras and Aristotle believed that there is no vacuum and this belief 
crystallized into the phrase : Nature abhors a vacuum. For instance, when a 
glass cyHnder, closed at one end, is filled with water ; then closed at the open end 
with the hand ; turned upside down ; and the hand removed while the open end of 
the cyHnder is under water, the water remains in the cyHnder. The rise of water 
in pump barrels was explained by the same hypothesis. When it was found that 
water could not be pumped higher than about 34 ft., it followed that the hypothesis 
required modification, for nature's horror of a vacuum obviously could extend 
only to the equivalent of 34 ft. of water. 

The pressure of the air.— In 1644, E. TorriceUi,^ a pupil of G. Galilei, pubHshed 
an account of an experiment which puzzled the philosophers of the time because 
they were obsessed by the hypothesis that nature abhorred a vacuum. 

In E. Torricelli's experiment, a glass tube — about four feet long, and closed at one end 
■ — was filled with mercury, the open end was closed with the thumb, and the tube inverted 
so. that, when the thumb was removed, the open end was immersed in mercury . No air was 
allowed to enter the tube during the operation. Instead of the mercury remaining suspended 
in the tube, the column of mercury fell to such an extent that its height above the surface 
of the mercury in the dish was nearly 30 inches, or 760 mm. The vacuous space in the tube 
above the mercury is called Torricelli's vacuum. 

Nature's horror of the vacuum at the top of the tube extended only to the equivalent 
of 30 inches of mercury. It did not appear probable that nature should have a 
particular whim of this character, and E. Torricelli suggested the alternative hypo- 
thesis that the column of mercury was maintained by the air pressing on the surface 
of the 7nercury in the outer vessel. B. Pascal, in his New experiments concerning the 
vacuum (1647), argued that since mercury is nearly 13 J times as heavy as water. 


30 inches of mercury will be equivalent to 34 ft. of water, and he accordingly- 
repeated E. Torricelli's experiment with a tube 46 ft. long, using water instead of 
mercury. He obtained a column of water 34 ft. long. When the experiment was 
repeated with other liquids, he found, in every case, that the height of the column 
was inversely as the density of the liquid. Hence, it was inferred that the height 
of the column of mercury is a measure o{ the pressure of the atmosphere, and that 
fluctuations in the pressure of the air are accompanied by a corresponding rise or 
fall in the column of mercury. R. Boyle (1665) apphed the term barometer to 
Torricelli's instrument — from the Greek ^dpo^, weight ; and fxirpov, a measure. 
In 1647, B. Pascal persuaded M. Perier to repeat Torricelli's experiment at the 
bottom and at the summit of the mountain Puy-de-D6me. On September 23rd, 
1648, M. Perier wrote that the result nous ravit tous d'admiration et d' etonnement, 
for the mercury sank lower in the tube the higher up the mountain the vessel was 
carried. This confirmation of what was anticipated by Torricelli's hypothesis was 
taken to prove that the pressure of the air per sq. cm. is greater at the bottom 
than on the top of the mountain, and not as Aristotle and his followers would teach 
that Nature has a greater horror of a vacuum at sea-level than at higher altitudes. 
In a posthumous work, On the weight of the mass of air, published in 1663, B. Pascal 
summarized arguments which proved conclusively that all those effects, previously 
attributed to Nature's horror of a vacuum, are really produced by the pressure, that is, 
by the weight of the air. 

After the discovery of Torricelli's vacuum a group of philosophers — Thomas Hobbes, 
Franciscus Linus,* etc. — refused to abandon a favourite hypothesis they had formed that 
the world is everywhere full and a vacuum is impossible. They were called plenists in 
contradistinction to the vacuuists — O. von Guericke, B. Pascal, Robert Boyle, ^ etc. — who 
believed that a vacuum was possible, and capable of being obtained by certain physical 
processes. A controversy followed, not always in the choicest of language ; thus, Thomas 
Hobbes, addressing Drs. Ward and Wallis, said : 

But I here dismiss you both together. So go your ways, you uncivil Ecclesiastics, 
inhumane Divines, Dedoctors of morality, unasinous Collegues, egregious pair of 
Issachars, most wretched Vindices and Indices Academiarum, and remember Vespasian's 
law {maledici senatoribus non opportere ; remaledicere fas et civile esse) that it is uncivil 
to give ill language first, but civil and lawful to return it. 

The facts finally conquered an erroneous hypothesis. 

Units o! pressure. — The pressure of the air in any given locality varies within 
comparatively narrow limits. The normal or standard pressure of the atmosphere 

is equal to the weight of a column of mercury of unit area, and 760 mm. high. This 
pressure is sometimes called " one atmosphere." It is merely necessary to know 
the height of the barometric column to know the weight or pressure of the air per 
unit sectional area. The standard corresponds with a weight of (76 X 13*596 =) 
1033-3 grms. per sq. cm., or 14' 7 lbs. per sq. ip. The word pressure is generally used 
in preference to weight, because air, like all other fluids, not only presses down- 
wards, but also equally in all other directions. 

The selection of the atmosphere as the unit of pressure is quite arbitrary, and 
other units are used — e.g. the kilogram per sq. cm., and the pound per sq. in. The 
pressure of a dyne per sq. cm. was recommended by the International Physics 
Congress at Paris in 1900, because it is consistent with the C.G.S. system of units. 
This unit was called a barie ; a similar unit, the barad, was proposed by a com- 
mittee of the British Association in 1888, and there has been some controversy as 
to whether the unit had better be referred to a dyne per sq. cm. or to a pressure a 
million times greater. The density of mercury is 13*596, and in latitude 45° the force 
of gravity is equivalent to 980*6 dynes. Hence, a barometer column 76 cm. high 
will be maintained by a pressure equivalent to 76xl3-596=1033-3 grms., or 1033*3 
X980*6=l,013,300 dynes per sq. cm., or in round numbers, 10^ dynes per sq. cm. 
This number — called a megabar— may be inconveniently large, and a ten-thousandth 
part of 10^ is called a bar, hence, a bar is equivalent to 100 dynes per sq. cm. ; a 


centibar to one dyne per sq. cm.; and a millibar to 01 dyne per sq. cm. This 
unit, the millibar, has been recommended for recording barometer readings. One 
megabar is equivalent to 750 mm. of mercury, under standard conditions. The 
approximation is correct to one part in 5000. Since 13"596x980"6=13332 ; 
and one ten- thousandth of this is 1*3332, it follows that to convert centimetres 
into bars, multiply by 133'38 ; and, to convert bars into centimetres, multiply by 
0-0075. Since there are nearly 2-54 cms. in an inch, 133'33 X 2-54 =338-63, therefore 
to convert inches into bars, multiply hy 338'63. A pressure of one megabar is almost 
2 per cent, greater than a kilogram per sq. cm. ; and 1*3 per cent, less than the 
atmosphere unit. 

The extent of the atmosphere. — The air gets less and less dense at higher and 
higher altitudes, and I. Newton (1704) estimated air to be four times rarer at an 
elevation of about 7 J miles than at sea level ; 1,000,000 times rarer at a height of 
76 miles ; and 1,000,000,000,000,000,000 times rarer at an altitude of 228 miles ; 
and so on. If ^q be the pressure and Dq the density of air at sea level, E. Halley's 
formula 6 becomes 

Pressure of air at an altitude h = p^e, Po 

Variations in the value of the gravitation constant, g, and the rotation of the 
planet are neglected. Under actual conditions, the earth's atmosphere is in- 
cessantly agitated by convection currents — winds and storms — so that there is a 
continual transfer of air from one part to another. The limiting height at which 
the atmosphere is in convective equilibrium is about 29 kilometres, and the tem- 
perature falls roughly about 10° per kilometre as we ascend. Above this region, 
the temperature of the air is constant. It is indeed impossible to place a limit to the 
height the atmosphere extends. G. J. Stoney showed that, because the molecules 
of some gases attain certain high velocities, these gases are able to escape from the 
atmosphere of the earth and the other planets. At a height of 100 to 125 miles, there 
is sufficient air to ofEer enough resistance to the passage of meteorites to raise their 
temperature to incandescence. Whatever be the height, the weight of the normal 
barometric column (per square centimetre of mercury) measures the normal weight 
of a column of air of the same sectional area and extending from sea level upwards. 
B. Pascal (1663) appears to have been the first to calculate the total weight of all 
air about the globe. His estimate is 8,283889,440000,000000 hvres— where a livre 
is equivalent to 1 lb. 1 oz. 10^ dr. avoirdupois. 


1 S. Brown, Essays, Edinburgh, 1858; G. F. Rodwell, Chem. News, 9. 14, 26, 50, 242, 1864; 
10. 74, 1865 ; 11. 74, 1865. 

2 T. Gomperz, Griechische Denker, Leipzig, 1. 191, ]896 ; London, 1. 238, 1901. 
' E. Tomcelli, Opera geometrica, Firenze, 1644. 

* F. Linus, De corporum inseparabilitate, London, 1661 ; Thomas Hobbes, Collected Works, 
London, 1845. 

^ R. Boyle, An Examen of Mr. T. Hobbes, his Dialogues physicus de natura aeris, London, 
1662 ; Animadversions upon Mr. T. Hobbes' Probletnata de vacuo, London, 1674 ; A defense of 
the doctrine touching the spring of air proposed by Mr. Buyle in his new physico -mechanical experi- 
ments against the objection of Franciscus Linus, London, 1662 ; T. Hobbes, Lessons of the Principles 
of Geometry, Appendix to Elementorum philosophiae, London. 1655. 

« G. H. Bryan, Phil. Trans., 196. A, 12, 1901 ; G. J. Stoney, Of Atmospheres on Planets and 
Satellites, Dublin, 1897; E. Halley, Phil. Trans., 31. 116, 1723; B. Pascal, Traitez de Vequilibre 
des liqueurs et de la pesanteur de la masse de Vair, Paris, 1663. 

§ 2. The Influence of Pressure on the Volume of Gases — Boyle's Law 

At the bottom of all cosmic order lies the order of mathematics, the law that twice 
two is always four. — P. Carus. 

The quantity of matter in a gas is most frequently determined by the measure- 
ment of its volume. The volume of a gas is very sensitive to changes of pressure. 


and it is therefore very pertinent to inquire : What is the effect of variations of 
pressure on the volume of a gas ? About the time Pascal and Torricelli demonstrated 
the weight and pressure of the atmosphere, 0. von Guericke (1650) invented the air 
pump. The new instrument attracted much attention, and the effect of the 
*' vacuum " (reduced pressure) was tried on all kinds of animate and inanimate 
objects. In his memoir, Nova experirmnta physico-mechanica de vi aeris elasticce 
(London, 1660), Robert Boyle says that he placed a partially inflated lamb's bladder 
in the vacuum produced by the air pump, and noticed that the bladder became 
fully distended to its former size. Boyle thus established the important fact that 
the less the pressure exerted upon a given mass of air, the greater its volume. 
In 1661, Boyle continued his work on the elasticity or spring of air, as he called it, 
and stated that R. Townley,^ after reading about Boyle's experiments on the 
determination of the density of air from the height of a colimin of mercury which 
it supports, propounded the view that " the pressures and expansions are in 
reciprocal proportions." On August 2nd, 1661, R. Hooke made some experiments 
which confirmed Townley's hypothesis, and W. Croone and R. Boyle, at a meeting of 
the Royal Society on September 11th, 1661, gave an account of some experiments 
on the same subject. In his Defense against lAnus (London, 1662), Robert Boyle 
pubhshed an account of the experiments which clearly estabhshed R. Townley's 
hypothesis. Accurate experiments of this nature, said Boyle, " have not been 
previously made (that I know) by any man." Boyle's result can be expressed in 
words : the volume of a gas kept at one uniform temperature varies inversely 
as the pressure. This is Boyle's Law. Some years afterwards, E. Mariotte, in 
his Discours de la nature de Vair (Paris, 1679), reported analogous results which he 
and M. Hubin obtained in 1676 by means of an apparatus similar to that employed 
by Robert Boyle, which led him to take it pour une regie certaine ou hi de la 
nature, que Vair se condense a proportion des poids dont il est charge, and thus to 
confirm R. Boyle's deduction made fourteen years earlier. On the Continent, ignoring 
a priority of at least fourteen years, the law is sometimes improperly designated 
la hi de Mariotte, or Mariottesches Gesetz. At the time of the discovery of the law, 
air was the only gaseous body known, and therefore the accuracy of the law was 
established by Boyle and Mariotte for one body only. The law of Boyle may 
therefore be expressed : The product of the pressure and the volume of a gas kept 
at one uniform temperature is always the same. Or, for a given mass of air, 
pv = constant. The numerical value of the constant, of course, depends upon what 
units are selected for representing the pressures and volumes. Pressures may be 
expressed in atmospheres, miUimetres of mercury, pounds per square inch, etc. ; and 
the volume in litres, cubic centimetres, cubic feet, etc. Boyle's law assumes yet 
another guise. If pi be the pressure of a gas occupying a volume Vi ; and p, the 
pressure when the volume is v, then, since the products pv and piVi are equal to 
the same constant, they are equal to one another. Consequently pv=piVi. If any 
three of these magnitudes be known, the fourth can be calculated directly. A large 
number of measurements are summarized in these formulae, any one of which, 
indeed, contains the essence of all Boyle's observations condensed into a simple 

Example.— A eudiometer holds 4-5 litres of gas when the barometer reads 755 mm. 
What will be the volume of the same body of gas when the barometer stands at 760 mm. ? 
Here, pi = 755, Vi=4:-5, p = 760, hence, ^=4-47 litres. The most common problem is to 
calculate— reduce— the volume of a gas at any observed pressure, to the correspondmg 
volume at normal pressure, 760 mm. Given 4-5 litres of gas at 755 mm. pressure, there is 
no need for any formula to calculate the corresponding volume at 760 mm. The Pressure 
760 mm. is greater than 755 mm., hence the volume will be less, hence miUtipJy 4-5 by the 
fraction i^^ and the result is 4*47 litres. 

When the volume of gas, collected over mercury, is to be measured when the 
pressure of the atmosphere is 760 mm., and the difference in the levels of the 
mercury in the gas jar and in the pneumatic trough is 56 cm., it foUows that the 


pressure of the gas in the narrow tube is 760 mm. less 560 mm. =200 mm. When- 
ever practicable, of course, the mercury inside and outside is brought to the same 
level before the gas is measured. 

Suppose that the confining liquid is water, not mercury. Water is frequently 
used when the gases are not appreciably soluble in that liquid. Suppose that the 
external pressure is 760 mm. (barometer), and there is a difference of 10 cm. between 
the level of the water exposed to the gas, and the level of the water exposed to the 
air. The weight of 10 cm. of water is not the same as the weight of 10 cm. of mercury. 
Mercury is 13'596 times as heavy as water, hence, a 10 cm. column of water is equi- 
valent to the weight of a column of mercury 10-f-13*596 or 0-74 cm. or 7'4 mm. 
high. The pressure of the gas is therefore 760 — 7*4 = 752-6 mm. But water vapour 
exerts a definite pressure at any given temperature, and a still further reduction 
must be made if we want the pressure actually due to the gas and not to the mixture 
of vapour and gas. This will be investigated later. 

Test for the equilibrium of gases. — If the gas be confined under such conditions 
that the product pv at any fixed temperature is not con- 
stant, the system will not be in a state of equilibrium. 
If a gas were confined in a cyhnder with a sliding piston 
moving without friction and if the constant in Boyle's 
equation be p (in atm.) v (in litres) =12, then, if the piston 
supports a weight of 6 atms., the gas will expand or con- 
tract until the product pv satisfies the test. Consequently, 
Boyle's law describes the necessary condition for the 
volume and pressure of a gas to be in a state of equi- 
o Pressures Ubrium whcu thc temperature is invariable. In practice 

Fig. 1. — Duhem's Ex- there is no such thing as a frictionless piston, and if 
periment. Boyle's law were to be tested in a real cylinder an allowance 

would have to be made for the friction of the piston by putting 
an extra weight a on the descending piston and a less weight ft on the ascending 
piston ; Boyle's law would then be {p4-a.)v or (p-\-ft)v is equal to a constant. 

P. Duhem (1902) ^ has used an interesting illustration. The dotted curve, Fig. 1, repre- 
sents the relation between pressure and volume as defined by Boyle's law. If the volume , 
corresponding with any given pressure be observed when the rising piston has come to rest, 
the observed volume will appear to be less than that corresponding with the pressure as 
defined by Boyle's law, because friction will prevent the piston rising to the point corre- 
sponding with the equilibrium position on the dotted curve. Similarly, on a descending 
piston, friction prevents the volume attaining that indicated on the equihbrium curve. 

The friction thus corresponds to what J. W. Gibbs (1876) called the passive re- 
sistance of a system to assume a state of equilibrium. The nature of the passive 
resistance can here be recognized, but in some cases we feel sure that something 
analogous retards the movement of a system to the condition called stable equi- 
librium, although we know nothing of the character of the passive resistance or 
hysteresis— from uo-rcpew, I lag behind — which opposes the change. 


1 G. F. Rodwell, Che7n. News, 9. 14, 26, 50, 242, 1864 ; 10. 74, 1865 ; 11. 74, 1865. 

2 P. Duhem, Traite elementaire de mecanique chimique fondee sur la thermodynamique, Paris, 
1897 ; Theorie thermodynamique de la viscosite, dufrottement, et desfaux equilihres chimiques, Pari.<?, 
1896; Thermodynamique et chimie, Paris, 1902; J. W. Gibbs, Trans. Connecticut Acad., 3. 108, 
343, 1876-8. 

§ 3. Deviations from Boyle's Law 

Experimentally we do not know of any gas behaving in strict conformity to the law of 
Boyle ; but in the case of many gases, and of nearly all gases at very high temperatures, 
the deviation from uniformity is very slight.- — ^J. B. Stallo. 

The pressures used by Boyle extended over a range varying from 3 cm. to 300 
cm. of mercury. It is hazardous to infer that because the product pv is constant 



over a limited range of pressures, it will remain constant for pressures widely different 
from those actually measured. The method of measurement used by R. Boyle, 
though excellent for its time, is now considered somewhat crude. In the middle of 
the eighteenth century, P. van Musschenbroeck (1729), J. H. Sulzer, and J. Robinson 
tried to find if Boyle's result could be extended to all pressures, but with no very 
definite results. In 1799, M. van Marum i called attention to the deviation of 
ammonia from Boyle's law at high pressures. H. C. Oersted and C. Suensson (1826), 
and C. Despretz (1827) extended the observations to other gases, and it was found 
that the easily condensable gases like ammonia, hydrogen sulphide, and cyanogen 
began to deviate appreciably from Boyle's law at pressures exceeding two atmo- 
spheres, and with air, the constancy of the product began to fail at pressures exceed- 
ing 20 atm., for it diminished with increasing pressures. Similar conclusions 
were estabHshed for other gases by F. J. D. Arago and P. L. Dulong (1831) and by 
C. S. M. PouiUet (1844). 

Later on, many careful investigations were made by H. V. Regnault (1847), 
J. 0. Natterer (1850-4), L. Cailletet (1870-9), E. H. Amagat (1869-93), and others, 
to find if the simple law of R. Boyle correctly describes the l3ehaviour of gases at 
pressures far removed from the normal pressure of the atmosphere — 76 cm. of 
mercury. The general results show that no two gases behave precisely in the same 
way. The deviations for many gases are significant. By differentiating the re- 
lation pv = constant, Jc, or rather v = k/p, dv/dp = — k/p'^, and if k be taken unity, 
and j9 = 2, 3, 4, . . . be substituted. 

dp p^' 


4' 9' 16' 

meaning that the greater the pressure to which a gas is subjected the less the 
corresponding decrease in volume, — dv, for any subsequent increase of pressure. 
With most gases, the concentration increases more, that is, the volume increases 
less than Boyle's law describes ; and at high 
pressures, the concentration increases less, that is, p 
the volume is greater than Boyle's law indicates. 
This is illustrated by plotting Boyle's law. 50 
Boyle's law, when graphed, furnishes the con- 
tinuous curve shown in Fig. 2. This curve is 40 
a rectangular hyperbola. The deviations with 
nitrogen from this ideal condition are indicated 30 
by the dotted line in the same Fig. 2. If it 
were not for this phenomenon, the density of 20 
the gas would increase so that while oxygen at 
one atm. pressure weighs about 0*0014 grm. per 'o 
C.C., at a pressure of 3000 atm. the gas would be 
four times as heavy as water, and at 10,000 atm. °^ 
pressure over 13 times as heavy as water. 

According to Boyle's law, the volume of a gas 
should diminish indefinitely as the pressure is • j c -i. 1 

increased, and in time the volume would approach zero, or become indehnitely 
small. This is absurd. Pressure can diminish only the space between the mole- 
cules and not the actual substance of the molecules. Hence, if h denotes the 
volume occupied by the molecules the changes in the volume of the gas with 
variations of pressure will be represented by p{v-h) ^constant, not by ^v=constant. 
It does not follow that h represents the actual volume of the space occupied by the 
matter in the molecules. The effect of the volume of the molecule on the compressi- 
bility of a gas was dimly recognized by D. Bernoulli, 1738 ; and by M U . Lomanos- 
soff, 1750 ; it was studied by A. Dupre, in 1865 ; and by J. D van der \\ aals in 1872. 

In his important Memoires sur relasticite et la dilatabilUe des flukes jusqu aux 









Boyle's Law)\ 


- Nitrogen 




— - 


20 30 4.0 

YiQ,. 2. — Volume : Pressure Curves. 



trh hautes pressions, embodying the results of work extending from 1878 to 1893, 
E. H. Amagat showed that while the product pv remains fairly constant at low 
pressures for many gases, the numerical value of pv changes in a remarkable manner 
as the pressures increase in magnitude. E. H. Amagat's measurements for carbon 
dioxide show that the product pv is not constant, for when 

p ' 

. 1 







1000 atms. 

pv . 

. 1 








Notice how the product pv at first diminishes in magnitude and then steadily 
increases. This is brought out very clearly on plotting the numbers. If the 
products pv were constant for all values oip,we should get the straight line, dotted 
and marked ideal gas line in Fig. 3 ; with carbon dioxide, however, the curve descends 
below the line for an ideal gas, and then steadily rises, passing above the ideal gas 
line when the pressure is nearly 500 atmospheres. 

The curves for hydrogen, helium, argon, and neon, at ordinary temperatures, 
do not descend below the ideal gas line, but take a path resembhng the hydrogen 

line shown in Fig. 3. However, even 
'•®o 1 I i 1 \ ^ i I ^^ these gases exhibit the same peculiar 

behaviour at lower temperatures. 
Thus, according to H. K. Onnes and 
C. Braak (1907), with hydrogen at 
—140°, the product pv reaches a mini- 
mum when the pressure is about 25 at- 
mospheres ; at —195°, 45 atmospheres ; 
and at —213°, 51 atmospheres. In 
1886, C. Bohr reported that oxygen 
behaved in a peculiarly abnormal 
manner at a pressure of about 0*7 
mm. of mercury. The pressure- 
volume curve gaVe an abrupt change 
of direction which was ascribed to 
the transformation of oxygen into 
another variety ; but some careful 
measurements by Lord Rayleigh 
(1907) and M. Thiesen (1901) indicate 
that the statement is probably 
summarize these results at a constant 































tre p 





Fig. 3. — pv-Pressure Curves (Amagat). 

based upon a mal-observation. To 
temperature : 

(1) With small pressures, the product pv decreases with increasing pressure 

showing that the volume of gas, at relatively small pressures, is less than 
is described by Boyle's law. At very low pressures, the gas will follow 
Boyle's law pv = piVi. Lord Rayleigh (1901-2) found no appreciable 
variation with oxygen, hydrogen, and nitrogen between O'Ol and 1*5 mm., 
showing that between these pressures the deviations from Boyle's law are 
too small to be detected. 

(2) With large pressures the product pv increases with increasing pressure, 

showing that the volume of the gas, at relatively great pressures, is greater 
than is described by Boyle's law. 

(3) All gases, in consequence, show a minimum value for the product pv. At 

0°, for example, the minimum value of pv for air and nitrogen occurs at 
100 atm. pressure ; for oxygen at about 200 atm. ; for carbon dioxide, 
at about 35 atm. ; and for ethylene at about 42 atm. The pressure 
corresponding with the minimum depends on the nature of the gas and 
on the temperature. The minimum is less prominent with the more 
permanent gases than with the more condensable gases. 


Gases which obey Boyle's and Charles' laws under ordinary atmospheric con- 
ditions usually remain gaseous at comparatively low temperatures and are accordingly 
called permanent gases. 


1 M. van Marum, Gilbert's Ann., 1. 145, 1799 ; P. van Musschenbroeck, Elements phyaicce, 
Lugduni Batavorum, 1734 ; H. C. Oersted and C. Suensson, Edin. Journ. Science, 4. 224, 1826 ; 
- Omluftens sammentrykkelighed, Forosg over den Mariotteske Lov, Kaobenhavn, Oversigt, 13, 1825; 
C. S. M. Pouillet, rMments de physique, Paris, 1. 327, 1844 ; Compt. Rend., 24. 915, 1847 • 
J. Robinson, System of Mechanical Philosophy, Edinburgh, 3. 637, 1822 ; C. Despretz, Ann 
Chim. Phys., (2), 34. 335, 443, 1827 ; Cmipt. Rend., 14, 239, 1842 ; 21. 216, 1845 ; J. 0. Natterer, 
Sitzber. Akad. Wien., 5. 351, 1850 ; 6. 557, 1850 ; 7. 557, 1851 ; 12. 199, 1854 ; Liehig'a 
Ann., 54. 254, 1845; Pogg. Ann., 62, 139, 1844; 94. 436, 1855; L. Cailletet, Compt. 
Rend., 70. 1131, 1870; 74. 1282, 1872; 75. 77, 1271, 1872; 90. 210, 1880; 88. 61, 1879; 
Ann. Chim. Phys., (5), 19. 386, 1879: J. H. Sulzer, Mem. Acad. Berlin, \\&, 1753; F. J.D.Arago 
and P. L. Dulong, Mem. Acad., 10. 193, 1831 ; Ann. Chim. Phys., (2), 43. 74, 1830 ; Bull. Soc. 
Encour., 29. 295, 1836 ; H. V. Regnault, Mem. Acad., 21. 1, 329, 1847 ; 26. 229, 1862 ; Compt. 
Rend., 13. 1077, 1841 ; Ann. Chim. Phys., (3), 4. 5, 1842 ; D. Bernoulli, Journ. Phys., (3), 8, 521, 
1899 ; Compt. Rend., 128. 1229, 1899 ; J. D. van der Waals, Die Continuitdt des gasformigen und 
fliissigen Zustaiides, Leiden, 1873 ; A. Dupre, Theorie mecanique de la chaleur, Paris, 1869 ; Compt. 
Rend., 56. 960, 1863 ; 57. 774, 1863 ; E. H. Amagat, Compt. Rend., 68. 1170, 1869 ; 71. 67, 1870 ; 
73. 183, 1871 ; 74. 1299, 1872 ; 75. 479, 1872 ; 77. 1325, 1873 ; 82. 914, 1876 ; 85. 27, 139, 1877 ; 
87. 342, 1878 ; 88. 336, 1879 ; 89. 437, 1879 ; 90. 995, 1880 ; 91. 428, 1880 ; 93. 306, 1881 ; 94. 
847, 1882 ; 95. 281, 638, 1882 ; Ann. Chim. Phys., (4), 28. 274, 1873 ; (4), 29. 296, 1873 ; (5), & 
270, 1876 ; (6), 11. 520, 1887 ; (5), 19. 345, 1880 : (5), 22. 353, 1881 ; (5), 28. 456, 464, 480, 500, 
1883 ; Archiv, Sciences Geneve, (4), 35. 169, 1869 ; (4), 40. 320, 1871 ; (4), 49. 246, 1873 ; (5), 8. 
270, 1876 ; Lord Rayleigh, Phil. Trans., 196. 205, 1901 ; 198. 417, 1902 ; Zeit. phys. Chem., 37. 
713, 1901; 41. 71, 1902; 52. 705, 1905; Proc. Roy. Soc, 73. 153, 1904; M. Thiesen, Ann. 
Physik, (4), 6. 280, 1901 ; C. Bohr, Wied. Ann., 27. 459, 1886 ; H. K. Onnes and C. Braak, 
Comm. Lab. Phys. Leiden 97, 99, 100, 1907. 

§ 4. Dalton's Law of Partial Pressures 

Accurate and systematic investigation has brought to light the infinite complexity of 
nature ; the fineness of the dovetailing of every event into many others ; the never-ending 
response of all things to changes in the conditions that encompass them ; the imiversal 
orderliness of natural occurrences ; and the absolute interdependence of cause and effect. 
— M. M. P. MuiR (1894). 

When two gases, which do not act chemically on one another under the con- 
ditions of the experiment, are brought together, the gases mix intimately, by diffusion, 
so as to form a homogeneous mixture. Furthermore,^ John Dalton (1802) found 
that each gas seemed to exert the same pressure as if it occupied the space alone, and 
the total pressure of the mixture of gases was the sum of the several pressures due 
to each gaseous component of the mixture. If P be employed to denote the total 
pressure of a mixture of gases, and^i the partial pressure exerted by one of the gases, 
P2 the partial pressure exerted by another gas, pg the partial pressure of a third gas, 
Dalton's discovery means that T P =i?i +i?2 + Pa + • • • ^^ words, in a mixture 
of gases which exert no physical or chemical action on one another, each gas exerts 
the same pressure as if it alone occupied the entire vessel, and the total pressure 
is the sum of the partial pressures due to each of the gases. This is Dalton's law 
of partial pressures. If the four volumes of nitrogen and one volume of oxj^gen m 
the atmosphere be under normal pressure, the nitrogen gas will sustain a pressure 
approximately 608 mm. and the oxygen gas 152 mm. of mercury. J. Dalton added : 

Since two gaseous fluids which exert neither attraction nor repulsion on one another, 
distribute themselves so that their imited pressure is equal to the pressure of the atmo- 
sphere, all the components of the atmosphere are arranged together at a given pressure and 
temperature, and by a paradoxical though true disposition, each of them occupies aU the 
space destined for the aggregate. 


It might be added that Dalton's partial pressure law is quite independent of Boyle's 
law, and can be extended to mixtures of any number of gases. 

Examples.— (1) Moist hydrogen gas is confined over water under a pressure of 747-2 mm. 
of mercury at 15 "3°, the partial pressure of water vapour at that temperature is J 2*9 mm. of 
mercury. Then, from Dalton's law of partial pressures, it follows that the hydrogen gas 
itself is under a partial pressure equivalent to 747*2 less 12*9, or 734-3 mm. of mercury. 

(2) If atmospheric air contains a mixture of four volumes of nitrogen and one volume 
of oxygen, show that if the manometer records a pressure p^ the partial pressure of the 
oxygen gas will be ^jo, and of the nitrogen gas ^p. 

(3) If a moist gas of volume Vi be confined in a vessel at a pressure p^, show that the 
volume V of dry gas at normal pressure, 760 mm., and the volume v^ of the water vapour 
at normal pressure, are respectively v=v^{p■^^—f)|lQ(i, and V2=ViflJQ0, where / denotes 
the vapour pressure of the water at the temperature of observation. 

There are many reasons for supposing that the molecules of a substance exert 
some kind of attraction on one another. This intermolecular attraction gives rise 
to phenomena of cohesion, viscosity, capillarity, surface tension, etc. The inter- 
molecular attraction is probably very powerful in solids, weaker in liquids, and very 
small with gases ; but it is highly probable that the molecules of nearly all gases 
do exert some attractive influence on one another, and the gases, in consequence 
of this physical action, " deviate " from Dalton's law to an extent dependent upon 
the magnitude of the intermolecular attraction. Many mixtures of gases show 
slight, but not marked deviations from the law, e,g. carbon dioxide and sulphur 
dioxide ; hydrogen with air, and with nitrogen ; etc. 

P. Fuchs 2 has investigated the change in volume which occurs on mixing chemi- 
cally indifferent gases — ^nitrogen with nitrous oxide, carbon dioxide, or oxygen ; 
nitrous oxide with carbon dioxide or oxygen ; and oxygen with carbon dioxide. 
In every case there is an expansion which is greater the more the two components 
differ in physical properties. The change in volume does not correspond with the 
ratio of the two gases, but reaches a maximum which is beyond the 1 : 1 ratio, 
so that the maximum change occurs with mixtures containing more than 50 per 
cent, of the gas with the lower critical temperature ; and the maximum lies nearer 
to the 1 : 1 ratio, the more the components resemble one another. The change in 
volume 8v is qualitatively but not strictly quantitatively represented by J. D. van 
der Waals' equation 


x(\-x) {^^^^^^ - (h + h- h,)) 

where x denotes the number of gram-molecules of the one gas, and (1 ~x), of the 
other ; ai represents the attraction constants of the molecules of the one com- 
ponent ; a2, of the other component ; and ai2, of the molecules of the different 
components for one another ; bi, 62? ^^^ ^12 represent the corresponding volume 
constants. Accordingly, the theoretical results agree more closely with the 
observed results when an allowance is made for the effect of the attraction of the 
molecules for one another. 

J. Dalton's law is thus a limiting law for ideal gases. A. Leduc ^ prefers to state 
the law for actual gases in the form : The volume occupied by a mixture of gases is 
equal to the sum of the volumes which the component gases would separately occupy 
at the same temperature and pressure as the mixture. If two gases, originally at the 
same pressure, are mixed so that the temperature and total pressure remain unaltered, 
the pressure of the mixture can be calculated if the coefficients of deviation from 
Boyle's law. Ay be known between the common pressures p and pi for the mixture 
and for each of the two gases, where 

V 739^9 A V9. — V-i ' 

^2'^2 J^V^-V\ 

There is usually found to be a slight increase of pressure on admixture which is 
scarcely measurable with the less condensable gases. The value of A at 16° between 


1 and 2 atm. is 0*000143 for a mixture of nearly equal volumes of carbon and sulphur 
dioxides ; 0*000005 for air ; —0*000002 for equal volumes of hydrogen and oxygen. 
The law had been applied to test if chemical action occurs on mixing certain 
gases, e.g. to find if any sign of chemical action occurs when nitric oxide (NO) is 
mixed with nitrogen peroxide (NOg) resulting in the formation of nitrogen trioxide 
(N2O3). It is assumed that if no chemical combination takes place, the mixture 
will obey Dalton's law, and conversely.^ The conclusion can be valid when it has 
been shown that the molecules of the two gases exert neither attraction nor 
repulsion upon one another. If they did, the test might lead to wrong conclusions with 
respect to chemical action. A slight contraction, for instance, might be evidence 
of molecular attraction, not of chemical combination. 


1 J. Dalton, Mem. Manchester Lit. Phil. 80c., 5. 635, 1802 ; Ann. Chim. Phys., (1), 44 40 
1802. ■ ' 

2 P. Fuchs, Zeit. phys. Chem., 92. 641, 1918 ; J. D. van der Waals, Binare Gemische, Leipzig, 

3 A. Leduc, Compt. Rend., 123. 805, 1896 ; 126. 218, 1859, 1898 ; A. Leduc and P. Sacerdote, 
ib., 126. 218, 1853, 1898 ; A. Leduc, Recherches sur les gaz, Paris, 105. 820, 1898 ; D. Berthelot 
and P. Sacerdote, Compt. Rend., 128. 820, 1899 ; D. Berthelot, ib., 126. 954, 1030, 1415, 1703, 
1877, 1898; 128. 1159, 1899. 

* H. B. Dixon and J. D. Peterkin, J own. Chem. Soc., 75. 613, 1899. 

§ 5. The Laws of Nature 

We must confess that physical laws have greatly fallen off in dignity. No long time ago 
they were commonly described as the Fixed Laws of Nature, and were supposed sufficient 
in themselves to govern the universe. Now we can only assign to them the humble rank 
of mere descriptions, often erroneous, of similarities which we believe we have discovered. 
—J. H. POYNTING (1899). 

Nature, always working by law, is always consistent, always inexorable ; her laws are 
invariable.- — ^A. Simmons. 

This is a convenient place to further emphasize the meaning of the term " law " 
in chemistry. The laws of a country may be the enactments of a ruling power, 
the ukases of a czar, or the regulations of the police superposed upon a people 
compelling them to act in particular ways, but it is of course absurd to say that 
Dalton's law and Boyle's law must be obeyed, implying that these laws are com- 
mands imposed upon gases which they are compelled to obey. The laws of nature 
describe, they do not compel. A substance does not act in a particular way because 
there is a law, but the law originated when it was found that substances acted 
in that particular way. Consequently, law is a useful term which the careless 
sometimes personify ; it is a figure of speech, and is employed by scientific men 
purely in a metaphorical sense. The term has led to some confusion, for it has 
led to the belief that the uniformity described by the law has been imposed on 
nature by the will of a rational being — God himself. As previously indicated, a 
law in science is a kind of summary of the present state of our knowledge of the 
phenomena described by the law, and it is always subject to revision with the 
growth of knowledge. Laws do not necessarily establish facts. Consequently, 
the term would be replaced by another word, if we could think of a better. 
Rule would perhaps lead to less misunderstanding. The German equivalent-- 
Gesetz, statute — is perhaps worse. A law of nature can have authority only in so 
far as it is based on facts. As indicated previously, the term " law of nature " is 
applied to a comprehensive generalization which " methodically and systematically 
describes certain natural phenomena." The laws of chemical combination describe 
what the elements do under definite conditions ; and generally, the laws of 
chemical and physical phenomena are collocations of those circumstances 


which have been found by experiment and observation to accompany all 
chemical and physical changes included in the statement of the law. The test 
of the " law " is that the statement holds good without exception. A broken 
law, said J. H. Poynting, is a false description. 

It is sometimes said that a law of nature has never been disproved ; this can 
only mean that if a law of nature is disproved, it ceases to be a law. The common 
meaning attached to the saying, " The exception proves the rule," is wrong, and 
it is an instance of confusion arising from the double meaning of words. In the old 
Latin form, Exceptio prohat regulam, the word prohat means tests, just as to-day 
proving wines means testing them. The proverb therefore meant that the apparent 
exception furnishes a means of trying, testing, or proving the rule, and if the ex- 
ception cannot be explained, then the rule breaks down, for the exception disproves 
the rule. The exception annihilates the rule, for, said J. W. Ritter in 1798, a law 
must be abandoned immediately a real exception is discovered — it is no longer a law. 

When the exact conditions are set up, the law describes the phenomenon without 
variableness or shadow of turning. The law is then regarded as an objective power. 
This power is called a force, and further, the force is said to be the cause of the 
phenomenon. Thus gravitation is regarded as an attractive force causing one 
particle to attract every other particle in the universe ; chemical affinity is regarded, 
in this sense, as a selective chemical change. If therefore we find a gas deviating 
from Boyle's law, or a mixture of gases " disobeying " Dalton's law, the alleged 
laws may be false, incomplete, or imperfect descriptions, or some perturbing influence 
is at work which masks the simple phenomena described by these laws. 

§ 6. The Influence of Temperature on the Volume of Gases — Charles' 


According to the schools of philosophy, it has been proved that the effect of cold is to 
make bodies contract while heat makes them expand.- — G. Galilei (1615). 

The expansion of air by heat has long attracted the attention of chemists. Hero 
of Alexandria (c. 117 B.C.), G. B. Porta (1616), C. Drebbel (1608), and G. Galilei 
(1615) experimented on the subject. H. Boerhaave considered the effect of tempe- 
rature on the volume of gases, and, in his Elementa chemice (Lugduni Batavorum, 
1732), he stated that when air is heated, it becomes so rare that -neither the measure 
nor the limit of its dilation has been yet discovered ; and added : 

Air of unequal masses but of the same density, is always expanded in the same measure 
by the same degree of fire ; so that these expansions in the same density of air, by a constant 
law of nature, are always proportional to the augmentations of heat. 

Influence of temperature on the volume of gases — pressure constant. — In 1790, 
Joseph Priestley concluded " from a very coarse experiment " that " fixed and 
common air expanded alike with the same degree of heat ; " J. Dalton, in 1801, 
inferred from his experiments : " Upon the whole, I see no sufficient reason why 
we may not conclude that all elastic fluids, under the same pressure, expand 
equally by heat ; " and J. L. Gay Lussac,i in 1802, quoted some experiments in 
support of the generalization : The same rise of temperature produces in all gases 
the same increase in volume, provided the pressure and mass be kept constant. 
This law is generally designated Charles' law, in honour of J. A. C. Charles, who, 
according to Gay Lussac, made some crude experiments on the subject fifteen 
years before Gay Lussac's publication. Some call this relation Ga?j Lussac' s law. 
It might, perhaps, with more propriety be called Volta's law, because A. Volta,2 
described it in his Memoria sulla uniforme dilatazione deiVaria, in 1793. 
G. Amontons had an inkling of this law in 1702. 

The increase in volume which occurs when one litre of nitrogen at 0° is heated 
in a suitable vessel is shown in the following table (R. Chappius, 1888) : 

Table I. — Thermal Expansion of Nitrogen . 


Temperature 0«*. 

Volume V litres. 

Expansion per litre per 




The numbers in the last column — called the coefficients of thermal expansion — 

mean that the volume ^; of a litre of nitrogen, when heated through 6° can be repre- 
sented very closely by the expression : v = {l +0*003676^) htres. In other words, 
nitrogen increases 0*003676, or very nearly 273rd part of its volume at 0° for every 
degree rise of temperature. More generally, if Vq be used to denote the volume of gas 
at 0°, we have, instead of the preceding expression, v = '^0(1 + 273^)' otv = VQ{l-\-aB). 
This is very nearly true for most of the common gases, and it therefore represents 
a condition which must be satisfied by the temperature and volume of a gas, 
under constant pressure, in order that the system may be in stable equihbrium. 

While solids and liquids have their own characteristic coefficient of expansion, 
gases have nearly the same coefficient of thermal expansion. This is the meaning 
of Charles' law. The coefficients of thermal expansion (pressure constant) for 
the gases run something like this for one atmosphere pressure and variations of 
temperature between 0° and 100° : 

Air . 

. 0-003671 

Hydrogen . 

. 0-003661 

Carbon dioxide 

. 0-003728 

Carbon monoxide . 

. 0-003669 

Sulphur dioxide . 

. 0-003903 

Nitrous oxide 

. 0003719 

These numbers are close enough to -^^ for most practical purposes. In general, 
the more easily a gas is liquefied and the greater its molecular weight, the greater 
the deviation from the constant 0*003665 found for air— witness carbon dioxide, 
0-003728; hydrogen bromide, 0-00386 ; etc. 

For every degree centigrade the temperature falls, the volume of the gas 
decreases by 273rd. If -^^^id part of a gas be taken away 273 times, no more gas 
remains. This is illustrated by plotting the above equation. 

If the temperature be less than —273°, the gas would have a negative volume, that is, 
a volume less than nothing ! If the temperature be —273°, the gas would occupy no voluine ! 
rt is impossible to imagine a substance occupying no space, but such is a logical conclusion 
from Charles' law. Where is the fallacy ? Whenever a natural process is represented by 
mathematical symbols, it is well to remember that the artificial statement often expresses 
more than actually obtains in nature, because, in the physical world, only changes of a certain 
kind occur. We must therefore limit the generality of the mathematical expression. 
Charles' law includes a simplifying assumption. The apparent volume of a gas may be 
resolved into at least two parts ; (1) the volume occupied by the molecules of the gas ; and (2) 
the space betvjcen the molecules. If b denotes the space occupied by the molecioles, and v 
the observed volume of the gas, the space between the molecules will be represented by 
V —b. Although for the sake of simplicity, we assiune v to represent the total volume occupied 
by the gas, Charles' law refers to v—b, that is, to the space between the molecules, and 
in that case, the conclusion that v=0 when the temperature is -273° involves no absurdity. 
Moreover, the gas would liquefy before the temperature -273° was attained, and the simple 
gas law of Charles would not then be applicable. 

It has been urged that J. L. Gay Lussac's statement of Charles' law means that 
the increase in the volume of a gas at any temperature, for a rise of 1°, is a constant 
fraction of its initial volume at 0°— in symbols, v=ro{l-\-ad) ; while J. Dalton's 
statement of the law means that the increase in the volume of a gas at any tempera- 


ture, for a rise of 1°, is a constant fraction of its volume at that temperature— in 
symbols dvjdd^av ; hence by integration, v=VQe^^ . If the latter expression be 
expanded, v=Vo(l+ct0+Ja2^2_|_ ^ ^ j^ and if the second and higher powers 
be outside the range of measurement, the two statements amount to the same 
thing. R. Mewes and L. Neumann 3 proposed to replace v=VQ{\-\-a6) by 
v=x-\-{vQ—x)(\-\-a^), or approximately v=VQ(\-\-a)^. The results at ordinary 
temperatures are good, but they become less accurate with decreasing tempera- 
tures. The discrepancies are in fact attributed to errors in the measurements at 
low temperatures which are introduced by surface condensations, etc. 

Influence of temperature on the pressure of a gas — volume constant.— About 
1682, R. Boyle made some experiments on the influence of " cold and heat " on 
the pressure, or the spring of air, as he called it, and found that the effect of the 
greatest degree of cold he could produce did not " weaken the spring by anything 
near so considerable as one would expect." The subject did not attract much 
attention until G. Amontons (1702-3) ^ published two memoirs in which he 
demonstrated that equal masses of air, measured at the same initial pressure, 
acquire equal increments of pressure when heated to the boihng-point of water 
provided the volumes are maintained at their initial value ; and if the pressure of 
the air before heating be doubled or tripled, the additional pressure produced when 
the air is heated to the boiling-point of water is likewise doubled or tripled. Other- 
wise expressed, the ratio of the total pressures {jp and f{) of air at two definite 
temperatures {T and Tj), and kept at a constant volume, has always the same 
value R and is independent of the initial pressure. In symbols, fjT =pilTi ; which 
can be written p = RT, where R is the constant of proportion. In words, the same 
rise of temperature produces the same increase of pressure provided the volume 
and mass of the gas be maintained constant. This relation might be called 
Amontons' law. It can be very simply deduced from Charles' and Boyle's laws, 
expressed in an analogous form, p=Po{l -\-27 3^)> or f = Po{l -{- Pd) , where 
/3 denotes the coeflB.cient of increase of pressure (volume constant). J. L. 
Gay Lussac thought that all gases had the same values of a and j3 ; and it was 
thought that a = j3. More exact measurements have shown that neither statement 
is true. The coefficient j3 for the above-named gases between 0° and 100° are : 


. 0-003665 

Hydrogen .... 

. 0-003663 

Carbon dioxide 

. 0-003688 

Carbon monoxide 

. 0-003845 

Nitrous oxide 

. 0-003676 

Absolute zero. — J. Amontons (1703) ^ argued that air would exert no pressure 
at all if it were cooled below freezing-point of water to about 2 J times the range of 
temperature between the freezing- and boihng-points of water. In 1779, J. H. Lam- 
bert ^ repeated Amontons' experiment and estimated that air would occupy no volume 
at all, if cooled to — 270° ; more accurate measurements make this temperature — 273°. 
This temperature, —273°, is supposed to be a non ultra plus, or limiting temperature 
— the nadir or lowest possible temperature — a kind of primum frigidum. Hence, 
—273° is sometimes called the absolute zero ; and temperatures reckoned from this 
zero are called absolute temperatures. U Association Internationale de Froid "^ 
recommended that the letter K — from Lord Kelvin — be employed to denote absolute 
temperatures so that 0° C. =32° F. =273° K. On the absolute scale of temperatures, 
0° C. will be 273° K. If J be employed to denote the temperature on the absolute 
scale, and 6 the temperature on the centigrade scale, we have T=273+^. Hence, 
if V be the volume of a gas when the absolute temperature is T, and Vi the volume 
when the temperature is Jj, from the preceding equation (3) v : Vi=T : Tj, which 
is but another way of stating Charles' law. The volume of a gas varies directly 
as the temperature, so that v=RTy where R is the constant of proportion. The 


arbitrary nature of the absolute zero deduced from the coefficient of thermal ex- 
pansion of air, will appear when it is remembered that a similar train of reasoning 
would furnish —5000° as the absolute zero, if the coefficient of expansion of mercury 
were made the standard. It must be remembered, however, that the coefficient 
of thermal expansion of all gases, unlike liquids and solids, has nearly the same 
value ; and further, the gaseous state probably represents the simplest form in 
which matter can exist. There are, however, other reasons for selecting —273° as 
the absolute zero which are discussed in works on thermodynamics. 

The combined influence of temperature and pressure on the volume of a gas. — 
According to Boyle's law, the volume of a gas varies inversely as the pressure, so 
that if a pressure pi and volume t'l change to a volume a; at a pressure p2, then, 
from the relation ;Pi^i=j32^ (Boyle's law). Again, according to Charles' law, the 
volume of a gas varies directly as the absolute temperature, so that if a gas whose 
volume is a? at a temperature Ti changes to a volume V2 when the temperature rises 
to T2, we have from the above relation, xT2=V2^i- On substituting the value of 
X from the preceding relation 

If 2>2j ^2j ^2 represent the volume of the gas under standard conditions of tem- 
perature and pressure, f^^'^jT^ will have a constant numerical value, say R ; and it 
follows at once that when both temperatures and 
pressure vary, the effect on the volume will be 
given by the equation pv=RT, where R is the 
constant of proportion — generally called the gas 
constant. An equation which attempts to express 
the relation between the pressure, temperature, 
and volume of a gas is sometimes called the equa- 
tion of state — Zustandsgleichung, or equation 
caracteristique — or the characteristic equation or 
the gas equation. The equation of state is applic- 
able to ideal gases. If an arbitrary value be 
assigned to the constant R, and corresponding 

values of p and v be plotted for a series of values 

of T, say T=l,2,3, . . ., a series of curves. Fig. vo/umes 

4, are obtained. These curves may be supposed ^^^^ 4._Surface showing the Rela- 

to have been drawn on a surface abed. While a tion between the three variables : 

plane suffices for showing the relation between two Temperature, Pressure, and 

variables, a surface in three dimensions is needed Volume of Gases. 

for three variables. These formulae are used a 

great deal in calculations involving the variations in the volumes of gases 

owing to variations in temperature and pressure. For mstance, m reducmg tne 

volume of a gas at any observed temperature and pressure to the con-espondmg 

volume at the standard or normal pressure and temperature-O C and 7bu mm. 

pressure — often represented by n.p.t., or N.P.T., or S.T.P., or S..L., b.r. 

ExAMPLE.^(l) If a gas measures 170 c.c. at a pressure of 735 mm. "mercury, and a 
temperature of 15°, what is the volume of the gas at normal temperature ajd Pye^"je . 
Here it is required to find v in the preceding formula where p = 7bO ; l -£ia, ^i ^ » 
Vi = 170; and ^1 = 735; hence, iy = ||i X^^^f X 170 = 155-8 c.c. ^^lo-dor at 

(2) Show that 13-8 c.c. of a gas at 747^6 mm. pressure at 19° reduce to 12 4 c.c. at 
760 mm. and 0°. 

Approximations can be used for general calculations,8 and books on gas analysis 
have tables for converting unit volume at 6° and pressure p X '/''^'''^^/r^^ 
standard conditions. It will be observed that the fraction T/i o lor tf ^^^^o^^ 
(273 + d)l{213 + <9o), and if ^o be 0°, the fraction reduces to 1 f oyyr^, or 1 +^Y:f^^J^^' 

The numerical value of the gas constant R.-The numerical value of i2 depends 

VOL. I. 


upon the units of pressure and volume ; if unit mass of gas be taken, the value of 
R will depend upon the molecular weight of the gas. If one gram-molecule be taken, 
j)V=RT, and if n gram-molecules be taken, j)v=nRT. If the litre he taken as the 
unit of volume and the atmosphere as unit of pressure, and since a gram-molecule of 
gas occupies nearly 22*4 litres at 0° and under one atmosphere pressure, 1 X 22*4 
=1 X 22x273 ; or i2=0082 litre-atmosphere. If the gram and cubic centimetre be 
taken as unit, it follows that if % represents the volume of a gram-molecule of any 
gas at n.p.t., i;i=22,400 c.c. ; pi is 1033-3 grms. per sq. cm. ; and Ti=273. Hence, 
22=pv/T = (1033-3 x22,400)/273 = 84,760 gram-centimetres of energy. From 
measurements of the mechanical equivalent of thermal energy, it is known that 
one gram-centimetre of mechanical energy is equivalent to 42,650 calories. Hence, 
R=:pvlT = l'd cals., or 2 cals. nearly. 


^ J. Dalton, Mem. Manchester Lit. Phil. Soc., 5. ii, 695, 1802 ; J. L. Gay Lusaac, Ann. Chim. 
Phys., (1), 43. 137, 1802 ; J. Priestley, Experiments and Observations on Different Kinds of Air, 
Birmingham, 1777- 

* A. Volta, Annali di Chimica, 4. 227, 1793 ; reprinted in J. Guareschi's Legge della dilatazione 
dei gaz di Alessandro Volta, Turin, 1914 ; G. Amontons, Mem. Acad., 50, 1703. 

» R.Mewes and L. Neumann, Zeit. Sauerstoff Stickstoff Ind., 11. 13, 1919; T. Box, Practical 
Treatise on Heat, London, 1876. 

* E. Mach, Die Principien der Wdrmelehre, Leipzig, 1900 ; G. Amontons, Mtm. Acad., 50, 

* J. Amontons, Mem. Acad., 50, 1703. 

•^ J. H. Lambert, Pyrametrie, Berlin, 29, 40, 74, 1799. 
' Chem. Ztg., 35. 3, 1911. 

8 G. J. Stoney, Proc. Roy. Soc. Dublin, (2), 6. 387, 1890; Wa. Ostwald, Zeit. angew. Chem., 
32. 359, 1919. 

§ 7. Deviations from Charles' Law 

Nature abhors the straight line.- — R. Ross (1914). 

We have already seen that the coefficients of thermal expansion of all gases 
are only approximately the same. The coefficients for the individual gases differ 
a little among themselves as indicated above. The variation in the coefficient of 
thermal expansion at temperatures and pressures not far removed from normal 
atmospheric temperatures and pressures, is not very marked, and for regular gas 
calculations can be ignored. It remains to indicate the variation, if any, in the 
coefficient of thermal expansion with large variations of temperature and pressure. 
H. Flaugergues (1825) showed that the coefficient of expansion of moist air is rather 
larger than that of dry air. Charles' law was also tested by P. L. Dulong and A. T. 
Petit (1815), F. Rudberg (1837), H. V. Regnault (1841), G. Magnus (1842), E. H. 
Amagat (1873), P. Jolly (1874), P. Chappius (1888), H. K. Onnes and M. Boudin 
(1900), etc.i The more important results are as follows : 

(1) The exact coeflacient depends on the nature of the gas.— H. V. Regnault, 
about 1850, proved that different gases have not the same coefficients of thermal 
expansion, as Charles' law assumes, but that each gas has its own specific constant. 
For ordinary calculations, particularly with gases which cannot be liquefied in the 
neighbourhood of atmospheric temperatures, the coefficient is taken to be 
a=^ = ^. 

(2) The influence of pressure. — The coefficient of expansion of most gases is 
increased by augmenting the pressure of a gas until a maximum value is attained, 
after that, the coefficient diminishes with increased pressure. For instance, E. H. 
Amagat (1893) found that the coefficients of expansion of carbon dioxide at tem- 
peratures between 50° and 60° assumed the following values when the pressure 
changed from 30 to 1000 metres of mercury : 







1000 metres 


. 00069 









Carbon dioxide thus shows a marked variation in the coefficient of thermal expansion 
at high pressure. In agreement with these facts, the coefficient also diminishes as 
the pressure is reduced, even as low as 0"077 mm. of mercury. The variation is 
not so marked with gases like nitrogen, oxygen, and hydrogen which are not easily 
condensed to the liquid condition. The general result of H. V. Regnault's and 
Amagat's work is to show that if a gas is more compressible than is represented by 
Boyle's law, the coefficient of thermal expansion is increased by pressure ; and 
conversely for gas less compressible than is indicated by Boyle's law, the coefficient 
of thermal expansion decreases with an increase of pressure. The value p which 
furnishes the greatest coefficient of thermal expansion is that same value of j) 
which gives the minimum product pv. At ordinary temperatures, therefore, hydro- 
gen and helium do not exhibit this variation in the value of their coefficients of 
expansion. With these gases, the coefficient of expansion steadily diminishes with 
increasing pressure ; although even these resemble other gases if the temperature 
be low enough. Consequently, at high enough pressures^ when the minimum pv is 
reached, the coefficient of thermal expansion of all gases decreases with an increase of 

(3) The influence of temperature. — The general effect of raising the temperature 
is to lower the coefficient of expansion. For instance, Hirn (1862) found that for 
water vapour from 0° to 

118-5° 162° 200° 246-5° 

Coefficient of expansion . 0-004187 0-004071 0-003938 0-003799 
Similarly, L. Troost and P. Hautefeuille (1876) found the coefficient for silicon 
tetrachloride fell from 0-00449 between 100° and 125° to 0-00399 between 125° and 
180° ; while between the same temperatures 
the coefficient for carbon tetrachloride fell 
from 000470 to 0-00414 ; and for phosphorus 
trichloride, from 0-00489 to 0-00417. 

The changes in the coefficient of expansion 
with increasing pressure become less and less 
as the temperature is raised, and finally dis- 
appear. So does the minimum value of the 
product pv become less and less marked as the 
temperature is raised. The gradual flattening 
of the carbon dioxide curves as the temperature 
rises from 40° to 100° is brought out very clearly 
in Fig. 5. All gases exhibit a minimum value fiq. 5.— Amagat's pv-T — Curves for 
for pv. The pressure required for a mini- Carbon Dioxide, 

mum depends on the temperature as well 

as on the nature of the gas. The minimum is most marked when the gas is near 
its temperature of liquefaction. If the temperature is much above this critical 
point, the minimum is very small— with hydrogen the minimum is inappreciable 
at 0°— Fig. 3. All other gases show a minimum at ordinary temperatures. Hence, 
H. V. Regnault, who discovered this peculiarity of hydrogen, was led to say ironically 
that hydrogen is a gas plus que parfait—a. gas more than perfect ; but hydrogen 
also shows a minimum at reduced temperatures. Similar remarks apply to helium 
and neon. 


1 H. Flaugergues, Gehler's Physik. Worterbuch, 1. 637, 1825 ; P. L. Dulong and A T Petit. 
Ann. Chim. Phys., 7. 117, 1815; F. Rudberg, Fogg- ^^^^ 41- 271, 1837; ^. 119, 1W8; U. 
Magnus, ib., 55. 1, 1842; H. V. Regnault, Mem. Acad., 21. 25, 1841 ; A7in ^?*'"-./^y^-» j^), 5. 
52, 1842 ; E. H. Amagat, ib., (4), 28. 274, 1873 ; L. Troost and P. Hautefeuille. tb (6). 7. 464. 
1876 ; P. Jolly, Pogg. Ann. Jubelbd., 82, 1874 ; P. Chappius, Arch Sciences Phys. Oefu, (3). ^J. 
5. 153, 248, 1888; H. K. Onnes and M. Boudin, Versl. Akad. ^'"^'^'I^'J' 224^^/ f- t*' 
Amagat, Ann. Chim. Phys., (6), 29. 68, 1893; G. A. Hirn, Cosmos, 22. 283, 413, 734, 1803; 
Theorie micanique de la chaleur^ Paris, 607, 1862. 




§ 8. The Critical State of Gases 

The ordinary gaseous and liquid states are only widely separated forms of the same con- 
dition of matter, and may be made to pass into one another by a series of gradations so 
gentle, that the passage shall nowhere present any interruption or break of continuity. 
Gas and lic(uid are only distinct stages of a long series of continuous physical changes.- — 
T. ANDREW.S (1869). 

The fact that some elements occur as gases, others as liquids, and yet others as 
solids is a mere accident of temperature or pressure. Similar remarks apply to 
chemical compounds which do not decompose when the temperature is augmented. 
If the prevailing atmospheric temperature were 100° higher than it is, water would 
be a gas ; and if 100° lower, water would be a solid. Similarly, if the atmospheric 
pressure were ten times as great as it is, chemistry books would describe sulphur 
dioxide and many other so-called gases either as liquids or solids ; while if the 
pressure were much less than it is, many so-called liquids would be styled gases. 
Every substance is potentially solid, liquid, and gas. The solid, liquid, and gaseous 
states of matter are merely phases assumed by virtually all kinds of matter as the 
temperature rises from absolute zero upwards. The three forms which the elements 
and their compounds can assume are called the three states o£ aggregation. The 
three states of water are : Gas above 100° ; liquid between 100° and 0° ; and solid 
below 0° under ordinary atmospheric pressure. These facts are symbolized : 

0° 100° 

Waterice — Waterjiquid ^Watergteam (760 mm.) 

Investigators who have special facilities for working at high temperatures, report 
that gold has a melting point, 1062°, and a boiling point, 2530°, or : 

1062° 2530° 

Goldsoiid ^ Goldiiquid ^ Goldgas (760 mm.) 

Similarly, those working in laboratories specially equipped for measurements at 
low temperatures, report that oxygen has a melting point, —227°, and a boiling point, 
-182-5°, or 

— 227° - 182-5° 

Oxygensoiid — Oxygenuquid ^ OxygeUgas (760 mm.) 

M. Faraday (1819) ^ emphasized the fact that when any form of matter passes 
from the solid to the liquid state, or from the liquid to the gaseous state, its physical 
properties diminish in number and variety. Thus, solids in becoming liquids lose 
their hardness, crystalline form, etc. ; and in passing to the gaseous state, the 
phenomena are still more marked, thus, all gases have nearly the same coefficient 
of thermal expansion. " The varieties of density, hardness, opacity, colour, elas- 
ticity, and form which render the number of solids and fluids almost infinite, are 
in gases supplied by a few slight variations in weight, and some unimportant shades 
of colour." 

The critical state. — T. Andrews demonstrated in his paper On the continuity of 
the gaseous and liquid states of 7natter,^ in 1869, that if gaseous carbon dioxide be 
gradually compressed in a vessel suitable for the observation, the volume diminishes 
more rapidly than would occur if Boyle's law correctly described the behaviour of 
the gas ; and when the pressure attains a certain value, the gas begins to liquefy. 
A further decrease in the volume does not change the pressure, but only increases 
the quantity of gas liquefied. At length, when all the gas has liquefied, a large 
increase of pressure only causes a minute decrease in the volume of the liquid, since 
liquids in general undergo but a small change of volume on compression. 

If the experiment be made with carbon dioxide at 0°, the gas commences to 
liquefy when the pressure has attained 35'4 atmospheres ; if at 13'1°, liquefaction 
commences at 49"8 atmospheres pressure ; if at 30°, 70 atmospheres pressure ; 
while if the temperature exceeds 31°, or, more accurately, 31' 35°, no pressure, 
however great, will liquefy the gas. Other gases exhibit analogous phenomena. This 



is in agreement with M. Berthelot's conclusion ^ in 1850 that pressure will not 
liquefy gases under all conditions of temperature. For each gas there is a particular 
temperature above which Uquefaction is impossible however great be the applied 
pressure. Andrews called this the critical temperature of the gas ; the corresponding 
pressure, or the critical pressure, is the least pressure which will liquefy the gas at 
the critical temperatures ; the volume of unit mass of the substance at the critical 
temperature and pressure is the critical volume ; and the reciprocal of the volume 
is the critical density. Consequently, a substance at the critical temperature and 
pressure is at its critical density, and is said to be in its critical state. The critical 
constants of a few substances are indicated in Table II., where the atmosphere is 
the unit of pressure ; and the volume refers to one gram of the gaseous 
substance in litres at 0°, and 760 mm. is the unit of volume ; and the density is 
referred to water at 4°. 

Table II. — Critical Constants of some Gases. 










Hydrogen . . . . . 





Nitrogen . 










Carbon dioxide . 





Nitrous oxide 





Sulphur dioxide . 










Carbon disulphide 





Air . . . 















Ammonia . 










Acetylene . 





Ethylene . 





Tin tetrachloride 





Hydrogen chloride 





Nitric oxide 










In 1883, J. Dewar * showed that the ratio of the critical temperature to the 
critical pressure of many gases is nearly proportional to the molecular volume, 
and that the quotient TclVe for the common gases generally lies between 3 J and 5. 
Other dependent relations have been indicated by E. Aries and W. K. Fielding. 

T. Andrews' critical temperature was forshadowed by D. I. Mendeleeff in 1861 
in a paper on the expansion of liquids above their boiling points,^ when he said : 

The absolute boiling point of a liquid is the temperature at which the cohesion and 
heat of vaporization become zero. At this temperature, the liquid changes to vapour 
regardless of pressure and volume. 

D. I. Mendeleeff's absolute boiling point thus corresponds with T. Andrews' critical 
temperature. D. I. Mendeleeff estimated the absolute boiling point of water to be 
580°, and of ethyl alcohol, 250°. It is interesting to notice the influence of tem- 
perature on carbon dioxide, partly liquid, partly gaseous. Observe the upper surface 
of the ^as confined in a glass tube containing partly liquefied carbon dioxide 
over mercury at 18°. The surface of the liquefied gas has a sharply defined meniscus. 
On raising the temperature, the meniscus of the liquid becomes flatter and flatter 
until, at 31-35°, the surface of the Uquid seems to disappear. The sharp line of 
demarcation between the liquid and gas vanishes at the critical temperature In 
the words of T. Andrews, as the temperature of the liquefied gas approaches 31 : 

The surface of demarcation between the liquid and the gas became fainter, lost its 


curvature, and at last disappeared. The space was then occupied by a homogeneous fluid, 
which exhibited when the pressure was suddenly diminished, or the temperature slightly 
lowered, a peculiar appearance of moving or flickering striae throughout the entire mass. 

At 40°, the tube contains a homogeneous gas. Liquid carbon dioxide cannot exist 
at this temperature, however great the pressure. Small tubes of liquid carbon 
dioxide for illustrating the phenomena by lantern can be obtained. Thin sections 
of quartz found in many granites contain cavities with liquid carbon dioxide which 
can be seen to pass through the critical point when the sections are warmed on the 
stage of a microscope. 

A blue opalescent mist appears in the tube before the meniscus of the liquid 
can be detected when the temperature of the gas is gradually lowered. The converse 
series of changes occur on heating. According to D. KonowalofE (1902),® the 
critical opalescence is due to the scattering of light by fine particles of liquid 
spontaneously formed about dust particles ; or, according to M. von Smoluchowsky 
(1908), to accidental aggregations of molecules produced by molecular collisions. 
The appearance of the blue mist is connected with slight disturbances which have 
been observed in the equation of state when applied to observations in the neigh- 
bourhood of the critical temperatures. P. de Heen (1888) argues that there are two 
kinds of molecules — molecules liquidogeniques, and molecules gasogeniques — and that 
the former can persist in the vapour phase. I. Traube (1892) and P. Villard favoured 
this view. If P. de Heen means that the pressure of a saturated vapour of a pure 
substance, like that of a mixture, depends upon the relative masses of liquid and 
vapour phases, the hypothesis is contrary to all experience. This was emphasized 
by G. G. Stokes and M. Prud'homme. According to M. von Smoluchowsky, the 
ceaseless to-and-fro agitation of the molecular particles of a gas will produce, 
spontaneously and continuously, minute inequalities in the density of different 
parts. A given cube of dimensions /x, for example, will contain sometimes 
more and sometimes less molecules. Usually these differences are inaccessible to 
measurement. The case is different when the fluid is not rarefied such as occurs when 
it is near the critical state. There is then a permanent condition of fine-grained 
heterogeneity where contiguous regions of notably different density are almost in 
equilibrium. Owing to molecular agitation, the denser swarms of molecules break 
up slowly, and, at the same time, others are forming elsewhere. The opalescence is 
produced by the molecular swarms causing a lateral diffraction of the light. The 
fluctuations of density increase as the compression increases and are very much more 
accentuated with a compressed gas than with a gas of normal density. The smaller 
the aggregates, the shorter the wave length of the light undergoing diffraction. 
Hence the opalescence may appear blue. M. von Smoluchowsky's theory has been 
extended by A. Einstein and confirmed by the work of H. K. Onnes and W. H. 
Keesom. It has also been applied to explain the opalescence of liquid mixtures in 
the neighbourhood of the point of critical miscibility, and the blueness of the sky. 

The relation between the pressure and the volume of, say, carbon dioxide, at 
different temperatures — T, Tq, Tj, T2 — is represented diagrammatically in Fig. 6. 
The portion of the curve K^T^, or K^Ti, represents the behaviour of the gas when 
liquid is present ; the portion K2M2, or KiMi, the behaviour of the gas in the 
presence of its own liquid and M2?^2» ^^ -^i?1j ^^^ behaviour of the liquid when no 
gas is present. It will be observed that K2^2 o^ ^1^1 i^ ^^^ ^^ of constant 
vapour pressure which is horizontal with the v-axis. It illustrates in a graphic 
manner the well-known law : At any fixed temperature, the pressure of a gas in 
the presence of its own liquid is always the same. The curve TqKqPq represents the 
relation between pressure and volume at the critical temperature ; and the curve 
T, the relation between p and v at a temperature when the gas does not liquefy. 
The line K^KiK^B represents the condition under which the gas, compressed at the 
stated temperatures T^, Tj, and T2, begins to liquefy, and hence it is the curve for 
saturated vapour, and also the curve for the liquid at its vaporization temperature ; 
it is not quite accurately called the dew curve, or ligne de rosee, because a gas 





under a gradually increasing pressure, jfirst shows signs of liquefaction under con- 
ditions represented by a point on this line ; similarly, the line K^M^M^A is called 
the boiling curve, or Ugne d' ebullition, because a liquid under a gradually diminishing 
pressure first shows signs of vaporization under conditions represented by a point 
on this line. Note also that the lines K^A, KqB, and KqPq divide the plane of the 
paperinto three regions. Everypointto the right of BKqPq represents a homogeneous 
gas ; every point in the region AKqB represents a heterogeneous mixture of 
gas and liquid ; and every point to the left of AKqPq, a homogeneous liquid. The 
gas in the region KqBVTq is below its critical temperature, and, in consequence, is 
sometimes called a vapour as distinct from a gas. The diagram, Fig. 6, thus repre- 
sents the conditions of equilibrium of a liquid or gas under different conditions of 
temperature, pressure, and volume. 

The continuity of the liquid and gaseous states.— It is interesting to 
note historically that C. Caignard de la Tour (1822),7 long before Andrews' experi- 
ment, noticed that when a liquid is heated 
in a sealed tube there is a definite tem- 
perature at which the surface of separa- 
tion between the gas and liquid disappeared 
and the whole contents of the tube become 
homogeneous. C. Caignard de la Tour's 
experiments thus demonstrate that the 
critical temperature is the upper limit of 
the liquid state ; and Andrews' experiments 
prove that the critical temperature is the 
lower limit to the homogeneous gaseous 
state. The passage from the one state to 
the other proceeds in a continuous manner. 
The liguid and gaseous states are con- 
tinuous, not abrupt. The properties — 
density, surface tension, viscosity, refractive 
power, heat of vaporization, compressibility, 
etc. — of a liquid gradually lose their distinctive character as the temperature is 
raised, until, at the critical temperature, the properties of liquid and gas are the same. 

There is no evidence of a change in molecular structure when, say, carbon dioxide 
passes from one state of aggregation to another ; nor is there any evidence of a poly- 
merization of the molecules when the common gases condense to Hquids. Nitrogen 
peroxide, water, and some other substances, however, do appear to polymerize and 
form compound molecules on passing from the gaseous to the liquid state of aggression. 
The properties of the condensing gases do not then exhibit that continuity 
shown by carbon dioxide and other gases which do not polymerize or dissociate. 

The difference between liquids and gases below the critical temperature seems 
to be a question of molecular attraction. If the molecules of a substance in the 
liquid state have essentially the same motions as in the gaseous state, the specific 
heat of a vapour should be nearly the same as that of the corresponding liquid. 
This is by no means the case. For example, the specific heat of liquid mercury is 
twice as large as that of the vapour; and the specific heat of liquid water is three times 
that of steam. There is, however, usually less difference between the specific heats, 
densities, and coefficients of thermal expansion of solids and the corresponding hquids. 

The condensation of binary mixtures of gases. — In a posthumous memoir 
presented to the Royal Society in 1886, T. Andrews » showed that some extra- 
ordinary phenomena occur when certain binary mixtures of gases are subjected to 
a gradually increasing pressure. A mixture of 6 parts of carbon dioxide and one 
of nitrogen commences to liquefy at 3-5° under a pressure of 48*3 atm. Here 
nitrogen condenses to a liquid at a temperature nearly 150° higher than its critical 
temperature, and at 102 atm. pressure, the whole of the nitrogen hquefies along with 
the carbon dioxide. The individual properties of the gases are thus profoundly 


Fig. 6. — Pressure - Volume Curves 
Carbon Dioxide. 




modified in the presence of other gases which are supposed to be chemically 
indifferent. The conception which has crystallized from Dalton's law of partial 
pressures, namely, that the two components of a mixture of gases are perfectly 
independent of one another, each preserving its own individuality, and each 
behaving as if it were an isolated individual, is quite erroneous. The explanation 
turns on the existence of a definite relation between the composition of a condensed 
liquid and of the vapour during, say, the distillation of a binary mixture of two 
volatile liquids which exert no chemical action on one another. L. P. Cailletet dis- 
covered this remarkable phenomenon during his Experiences sur la compression 
des melanges gazeux in 1880. If a mixture of one volume of air and nine volumes 
of carbon dioxide be subjected to a gradually increasing pressure at about 2°, the 
gas begins to liquefy at a pressure of about 72 atm.; and on increasing the pressure, 
still keeping the temperature constant, the liquid again passes into the gaseous 
state when the pressure reaches 149 atm. ; and the liquid does not reappear again 
however great the pressure. If the pressure at which the liquid appears and 
disappears be plotted with the corresponding temperature, we get the dew curve 
BKC, Fig. 7. For the same abscissa Ti, there are two ordi- 
nates, pi and p2, between which the mixture is in a hetero- 
geneous condition. At temperatures above Tq, no condensa- 
tion will occur at all ; below Ti, only normal condensation 
takes place ; at temperatures between Ti and Tq, both 
normal and retrograde condensation as P. Kuenen (1893) 
named the phenomenon, will occur. The dotted line AC repre- 
sents the boiling curve ; above AC, the system will be in the 
liquid state. K corresponds with the critical temperature of 
Fig. 7.— Diagrammatic, the mixture ; C is called a plait-polnt. For mixtures of two 
gases, therefore, (1) there is a critical zone of temperature above 
which complete or partial liquefaction is impossible. (2) Within the temperature of 
the critical zone itself a part of the mixture can be brought by pressure to the liquid 
state, and in the region of retrograde condensation, condensation is produced by 
diminution of pressure, and evaporation by an increase of pressure. The phenomena 
with mixtures thus appears quite different from what obtains with single gases. (3) 
Below the temperature of the critical zone, the whole of the mixture can be 
liquefied by pressure. 

The phenomenon occurs only with mixtures of a certain composition ; above 
and below these limits, the dew curves are quite normal. The curves can be taken 
in three dimensions with the three variables — pressure, volume, and temperature. 
The two dew points of a given mixture approach one another as the temperature 
rises. Thus, F. Caubet (1901) found with a mixture of 74' 58 per cent, of carbon 
dioxide and 254:2 per cent, of sulphur dioxide gases : 

Table III. — Betrogbade Condensation of Mixtures of Carbon Dioxide and 

Sulphur Dioxide. 











Volume of liquid. 


Volume of liquid. 


Volume of liquid. 














































One asterisk * represents the first dew point — ler point de roaie ; and two asterisks ** 
the Becond dew point — 2e jioinl de rosee. 


L. P. Cailletet and E. Mathias (1866) » found empirically that the mean values 
of the densities of a liquid, D^, and of its saturated vapour, Z)^, at a constant 
pressure, vary with the temperature in a very simple manner. If the densities be 
plotted with the temperature, a closed curve AKB, Fig. 8, is obtained. The 
mean values of the densities of the co-existing liquid and vapour, plotted against the 
temperatures, fall on a straight line, KC, Fig. 8. The density of the liquid decreases 
while that of the vapour increases as the temperature rises, until, at the critical 
point, K, the two densities are equal to the critical density. Hence, the rule was 
called the hi du diametre rectiligne, or Cailletet and Mathias' law of rectilinear 
diameters. According to this empirical rule, the mean 
values of the densities of a liquid and of its saturated 
vapour is a linear function of the temperature ; so that 
if D represents the mean value of the two densities, 
D=a-{-bd, where a and h are constants, and 6 denotes the 
temperature on the centigrade scale. For argon, the equa- 
tion of the mean density curve is Z)=0'209 56 —0-00262 35^ ; ~'^°° 
and in cases where the curve has a slight curvature, the - '^o' 

equation D=a-\-hd-}-cd^ usually represents the observed ^ 

results. The law has been tested with carbon dioxide, o o 4. q oa 

sulphur dioxide, nitrous oxide, hydrocarbons, alcohols, car- Yiq. 8. Variations in the 

bon tetrachloride, tin tetrachloride, oxygen, helium, xenon, Densities of Co-existing 
etc. In all cases the empirical law was found to be re- Liquidand Gaseous Oxygen, 
markably exact, except in the case of those substances — 

e.g. the alcohols, fatty acids, etc. — which are known to exhibit molecular aggrega- 
tion. The curvature of the line is taken to indicate molecular association, although 
the absence of curvature does not necessarily mean that molecular association is 
absent. E. Mathias and H. K. Onnes' results for oxygen are indicated in Fig. 8 ; 
the curve is plotted from the following observations : 

Temperature . . -2104° -1820° -154-5° -1402° -1299° -123-3° -120-4** 

Density of liquid, jD< . 1-2746 1-1415 0-9758 0-8742 0-7781 0-6779 0-6032 

Density of vapour, A' . 0-0001 0-0051 0-0385 0-0805 0-1320 0'2022 0-2701 

Mean density, D . . 06373 05733 05072 0-4773 0-4550 0-4400 0*4366 

The mean densities calculated from the linear expression, Z)=0- 1608— 0*002265^, 
do not deviate from the observed values more than ± 0*003. The law does not hold 
good for substances whose molecules in the liquid and gaseous states have a different 


1 B. Jones, The Life and Letters of Faraday, London, 1. 308, 1870. 

2 T. Andrews, Phil. Trans., 159. 575, 1869 ; B. A. Eep., 76, 1861 ; W. A. Miller, Chemical 
Physics, London, 1863. 

3 M. Berthelot, Compt. Bend., 30. 166, 1850. 

* J. Dewar, Phil. Mag., (5), 18. 210, 1884 ; Nature, 28. 561, 1883 ; E. Arifes, Compt, Rend., 
166. 193, 1918 ; W. R. Fielding, Chem. News, 117. 379, 1918. 

5 D. I. Mendeleeff, Liehig's Ann., 119. 1, 1861. 

« D. Konowaloff, Ann. Physik, (4), 10. 360, 1903; (4), 12. 1160, 1903; L Traube, tb., (4), 
8. 289, 1902; M. von Smoluchowsky, ib., (4), 25. 205, 1908; Bull. Acad.. Cracow, 1057, 1907; 
P. de Heen, Recherches touchant la physique comparee et la theorie des liquides, Paris, 1888 ; Bull, 
Acad. Belgique, (3), 25. 695, 1893 ; P. Villard, Ann. Chim. Phys., (7), 10. 429, 1897 ; A. Einstein, 
Ann. Physik, (4), 33. 1275, 1910; (4), 36. 1572, 1910; W. H. Keesora, ib., (4), 35. 591, 1911 ; 
H. K. Onnes and W. H. Keesom, Comm. Lab. Phys. Leiden, 104, 1908 ; Lord Raylcigh, Phil. 
Mag., (4), 41. 107, 1871 - (5), 47. 375, 1899 ; M. Prud'homme, Journ. Chim. Phys., 14. 445, 

' C. Caignard de la Tour, Ann. Chim. Phys., (2), 21. 127, 178, 1822 ; (2). 22. 140, 

«'t. Andrews, Phil. Trans., 178. 45, 1887 ; L. P. Cailletet, Compt. Rend., 90. 210, 1880 ; Journ. 
Phys., (1), 9. 192, 1880 ; (2), 2. 389, 1883 ; J. P. Kuenen, Phil. Mag., (5), 40. 173, 189o ; Zext. 
phys. Chem,, 11. 38, 1893 ; 24. 667, 1897 ; F. Caubet, ib., 40. 257, 1902 ; Liquefaction des melanges 


gazeux, Paris, 1901 ; J. Dewar, Proc. Boy. 8oc., 30. 538, 1888 ; P. Duhem, Journ. Phys. Chem., 1. 
273 1897. 

» E. Mathias and L. Cailletet, Journ. Phys., (2), 5. 679, 1886 ; Compt Rend., 102. 1202, 1886 ; 
104. 1563, 1887 ; E. Mathias, ib., 112. 85, 1891 ; Ann. Fac. Sciences Toulouse, (1), 6. 1, 1892 ; 
S. Young, Phil. Mag., (5), 33. 263, 1892 ; (5), 50. 291, 1900 ; Journ. Chem. Soc., 59. 37, 126, 
929, 1891 ; K. Tsuruta, Phys. Rev., (1), 10. 116, 1900; E. Mathias and H. K. Onnes, Comm. 
Lab. Phys. Leiden, 117, 131, 1911. 

1" P. E. Guye, Arch. Sciences Oenhve (3), 31. 176, 1894 ; E. Mathias, Le point critique, des 
corps purs, Paris, 1904. 


§ 1. Gay Lussac's Law of Combining Volumes 

Omnia mensura et numero et pondere disponsuisti — Thou hast ordered all things in 
measure, and number, and weight. — Liber Sapientiae. 

Not very long after John Dalton had directed the attention of chemists to the 
relations subsisting between the weights of bodies which combine in different 
proportions, J. L. Gay Lussac i established a similar correspondence between 
volumes of combining gases. A. von Humboldt, the naturalist and eirplorer, 
collected samples of air from different parts of the world, and with the aid of 
J. L. Gay Lussac, analysed the different samples with the idea of finding if the 
composition of air was variable or constant. J. L. Gay Lussac used Cavendish's 
process — explosion of a mixture of air and hydrogen gas. As a preliminary, A. von 
Humboldt and J. L. Gay Lussac investigated the proportion by volume in which 
hydrogen and oxygen combine, and found the ratio of hydrogen to oxygen, by 
volume, to be nearly as 2 : 1. If either hydrogen or oxygen was in excess of these 
proportions, the excess remained after the explosion, as a residual gas. A. von 
Humboldt and J. L. Gay Lussac (1805) found : 

Vols, of oxygen. 

Vols, of hydrogen. 

Vols, of residue. 



101*3 hydrogen 



101-7 oxygen 

After making corrections for impurities, etc., in the gases, J. L. Gay Lussac and 
A. von Humboldt stated that " 100 volumes of oxygen required for complete satura- 
tion 199-89 volumes of hydrogen, for which 200 may be put without error." 
A. Scott (1893) found, as the result of twelve experiments on the volumetric com- 
position of water, that oxygen and hydrogen combine very nearly in the ratio 
1 : 2-00245 by volume. 

Struck by the simplicity of the relation thus found, J. L. Gay Lussac (1808) 
followed up the subject by numerous experiments with different gases. As a 
result, in his Memoire sur la comhinaison des substances gazeuses, les unes avec les 
autres (1809), he concluded that " gases always combine in the simplest proportions 
by volume." For instance, one volume of hydrogen combines with one volume 
of chlorine forming two volumes of hydrogen chloride ; two volumes of hydrogen 
combine with one volume of oxygen forming two volumes of water vapour (which 
condenses to liquid water if the temperature be below 100°). 

There are slight deviations with the gases which show deviations from the 
laws of Boyle and Charles, but the experimental results are such as to leave no 
doubt that J. L. Gay Lussac's generalization is valid, and accordingly, we define 
Gay Lussac's law : when gases react together, they do so in volumes which bear 
a simple ratio to one another, and to the volume of the gaseous product of the 
action. It is assumed, of course, that the initial and final products of the reaction 
are under the same conditions of temperature and pressure. 

The remarkable way in which elements unite by weight was traced to a 
peculiarity in the constitution of matter ; so here, we are tempted to make a 
similar quest. It follows at once (1) if elements in a gaseous state unite m simple 



proportions by volume, and (2) if the elements also unite in simple proportions 
by atoms, then the number of atoms in equal volumes of the reacting gases must 
be simply related. With John Dalton, in his A New System of Chemical Philosophtj 
(Manchester, 1808), let us make a guess. Assume that equal volumes of the 
different gases under the same physical conditions contain an equal number 
— say n — of atoms. Then, when two volumes of hydrogen react with one volume 
of oxygen to form two volumes of steam, we have 2n atoms of hydrogen reacting 
with r? atoms of oxygen to form 2n " compound atoms " of steam. Hence, two 
atoms of hydrogen react with one atom of oxygen to form two " compound atoms " 
of steam. In that case, every atom of oxygen must be split into half to make 
two " compound atoms " of steam. This contradicts the fundamental postulate 
of the atomic theory expressed in John Dalton's aphorism : " Thou knowest no 
man can split an atom," meaning that atoms are assumed to be indivisible in 
chemical reactions. 2 Similar contradictions are encountered in nearly every case 
of combination between gases, hence J. Dalton regarded J. L. Gay Lussac's law 
as untenable : Equal volumes of homogeneous gases, under like conditions of tem- 
perature and pressure, do not contain the satne number of atoms. There is such a marked 
uniformity in the deportment of elementary and compound gases with respect to 
variations of temperature and pressure, that it is not very probable any essential 
difference will be found in their constitution. 


^ J. L. Gay Lussac and A. von Humboldt, Journ. Phys., 60. 129, 1805; J. L. Gay Lussac, Mem. 
Soc. Arcueil, 2. 207, 1809 ; Alembic Club Reprints, 4, 1893 ; A. Scott, Phil. Trans., 184. 543, 1893. 

2 W. C. Henry, Memoirs of the Life av^, Scientific Researches of John Dalton, London, 1854 ; 
Alembic Club Reprints, 4, 1893. 

§ 2. Amadeo Avogadro's Postulate 

Advances in knowledge are not commonly made without the previous exercise of some 
boldness and licence in guessing.— W. Whewell. 

J. J. Berzelius i thought it strange that J. L. Gay Lussac should have contented 
himself with having determined the combining ratios of gaseous substances, and 
should make no attempt to extend his discovery. Clearly with J. Dalton the faculty 
of speculation was predominant, and with J. L. Gay-Lussac experimentation. 
An epoch-making memoir entitled, Essai d'une maniere de determiner les masses 
relatives des molecules elementaires des corps, et les proportions selon lesquelles elles 
entrent dans les combinaisons, was published in 1811 by Amadeo Avogadro,^ an 
Italian physicist. In his memoir A. Avogadro said — 

J'ai propose \ine hypothese pour expliquer le fait decouvert par M. Gay Lussac, que 
les volumes des substances gazeuses qui se combinent entre elles, et des gaz composes qui 
en r68\iltent, sont toujours dans les rapports tr^s simples entre eux. 

He pointed out that the difficulty with Dalton's hypothesis can be avoided if we 
distinguish clearly between elementary atoms and the small particles of a gas. 
Assume that the small particles of a gas are aggregates of a definite number of 
atoms ; then, using A. Avogadro's own words : 

Les molecules constituantes d'un gaz simple quelconque, c'est-a-dire celles qui s'y 
tiennent a une distance telle a ne pouvoir exercer leur action mutuelle, ne sont pas form^es 
d'une seule molecule el6mentaire mais r6sultent d'un certain nombre de ces molecules 
r6unies en une seule par attraction. 

A. Avogadro called these aggregates molecules, in order to distinguish them from 
the ultimate atom. His actual term was molecules constituantes or molecules 
integrantes — the former term was used for molecules of elements, the latter for 



molecules of compounds. The one term molecule (the diminutive form of the 
Latin word jnoles, a mass) is now applied to both Avogadro's inolecules constitiiantes 
and molecules integr antes. Each molecule of an elementary gas is supposed to 
contain the same number and kind of elementary atoms. What J. Dalton called 
atoms A. Avogadro called molecules elementaires. The word " atom " does not 
occur in the latter's memoir. The modern meanings of the terms atom and 
molecule were clearly stated by A. M. Ampere 3 in 1832, and by A. Gaudin in the 
same year. Some years later these distinctions were emphasized by A. Laurent 
(1846) and employed in his posthumous book Methode de chiinie (Paris, 1854). 
A. M. Ampere used the term particle for an aggregate formed by the juxtaposition 
of molecules. He said : 

In the passage from liquid to the gaseous state, the molecules are separate from one 
another ; and conversely, in passing from the gaseous to the liquid state, the molecules 
are drawn together. In the passage from the liquid to the solid state, I think that two 
or more molecules are drawn together. Mechanical forces alone can separate the particles ; 
chemical forces are required to split the molecules. 

For the sake of simplicity, assume that each molecule of hydrogen gas is com- 
posed of two atoms of hydrogen, and make a similar assumption for oxygen gas ; 
and assume with A. Avogadro that equal volumes of all gases, at the same 
temperature and pressure contain the same number of molecules. This 
postulate is now known as Avogadro's hypothesis. In A. Avogadro's own words : 

L'hypothese qui se presente la premiere a 6gard et qui parait mSme la seule admissible, 
est de supposer que le nombre des molecules integrantes dans la gaz quelconque est toujoura 
le meme a volume 6gal, ou est toujours proportionnel aux volumes. . . . 

Suppose that two volumes of hydrogen contain 2w molecules of hydrogen, then one 
volume of oxygen will contain n molecules. These react to form 2w molecules of 
steam — each molecule of steam contains two atoms of hydrogen and one atom of 
oxygen. The idea can be more clearly illustrated by means of the subjoined 
diagrams. Each square represents one volume of a gas. Each volume contains 
n molecules. We do not know the numerical value of n, but, for the sake of 
simpUcity, take n=4. It makes no difference to the final conclusion what 
numerical value we assign to n. Then we have : 


t ••! 

Hence, although the atoms of oxygen cannot be split, yet a 2-atora molecule of 
oxygen can be subdivided so that one atom of oxygen enters the composition of 
each of two molecules of water. Again, with hydrogen and chlorine. 




8 <P 

a, . t <SG> 


Diagrams similar in principle to these were used by M. A. Gaudin about 1832 in 
his Recherches sur la structure intinie des corps inorganiques dejinis. It must not be 
supposed for one moment that what may be called Gaudin's diagrams are intended 
as pictures of the actual molecules. They are to be regarded as aids to the under- 
standing of how Avogadro's hypothesis has led chemists to conclude that the mole- 
cules of gaseous elements are really compounded atoms, and how Avogadro's 
hypothesis reconciles the observed volume relations during the combination of 
gases with the atomic theory. 

It has been assumed for the sake of simplicity, that the molecule of water con- 
tains three atoms, and that each molecule of hydrogen and oxygen contams two 
atoms. As a matter of fact, all we can infer from the observed facts is that the 
molecule of oxygen is split into halves, and, in the absence of evidence to the contrary, 


we assume for every substance the simplest molecular structure consistent with the 
observed facts. 

A. Avogadro extended J. Dalton's atomic hypothesis and adapted it particularly 
to^gases. We owe to the former the conception of two orders of minute par- 
ticles : (1) the atom or unit of chemical exchange ; and (2) the molecules are the 
smallest particles oi an element or compound which exist free in a gasr 
This definition of a molecule is usually extended into the less satisfactory definition : 
A molecule is the smallest [particle of an element or compound which exists 
in a free state ; otherwise expressed, the molecules of an element or compound 
are particles so small that the specific properties of the substance depend 
upon the particles remaining intact. Hence, if molecules be subdivided the parts 
no longer have the specific properties of the original substance. If the molecules 
of steam, H2O, be subdivided, two atoms of hydrogen and one atom of oxygen 
would be formed per molecule ; the atoms unite in pairs to form molecules. 

Diatomic molecules for gaseous chlorine, hydrogen, and oxygen at ordinary 
temperatures furnish a satisfactory explanation of what we know to-day, but it 
is possible that at some future date, the evidence will compel us to consider these 
molecules to be tetra- or hexa-atomic. This will not materially afiect the principle 
as indicated above. The molecule of mercury is supposed to be monatomic ; and 
the molecule of sulphur, hexatomic. 

In 1814, A. M. Ampere advocated views similar to those of A. Avogadro, but he 
compHcated the latter's simple hypothesis by an unsuccessful attempt to apply 
his conception of molecules to crystalUne solids. Avogadro considered Ampere's 
extension unjustifiable. A. M. Ampere clearly emphasized the hypothetical nature 
of A. Avogadro's conception in a letter to M. le Comte Berthollet in 1814, when he 
said : If the consequences of the hypothesis be confirmed by further experiments, 
and the hypothesis be in agreement with known facts, elle pourra acquerir un degre 
de prohdbilite qui approchera de ce qu^on nomme en physique la certitude. Increasing 
knowledge has made A. Avogadro's hypothesis more and more probable ; it has 
been tested in hundreds of experiments, and never found wanting. The hypothesis 
has done such good service in giving a rational explanation of many different 
phenomena that it has been accepted as a fundamental truth. It gave chemists a 
clear definition of the atom, a method of determining the relative weights of the 
atoms, and of estimating the number of atoms in the molecule. 


1 J. J. Berzelius, Essai sur la thiorie des proportions chimiques et sur Vinfluence chimique de 
V ehctricite, Paris, 14, 1819 ; A. N. Meldrum, Avogadro arid Dalton, Edinburgh, 14, 1904. 

2 A. Avogadro, Jmirn. Phys., 73. 58, 1811; 78. 131, 1814; Alembic Club Reprints, 4. 
1893 ; Mem. Accad. Torino, 26. 440, 1864 ; J. Guareschi, Amadeo Avogadro e la teoria molecolare, 
Torino, 1901. 

8 A. M. Ampere, Bibl. univ. Geneve, 49. 225, 1832; Ann. Chim. Phys., (1), 90. 43, 1814; 
(2), 58. 432, 1835 ; M. A. Gaudin, ib., (2), 52. 113, 1833 ; Becherches sur le groupement des atomes 
dans les moliculeset surles causes les plus intimes des formes cristallines, Paris, 1847 ; L^ architecture 
du monde des atomes, Paris, 1873 ; E. Grimaux, Thiories et notations chimiques, Paris, 1884 ; E. 
Erlenmeyer, Zeit. Chem., 6. 610, 1863. 

§ 3. The Relative Weights of the Molecules 

In order to bring into harmony all the branches of chemistry, we must have recourse 
to the complete application of the theory of Avogadro in order to compare the weights and 
the numbers of the molecules. — S. Cannizzaro. 

John Dalton in 1807 raised the query : " Are there the same number of particles 
of any elastic fluid in a given volume and under a given pressure ? " It is curious 
that in answering " No," J. Dalton ^ abandoned an hypothesis which afterwards 


proved to be one of the most fruitful suggestions in the development of chemistry, 
for, under the name of Avogadro's hypothesis, it has correlated what appeared antago- 
nistic and contradictory, and has harmonized what seemed discordant and confused ; 
it made Dalton's atomic hypothesis a clear, intelUgible, and fertile theory. As 
C. A. Wurtz said in his The Atomic Theory (London, 1880), had it not been for this 
development, J. Dalton's hypothesis was in a fair way of being sentenced to steriUty 
and oblivion. A fellow countryman of A. Avogadro, namely S. Cannizzaro, seems 
to have seen, more clearly than any other, the importance of A. Avogadro's hypo- 
thesis in putting J. Dalton's on a firm basis. 

S. Cannizzaro' s ideas were first pubHshed in a letter to S. de Luca embodying 
a Sketch of a Course of Chemical Philosophy ^'^ given in the Royal University of Geneva 
in 1858. Before S. Cannizzaro published his paper, rank confusion prevailed in 
chemical Hterature. The terms atomic weight, molecular weight, and combining 
or equivalent weight were used and abused in every conceivable way. J. B. A. 
Dumas lost faith in the atomic theory and wrote in despair : 

Si j 'en etais le maitre j 'eff acerais le mot atome de la science, persuade qu'il va plxis loin 
que I'experience : et jamais en chimie nous ne devons aller plus loin que I'exp^rience. 

Avogadro's hypothesis was necessary for salvation ; it lay dormant in chemical 
literature for nearly half a century ; S. Cannizzaro brought the awakening, and 
showed chemists that the atom must be defined with reference to A. Avogadro's 
molecule. After reading S. Cannizzaro's pamphlet, Lothar Meyer (1860) thus 
describes his own conversion : " the scales fell from my eyes, my doubts disappeared, 
and a feehng of tranquil security took their place." A. Avogadro's hypothesis 
was thus made the basis of the current theory of chemistry. 

By definition, the relative density of a gas is a number which represents how 
much heavier any volume of the gas is than an equal volume of the standard gas — 
generally hydrogen — measured at the same temperature and pressure — generally 
at 0° and 760 mm. pressure. Thus, the relative density of steam is 8*95. This 
means that any volume, say a litre of steam, is nearly nine times as heavy as the 
same volume of hydrogen. 

Strictly speaking, the density of a gas is the weight of 1 c.c. of the gas at 0° and 760 nmi. 
The density of a gas is usually expressed in terms of a litre of the gas because the number 
representing the weight of 1 c.c. would be inconveniently small. One litre of hydrogen 
at n.p.t. weighs very nearly 0*0896 grm. " So important is this standard weight-unit," 
said A. W. Hofmann in hia Introduction to Modern Chemistry (London, 1865), " that a name 
is needed to denote it." He suggested crith {Kpie-n, a barley com, or small weight) to 
denote the weight of a litre of hydrogen at n.p.t. The weight of the same volume of 
oxygen would then be 16 criths, of nitrogen 14 criths, etc. The term has now dropped 
out of use, although for a time it served a useful purpose. 

By Avogadro's hypothesis, equal volumes of gases, under like conditions of 
temperature and pressure, contain the same number of molecules, consequently, 
the specific gravity or relative density of a gas is proportional to its molecular 
weight. Let n represent the number of molecules in a volume v of each of two 
different gases, and if the molecules of each gas are all alike with the respective 
molecular masses M^ and i/g, then the one gas will have a mass nMi and the other 
a mass wMg. Let the densities of the respective gases be Di and D.^y then since 
density denotes the mass of unit volume, D^-.D^ — nM-^v ; nM^jv ; that is, 
Z>i : i>2=^i : ^2 or 

Di Ml Ml Mg Q. 

Drw^DTD, ' ' ' • ^^ 

or the relative densities of any two gases are proportional to their respective mole- 
cular weights ; and the quotient of the molecular weight by the density is the 
same for all gases. It is convenient to employ the term molecular volume for the 
quotient obtained by dividing the molecular weight M of a gas by its relative density 


D\ consequently, from the second of equation (1), the molecular volumes of all 
gases are the same. 

If we accept this deduction, it enables us to determine the molecular weights of 
gases, once we have fixed an arbitrary standard for the density. Cannizzaro's 
unit, hydrogen==2, is frequently taken as the standard, or else hydrogen unity, 
that is, as S. Cannizzaro expressed it, " the quantity of hydrogen contained in a 
molecule of hydrogen chloride " is taken as unity. The determination of the 
molecular weight of a gas is thus reduced to a laboratory measurement — the 
determination of the relative density of the gas. Methods for measuring vapour 
densities are outlined later. 

It has been shown within certain limitations, that the numerical values for the 
molecular weight and relative density of a gas referred to the standard hydrogen , 2, 
are the same. That is, 

Molecular weight = Relative density (H2=2) . . • (2) 

For example, the observed density of steam is 18 (H2=2), the molecular weight ot 
steam is therefore 18 likewise. Again, if the relative density be referred to the 
standard hydrogen unity, or oxygen 16, the relative density is half the molecular 
weight ; or the molecular weight is twice the density. 

Molecular weight =: 2 X Relative density (H=l) . . • (3) 

For instance, the density of steam is 9 (H=l), the molecular weight is therefore 
twice 9 or 18 as before. When the relative density is referred to oxygen 32, as is 
common in recent years, it is virtually assumed that there is an imaginary gas whose 
relative density is unity ; and to avoid the hypothesis implied in the term molecular 
weight, the term molar weight is applied to the relative density of a gas referred 
to oxygen 32. 

If the relative density be determined, as is frequently the case, with reference to 
the standard air unity, then, since the density of air with respect to hydrogen is 
28-75 (H2=2) ; or with reference to oxygen 28-98 (02=32), it follows that 

Molecular weight=28- 75 X Relative density (Air unity) . . (4) 

For example, the relative density of cyanogen is 1-806 (air unity), the molecular 
weight is therefore 1-806x28-75=51-92 (H2=2). This is in agreement with the 
formula C2N2. 

It is unfortunate that these different units are employed, even though all give the 
same final result. It shows the necessity for clearly understanding the particular 
meaning of terms employed before elaborating an argument. The method of deter- 
mining the relative density of a gas by weighing a globe full of gas and then full 
of air, led to the use of air as a standard of reference. Thus, J. L. Gay Lussac (1815) 
found a 2J-litre globe weighed w-\-2'l^ grms. when filled with air, and w+4-946 
grms. when filled with cyanogen ; consequently the relative density of cyanogen, 
air unity, is 4-946/2-738=1-806. The custom of referring all gas densities to air 
as a standard was gradually adopted. The system has been shown in recent years 
to be faulty when very accurate results are required because there are undoubtedly 
slight variations in the composition of air, and this causes the density of air — ^the 
standard of reference — to vary in a corresponding manner. 

If the specific gravity of a gas is to be referred to water as standard^ the relative 
density, air unity, is multiplied by the weight of one c.c. of air, viz. 0-001293; 
by 0-00008996 if hydrogen unity be the standard; and by 000004469 if oxygen be 32. 
Thus, the relative density of carbon dioxide is 1-57 (air unity) ; 22 (hydrogen 
unity) ; and 44 if oxygen 32 be the standard. Hence, also, the specific gravity 
with respect to water as standard is 1-57x0-001293=0-00203. 

It will be noted that if W denotes the weight of a Htre of a gas of molecular weight ilf , 
and D denotes the relative density, air lanity, >r = |M X 0*08996 ; D=M/2S15, and 
therefore 100{W —D)=M, or the molecular weight of a gas, is nearly 100 times the difference 
between the weight of a litre of the gas at n.p.t. and the relative density of the gan, air unity. 


Returning to S. Cannizzaro's important paper, S. Cannizzaro gave the following 
numbers, among others, for the densities of the different gases referred to hydrogen 
taken as 2, or to a semi-molecule of hydrogen taken as unity : 

Relative densities. 

Hydrogen •••....., 2*0 

Oxygen [ [ 32-0 

Chlorine ......... 71.q 

Nitrogen 28*0 

Water vapour •••..... Ig'O 

Hydrogen chloride . . . . , . . 3g.5 

If, therefore, the molecules of hydrogen, oxygen, nitrogen, and chlorine contain 
two atoms, the atomic weights of these gases will be half the respective molecular 
weights. Hence, making a selection from S. Cannizzaro's tables : 

Table I. — S. Cannizzaro's Table of Atomic Weights. 


Relative density of gas. 

Atomic Weight, or Density -r 2. 

Hydrogen . 




In the case of compounds, if the molecule of hydrogen chloride contains an 
atom of chlorine and an atom of hydrogen, the molecular weight will be 35*5+1 
= 36' 5; and the molecule of water vapour containing two atoms of hydrogen and 
one atom of oxygen — or, as A. Avogadro (1811) expresses it, une demi-molecule 
d'oxygene avec une molecule ou, ce qui est la mefne chose, deux demi-molecules d'hydrogene 
— will have a molecular weight of 16+2=18. Hence, given the molecular weight of 
a compound gas, and the weights of the atoms of all but one of the elements, it is 
possible to compute the weight of the atom or atoms of that element in the molecule 
in question. The 7nodus operandi will be discussed in two later sections. 

A. Avogadro explicitly guarded against the assumption that the number of 
constituent atoms in the molecule of a gas must always be 2. There is really nothing 
in the facts to justify the assumption that the atoms themselves are simple particles. 
For all we know to the contrarj^, the atoms may be clusters of n particles. Indeed, 
we shall soon review some cogent evidence which has led many chemists to abandon 
Newton's solid impenetrable atoms, and to infer that Dalton's atoms are not 
nature's irreducible minima. Even if this inference be valid, each cluster of 
n particles which forms an atom has a definite weight — atomic weight — and enters 
into and is expelled from chemical combination as if it were a simple particle. If an 
atom be a cluster of particles, each cluster, so far as we can tell, has up to the present 
time behaved in chemical reactions as if it were an individual particle. The actual 
weight of a molecule is certainly not the molecular weight. When it is said that 
the molecular weight of hydrogen chloride is 36' 5, this number simply means that 
we have conventionally agreed to fix the molecular weight of, say, oxygen as 32 
units, and that the molecular weight of hydrogen chloride is to that of oxygen as 
36-5: 32. Consequently, like atomic weights, molecular weights denote ratios, 
they are relative not absolute numbers. 

To deduce Avogadro' s law from the relation between the relative densities and the 
molecular weights of the gases. Let Mj and M^ denote the weights of the molecules 
of two gases— A and B respectively ; further, let n^ and n^ respectively denote the 
number of molecules in unit volumes of the two gases. The weights of unit volumes 
{i.e. the densities) of the two gases will be ilf i??i and Mg^s- The observed fact is 
that the molecular weights {M^ and M^) of the gases are proportional to the densities 
(Mi^i and M^n^) of the gases ; or M^ni : ilf 2^2=^1 ' ^2» from which it follows 

VOL. I. N 


that in unit volumes of the two gases ni=n2. This is the symbolic way of stating 
Avogadro's rule. Hence, it has been claimed that Avogadro's postulate can be 
deduced from the relation between the molecular weights and the densities of two 
gases. It is easy to be misled by the apparent precision and rigorous accuracy 
conveyed to the mind by reasoning expressed in mathematical symbols, and to 
assume that the conclusions of such reasoning are certainties. Some affirm, on 
the strength of the simple demonstration just indicated, that " Avogadro's hypo- 
thesis is true." The reasoning is perfectly sound, but what about the premises, 
or statements upon which the reasoning is based ? Avogadro's method for the 
determination of molecular weights tacitly assumes that the hypothesis is true. 
Hence, if the mathematical demonstration be employed to prove that Avogadro's 
hypothesis is true, the argument proceeds in a vicious circle. It is assumed in 
the premises what is " proved " in the demonstration. A conclusion proved by 
mathematics cannot be any more certain than the premises on which the reasoning 
is based. 


1 J. Dalton, A New System of Chemical Philosophy, Manchester, 1808. 

2 S. Caimizzaro, Nuovo Cimento, 7. 321, 1858; Journ. Chem. Soc., 25. 941, 1872 ; Ostwald's 
Klassiker, 30, 1891 ; Alemhic Clvh Reprints, 18, 1858 ; E. von Meyer, Journ. prakt. Chem., (2), 
83. 182, 1911 ; L. Meyer, Ostwald's Klassiker, 30, 1891 ; J. B. A. Dumas, Lerons sur la philosophic 
chimique, Paris, 1836. 

§ 4. The Formulse of Compounds 

Avogadro's hypothesis affords a bridge by which we can pass from large volumes of 
gases, which we can handle, to the minuter molecules, which individually are invisible and 
intangible.— W. A. Shenstone. 

Since S. Cannizzaro's time, an enormous number of molecular weights have been 
determined by the vapour density method. If the molecule cannot be decomposed, 
it must be assumed that it is composed of one kind of matter only. If the substance 
is compound, it must be analysed so as to find the ratio, by weight, of its component 
elements referred to the oxygen standard (16). For instance, suppose that the 
analysis of a gaseous compound furnished : Nitrogen, 82* 35 per cent. ; hydrogen, 
17'65 per cent. Using S. Cannizzaro's data, if hydrogen has an atomic weight 
of unity and nitrogen 14, the compound has the equivalent of 17* 6 5-^1, or 17*65 
hydrogen atoms for every 82* 35^14 nitrogen atoms; or 5'9 nitrogen for every 
17*65 hydrogen atoms. By hypothesis we cannot have fractions of atoms. The 
nearest whole numbers are 3 hydrogen atoms for one nitrogen atom. Since the 
sum of the atoms in the compound must represent the molecular weight, it follows 
that the molecular weight must be 3n-\-lin, that is, the molecular weight is 17x1 ; 
17x2 ; 17x3; . . . ot I7n. The formula is NnHsn. We can get no further until 
we know the molecular weight. If the vapour density of the compound (hydrogen 
=2) be 17, the molecular weight is 17. Hence, 17=17w, or n=l. The compound 
analysed can therefore be represented by the formula NH3. 

Examples.- — (1) E. W. Morley (1895) found, in some careful experiments on the synthesis 
of water: Hydrogen used, 3-7198 grms. ; oxygen used, 29'5335 grms. ; water formed, 
33-2530 grms. That is, one part by weight of hydrogen combines with 7-94 parts by weight 
of oxygen to produce 8-94 parts by weight of steam. A molecule of steam must contain n 
atoms of hydrogen, because parts of an atom do not take part in chemical changes. Hence, 
n parts by weight of hydrogen per 7-94« parts by weight of oxygen give a molecule of 
steam of weight 8-94w. This all follows from the atomic theory. To apply Avogadro's 
hypothesis, with Cannizzaro's standard, the density of the steam must be determined. 
It lies between 16 and 20. It is difficult to determine the number exactly. If n = l, the 
density of the steam molecule will be near 8-94. This does not agree with the observed 
density 16 to 20. If n~2, the density of the steam will be 17*88 ; and if n = 3, the density 
of steam will be 26'82. Hence, w=2. This means that each molecule of water vapour 



contains 2 atoms of hydrogen, atomic weight 1, and one atom of oxygen, atomic weight 
15-88 ; or if we make our imit oxygen = 16, the atomic weight of hydrogen will be 1008. 

(2) Two different compounds have the same ultimate composition, namely : carbon 
92-31 per cent., hydrogen 769 per cent., but the one has a relative density 26, and the 
other a relative density 78 (H=2). What is the formula of each compoimd ? There are 
92-31^12=7-7 carbon atoms per 7-7-^1=7-7 hydrogen atoms; but we cannot have 
fractions of atoms, hence dividing by 77 we get the ratio 1:1. That is, the formula of 
the compoimd is CnHn- The molecular weights of this series of compoimds is (12 + l)n 
or 13«. If w = 2, the molecular weight will be 26. Hence, one of the compounds is CjHj, 
and the other is CgHg. 

In calculating formulae for substances which cannot be vaporized, and one of 
the methods to be described later cannot be applied, it is usual to assume that the 
molecule has the simplest formula. In that case the formula is said to be empirical. 
Some prefer to use the term formula weight in place of molecular weight when 
the actual molecular weight has not been determined. The formula weight, like 
the molecular weight of a compound, is the sum of the atomic weights of the 
elements represented in the known or assumed formula of the compound. 

Examples.- — (1) 10 grams of pm-e tin when oxidized in air gave 12-7 grams of oxide. 
What is the formula of tin oxide ? The atomic weight of tin is 119, and of oxygen 16. 
Hence, the ratio : Tin : oxygen = 10 ^119 : 2-7^16=0-084 : 0-17 = 1 : 2. The formula is 
therefore written SnOg, although there is nothing to show why it is not Sn204 ; SngOg ; 
. . . Snn02n- 

(2) A sample of crystallized sodium carbonate furnished on analysis 37-2 per cent, 
of NaXOg, and 62-8 per cent, of HgO. What is the formula of the compound ? The ratio 
NaaCOg: H2O = 37-2^106 : 62-8-M8=0-35: 3-49 = 1 : 10. Hence, the formula is taken 
as NagCOj.lOHgO, although there is nothing to show why it is not some multiple of this, 
say, iwNaaCOg.lOwHaO. 

(3) A. Jones (1892) analysed a sample of electric calamine, and found : Silica, SiOj, 
25-33; zinc oxide, ZnO, 67-15; and water, HjO, 7*47 per cent. Show that this 
corresponds verj'^ nearly w?th the formula Zn2Si04.H20. 

(4) W. F. Hillebrand and W. H. Melville (1892) analysed some crystals obtained by the 
action of sulphuric acid on uranium oxide, and found : UO2, 53-99 ; SO 3, 36*95 ; HgO, 
14-13 per cent. Show that the molecular ratio of these three constituents is 1 : 2 : 3-94, 
and that this corresponds with the formula 1102(803)2, 4H2O or 11(804)2. 4H2O. 

(5) G. Femekes (1902) analysed a salt obtained by treating a solution of mercuric 
chloride with potassium ferrocyanide, and found : Potassium, 15-82 per cent. ; mercury, 
40-63 ; iron, 11-45 ; and cyanogen, CgNg, 31-78. Show that the simplest empirical formula 
for the compound is K2HgFe(CN)8. 

§ 5. The Relative Weights of the Atoms 

Atoms are so inconceivably little that their aggregates are alone the ostensible subject 
of experiments.- — S. Brown. 

It has abeady been stated that the conceptions molecular weight and atomic 
weight are quite independent of our theories about the nature of atoms and mole- 
cules ; nor are the conceptions much affected by the actual weights of the atoms 
and molecules because the terms under consideration are definite expressions of 
Avogadro's hypothesis coupled with observed facts. It might therefore have been 
misleading to head this paragraph : Weighing the Atoim. There are reasons for 
supposing that the molecular weight of some compounds in the liquid or sohd con- 
dition is a multiple of the molecular weight of the same substance in the gaseous 
condition. The molecule of steam approximately corresponds with the formula 
H2O ; but in liquid water there are reasons for supposing the molecule is either 
(H20)3 or (H20)4, that is, the formula for hquid water is not HgO, for it contains 
molecules corresponding with H4O2, HeOa, or H8O4. . _ ^ , , .1 . • 

Refer back to the difficulty in fixing the atomic weight'of carbon from the ratio 
of the weights of carbon and oxygen in the two oxides of carbon which we encountered 
in applying J. Dalton's atomic theory. Suppose that we do not know the atomic 



weight of carbon, but that we do know the composition of a number of volatile 
carbon compounds as well as their relative densities or molecular weights, Table II. 

Table II. — ^Molecular Weights oe Some Carbon Compounds. 

Volatile compound of carbon. 

Composition by weight. 


1 Amount of carbon 
1 per molecule. 

Carbon monoxide 

Carbon 12 ; oxygen 16 



Carbon dioxide . 

Carbon 12 ; oxygen 32 


i 12 

Methane .... 

Carbon 12 ; hydrogen 4 


1 12 

Ethylene .... 

Carbon 24 ; hydrogen 4 


.12X2 = 24 

Propylene .... 

Carbon 36 ; hydrogen 6 


12X3 = 36 

Carbon disulphide 

Carbon 12 ; sulphur 64 


1 12 

The smallest weight of carbon in a molecule of any of its known compounds is 
12, and consequently this number is assumed to be the atomic weight of carbon. 
The atomic weights of a great number of the elements have been determined in a 
similar manner. 

The determination of atomic weights. — According to J. Sebelin's Beitrluje 
zur Geschichte der Atotngewichte (Braunschweig, 1884), when J. J. Berzelius was 
asked how he was able to get such excellent analyses, analyses which have been the 
admiration of generations of chemists, he answered : 

Try to find that method of analysis in which the accuracy of the result is least dependent 
upon the skill of the operating chemist ; and when this method has been selected, consider 
what unavoidable conditions are present which are likely to affect the result with errors ; 
and then ascertain whether the errors will increase or diminish the result. Then make 
another determination in which the opposite effects can alone be produced. If the two 
results are the same, the determination was correct. 

The actual method used in finding the atomic weight of an element really 
requires : 

(1) An-exact analysis of a series of compounds containing the given element ; and 

consequently the compounds investigated must be such as lend themselves 
to exact analysis, and which can be prepared in a highly purified condition. 

(2) It is an advantage if the compound be volatile without decomposition, so 

that its vapour density can be determined. There are several other 
methods of computing the molecular and hence also the atomic weights of 
the different elements ; and in several, the compound need not be 
volatile. Fortunately, atoms and molecules possess other qualities besides 
mass, which are dependent upon their atomic weights and which can be 
readily measured. Some of these will be described later. 

(3) The smallest proportion of the element under investigation contained in all 

the compounds whose molecular weights are known is finally selected as 
the atomic weight of the given element. 

J. A. Wanklyn (1894) ^ once claimed to have discovered a series of hydrocarbons, 
one member of which contained carbon 102 parts by weight, and hydrogen 17 parts, 
and had a vapour density of nearly 116 (hydrogen 2). Assuming the atomic weight 
of carbon is 12, and of hydrogen 1, these numbers give formula C8.5H17. If this 
statement had been corroborated, and we were quite sure that Wanklyn's hydro- 
carbons were not mixtures, it would be necessary to make the atomic weight of 
carbon = 6, and write the formula of the compound in question C17H17, and this 
in spite of the fact that thousands of compounds of carbon are known, and all agree 
with the number 12 for the atomic weight of carbon. The formula of carbon 
monoxide— CO— would then be written CgO, etc.— but J. A. Wanklyn's claim has 
never been established. 

These remarks emphasize the importance of examining as large a number of 
volatile compounds as possible when fixing the atomic weight of an element. The 


importance of this principle was recognized as early as 1859, for F. A. Kekule then 
wrote : 

It is of exceptional importance for chemists to determine the relative masses of particles 
which are not subdivided in chemical reactions. In order to determine the atomic or 
molecular weights of the elements and their compounds with some degree of probability 
It IS necessary to investigate a very great number of compounds and a very great number 
of chemical reactions. 

If only a small number of compounds be examined, there is always a possibility, 
and perhaps a probability, that the actual minimum weight does not occur amongst 
the set of compounds taken. It follows, therefore, that the atomic weight of an 
element is the least amount of that element— relative to the standard 
oxygen, 16— which is present in any molecule of all its known volatile 
compounds. The value so obtained is the maximum possible value ; the real value 
may afterwards prove to be a submultiple of this. The atomic weight must be 
equal to a whole multiple or submultiple of its combining weight. Owing to the 
fact that the molecular weights of so many volatile compounds of carbon are known, 
it is not very probable that the atomic weight of carbon is less than. 12. 

1 J. A. Wanklyn, Chem. News., 70. 89, 147, 1894 ; F. A. Kekule, Liehig's Ann., 106. 129, 1858. 

§ 6. Methods for Measuring the Vapour Densities of Gases, and of Volatile 

Liquids and Solids 

The history of science shows that even during that phase of her progress in which she 
devotes herself to improving the accuracy of the numerical measurement of quantities 
with which she has long been familiar, she is preparing the materials for the subjugation 
of new regions, which would have remained unknown if she had been contented with the 
rough methods of her early pioneer.— J. C. Maxwell. 

When determinations of molecular weights are made to decide between quantities 
widely different, minor corrections, necessary for exact values, are not required. 
For instance, if chemical analysis showed that the molecular weight of a compound 
is some multiple of 20, then a molecular weight of 83, by vapour density methods, 
indicates that 4x20=80 is the molecular weight of the substance. With ordinary 
vapour density determinations, therefore, the weight of 22 '4 litres of the gas or 
vapour at 0° and 760 mm. is to be computed from measurements with hydrogen = 2 
or oxygen = 32 as standards of reference. No new principle is involved. If an 
intermediate value between two possible values for the molecular weight of a sub- 
stance is consistently obtained, there is a disturbance — ^possibly association or 
dissociation — which must be investigated more closely. 

These remarks do not apply when the molecular weights of gases are estimated 
from their densities in order to serve as a control for the atomic weights. The 
densities are then determined with as great an accuracy as possible. In the fourth 
century B.C., Aristotle made an unsuccessful attempt to determine the weight of 
air contained in a bladder ; and, about 1632, G. Galilei established the fact that air 
has weight ; R. Descartes (1638) said that G. GaUlei's primitive method of weighmg 
air rCest pas mauvaise. Robert Boyle, in his Hydrostatics (Oxford, 1666), gives the 
specific gravity of air 0-00125— with water unity as standard. The air of tartar, which 
consists of a mixture of carburetted hydrogen and carbonic acid gases, was weighed 
in a bladder by S. Hales,i and he compared the weight so obtained with that of 
the same bladder filled with air ; F. Hauksbee determined the specific gravity of the 
mixture of carbon dioxide and nitrogen obtained by passing air over red-hot iron. 
The specific gravity of these mixed gases was so near that of air that the ex- 
perimenters, by their methods, did not establish a difference. J. Mayow supposed 



but did not prove that the nitro-igneous constituent of air was heavier than the resi- 
dual air from which it was separated. H. Cavendish, however, in his Exferiinents 
on Factitious Airs, in 1766, first estabhshed the difference in the specific gravities 
of air, carbon dioxide, and hydrogen, and this has been cited as the first conclusive 
proof of a pluraUty of elastic fluids. J. Priestley tried to weigh the different 
kinds of air in glass flasks by the displacement of water in a pneumatic trough, 
but the drops of water which adhered to the inside of the flask introduced too 
many errors. J. Priestley then used a bladder, and added that although the 
determination " cannot be done with precision in a bladder, as used by Mr. 
Cavendish, because the degree of distension cannot be measured with much accuracy, 
yet the circumstance is more than counterbalanced by being able to change the 
air, with compressing the bladder, without wetting it." J. Priestley found the 
bladder filled with — 

Phlogisticated air weighed 
Nitrous air 
Common air . 
Dephlogisticated air 




The early chemists apparently thought the determination of the density of a 
gas to be so simple an operation that details would be redundant ; and they con- 
sidered it was necessary merely to weigh a bladder or a flask first evacuated, and then 
filled with the required gas. Towards the end of 1779, F. Fontana 2 devised a much 
better method of measuring the specific gravities of different gases. 

The stoppered globe A, Fig. 1, of known capacity is unscrewed from the gas stoppered 
receiver B, exhausted, weighed, and again screwed to the receiver ; meanwhile, the 
receiver is filled over a mercury pneumatic trough with the gas 
under investigation. The stopcocks are opened, the cylinder 
B depressed in the mercury until the surface of the mercury 
is the same inside and outside the cylinder. The stopcocks are 
then closed, the difference of the two weighings is taken to 
represent the weight of the gas in the globe. This result, divided 
by the capacity of the vessel expressed in cubic inches, gives the 
weight of a cubic inch of the gas in question. 

J. B. Biot and F. J. Arago (1806) determined the density of 
undried gases by means of a globe between 5 and 6 litres 
capacity. The results were corrected for the air displaced 
by the globe ; the residual air in the evacuated flask ; the 
cubical expansion of glass ; and the hygroscopic moisture 
in the gas. The results were reduced to normal tempera- 
ture and pressure, to sea-level, and to a latitude of 45°. 
For a long time J. B. Biot and F. J. Arago's measurements 
were considered to be a model for the work of others. 
J. J. Berzelius and P. L. Dulong (1820) and J. B. A. Dumas and J. B. J. D. 
Boussingault (1841) followed J. B. Biot and F. J. Arago's method, but they dried 
the gases. 

A new era was inaugurated by H. V. Regnault in 1847. He introduced many 
vital improvements in J. B. Biot and F. J. Arago's procedure — chiefly in the use of 
a counterpoise balloon, and in the filling and exhausting of the globes while they 
were surrounded by a bath of melting ice. Modern work follows closely on the 
lines marked out by H. V. Regnault. Every known precaution which will conduce 
to the accuracy of the result is taken : (1) Attention is paid to the extreme purifica- 
tion of the gases to be measured ; (2) the difference in the buoyancy in air of the 
weight and of the substance to be weighed is eliminated by reducing the weighings 
to the vacuum standard ; (3) Lord Rayleigh's correction (1893) for the difference 
in the volume of the evacuated and filled balloon holding the gas is applied ; (4) the 
expansion of the glass with variations of temperature is considered ; (5) corrections 
are made for the deviations of the gas under investigation from Boyle's and Charles' 

Fig. 1. — Fontana's Ap- 
paratus for Measuring 
the Density of Gases. 


laws ; (7) an allowance is made for a slight condensation of gas on the inner walls 
of the measuring vessel ; etc. In measuring the relative density of a substance 
which is gaseous at ordinary temperatures, three methods are available : 

A. Weighing a known volume of the gas. The balloon method was worked out by 
H. V. Kegnault (about 1847), and it has been much used in more recent work, where 
the general tendency has been to reduce the size of the balloons. H. V. Regnault 
worked with balloons about 10 litres capacity ; E. W. Morley (1896) 3 with balloons 
8-21 litres capacity; A. Leduc (1897), 23 litres ; Lord Rayleigh (1888-95), 
1-8 litres ; P. A. Guye and C. Davilla (1905) used globes of capacity 0-38 to about 
0-82 litre for nitric oxide ; E. P. Perman and J. H. Davis (1906), 0'5Htre ; and R. W. 
Gray (1905), 0-267 litre. The determinations made with small balloons are quite 
as concordant among themselves as those made with balloons of larger volume. 

In this method the glass globe of volume v is counterpoised on the balance by a second 
tare balloon of approximately the same volume so as to eliminate corrections necessary 
for the buoyancy of the air. By repeated exhaustions and re-fillings, the balloon is filled 
with the gas under investigation. The temperature and pressure are respectively 6 and p. 
Let w denote the difference between the weights of the full and empty balloon. The volume 
Vq of the gas at 0° and 760 mm. pressure is first calculated in the ordinary manner^: 

( p \( 273 \ 0-3592t;« 

From Avogadro's hypothesis the molecular weight of a gas represents the weight 
of 22*3 litres of a gas if hydrogen = 2 be taken as the standard. Consequently, 
if w grams of a gas occupy v^ c.c. at 0° and 760 mm. pressure, 22,300 c.c. will weigh 
22,300«^-^^i grms., and this represents the molecular weight, or the uncorrected 
relative density of the gas, hydrogen = 2. For a high degree of accuracy, it is of 
course necessary to include correction terms as indicated above. 

Examples. — (1) 585 c.c. of carbon dioxide measured at 18° and 756 mm. pressure 
weighed 1 '076 gram. What is the molecular weight of the gas ? 685 c.c. of gas become, 
at 0° and 760 mm., 546-1 c.c. Hence, the molecular weight is 22,300 X l-076-f546-l=43'9. 

(2) H. V. Regnault (1845) filled a 10-litre globe with air at a pressure of 761-19 mm. 
at the temperature of melting ice. In addition to the tare balloon 1487 grms. were required 
to balance the globe. The globe was then exhausted to a pressure 88-43 mm., and 
14-141 grms. were now required to restore equilibrium. The globe was then filled with 
dry oxygen at 0° and 750*22 mm. pressure, 0-172 grm. was needed in addition to the tare 
to balance the globe. The globe was then exhausted to 4-59 nun. pressure and weighed, 
again 14-033 grms. were required. The globe lost 12-654 grms. of air at 761-19 — 8-43 
= 752-76 mm. pressure and 0°. This corresponds with 12-776 grms. of air at 760 mm. 
Similarly with the oxygen : 14-1281 grms. at 760 mm. and 0°. The weights refer to equal 
volumes, and therefore the relative density of the oxygen (air unity) is 14-1281^12-776 
= 1-10563. 

B. Measuring the volume of a known weight of the gas. — The volume occupied by a 
known weight of gas is measured in a suitable voluminometer, and the gas required to 
fill the balloon is weighed in another vessel either («) by finding the loss of weight 
due to the escape of gas from the generating apparatus, or {h) by absorbing the gas 
in suitable apparatus. In the former case, given the temperature and pressure of 
the confined gas, the capacity of the balloon, and the loss of weight in the vessel 
from which the balloon was filled, the density follows directly. This method was 
used by E. W. Morley (1896) for hydrogen, and by A. Jaquerod and A. Pintza (1904) 
for sulphur dioxide. In a variation of this procedure, the measuring vessel is filled 
with the purified gas and its temperature maintained at 0°, while the pressure 
(approximately 760 mm.) is determined. The gas is then absorbed in a suitable 
apparatus previously evacuated and connected with the voluminometer by a tightly 
fitting joint. The weight of the absorbed gas completes the required data. This 
method was used by P. A. Guye and A. Pintza (1904-5) for nitrous oxide, carbon 
dioxide, and ammonia ; by E. P. Perman and J. H. Davis (1906) for ammonia ; 
and by R. W. Gray and E. P. Burt (1909) for hydrogen chloride. 

0. Measuring the buoyancy of the gas in atinosfheres at a known pressure.— 
A good analytical balance will indicate 00001 grm. when carrying a load of 100 grms. ; 


the balances used for assaying will indicate O'OOOOl grm. with a load of 10 grms. 
The sensibility of instruments for detecting variations of mass has been so sharpened 
that the latest form of micro-balance will carry a maximum load of 5x10"^ grms., 
and is sensitive to 3*3x 10"^ grm. The weighing of minute masses is called micro- 
weighing.* Probably the first micro-balance was made by E. Warburg and 
T. Ihmori in 1886. The beam of this balance was made of thin quartz rods to which 
were cemented razor knife-edges ; the deflections of the beam were read from a 
mirror and scale without taring the weights. The sensitiveness of this balance 
was about the same as the assay balances. In 1906, W. Nernst and E. H. Riesenfeld 
devised a torsion micro-balance in which a quartz fibre was cemented to the prongs 
of a vertical brass fork ; and a thin glass rod likewise fixed horizontally to the quartz 
fibre. One end of the rod is intended to serve as a pointer on a silvered scale, and the 
other carries a tiny pan. The load causes a slight torsion of the quartz fibre. The 
maximum load is 2 mgrm., and the lower limit of sensibility is 5xlO~^ grms. In 
another type, the principle of Archimedes is applied, and a gas manometer takes 
the place of a set of weights. This apparatus was improved by B. D. Steele and 
K. Grant (1909), and W. Ramsay and R. W. Gray (1911), so that a weight 00000001 
grm. can be accurately determined. In some of the improved forms a still greater 
sensibility has been attained. In this way, the density of less than 0"75 cubic 
millimetres of the emanation from radium has been determined, and the corre- 
sponding molecular weight computed from the result. This is a triumph of manipu- 
lative skill. 

If air at the same temperature and pressvire as the ambient atmosphere be confined in 
a quartz bulb, it will apparently weigh nothing, but if the outside air be reduced in density, 
the air inside the quartz bulb will appear to have a positive weight. Given the pressure 
of the ambient air, the weight of the confined air can be readily calculated as indicated 
in the subjoined example. The beam of the instrument is a framework of thin rods of 
silica arranged to swing on a central knife-edge resting on a central support. A scale-pan 
or bucket and a sealed air bulb of known volume are suspended from the framework by 
quartz threads, and coiinterpoised by a weight. All is enclosed in an air-tight metal case 
fitted with a mercury gauge. A mirror reflects a beam of light from the window to a 
scale a few feet away. The tube containing the gas \mder investigation is placed in 
the bucket, and the pressure noted at which the beam is in equilibrium. This is indicated 
by the spot of reflected light. The bulb containing the gas is broken, and all the glass 
splinters are placed in the bucket. The gas is removed by evacuating the metal case a 
few times. The pressure of the air again required to bring the spot of light to equilibrium 
is noted. Suppose, by way of example, that the pressure of the air required to bring a tube 
of xenon gas in the equilibrium position be 70 mm. ; and similarly the empty tube, 52-9 mm. 
The difference, 17*11 mm., corresponds with a weight 608 millionths of a milligram. A 
correction is required for differences in the weight of the glass vessel at pressures of 70 mm. 
and 52*9 mm. It is 15 millionths mgrm. Again, the effect of the reduced pressure on the 
buoyancy of the glass bulb and the silver counterj^oise is different. By substituting a 
counterpoise of silica the difference was found to be 91 millionths of a mgrm. Hence the 
weight of the gas in question is 608 — 91 + 15 = 532 millionths of a milligram. 

The micro-balance has been used by F. W. Aston (1914) to compare the densities 
of two gases. The gas to be investigated, density Z), is admitted to the balance 
case, and the pressure p determined at which the balance beam is in a given position. 
The corresponding pressure pi for a gas of known density Dj, say, oxygen is then 
determined. The densities of the two gases D and Di are inversely proportional 
to the pressure p and pi, or the density D of the required gas is PiDi/p. 

The vapour density of solids and liquids which can be vaporized without 
decomposition can be obtained by the following methods : 

A. Weighing a known volume of the vapour.— In J. B. A. Dumas' process (1826) ^ 
the substance is vaporized in a weighed glass bulb at atmospheric pressure. The 
bulb is then sealed up, and the weight of the vapour determined. The capacity of 
the bulb is then measured. From the resulting data, the vapour density of the 
gas follows directly. 

Example. — ^Tbe following data were obtained by H. E. Roscoe (1878) for vanadium tetra- 
chloride : Weight of globe filled with air (9°, 760 mm.), 24-4722 grams ; weight of sealed 


globe (9°, 7G0 mm.), 25'0102 grams ; temperature of bath when sealing the globe, 215" ; 
barometer when sealing the globe, 762 mm. ; and the weight of bulb full of water, 194 grams. 
The globe held less, 24-4722 = 169*5 grams of water at 9°. This represents very nearly 
169-5 c.c. of water, or the capacity of the globe is 1695 c.c. The apparent weight of the 
substance at 9° is 25-0102 —24-4722 =0-538 gram. The empty globe was buoyed up, during 
weighing, by its own bulk of air at 9° and 762 mm., and since 1 c.c. of air weighs 0-001293 
grams, 169*5 c.c. of air at 9° and 762 mm. weigh at (0-001293 X 169-5 x 273 X 762)-i-(760 
X 282) =0-213 gram. This added to 0-538 gram, gives 0-751 gram, the weight of the vapour 
in the globe at the time of sealing. The 0-751 gram of vapour occupied 169-5 c.c. at 215** 
and 762 mm. pressure, or 95*10 c.c. at 0° and' 760 mm. pressure. Hence, 22,300 c.c. of 
vapour at normal temperature and pressure weigh 176*1 grams. This number also repre- 
sents the molecular weight of vanadium chloride. 

Vessels made of porcelain have enabled H. St. C. Deville and L. Troost (1858), 
H. E. Koscoe (1878), and others to determine vapour densities by this process at 
temperatures far exceeding those at which even hard glass softens. The objection 
to Dumas' process is the amount of material which has to be vaporized in order 
to drive out the air from the bulb. This waste is avoided in the two succeeding 
methods — Hofmann's and Meyer's processes. By using porcelain or platinum 
vessels, Dumas' process has been employed for bodies volatilizing at high 

B. Measuring the volume of a known weight of the vapour. — J. L. Gay Lussac 
(1811) showed that the vapour density of a substance can be determined by 
measuring the volume of a known weight of the vapour in such a way that the 
volatile substance is confined in a small vessel of known capacity by means of 
mercury or any other substance which boils at a high enough temperature— 6.(7. 
Wood's fusible alloy. When the vessel has been heated the bath is removed. After 
cooling, the volume of the vapour at the highest temperature of the bath can be cal- 
culated from the weight of, sa.j, mercury remaining in the vessel. J. L. Gay Lussac's 
process was perhaps the oldest method used for measuring vapour densities. He 
placed a known weight of the substance under investigation in a graduated 
glass tube, about 40 cm. long, and filled with mercury. The tube dipped in 
mercury and was surrounded by a hot jacket so as to vaporize the substance. 
The temperature and volume of the confined vapour were measured. In A. W. 
Hofmann's process (1868) the measuring tube is over 760 mm. in length. 

Example. — -The following data were obtained for carbon tetrachloride, CCI4 : Weight 
of liquid in bulb, 0-3380 grm. ; the volume of vapour, 109-8 c.c. ; the temperature of vapour, 
99-5° ; the barometer, 7469 mm. ; and the height of mercury in tube, 283*4 mm. The 
pressure of the vapour is the barometric height less the weight of the column of mercury 
in the Hofmann's tube, that is, 746-9 — 283-4^463-5 mm. Hence, 0-3380 gram of vapour 
at 99-5° and 463-5 mm. pressure occupy 109-8 c.c, and 49-09 c.c. at 0° and 760 mm. Hence, 
22,300 c.c. of the vapour at normal temperature and pressure weigh 153-6 grams, and this 
number represents the molecular weight of carbon tetrachloride. 

A. W. Hofmann's process is useful when only a small amount of the substance 
is available for a determination ; and for a substance which decomposes when 
heated at a temperature in the vicinity of its boiling point at ordinary atmospheric 
pressures. In V. and C. Meyer's process (1877) « the apparatus is simphfied by 
measuring the volume of air displaced by a given weight of the substance 
vaporized in a suitable vessel. 

If the substance be vaporized too slowly, vapour will be carried forward with 
the expelled air, and be condensed, thu» reducing the volume of air (or gas) measured 
in the gas burette. It is considered that the vaporization vessel should be at least 
30° above the boihng point of the substance in order to secure rapid vaporization. 
If the gas be collected over water instead of over mercury and is filled with ordinary 
moist air instead of with dry air, a correction for the pressure of aqueous vapour may 
be applied. If air contains a per cent, of moisture ; and / denotes the pressure of 
aqueous vapour at the room temperature ; and p, the barometric pressure, the 
actual pressure of the confined gas is taken to be p-(l— r^o«)/ This refinement is 
usually ignored. 


Example." — -The vapoiir density of water was determined, and the following data were 
obtained. Xylene, boiling at about 138°, was used in the hot jacket E. It was found that 
the weight of the water in the stoppered tube was 0*0102 grm. ; the temperature of the 
gas in the biirette, 16*5° ; the barometer, 7038 mm. ; and the volume of gas, 16'6 c.c. 
The 16-6 c.c. of vapour at 16-5° and 7038 mm. becomes 14*496 c.c. at 0° and 760 mm. 
This is the volmne of 0*0102 gram of vapour. Hence, 22,300 c.c. of the vapour will weigh 
15*7 grams. This number represents the molecular weight of water vapour. 

V. Meyer's apparatus has been modified in various directions without altering 
the fundamental principle. J. S. Lumsden (1903) proposed a modification in 
which the increase of pressure was measured while the volume of the apparatus 
was kept constant. Glass vessels are suited for this determination only at com- 
paratively low temperatures ; vessels made of hard porcelain have been used by 
J. Mensching and V. Meyer (1886) for temperatures up to about 1500° ; platinum, 
platinum-iridium alloy, vitreous siUca by J. Dewar and A. Scott (1879), L. F. 
Nilson and 0. Pettersson (1886), and by J. Mensching and V. Meyer (1886) ; and 
vessels of iridium fined inside and outside with a magnesian cement, and heated 
in an electric furnace, enabled W. Nernst (1903) and H. von Wartenberg (1908) to 
measure vapour densities at temperatures as high as 1800° and even 2000°. 


1 S. Hales, Vegetable Staticks, London, 185, 1727 ; F. Hauksbee, Phil. Trans., 25. 2409, 1707 ; 
J. Mayow, De parte aerea igneaque spiritus nitro, Oxford, 1669 ; H. Cavendish, Phil. Trans., 55. 
141, 1766 ; J. Priestley, Experiments and Observations on the Different Kinds oj Air, London, 
2. 93, 1790. 

2 L. Cavallo, A Treatise on the Nature and Properties of Air, London, 422, 1781 ; J. B. Biot 
and F. J. Arago, Mem. Acad., 301, 1806; J. J. Berzelius and P. L. Dulong, Ann. Chim. Phys., 
(2), 15. 386, 1820 ; J. B. A. Dumas and J. B. J. D. Boussingault, ib., (3), 3. 267, 1841 ; Liebig's 
Ann., 40. 230, 1841 ; H. V. Regnault, Mem. Acad., 21. 25, 1841 ; Ann. Chim. Phys., (3), 5. 52, 
1842 ; P. A. Guye and C. DaviUa, Mem. Soc. Phys. Geneve, 35. 615, 1908 ; E. P. Perman and 
J. H. Davis, Journ. Chem. Soc., 90. 743, 1906 ; R. W. Gray, Journ. Chem. Soc, 87. 1601, 1905. 

3 Lord Rayleigh, Chem. News., 58. 52, 1888 ; Phil. Trans., 196. A, 205, 1901 ; 198. A, 417, 1902 ; 
Zeit. phys. Chem., 37. 713, 1901 ; 41. 71, 1902 ; 52. 705, 1905 ; Proc. Roy. Soc., 73. 153, 1904 ; 
E. W. Morley, Amer. Journ. Science, (3), 41. 220, 276, 1891 ; A. Leduc, Campt. Rend., 123. 743, 
1896 ; 125. 297, 571, 646, 1897 : 126. 413, 1898 ; Ann. Chim. Phys., (7), 15. 5, 1898 ; Journ. Phys., 
(3), 7. 5, 189, 1 898 ; P. A. Guye and A. Pintza, Mem. Soc. Phys. Geneve, 35. 594, 1908 ; A. Jaquerod 
and A. Pintza, ib., 35. 589, 1908 ; E. P. Perman and J. H. Davis, Journ. Chem. Soc., 90. 743, 
1906 ; R. W. Grav and F. P. Burt, ib., 96. 1633, 1909 ; W. Nernst, Zeit. Electrochem., 10. 629, 1904. 

* E. Warburg and T. Ihmori, Wied. Ann., 27. 481, 1886 ; 31. 100, 1887 ; H. Petterson, Ein 
neue Microunge und ihre Anwendung, Stockholm, 1914; F. W. Aston, Proc. Roy. Soc., 89. A, 
439, 1914 ; W. Ramsay and R. W. Gray, ib., 84. A, 53, 1911 ; B. D. Steele and K. Grant, ib., 82. 
A, 580, 1909; J. Kramer, Chem. Ztg., 41. 773, 1917; W. Nernst and H. Riesenfeld, Ber., 36, 
2086, 1903 ; W. Nernst, Golt Nachr., 2, 1902. 

* J. B.A.Bum&s, Ann. Chim. Phys., {2), 33. 341, 1826; C. W. Balke and E. F. Smith, Jowrn. 
Chetn. Soc, 94. 1043, 1908 ; H. St. C. Deville and L. Troost, C&tnpt. Rend., 46. 239, 1858 ; Liebig'e, 
Ann., 113. 42, 1860; H. E. Roscoe, Proc Roy. Soc, 27. 246, 1878; Ber., 11. 1196, 1878; 
A. W. Hofmann, Ber., 1. 198, 1867; J. L. Gay Lussac, Ann. Chim. Phys., (1), 80. 118, 1811 ; 
E. Ernyei, Zeit. anorg. Chem.., 25. 313, 1900. 

« V. Meyer and C. Meyer, Ber., 12. 2204, 1879 ; V. Meyer, ib., 9. 1216, 1876 ; 11. 2068, 
1878 ; A. Combes, Journ. Chem. Soc, 56. 571, 1889 ; J. S. Lumsden, ib., 83. 342, 1903 ; L. M. 
Dennis and H. Isham, Journ. Amer. Chem. Soc, 29. 18, 1907 ; J. Mensching and V. Meyer, Zeit. 
phys. Chem., 1. 145, 1887 ; J. Dewar and A. Scott, Proc Roy. Soc. Edin., 14. 410, 1887 ; L. F. 
Nilson and 0. Pettersson, Journ. prakt. Chem., (2), 33. 1, 1886; Zeit. anal. Chem., 27. 197, 1888; 
Ber., 17. 987, 1884; W. Nernst, Zeit. Electroch., 10. 629, 1904; H. von Wartenberg, Ber., 39. 
381, 1908. 

§ 7. The Struggle o! Avogadro's llypothesis for Recognition 

The first attempt at generalization seldom succeeds ; speculation anticipates experience, 
for the results of observation accumulate but slowly.- — J. J. Berzelius (1830). 

A. Avogadro's hypothesis had a long struggle for recognition in spite of the fact 
that his memoir was followed three years later by A. M. Ampere's note addressed 
to M. le Comte Berthollet and entitled, Sur la determination des proportions dans 
lesquelle.^ les corps se combinent, d'apres le nomhre et la disposition respective des 
molecules dont leurs particles integrantes sont composees (1814), advocating similar 


views. Half a century elapsed before the hypothesis was generally accepted. 
Among the many reasons for its failure was the fact that comparatively few sub- 
stances which could be vaporized were then known, and hence the molecular weights 
of but few compounds had been determined with precision. At that time, an 
accurate knowledge of the weights of the elements was considered to be the most 
pressing subject of investigation. J. Dalton's atomic theory had just been born, 
and accurate data were also needed before that theory could be utiUzed. Referring 
to J. Dalton's theory J. J. Berzelius said : 

I recognized that if the newly arisen light was to be spread, it would be necessary to 
ascertain with the utmost accuracy the atomic weights of all elementary substances. . . . 
Without such work, no light would follow the dawn. 

Something more than the mere accumulation of experimental data was necessary 
to find a method for determining the number of atoms in a molecule, in order that 
the atomic weight of the constituent elements could be obtained. Dalton 
pointed out that in fixing the atomic weight of oxygen with respect to hydrogen 
unity, he assumed that water is a binary compound of one atom of hydrogen and one 
of oxygen. If water be really a ternary compound containing two atoms of hydrogen 
and one of oxygen, it will be necessary to double the atomic weight of oxygen 
determined on the former assumption ; and if water contains two atoms of oxygen 
and one of hydrogen, the atomic weight of the oxygen would have to be halved. 
Similarly with other compounds. The uncertainties in the application of J. Dalton's 
atomic theory are due to the arbitrary nature of the assumption of the number of atoms 
in a molecule.^ 

W. H. WoUaston (1814).— In 1810, T. Thomson gave a list of the weights 
of various acids and bases which neutralized one another, and showed that these 
numbers were independent of the hypothesis of Dalton. W. H. WoUaston, in his 
paper A synoptic scale of chemical equivalents (1814), proposed to substitute the 
term equivalent in place of Dalton's atom. He claimed that his numbers were 
not warped by the uncertainties of the atomic theory, and that for practical pur- 
poses it is not necessary to know hypothetical atomic weights when equivalent 
weights are known. WoUaston thus proposed to do for the elements what J. B. 
Richter (1791-1802) had done for the acids and bases, and he accordingly used the 
term equivalent proposed by H. Cavendish in 1788. Starting with oxygen 10 
as the unit of reference, he found the equivalent of hydrogen to be TS — meaning 
that 1'3 parts of hydrogen unite with 10 parts of oxygen to form water. In this 
sense, equivalent weights are identical with combining weights. WoUaston, 
however, got into difficulties in deahng with substances like carbon with two com- 
bining weights, for he was obliged to assume that in carbon dioxide two equivalents 
of oxygen were united with one of carbon so that equivalents and combining weights 
were no longer the same. Since 7" 5 parts of carbon unite with 20 parts of oxy^gen 
by weight to form carbon dioxide, W. H. WoUaston said the formula of the compound 
is CO2 ; and because 75 parts of carbon unite with 10 parts of oxygen by weight 
to form carbon monoxide the formula is CO. W. H. WoUaston might just as 
arbitrarily have said 375 of carbon unite with 10 parts by weight of oxygen to form 
carbon dioxide, and the formula is accordingly CO ; and in carbon monoxide 7*5 
parts of carbon unite with 10 parts of oxygen by weight, and therefore the formula 
is C2O. Hence, W. H. WoUaston's equivalents leave the difficulty with J. Dalton s 
atoms just as it was. Ignoring this uncertainty, the former computed the 
equivalents of 12 elements, and 45 compounds from various analyses. A. Ladenburg 
(1869) 2 is severe on W. H. WoUaston, for he says that WoUaston's views involved 
a retrograde step, for W. H. WoUaston believed that he was deahng with un- 
ambiguous conceptions free from aU hypothesis ; and in introducing the term 
equivalent, he confused the conceptions of the equivalent and the atom, and 
rendered a vigorous struggle necessary before the two concepts could be ciaritied. 

J. J. Berzelius (1810-26).— J. J. BerzeUus seems to have regarded the investigation 
of the laws of combining proportions to be one of the most important objects 



of his life's work, and in a memorable work, Essai sur la theorie des frofortions 
chimiques et sur Vinjiuence chimique de relectricite, published in Sweden in 1814, and 
at Paris in 1819, he developed his conceptions of the atomic theory, and collected 
together the results of his arduous work on the combining proportions of the 
elements which he had published between 1810 and 1812. In his Lehrbuch der 
Cheynie (Dresden, 1825), J. J. Berzelius remarked that it did not matter much 
whether the particles which combine chemically be called atoms, molecules, chemical 
equivalents, or any other terms, but he preferred to use the term atom. In spite 
of Dalton's demonstration that Gay Lussac's law does not mean that equal 
volumes of elemental gases contain the same number of atoms, J. J. Berzelius used 
this erroneous Volumtheorie as a guide in determining the numerical values of the 
atomic weights of the elements which were gaseous at ordinary temperatures. He 
said, one volume of an elementary substance in the gaseous state corresponds with 
one atom, and he called the smallest particles Volumatome. This erroneous theory 
gave him satisfactory results in deducing the composition of water, and of the two 
carbon oxides ; and consequently also of the atomic weights of oxygen, hydrogen, 
and carbon. J. J. Berzelius (1818) was not so fortunate with some of the metallic 
oxides, particularly the sesqui-oxides. It is illogical, said he, to express the 
composition of a series of oxides of an element A by the formulae A2O2, A2O4, A2O6 . . ., 
and declared that in a series of compounds of the two elements one compound must 
always be represented as containing a single atom of one of the elements, and 
accordingly he wrote the formulae AG, AO2, AO3 ... If the simplest oxide were 
A2O2 Berzelius said that the atoms of the elements A would be divisible mechanically. 
J. Dalton saw the fallacy in this argument for fixing the number of atoms in a 
molecule of the solid oxides, but, led on by the erroneous argument, Berzelius 
(1818) wrote the formulae for ferrous oxide Fe02 instead of FeO ; and ferric oxide, 
FeOa instead of Fe203, and thus obtained numbers for the atomic weights of iron, 
chromium, etc., double the values of those now accepted. The chemical similarities 
of iron with chromium led him to symbolize chromic oxide CrOg by analogy with 
FeOa ; similarly he formulated the metal oxides Zn02, Mn02 • • • by analogy with 
FeOg. In 1826, however, he wrote — 

Assuming that chromic oxide contains three atoms of oxygen ; and that chromic 
anhydride contains six atoms, in forming neutral salts, chromic anhydride neutralizes an 
amount of base containing one-third as much oxygen as it itself contains. By analogy 
with sulphuric anhydride, and other anhydrides with three atoms of oxygen, it is most 
probable that chromic anhydride contains three atoms of oxygen to one of chromium, 
consequently chromic oxide will contain three atoms of oxygen to two of chromium, and 
the formulae for chromic oxide will be CrjOg, and for chromic anhydride, CrOg. The 
isomorphic law will then make ferric oxide, FcgOg, instead of FeOg ; and aluminium oxide, 
AI2O3, instead of AIO3. 

These considerations led J. J. Berzelius in 1826 to halve the atomic weights of the 
metals published in his 1818 Table of Atomic Weights. The following table 
represents atomic weights of a few elements selected from Berzelius' 1818 and 
1828 Tables, and recalculated for the standard oxygen 16 instead of 100 used by 
Berzelius. The modern values also are given by way of comparison. 

Table III. — Berzelius' Atomic Weights. 


































Berzelius never succeeded in deciding whether the binary compounds he analyzed 
contained two or more atoms per molecule. Without any rules to guide him, said 
J. B. A. Dumas (1832), but guided mainly by analogies, he fixed by intuition the 
atomic weights of a number of elements which subsequent experience has only 
tended to confirm. When all is said, however, this method is unsatisfactory, because 
it is arbitrary, and liable to be capriciously modified by each worker. In addition 
to his volume law, Berzelius also used Dulong and Petit's rule of specific heats, and 
Mitscherlich's isomorphic law to assist him in fixing the atomic weights of the 

J. B. A. Dumas (1826-37).— In 1826, J. B. A. Dumas 3 pubUshed an important 
memoir Sur quelques points de la theorie atomistique : 

The object of these researches is to replace by definite conceptions, the arbitrary data 
on which nearly the whole of the atomic theory is based. 

Dumas accepted the hypothesis of Avogadro as propounded by Ampere, namely, 
that equal volumes of gases contain an equal number of particles, and in the case 
of the simple gases, that these particles are subdivided during chemical reactions. 
J. B. A. Dumas thus recognized the importance of measuring the relative densities 
of gases and vapours, and he devised his well-known method for determining these 
constants. In 1832, he had determined the relative vapour densities of mercury, 
iodine, phosphorus, sulphur, arsenic, etc., and noted some irregularities with sulphur, 
mercury, phosphorus, and arsenic. He spoke of un demi-atome in the same way 
that Avogadro spoke of une demi-molecule. J. J. Berzelius appears to have been 
obsessed by his dualistic theory {q.v.), in which he assumed that the ultimate 
particles (molecules) of elementary substances cannot be split when they form a 
binary compound ; so that he wrote the formula of hydrogen chloride H2CI2 ; or 
else he misunderstood J. B. A. Dumas, owing to the confusion of the words atom 
and molecule, and was led to say (1826) : ^ "It is usually supposed that an 
hypothesis ought to be abandoned as soon as it leads to an absurd conclusion; " 
and if Avogadro' s hypothesis involves the subdivision of an atom, it must be 

In his Legons sur les philosophie chimique (Paris, 1837), J. B. A. Dumas appUed 
Avogadro's volume law — equal volumes, an equal number of atoms — to explain the 
formation of hydrogen chloride, HCl, and nitric oxide, NO, from the elementary 
gases, and he showed that the physical atoms must be spHt during the reaction. 
Hence, said J. B. A. Dumas, la chimie coupait les atonies que la physique ne pouvait 
pas couper, so that Avogadro's molecules are Dumas' physical atoms. Here 
again there was some confusion owing to the unfortunate use of the word atom 
in place of Avogadro's molecule. J. B, A. Dumas then went on to show that 
while Avogadro's volume law gives satisfactory values for the atomic weights 
of oxygen, nitrogen, chlorine, bromine, and iodine, difficulties are encountered 
with phosphorus, arsenic, mercury, and sulphur. For instance, ammonia is formed 
by the union of three volumes of hydrogen and one volume of nitrogen ; and 
phosphine, a similar gas, is presumably formed in a similar manner so that phosphine 
should be produced by replacing the nitrogen of ammonia by an equal volume of 
phosphorus gas. Consequently, it was argued that the density of phosphorus 
vapour ought to be 31*4 (hydrogen unity— in the original, oxygen 100 is the 
standard of reference) ; experiment gives a number twice as great ! A similar 
discrepancy was found with arsenic. This can only mean that equal volumes of 
the vapours of nitrogen, phosphorus, and arsenic do not contain the same number 
of atoms. Again, J. B. A. Dumas showed that about 200 parts of mercury unite 
with 16 parts of oxygen to form mercuric oxide, and therefore the atomic weight of 
mercury must be nearly 200 ; but judging from the density of mercury vapour, 
the atomic weight of this element is nearly 100. Consequently, le chaleur diviserait 
les particles du corps plus que V action chitnique. Equal volumes of gases sometimes 
contain an equal and sometimes an unequal number of atoms, and therefore the 


determination of the densities of vapours cannot be a trustworthy guide in evaluat- 
ing the atomic weights of the elements. The facts seemed to be against Avogadro's 
hypothesis and J. B. A. Dumas accordingly gave it up in despair. He then tried 
an application of Dulong and Petit's rule, but here again he was disappointed with 
the exceptions ; and finally, after trying Mitscherlich's isomorphic rule, he said, 
tout considers, la theorie atomique serait une science purement conjecturale, si elle ne 
s'appuyait pas sur Visomorphisine. 

W. Prout (1833) and A. Gaudin (1835).— W. Prout, in his work Chemistry . . . 
considered with reference to Natural Theology (London, 122, 1833), adopted the 
hypothesis of Avogadro's, viz. " under the same pressure and temperature, all 
bodies in a perfectly gaseous state contain an equal number of self-repulsive mole- 
cules," to explain the volume relations of hydrogen, oxygen and water, and of hydro- 
gen, chlorine, and hydrochloric acid. W. Prout's explanation is almost as clear 
as if it had been written to-day. From the observed results, said he, 

It follows irresistibly that every self -repulsive molecule of oxygen has been divided into 
two, and consequently must have originally consisted of at least two elementary molecules, 
somehow or other associated, so as to have formed one self-repulsive molecule. 

M. A. Gaudin, in his papers Recherches sur la structure intime des corps inorganiques, 
published in 1833, had previously pointed out that J. B. A. Dumas' difficulty with 
mercury and phosphorus could be explained by assuming that the mercury molecule 
is monatomic, and that of phosphorus tetratomic ; evidently J. J. BerzeHus did not 
like this mode of evading the discrepancy observed by J. B. A. Dumas, and con- 
sidered it to be nur ein Spiel der Phantasie, although M. A. Gaudin's suggestion is 
now generally accepted ; so also is Gaudin's happy use of the terms mono-, di-, tri-, 
. . . atomic for indicating the number of atoms in a molecule. 

Failures with Avogadro's hypothesis. — Towards the middle of the nineteenth 
century, as a result of these failures to apply Avogadro's hypothesis, the atom was 
abandoned by the majority of chemists as a discredited theory. In illustration, 
J. B.A.Dumas said in 1837: Si fen etais lemaUre,feffacerais le motatome de la science. 
WoUaston's equivalents were used, notably by L. GmeHn in his popular Handbuch 
der theoretischen Chemie (Frankfurt-am-Main, 1817-9). In the early editions of this 
book Gmelin used the term Mischungsgewichte — mixing weights — and in a later 
edition (1843), the term stoichiometric numbers in place of equivalents. Gmelin 
said, if an atom is the smallest quantity of a body which enters into combination, 
the equivalents must represent atoms ; the atomic notation of Dalton is based on 
hypothesis, equivalents are a reality. The inconsistencies involved in W. H. 
WoUaston's equivalents were thus ignored. The whole subject at this time (1840-50) 
was in a very confused state. In addition to the muddling of the terms atom, 
molecule, and equivalent, there were tables of atomic weights, equivalents, H. V. 
Kegnault's equivalents thermiques (1849) based upon Dulong and Petit's rule ; 
H. Kose's (1857) and J. C. G. de lAa.T\giidi,G' ^equivalents isomorphiques (1855) based upon 
Mitscherlich's rule ; and M. Faraday's electrochemical equivalents (1834). ^ Different 
chemists used different standards for their equivalent and atomic weights. The 
same chemical formula was used for different compounds, and different formulae 
for the same compound — for instance, F. A. Kekule in his Lehrbuch der organischen 
Chemie (Stuttgart, 1861) indicated nineteen different formulae which had been 
proposed for acetic acid. Inorganic chemists thus failed to establish the conception 
of an atom, but fortunately organic chemists had begun to see more clearly. 

C. F. Gerhardt and A. Laurent (184^56).— In 1842, in a memoir entitled Re- 
cherches sur la classification chimique des substances organiques, C. F. Gerhardt was 
groping for a method of distinguishing between equivalent and atomic weights, and 
he put forward some important views respecting the equivalents of certain elements 
taking part in organic reactions. He showed that when an organic reaction gave 
rise to water, carbon dioxide, carbon monoxide, or ammonia, the smallest amounts 
produced are those represented by the formulae H4O2, C2O4, C2O2, S2O4, NH3 


respectively, on the assumption that the equivalent or atomic weights are H=l, 
0=8, 0=6, S=16, N=14:. Hence, the quantities indicated by the formulae must 
represent an equal number of equivalents. It seems strange, said he, that no reaction 
in organic chemistry can give rise to less than a single molecule of water, H4O2, 
or carbon dioxide, C2O4 ; and that these quantities of gases occupy equal volumes. 
Consequently, H4O2 and C2O4 represent either one or two equivalents ; the former 
hypothesis fits the facts best, and therefore he argued that the equivalents of the 
elements 0=8, C=6, and S=16 should be doubled so that the preceding formulae 
can be written H2O, CO2, CO, SO2, and NH3 respectively. C. F. Gerhardt thus 
obtained numbers for the equivalents of the elements, hydrogen, oxygen, carbon, 
sulphur, and nitrogen in agreement with the atomic weights used by BerzeUus in 

C. F. Gerhardt also advocated the adoption of a common standard for comparing 
chemical formulae with one another, and he recommended the use of what is known 
as the two-volume standard : those quantities by weight which occupy two volumes 
when in the gaseous state and when the volume of atomic hydrogen is taken as unity. 
Hence, Avogadro's hypothesis is sometimes called the Avogadro-Gerhardt law. In 
a later part of his paper, Gerhardt showed that his notions of atomic weights, the 
volume theory, and equivalents were not clear, because he stated that all these 
concepts coincide. A. Laurent (1846), however, obtained a clear grasp of the 
meanings to be attached to these terms, and he adopted Gerhardt's happy idea that 
chemical formulae should represent comparable quantities ; he also adopted the 
two-volume standard, but in doing so he was obliged to admit that there are some 
exceptions — e.g. the vapour of ammonium chloride, and sulphuric acid — which 
seemed to correspond with a four-volume standard. The names of Laurent and 
Gerhardt are usually linked together ; it is, indeed, difficult to isolate the particular 
contributions made by each because they published a great deal jointly, and, being 
intimate friends, they probably discussed the whole subject together. A. Laurent's 
posthumous Methods de chimie (Paris, 1854) and C. F. Gerhardt's Traite de chimie 
organique (Paris, 1856), ^ did much to clear the conceptions of equivalents, atoms, 
and molecules ; and their definitions of these entities, and most of their formulae 
for organic and inorganic compounds are virtually in use to-day. Laurent repre- 
sented the union of hydrogen and chlorine by the equation (HH)-|-(C1C1) 
=(HC1)+(HC1) ; to-day we write, H2+Cl2=2HCl ; similarly the synthesis of 
water was symbolized, (HH)+(HH)+(00)=(HH)0-f (HH)0 ; to-day we write, 
2H2H-02=2H20. A. Gaudin (1832) employed special diagrams to symbolize these 

Several other systems of symbolizing chemical operations have been proposed. A. C. 
Brown (1867), for example, used Greek letters to represent different chemical action, thus 
(f> represented the replacement of hydrogen in a molecule by the radicle CHg.and if o denotes 
a molecule of ammonia, NH3, the symbol <f)a represented the substitution of one hydrogen 
atom in ammonia to form CH3NH2. In view of the great variety of chemical processes 
and compoimds, such a system would be more cumbrous and throw greater strains on the 
memory than the present system. B. C. Brodie proposed a new notation in his Calculus 
of Chemical Operations ( 1867), which he regarded as " a rigid expression of fact, independent 
of the atomic hypothesis. B. C. Brodie's system, however, involved assumptions even 
more drastic than the atomic theory, and the notation was so confusing that it died as soon 
as it was born. 

S. Caimizzaro (1857-8).— In 1857, S. Cannizzaro stated his belief : 

There are no exceptions to the universal law that equal volumes of f «««j;°"*^*j^^ 
numbers of molecules, and that the apparent exceptions wiU disappear when more searching 
experiments are made. 

He showed that the apparent exceptions to C. F. Gerhardt's two-volume law.pointcd 
out by A. Laurent, are not real, for the work of H. St. C. Dev.Ue (1857) has shown 
that ammonium chloride and sulphuric acid are decomposed by heat, and theretore 
the observed vapour densities are the densities of mixtures of the decomposition 


products and not of homogeneous compounds to which Avogadro's hypothesis alone 
refers. Similar conclusions were deduced independently and almost simultaneously 
by H. Kopp (1858) ^ and F. A. Kekule (1858). Immediately afterwards, S. Canniz- 
zaro pubUshed his celebrated Sunto di un corso di Jllosofia chimica fatto nella Reale 
Universita di Genova (1858), which placed Avogadro's hypothesis at the foundation 
of the system of chemistry which obtains to-day — witness, among other works, 
W. Nernst's popular Theoretische Chemie vom StandjmnJcte der Avogadroschen Regel 
und der Thermodynamik (Stuttgart, 1916). The atomic theory of the present-day 
chemistry is the work of many minds. In the words of G. Chrystal (1885) : 

Few scientific ideas spring up suddenly without previous trace or history ; a close 
examination always shows that the sprite was in the air before the Prospero came to catch 
him. . . . There are long periods in science in which great improvements were effected 
which cannot be traced to any individual, but seem to have been due merely to the working 
of the minds of scientific men generally upon the matter, one giving it this little turn, another 
that, in the main, always for the better. 


1 C. Graebe, Journ. prakt. Chem., (2), 87. 145, 1913 ; W. H. WoUaston, Phil. Trans., 104. 1, 
1814 ; T. Thomson, The Elements of Chemistry, Edinburgh, 1810 ; E. Hjelt, Berzelius—Liehig— 
Dumas. Ihre Stellung zur Eadikaltheorie, 1832-1840, Stuttgart, 1908 ; S. Cannizzaro, Historische 
Notizen und Betrachtungen iiber die Anwendung der Atomtheorie in der Chemie und uber die 
Systeme der Konstitutionsformeln von Verhindungen, Stuttgart, 1913 ; A. N. Meldrum, Avogadro 
and Dalton. The Standing in Chemistry of their Hypotheses , Edinburgh, 1904. 

2 A. Ladenburg, Vortrage vber die Entwickelungsgeschichte der Chemie in den letzten 100 Jdhren, 

» J B. A. Dumas, Ann. Chim Phys., (2), 49. 210, 1832 ; 50. 170, 1832. 

4 J. B. A. Dumas, Ann. Chim, Phys., (2), 33. 337, 1826. 

^ H. V. B-egnault, Cours elementaire de chimie, Paris, 1849 ; H. Rose, Pogg. Ann., 100. 270, 
1867; J. C. G. de Marignac, Archiv. Sciences Geneve, 2. 89, 1858; M. Faraday, Phil. Trans.. 124. 
77, 1834. 

^ A. Laurent, Ann. Chim. Phys., (3), 18. 266, 1846 ; A. Gaudin, Eecherches sur la structure 
intimes des corps inorganiques definis, Paris, 1832; Ann. Chim. Phys., (2), 52. 113, 1833; A. 0. 
Brown, Laboratory, 1. 37, 1867. 

' H. Kopp, Liebig's Ann., 105. 390, 1858 ; F. A. Kekule, ib., 106. 143, 1858. 

§ 8. Deviations from Avogadro's Law 

When a fact appears to be opposed to a whole train of deductions, it invariably proves 
to be capable of bearing some other interpretation. — Sherlock Holmes. 

It is sometimes said that a phenomenon " ought to take place," but it does not ; 
the phenomenon is then said to be abnormal or anomalous. These terms are not 
very happily chosen, and they are sometimes used rather carelessly ; they are not 
intended to imply that nature is erratic, arbitrary, and lawless. The words simply 
mean that in groping for the truth, an unexpected result has been obtained, which 
once stood, or now stands, challenging investigators to show how the unexpected 
should have been expected. In this sense it has been said that abnormal phenomena 
do not occur in nature. Some of the most treasured generalizations in science have 
been won by investigating the abnormal. This applies both in the laboratory and 
in the study. 

Abnormal vapour densities. — According to Avogadro's hypothesis, if the relative 
density of hydrogen be taken as unity, the quotient M/D=2, where M denoted the 
molecular weight, and D the relative density of the gas. Some puzzling exceptions 
to this rule were encountered during the early application of the hypothesis, for 
several substances do not conform to the ratio when molecular weights deduced 
by the ordinary chemical methods are employed, and, in consequence, these sub- 
stances were said to possess abnormal vapour densities. This led chemists to look 
upon Avogadro's rule with suspicion, and there were some controversies as to 
whether (i) substances with abnormal vapour densities really follow Avogadro's rule ; 


or whether (ii) substances with an abnormally low vapour density are dissociated 
into simpler molecules, and substances with an abnormally high vapour density 
are associated into more complex molecules. J. B. A. Dumas (1836) ^ thought 
that the abnormal vapour densities invahdated the hypothesis, while M. A. Gaudin 
(1833) considered that the alleged failure was due to a pecuHarity in the molecules 
of the gas, which, when taken into account, left the hypothesis quite valid. In- 
dependent proofs of the validity of M. A. Gaudin's inference are discussed later on 
when the particular substances are treated. As soon as M. A. Gaudin's interpre- 
tation had been demonstrated experimentally, Avogadro's hypothesis won its way 
into the heart of chemical science. 

There are two possible deviations with compounds ; the ratio 

Molecular weight , . , ., ^ 
— .^ — o — may be greater or less than 2 

when the density of hydrogen is taken imity. In the case of elementary gases, 
S. Cannizzaro (1858) showed that the atomic weight^ is equal to half the vapour 
density of the gas, if hydrogen 2 be the unit, or to the vapour density itself, ^/Z)=l, 
if hydrogen be unity. Here, again, there are two possible deviations : 

Atomic weight , , , ^t. -^ 

— =r rr— ^ — may be greater or less than unity 

when hydrogen unity is the standard of reference. The interpretation of the results 
in the two cases are similar. 

The molecules of the substance are decomposed or dissociated ; the molecules are 
actually less complex than corresponds with the simple chemical formulae, and the 
ratio MjD is greater than 2, or the ratio AjD is greater than unity. For example, the 
vapour density of steam is 9 (H=l), the molecular weight 18, and the ratio MID=2; 
at a very high temperature, there are reasons for supposing that the vapour density 
would be 6, and the ratio M/D would appear to be 3. This corresponds with the 
value of M/D on the assumption that the steam is dissociated into its elementary 
molecules : two volumes of hydrogen, and one volume of oxygen, so that the density 
of a mixture is involved and not that of a homogeneous substance as is required if 
Avogadro's rule is to be applied. The density of such a mixture will be (24-16)-^3 
=6 ; the assumed dissociation thus gives a number in agreement with observation. 
If the observed density were 8, this would represent a mixture with 33j per cent, 
of dissociated, and 66| per cent, of undissociated steam. The cases with phosphorus 
pentachloride, PCI5 ; ammonium chloride, NH4CI ; sulphuric acid, H2SO4 ; mer- 
curous chloride, HgCl ; nitrogen peroxide, N2O4 ; and hydrogen iodide, HI, are 
discussed later. With elementary gases, J. B. A. Dumas (1832) found that mercury 
vapour has a density of 100 corresponding with an atomic weight of 100, but the 
atomic weight deduced by chemical methods is 200, consequently v4/Z)=2 instead 
of 1. It is therefore assumed that the molecule of mercury vapour is monatomic 
and MID=2, while A/D=l. The cases with iodine, the metal vapours, etc., are 
discussed later. 

The molecules of the substance are associated or condensed ; the molecules are 
more complex than corresponds with the simple chemical formulae ; and the ratio 
M/D is less than 2, or the ratio A/D less than unity. The molecular weight of acety- 
lene, C2H2, is 26, the vapour density is 13, and the ratio M/D is normal. Benzene 
has exactly the same chemical composition, and its vapour density is 39 (H unity) ; 
if the molecular weights of the two gases be the same, the ratio M/D for benzene 
would be 0-67, but if benzene be more complex than acetylene, say (C2H2)3 or C,,H<,, 
the molecular weight of the complex molecule will be 78, and the ratio MJD becomes 
normal. Hence, for this and other reasons, benzene is regarded as if it were a 
product formed by the condensation of three molecules of acetylene. Phosphorus 
trioxide and pentoxide, and other examples, are discussed later. With elemental 
gases, J. B. A. Dumas (1832) found that the density of phosphorus vapour is 62*8, 

VOL. I. ^ 



and the atomic weight deduced by chemical methods, by analogy with nitrogen, 
is 31 "4, so that the ratio A/D is one-half. This is taken to mean that the molecular 
weight of phosphorus is not that equivalent to P2> ^^^ is rather equivalent to P4. 
Sulphur and arsenic are discussed later. 

The effect of changes in the molecular weight of a gas on the laws of Boyle and 
Charles. — The gas equation, 

must now be revised in order to allow for changes in the molecular weight of the gas 
when it changes from one state to another. Remembering that the density D of a 
gas is equal to the molecular weight M divided by the volume v, or M=Dv, the 
gas equation can be written, 

P _ n 

TD ~ T^D^ 

provided M=Mi. Let M, 7), and v respectively denote the molecular weight, 
density, and volume of the gas by one condition of temperature and pressure ; and 
Ml, Di, and %,the same constants for another condition of temperature and pressure, 
then, by substitution in a preceding equation, pvlMT=piVilMiTi. If the volume 
Vi at some standard temperature Ti and pressure pi be taken, the numbers pi, Vj, 
and Ti will always have one fixed value. Let R denote this constant value of 
PivJTi. The gas equation then assumes one of the forms : 


^£^^;oT,pv=^^RT',ov,pv = iRT 

where * stands in place of the ratio of the molecular weights of the gas in the two 
conditions, M/M^. If the molecules of the gas neither dissociate nor polymerize 
when the conditions change, M=Mi ; or pv=RT because i—\. Again, if the gas 
molecules polymerize or condense so that, say, two molecules combine together to 
form one molecule, there will be only half as many molecules in a given space as 
before : M=^Mi, and pv=\RT. If, however, the gas dissociates or decomposes 
so that each molecule of the gas forms two molecules of another gas or gases, then 
M=2Mi, and pv=2RT. Hence, the ordinary gas equation, pv=RT, is a special 
case of the more general relation, pv=iRT, where the numerical value of i 
indicates whether or not the gas keeps the same molecular concentration during 
the change. K i=l, there is neither dissociation nor polymerization ; if i be 
less than unity, the gas polymerizes ; and if i be greater than unity, the gas 
dissociates when the conditions are changed. 

The effect of deviations from Avogadro's hypothesis on Gay Lussac's law of 
volumes. — The molecular volumes of many gases are not all the same, and they 
thus exhibit small deviations from the law MID=2 (hydrogen unity). This is 
shown for a few gases at 0° and 760 mm. in the following table : 

Table IV.< — A Comparison of the Molecular Volumes of Some Gases. 



Observed density 


weight M. 


volumes M/D. 

Oxygen, Oj . 




Nitrogen, Ng . 




Carbon monoxide, CO 




Carbon dioxide, CO 2 . 




Methane, CH4 . 




Ethane, CjHe . 




Ethylene, C2H4 




Acetylene, C2H2 






In calculations involving gaseous volumes, the errors due to the deviations of 
the molecular volumes from the theoretical may be greater than the experimental 
errors. Instead of writing the reaction, 200+02=2002, in the form, 2 Vols. 
00+1 Vol. 02=2 Vols. CO2; it becomes necessary to write 2xO-994=l-988 
volumes of carbon dioxide, and the equation becomes 2 Vols. CO+1 Vol. 02=1 '988 
Vols. CO2. Similarly, with equations involving other discrepant gases. If the 
partial pressure of the deviating gas be less about 25 per cent., the discrepancy 
may be disregarded since the lower the partial pressure of the gas, the more 
nearly does it behave like an ideal gas. Thus, the lower the pressure confining 
carbon dioxide, CO2, at 20°, and of ethane, C2H6, at 0°, the more nearly do the 
molecular volumes approach the value 2 for ideal gases. 







760 mm. 

Carbon dioxide 

. 1-998 







. 1-998 






Correction of the ratio M/D for gases which deviate from Boyle's law.— It 

follows from Avogadro's hypothesis : (i) The molecular volumes — i.e. the quotients 
of the molecular weights M by the respective densities D — of all gases are the same, 
so that 3Ii : Di=M2 : D^, and (ii) the molecular weights of all gases are pro- 
portional to their densities, so that Mj : M^^D^ : D2. These deductions can be true 
only for gases in which the pressure is not affected by intermolecular attractions 
as is the case with gases which follow the simple gas laws. Densities calculated for 
gases which do not conform with Boyle's law do not agree satisfactorily with obser- 
vations unless the gases are attenuated or rarefied, thus showing that Avogadro's 
hypothesis is not strictly accurate with gases under normal pressure. Similarly, 
the experiments of H. V. Regnault 2 (1847) and others have shown that Boyle's 
and Charles' laws approach exactitude only when the pressures are very small. 
Gases approach the so-called ideal state when their pressures are reduced ; and, at 
the limit, when the pressures are indefinitely small, Avogadro's hypothesis is strictly 
valid. Otherwise expressed, the molecular volumes of all gases are exactly the 
same only when the gases are extremely rarefied ; and the limiting value of the 
ratio of the densities D^ and D^ of two gases will be equal to the ratio of their 
molecular weights M^ and M^ only when the pressures of the respective gases 
approach zero. The deviation of a gas from Boyle's relation Po%/pv=l, or 
I—Po^qIpv^^O, can be symbolized : 

^^l_Mo (1) 


where p^ and Vq respectively denote the atmospheric pressure and volume of the 
gas at 0° ; and p and v the corresponding values at some small pressure. For the 
so-called permanent gases, Kegnault's experiments show that the coefficient A 
is very nearly constant between one and six atmospheres pressure. Consider two 
gases under a very small pressure p ; let each be subjected to atmospheric pressure 
Pq when the volumes become respectively Vi and v^ ; then, Vi=vp{l—Ai)/po ; and 
V2=vp{l—A2}lj)Q ; and by division, 

h _ Izi^i " (2) 

v,-l-A, . • • • 

This means that the molecular volumes of the two gases under atmospheric pressures 
have the proportional values 1—Ai and 1— .42- Let Di and D2 denote the re- 
spective densities of the gases under atmospheric pressures— temperature constant 
—then, the ratio of the molecular weights Mi : M2 is equal to the ratio of the pro- 
ducts of their molecular volumes by the corresponding densities ; that is, to the 
ratio Di(l—Ai) : 2)2(1—^2) 5 or, 

Mi_Di{i-zAi) (3) 

M2~D2{1-'A2) • • • • V ' 



If A for the two gases be zero the expression reduces to that required by Avogadro's 
rule. The densities employed in calculations with formula (3) are the weights in 
grams of a normal litre of the respective gases ; the evaluation of the coefficients A 
is a problem for the physicists. A number of values have been determined, but the 
task is a difficult one, and is subject to some uncertainty since it involves an extra- 
polation of the pv and y-curve. The following values for A] between atmospheric 
and zero pressures are compiled from data by A. Leduc (1898), R. W. Gray and 
F. P. Burt (1909), and A. Jaquerod and 0. Scheuer (1908) 3 : 

Table V. — NumebicaIj Values of the Coefficent A. 





Hydrogen .... 

-0-00056 . 

Nitrous oxide . 




Hydrogen chloride 


Nitrogen . 


Sulphur dioxide 


Carbon monoxide 




Nitric oxide 


Ethane . 


Carbon dioxide . 


Methyl chloride 

+ 0-02468 

Ammonia .... 


Ether .... 


If oxygen be the standard gas with ^2=32, D2=1'4290, and ^2=0*00096, it 
follows that if the numerical values of the density D and the deviation a be known, 

Molecular weight=22-5739D(l— ^) 

The results computed by this method are in fair agreement with the values obtained 
by chemical processes. For example, with oxygen, 32, as standard 

Hydrogen. Nitrogen. Carbon monoxide. Nitric oxide. Methane. 

M (Chemical) . . 2-015 28-019 28-009 30-006 16-039 

M fPhysical) . . 2-016 28-020 28-000 30-010 16-032 

These results are in close agreement. This physical method thus rivals in accuracy 
the molecular weights of the permanent gases determined by chemical processes. 
There are not so many complications with physical methods as are involved in 
conductmg a series of chemical operations with pure substances. This physical 
method is known as D. Berthelot's limiting density method of determining 
molecular weights.^ The data required for the application of Berthelot's method 
are (i) the densities, and (ii) the compressibility of the gas under investigation ; 
and also (iii) the compressibility of the standard gas. 

With the more easily liquefiable gases, the coefficient A changes rapidly with 
changes of pressure, and consequently A cannot be assumed constant without 
sensible error. It is therefore necessary to use values for the coefficients A deter- 
mined for the variations of pressure near to those under which the density has to be 
determined. The available data are not sufficiently exact to enable the method to 
be used for accurate molecular weights of such gases, the coefficients A are usually 
too high, and the molecular weights correspondingly low. For instance : 

Carbon dioxide. 

Nitrous oxide. 



Sulphur dioxide. 

M (Chemical 

. 44-000 





M (Physical) 

. 44-013 





where the comparison is not so favourable. 

According to D. Berthelot,* the molecular weight 3f of a normal liquid is related 
with its critical density Z)„ critical pressure pc, and critical temperature Tc by the 

M = 22-4 

3-6 ' 273 




where 3" 6 represents the mean value of the actual to the theoretical density at the 
critical temperature for normal or non-associated liquids. E. Mathias has also 
shown that in accord with the law of rectilinear diameters, the critical density of a 
substance is related to the densities of the Uquid Di and of the saturated vapour Z>r 
at a temperature T by the expression 

n - I>i-J^v . or, ri ^ A 

when the temperature does not exceed the boihng point of the liquid under atmo- 
spheric pressure. Consequently, by substitution of the second of these equations 
in that of Berthelot,^ 

Molecular weight = 11-4 ^ ^ 

The molecular weights of substances which are liquid at ordinary temperatures, 
calculated by this expression, are often a little too high. For example — 

Cya SO2 CCI4 CS2 NH3 HjO SnCl* 

Calculated . . 50 GSl 152*3 73'4 19-2 251 252-4 

Formula weight . . 52 64 1538 76 17 18 260 


1 J. B. A. Dumas, Lecons sur la philosophie chimique, Paris, 1836 ; Ann. Chim. Phya.^ (2)» 
49. 210, 1832 ; (2), 50. 170, 1832 ; M. A. Gaudin, ih., (2), 52, 113, 1833 ; S. Cannizzaro, Nuovo 
Cimento, 8. 71, 1858. 

2 H. V. Regnault, Mem. Acad., 21. 329, 1847. 

3 R. W. Gray and F. P. Burt, Journ. Chem. Soc., 95. 1633, 1909 ; A. Jaquerod and 0. Scheuer, 
Mem. Soc. Phys. Nat. Geneve, 35. 659. 1908 ; A. Ledue, Ann. Chim. Phys., (7), 15, 6, 1898 ; (8), 
19. 441, 1910. 

4 D. Berthelot, Ccrmpt. Rend., 126. 954, 1898; Journ. Phys., (3), 8. 263, 1899; Zext. 
EleUrochem., 34. 621, 1904 ; Lord Rayleigh, Phil. Trans., 198, 417, 1902 ; Proc. Roy. Soc., 73. 
153, 1904 ; H. F. V. Little, Science Progress, 7. 504, 1913 ; G. Baume, Journ. Chim. Phys., 6. 
52, 1908 ; P. A. Guye, ib., 6. 778, 1908 ; 17. 141, 1919. ^ ,^ 

5 D. Berthelot, Compt. Rend., 128. 006, 1899; C. M. Guldberg, Zeit. phys. Chem., 32. 116, 
1900 ; E. Mathias, Le point critique des corps purs, Paris, 164, 1904. 

§ 9. Radicals or Radicles 

For the chemist, each molecular compound is proximately made up of less compound 
atoms which are indivisible by forces which can divide their product, and these m turn can 
be separated by chemical agents into simple atoms. — S. Bbown. 

In 1815, J. L. Gay Lussac,i after studying the properties of hydrocyanic acid, 
reported cyanogen (CNjg, to be "a remarkable example, and at present, a unique 
example, of a body which, although a compound, plays the part of a smgle body 
in its combinations with hydrogen and the metals." Indeed, if chemists did not 
know how to resolve cyanogen into its constituent elements, this compound would 
very probably be classed as an element, and further, it would probably be assigned 
a place in the halogen family of elements to be studied later. Since Gay Lussac s 
discovery a great number of similar groups of what might be called pseudo-elements 
have been found. For convenience, they are commonly called radicals ov, following 
the custom of the London Chemical Society, radicles. There have been periodic 
discussions on the spelUng of the term— radicle or radical. The latter is 
taken to be historically correct, and the former etymologically correct.- Ihe word 


radical was previously employed by G. de Morveau (1787) and by A. L. Lavoisier 3 
with a different meaning, for with A. L. Lavoisier a radicle could be a simple or com- 
pound body ; he says, le carhone est le radical de Vacide carhonique, and added that 
vegetable acids contain le radical oxalique, tartarique, etc. The definition : a radicle 
is a group of atoms which can enter into and be expelled from combination with- 
out itself undergoing decomposition, is virtually that given by J. von Liebig in 1838. 
Each radicle acts as an unchanging constant in a series of compounds ; and each can 
be replaced by an equivalent element or elements. In very few cases has it been 
possible to isolate the radicle, but the definition has nothing to say about the inde- 
pendent existence of radicles. " Radicles," said A. Kekul^ (1858), " are not firmly 
closed atomic groups, but they are merely aggregates of atoms placed near together 
which do not separate in certain reactions, but fall apart in other reactions." For 
convenience, the term radicle is sometimes applied to an atom in a compound 
which can be replaced by another atom or radicle without a further change in the 
nature of the compound ; in that case, the radicle is said to be a simple radicle, in 
contrast with compound radicles, which are groups of atoms. 

References . 

1 J. L. Gay Lussac, Ann. Chim. Phys., (1), 95. 136, 1815. 

2 Anonymous, Chem. News, 9. 143, 166, 191, 204, 1864; E. Divers, ib , 54. 36, 260, 1886; 
J. SpiUer, ib., 54. 83, 1886; H. G. Madan, ib., 54. 71, 1886 ; Nature, 33. 535, 1886; J. F. Heyes, 
ib., 33. 559, 1886. 

3 J. von Liebig, Liebig's Ann., 25. 113, 1838 ; A. Kekule, ib., 106. 129, 1858 ; A. L. Lavoisier, 
G. de Morveau, and A. F. de Fourcroy, Methode de nomenclature, Paris, 1787; A. L. Lavoisier, 
Traite elementaire de chimie, Paris, 1789. 

§ 10. The Atomic Weights of the Elements 

Every chemical element is regarded as having a distinct and definite nature of its own, 
which natm-e, moreover, determines all its activities.- — B. P. Browne. 

The ratio between the atomic weights of oxygen and hydrogen is the base-line upon 
which our entire system of atomic weights depends. — F. W. Clarke (1896). 

What are the best representative values for the atomic weights of the elements ? 

— The best available determinations of the value of the oxygen-hydrogen ratio give 
numbers ranging between r005 and 1'008 when the standard reference is oxygen 16. 
All measurements made by man are affected by unavoidable errors of experiment ; 
and measurements of the numerical value of all constants differ within certain 
Umits amongst themselves. It is convenient to select one representative value 
from the set of different observations ranging between the limits I'OOS and 1*008. 
The majority of chemists have agreed to let the International Committee of Atomic 
Weights decide what are the best representative values for the atomic weights of 
all the elements year by year. Hence, the generally accepted ratio for the atomic 
weights of hydrogen and oxygen is 1*008 : 16. Every time new and more refined 
methods of measurement are employed, a change— generally insignificantly small — ■ 
may be necessary. It must be recognized that the true atomic weights cannot be 
altered by the votes of the majority of the members of the International Committee 
of Atomic Weights.! There is an uncertain factor in the accepted values of the 
atomic weights, as there is in all our judgments. Aristotle was no doubt right, 
" Nothing can be positively known, and even this cannot be positively asserted." 
This doctrine, however, if rigorously applied, would paralyze all action. Accord- 
ingly, sound-minded people are accustomed to balance the evidence and then act. 
A careful consideration of all the available evidence considerably reduces the risk 
of error, and this method adopted by the Committee appears to be the most satis- 
factory solution of the problem. 

The atomic weights of the elements are indicated in the following table. The 
numbers are those recommended by the International Committee on Atomic 


Weights (1920). The atomic number, indicated in the same Table, will be 
discussed later. 

Table VI. — International Atomic Weights (1921). 0=16. 















Antimony . 




























Niobi\ma (Colum- 

Beryllium (Gluci- 









Niton (radium 













Nitrogen . 












Cadmium . 











Palladium . 
















Platinum . 








Potassiimi . 








Praseodymiiun . 




Chromium . 












Rhodium . 




Columbium (Nio- 

Rubidium . 
















Samarium . 








Scandium . 












Europium . 




























Strontium . 












Glucinum (Beryl- 

Tantalum . 








Tellurium . 




Gold . 












Thalliima . 




Holmium . 








Hydrogen . 












Tin . . . 








Titanium . 








T\mgsten . 
















Vanadium . 












Lead . 




Ytterbium (Neo- 









Lutecium . 












Zinc . 








Zirconium . 








For ordinary calculations involving the use of atomic weights, most of these 
constants, excepting chlorine (35-5), copper (63-5), and zinc (65-5), are rounded 
off to the nearest whole numbers. The elements just named are then assigned the 
constants indicated in the brackets. The atomic weight table made by J. J. 
Berzelius in 1826 has excited admiration on account of its accuracy, ^^'ith the 
standard 0=16, most of J. J. Berzelius' numbers are remarkably close to those we 
are using to-day. For instance, with the common elements : 

o. H. N. ci. 

Berzelius' atomic weights 16 1 14-15 35*47 

To-day's numbers 16 1-008 14 35-46 














This is a testimoDy to the accuracy of J. J. Berzelius' work and particularly so when 
the state of the knowledge of analytical chemistry in Berzelius' time is borne in 

Are atomic weights whole numbers ? — ^It must be added that although we 
are compelled to take the numbers as we find them, yet, the experimental errors 
involved in a complex operation are great, and these errors are sometimes so 
obscured by a cloud of auxiliary calculations that they are not always easy to detect. 
Consequently, G. D. Hinrichs (1893) suggests that the true atomic weight of an 
element must be regarded as a limit to which the observed values approach as the 
disturbing factors are eUminated.- It required a century of measurements on the 
density of atmospheric nitrogen before the presence of 1 per cent, of argon was 
detected therein. Accordingly, many chemists firmly believe that the rounded 
numbers are the best representative values of the atomic weights, and that the 
small deviations from the rounded numbers indicated in the International Table 
represent real, if unrecognized, errors of experiment ; M. Rudolphi (1901) also 
attributed the deviations of the atomic weights from whole numbers to the presence 
of small quantities of unknown elements whose properties are closely allied to the 
elements with which they are mixed. 

Why is oxygen 16 taken as the standard in preference to hydrogen unity ?— 
During the latter part of the nineteenth century, J. Dalton's (1803) standard 
hydrogen unity, was used for the atomic weights instead of oxygen 16. Hydrogen 
was selected as a standard for gas densities and atomic weights because it is the 
lightest element known. In determining atomic weights, it will be observed that 
one of them, say A, is arbitrarily fixed as a standard, and the atomic weights of the 
other elements are fixed through the relations B=liA ; C^^^k^B ; D=]c^C ; . . . 
where ^i, k2, k-^, . . . k^ are numerical ratios. Here, obviously, the numerical 
ratios referred to the element A as standard are : 

Hence, since each observed ratio k embodies unknown errors, the errors will accumu- 
late most on that particular ratio which is least directly connected with the standard 
of reference, A. Consequently, J. S. Stas (1860-65) pointed out, as J. J. Ber- 
zeHus (1818) did before him, that the determination of the atomic weight of an 
element should be connected with the standard as directly as possible. Very few 
compounds of the metals with hydrogen are suitable for an atomic weight deter- 
mination, while nearly all the elements form stable compounds with oxygen. Hence, 
if hydrogen be the standard, it is necessary to find the exact relation between the 
given element and oxygen, and then calculate what that relation would be on the 
assumption that the relation between hydrogen and oxygen is known. C. W. 
Blomstrand expressed similar ideas in his Die Chemie der Jetztzeit (Heidelberg, 
1869) ; he said the atomic weights of practically all the elements are compared with 
hydrogen through the intervention of oxygen. Hydrogen compounds — hydrides — 
are comparatively rare ; oxygen compounds^ — oxides — are common. Hence, the 
weight ratio between oxygen and hydrogen must be known with great accuracy 
since a small error becomes cumulative and it becomes serious in elements with a 
large atomic weight — e.g. with uranium, the experimental error is multiplied about 
15 times. Every improved determination of the relation between hydrogen and 
oxygen would then be followed by an alteration in the weight of every other element 
whose value, with respect to hydrogen as a standard, has been determined by the 
indirect process just indicated, for as J. J. Berzelius said in 1816, oxygen is a kind 
of nucleus about which chemistry has grown. The determination of the exact 
relation between hydrogen and oxygen appears to be more difficult than many 
other determinations, and hence, the majority of chemists think it better to refer 
the atomic weights of the elements to oxygen 16 as the standard instead of making 


the atomic weights depend on the more or less uncertain relation H : O. Hydrogen 
is a theoretical standard, oxygen is the real basis. The standard oxygen 16 is quite 
arbitrary. G. D. Hinrichs (1893) proposed carbon (diamond) =12 as the standard 
of reference. T. Thomson (1825) used oxygen 1 ; W. H. Wollaston (1814), oxygen 
10 ; J. S. Stas (1860-65), oxygen 16 ; and J. J. Berzelius (1830) oxygen 100 as 
standard. The latter number makes the atomic weights of many elements incon- 
veniently large, and if the atomic weight of oxygen be any whole number less than 
16, fractional atomic weights will be required. The use of the oxygen 0=16 unit 
involved the least change in the number in vogue when hydrogen unity was the 

This question of a standard is not of mere academic interest, because, in buying 
and selling ores on the percentage amount of contained metal, a difference in the 
atomic weight selected may involve appreciable differences in the estimated value 
of the ore. For instance, if oxygen be taken 16, the corresponding atomic weight 
of antimony is 1199, and of uranium 239'61 ; if hydrogen be taken as unity, these 
values become respectively 118'9 and 2 37 "6 5— differences of one and two units.3 


1 P. A. Guye, Journ. Chim. Phys., 14. 449, 1916 ; T. Renard, ih., 15. 541, 1917. 

2 G. D. Hinrichs, The True Atomic Weights of the Chemical Elements and the Unity oj Matter, 
St. Louis, 1894 ; E. W. Morley, Journ. Amer. Chem. Soc., 22. 57, 1900 ; J. S. Stas, Recherches sur 
les rapports reciproques des poids atomiques, Bruxelles, 1860 ; Recherches sur les lois des proportions 
chimiques, etc., Bruxelles, 1865 ; J. J. Berzelius, Ldrbok i Kemien, Upsala, 1818 ; W. H. Wollaston, 
Phil. Trans., 104. ], 1814 ; T. Thomson, An Attempt to establish the First Principles of Chemistry 
by Experiment, London, 1825; H. Collins, Gfiem. News, 119. 247, 1919 ; M. Rudolphi, Chem. Ztg., 
25. 1133, 1901. 

3 H. Erdmann, Zeit. anorg. Chem., 27. 127, 1901 ; T. W. Richards, ib., 28. 355, 1901 ; B. 
Brauner, Ber., 22. 1106, 1889 ; 24. 256, 1897 ; 26. 186, 1901 ; L. Meyer and K. Seubert, ib., 18. 
1089, 1885 ; W. A. Noyes, ib., 24. 523, 1891 ; W. Ostwald, Lehrbuch der allgemeinen Chemie, 
Leipzig, 1. 43, 1891. 

§ 11. The Relation between the Molecular Weights and the Volumes of 


The theory of molecules is an ideal conception placed by the mind like another Atlas 
underneath a measureless world of facts to give them intelligible cohesion and hold them up 
to view.— S. Brown. 

The molecular weight of any gas is numerically equal to the weight of any 
volume of the gas when the weight of an equal volume of hydrogen under the same 
physical conditions of temperature and pressure is 2. Two grams of hydrogen, 
taken as the standard, occupy 22*3 to 22*4 litres at normal temperature— 0°— and 
normal pressure— 760 mm. of mercury. Hence, it follows directly from Avogadro's 
hypothesis that the molecular weight of any gas, expressed in grams, occupies 
approximately 22*3 litres at 0° and 760 mm. pressure. Consequently, to find the 
molecular weight of a gaseous substance, weigh 22*3 litres of the gas at a convenient 
temperature and pressure ; calculate the corresponding volume at 0° and 760 mm. 
pressure, and calculate by proportion the weight of 22' 3 litres. 

Example.— A litre of gas at 20° and 730 mm. weighs 1-764 grams, what is the molecular 
weight of the gas ? By the method of calculation indicated in the next chapter, one litre 
of a gas at 20° and 730 mm. pressure contracts to 894-5 c.c. at 760 mm. and 0°. Hence, 
if 894-5 c.c. weigh 1-764 grams, 223 litres will weigh 43-97 grams. Hence the molecular 
weight of the gas is nearly 44. 

It must here be mentioned that the number 22' 3 is not quite right for all gases. 
Many gaseous molecules have a slight attraction for one another, so that the mole- 
cules are slightly more closely packed than is represented by Avogadro's hypothesis. 
The greater the intermolecular attraction, the greater the weight of 22*3 litres, and 


consequently, the less the volume of a molecular weight of the gas expressed in 
grams. Thus, experiment shows : 

Hydrogen. Oxygen. Nitrogen. Chlorine. Hydr^X" divide. (0°.'760 mm.). ^^-^J^' 

22-40 22-39 22*45 22-01 22-22 22-26 22-39 22-55 

The deviation from 22*3 can be neglected in ordinary chemical calculations. 

The molecular weight of a compound not only tells us a weight, but it also tells 
us that if the molecular weight be expressed in grams, the substance when gaseous 
will occupy 22-3 litres at 0° and 760 mm. Further, the molecular weight of a gas, 
expressed in kilograms, occupies, approximately, 22*3 cubic metres at 0° and 760 
mm. pressure. By mere chance, the number of avoirdupois ounces in a kilogram is 
35"26, which is very nearly the same as the number of cubic feet in a cubic metre 
(35' 31) — J. W. Richards.i The difierence is only one-seventh of 1 per cent. Hence, 
the molecular weight of any gas, expressed in avoirdupois ounces, occupies, 
approximately, 22*3 cubic feet at 0° and 760 mm. pressure. These factors are 
useful in calculations involving cubic feet, cubic metres, and Htres. 

1 J. W. Richards, Journ. Franklin Inst., 152. 109, 1901. 

§ 12. Chemical Equations and Chemical Arithmetic 

In his calculations, the chemist relies on the supposed numerical relations of the invisible, 
intangible, immeasurable particles he calls atoms. These relations have been determined 
by others in whom he has confidence, and the accuracy of these relations has to be accepted 
on faith.. — ^H. C. Bolton. 

The molecular weight of an element or compound is the sum of the atomic 
weight of each of the atoms of the constituent elements. — Let a molecule be com- 
posed of rii atoms of one element, n2 atoms of another, n^ atoms of a third, and so on ; 
further, let Ai, A^, A^, . . , denote the atomic weights of the respective elements, 
then the molecular weight of the compound will be Wi^i+^2^2+^3^3+ • • • 
For example, with the approximate atomic weights, the molecular weight of 
hydrogen, H2, is 2 ; of water, H2O, 18 ; of sulphuric acid, H2SO4, 98 ; and of ferrous 
ammonium sulphate, reS04.(NH4)2S04.6H20, 392 — since the summation furnishes 
56+32+4xl6+2(14+4)+32+4xl6+6(2+16) = 392. 

The process or art of calculating the numerical relations of the elements and their 
compounds is sometimes called stoichiometry — from the Greek a-roix^Ta, a fundamental 
constituent ; furptw, I measure. The term appears to have been devised by J. B. Richter, 
in his book, Anfangagrunde der Stochyometrie oder Messkunst chymischer Elementes (Brealau, 
1792-3), or, The rudiments of stoichiometry or the numerical relations of the chemical elements, 
for that branch of chemistry which deals with the numerical proportions in which sub- 
stances combine. To-day the term is sometimes extended to comprise molecular and 
atomic weight determinations and also the general measurable properties of solids, liquids, 
and gases ; solutions and mixtures ; etc. — witness, S. Young, Stoichiometry (London, 1908). 

When the initial and final products of a chemical reaction as well as the com- 
position and proportions of the molecules concerned in the reaction are known, the 
facts can usually be symbolized or abbreviated into a kind of shorthand expression 
which takes the form of a chemical equation. There are some limitations which 
will be described later. 

The equation indicates the nature of the different substances concerned in the 
reaction ; as well as the proportions of the different substances which occur in 
the initial and final products of the reaction. — For instance, when mercury is 
heated in air and mercuric oxide, HgO, is formed, the reaction can be represented 
in symbols : 2Hg+02=2HgO. We here ignore the nitrogen of the air because, 
so far as we can tell, it plays no direct part in the chemical reaction. Similarly, 


when mercuric oxide is heated to a high temperature, it decomposes, forming 
metallic mercury and oxygen. In symbols, 2HgO=2Hg+02. The symbol = or 
-> is used instead of the words " produces " or "forms," and the symbol + is used 
for " together with " on the right side of the = sign, and for " reacts with " on the 
left side. The latter equation reads : *' Two molecules of mercuric oxide, on decom- 
position, produce a molecule of oxygen and two molecules of monatomic mercury.'* 
The number and kind of the atoms of the two sides of the equation must always 
be the same (persistence of weight). 

The eauation indicates the proportions by weight of the substances concerned 
in the reaction. — The atomic weight of mercury is 200, and the atomic weight of 
oxygen is 16, hence, the molecular weight of mercuric oxide is 216, and of oxygen 
32. The latter equation can therefore be read : "432 grams (ozs. or tons) of mer- 
curic oxide in decomposing form 32 grams (ozs. or tons) of oxygen gas and 400 
grams (ozs. or tons) of metallic mercury." Hence, the chemical equation can be 
employed in all kinds of arithmetical problems dealing with weights of substances 
formed or produced. 

Examples.- — (1) How much mercuric oxide is required to furnish 20 grams of oxygen 
gas ? Write down the proper equation ; write 432 below the mercuric oxide, and 32 below 
the oxygen. We are not concerned with the mercury in this problem. Since we read from 
the equation : 32 grams of oxygen are furnished by 432 grams of mercuric oxide, one gram 
of oxygen will be furnished by 432-^32 = 13-5 grams of mercuric oxide; and 20 grams 
of oxygen will come from 20 X 13*5=270 grams of mercuric oxide. 

(2) Show that 2f grams of oxygen and 27J grams of mercury can be obtained theo- 
retically from 30 grams of mercuric oxide. Obviously, 432 grams of mercuric oxide will 
give 32 grams of oxygen, therefore 30 grams of mercuric oxide will give 2| grams of oxygen. 

The equation indicates the proportion by volume of the gases concerned in the 
reaction. — We have seen in the preceding section that if we express 

-, , , .... Volume at 0" and 760 mnx, 

Molecular weight in per molecular weight. 

Grams . . . 22*3 litres 

Kilograms 22-3 cubic metres 

Ozs. (avoir.) 22*3 cubic feet 

Consequently, the idea conveyed by the equation, 2HgO=02+2Hg, can be 
expressed in these words : '' 432 grams (kilograms or ozs.) of mercuric oxide will 
furnish 32 grams (kilograms or ozs.) of oxygen, or 22' 3 litres (cub. metres or cub. ft.) 
of oxygen gas at 0° and 760 mm. and 400 grams of mercury." 

Examples.— (1) What volume of oxvgen will be obtained by heating 30 grams of 
mercuric oxide ? 432 grams of mercuric oxide wiU furnish 30 x22•3-^432 = l•55 htres of 
oxygen gas at 0° and 760 mm. pressure. x rvo j 

(2) How much mercuric oxide will be needed for 10 cub. ft. of oxygen gas at Mid 
760 mm. pressure ? Here 22-3 cub. ft. of the gas come from 432 ozs. of mercuric oxide, 
hence, 432 X 10-^22-3 = 193 ozs., or 12 lbs. 1 oz. of mercuric oxide are required. 

It wiU be observed that in these examples it has been assumed that the reactions 
go to an end. This is an idealized imaginary condition which rarely obtams in practice 
where other factors— temperature, concentration, unequal mixing, etc —introduce 
disturbances. In practice, there are nearly always some losses, and the actual 
vield is X per cent, of that theoretically possible on the assumption that the ideaUzea 
equation is the limit or goal of perfection. In order to make sure that a reaction 
will proceed to an end, y per cent, excess of the initial products may be reqmred. 
Each reaction, in this respect, has its own specific character For example^ tne 
formation of nitric acid, HNO3, by heating sulphuric acid H2SO4, with^s^um 
nitrate, NaNOg, is represented by the equation: 2NaN03+H2bU4--liiNU3 
+Na2S04, where 170 parts of sodium nitrate apparently require 98 parts ot sul- 
phuric acid to produce 126 parts of nitric acid. The manufacturer, l^o^e^^'^^^ 
found by a process of trial and failure that under his conditions, an excess of about 
80 more parts of sulphuric acid are needed to convert the 170 parts of sodium 



nitrate into nitric acid. The equation would then be more correctly written : 
on both sides of the equation does not cancel out when the reaction is applied 
under industrial conditions. This, however, makes no difference to the general 
principles of chemical arithmetic here discussed. If the limitations of the stoichio- 
metrical rules be not appreciated by the industrial chemist, his work will be 
considerably hampered. In general, the rigid appHcation of fixed (scientific) 
principles, without a due appreciation of their limitations, is disastrous in the 
application of scientific methods in industrial work where success is estimated, not 
by the profoundness of a theory, but by the results achieved, or dividends secured. 

§ 13. The Relation between Atomic and Combining Weights — Valency 

Die Valenz nur ein Ausdruck des Gesetzes der multiplen Proportionen i^.- — C. W. 
Blomstrand (1869). 

Each atom carries into its combinations two things : first, its own proper energy ; and 
second, the faculty of expending this energy in its own way, in attaching other atoms to 
itself, not indiscriminately, but definite atoms and in definite numbers.- — C. A. Wubtz 

Observation shows that the relative combining weights of oxygen and hydrogen 
are very nearly as : H=8 : 1 ; and that the atomic weights of oxygen and hydrogen, 
deduced from the atomic theory and Avogadro's hypothesis, are very nearly as 
: H=16 : 1. In fine, the atomic weight of oxygen is twice its combining weight. 
For carbon in carbon dioxide, the combining weight is 3, while the atomic weight 
of carbon is 12, that is, the atomic weight of carbon is four times the combining 
weight. In the case of hydrogen and chlorine, the atomic and combining weights 
are the same. In A. W. Hofmann's Introduction to Modern Chemistry (London, 
1865), it is emphasized that the atomic weight of an element represents the 
minimum quantity of an element which can take part in forming a molecule of 
a compound ; the equivalent, or combining weight of an element, represents the 
minimum quantity of an element which is required to fix one atom of hydrogen 
taken as a standard ; and the valency or valence {valens, worth), or the atom-fixing 
power of an element, represents the number of times the combining or equivalent 
weight is contained in the atomic weight. In illustration, 

Hydrogen. Chlorine. Oxygen. Nitrogen. Carbon. 

Atomic weight . . 1 35*5 16 14 12 

Combining weight . 1 35"5 8 4*67 3 

Valency ... 1 1 2 3 4 

Consequently, as a first approximation, 

Atomic weight 

: Valency. 

Combining weight 

Elements, however, may have more than one equivalent or combining weight, and 
since the atomic weight remains constant, an element may have more than one 
valency. Consequently, an atom not only has the power of fixing an atom of 
another element, but, under definite conditions, it has a definite number of such 

Although valency is primarily a number or a numerical ratio, the term is also 
used to express a general characteristic of the elements. The valency o£ an element 
(or radicle) represents the general property of an atom (or radicle) to combine 
with a certain definite number o! other atoms (or radicles). In order to avoid 
confusing valency a number with valency a property, some restrict the use of the 
term so that valency is reserved for the property, and valence for the number ; thus, 
mercury is an element with a valency of one or two, and in mercuric chloride, HgCl2, 


mercury has a valence of 2, and in mercurous chloride, HgCI, a valence of one. 
This suggestion is good when there is any risk of confusion. 

The meaning of valency can be represented another way. Numerous observa- 
tions indicate that there is generally a limit to the number of atoms which can unite 
with a given atom, so that the atoms of an element appear to differ from one another 
with respect to the number of other atoms with which they habitually combine ; 
valency may then be regarded as representing a habit of an element for combination ; 
it has nothing to do with the force holding the atoms together. The valence of an 
element is obtained by finding — directly or indirectly — how many atoms of hydrogen 
can combine with or be replaced by an atom of the given element. The valence of 
hydrogen is always taken as unity. Hence the definition : The valence of an element 
is a number which expresses how many atoms of hydrogen, or of other atoms 
equivalent to hydrogen, can unite with one atom of the element in question. 
Strictly speaking, valency is only applicable to those gases and liquids whose molecular 
weights have been determined ; and it is extended to solids by analogy with gases. 
We do not know the molecular weights of solids, and we therefore do not know if 
the valency concept can be extended to solids ; it may possibly require modification. 

Chemical affinity and valency are both pecuUar but essentially different pro- 
perties of the atom, and they must not be confounded. The terms, however, are 
sometimes used synonymously, since valency could not be manifested between two 
elements which have no affinity for one another. Affinity refers to the act of 
chemical combination ; valency governs the form of chemical combination. The 
intensity of the chemical energy displayed by hydrogen, oxygen, nitrogen, and 
carbon, in the act of combining with chlorine, is very different — chlorine unites with 
hydrogen with great avidity ; with carbon the action is so sluggish that it requires 
a powerful stimulant ; while, the union of chlorine with oxygen and nitrogen, is so 
difficult that it can only be effected indirectly, not directly. On the other hand, 
however vigorous the act of combination, the hydrogen atom is so constituted that 
it can unite with only one atom of chlorine, while carbon can unite with four, 
nitrogen with three, and oxygen with two. If the energy of the combination of 
chlorine with these four elements be represented by the amount of heat, evolved 
(+)or absorbed (— ) during the combination, the chemical affinity is approximately : 





Chemical affinity . 

, +22-0 



+5-2 xinite 








Very unstable. 


A. S. Couper (1858), one of the pioneers in clarifying our ideas about valency, dis- 
tinguished the two concepts by calling the former affinity of kind, and the latter 
affinity of degree. Affinity of kind, said' he, is the specific affinities manifested bythe 
elements the one for the other ; affinity of degree is the grades or limits of combina- 
tion which the elements display. 

According to the law of multiple proportions, the states of saturation of the 
elements chsmge per saltu7n ; so also according to the doctrine of valency the affinities 
of the elements are exhausted by stages. The two conceptions are not identical. 
According to the latter, each element has a capacity for saturation which is 
definite for a given combination, but which varies from element to element. In 
1858, S. Cannizzaro explained the difference by comparing the two series of chlorides : 
HgCl and HgClg ; CuCl and CuClg ; etc., and he added that the law of multiple 
proportions asserts that the quantities of an element contained in different molecules 
must be whole multiples of one and the same quantity ; but this law cannot foresee 
that one atom of the element is equivalent in one case to one atom of hydrogen and 
in the second case to two atoms of hydrogen. 

Nomenclature.— With hydrogen and chlorine, the atomic and conibiniug weights 
are the same, and the valency is unity. These elements are accordingly said to be 
univalent, or monads ; for similar reasons, oxygen is bivalent, or a dyad ; nitrogen 
is tervalent, or a triad ; carbon is quadrivalent, or a tetrad ; and so on to octovalent 


elements or octads. The valency of an element is frequently represented by 
attaching the necessary numbers, in dashes or Roman numerals, to the top right- 
hand corner of the symbol for the element, as suggested by W. Odling in 1855. 
Thus, the symbols ff and CP respectively mean that hydrogen and chlorine are 
univalent ; 0" means that oxygen is bivalent ; N^" means that nitrogen is ter- 
valent ; and C^ that carbon is quadrivalent. By collecting together a few com- 
pounds with their symbols the idea can be made clearer. 

























Some heptads and octads are known. Hence, the valency of all known atoms can 
be represented by an integer ranging from 0, 1, 2, ... to 8. The elements 
generally combine in such a way that an equal number of valencies are opposed to 
one another. 

No chemical compound is known to be formed by the union of the elements 
of the argon family, the so-called inert or noble gases. So far as our knowledge 
goes, these gases have therefore a zero-valency, and the elements appear to be 
non-valent. Any element existing free in a monatomic condition is non-valent in 
the sense that its atoms are not united with others by means of valency bonds ; 
but the two cases differ in that the maximum valency of the latter is n units, while 
that of the inert gases is zero. 

A few examples of radicles of different valency may be quoted : Monad radicles 
—OH, CN (generally written " Cy "), NO3, NH4 (sometimes written "Am"), 
COOH, etc. Dyad radicles — SO4, SO3, CO3, SiOa, etc. Triad radicles — PO4, 
FeCye, etc. Tetrad radicles — FeCyg, Si04, etc. There are some important hydro- 
carbon radicles — CH3, called methyl ; C2H5, ethyl ; C3H7, propyl ; C4H9, hutyl ; 
C5H11, amyl ; etc. The members of the group of hydrocarbon 'radicles with the 
general formula C^Hyw+i, are called the allcyl radicles. The members of the group, 
C„Hn-i radicles — CgHs, phenyl; C6H4.CH2, benzyl, etc. — are called the aryl 
radicles. There are also many other uni- and poly-valent hydrocarbon radicles. 

Structural, graphic, or constitutional formulae. — The valency of an element is 
sometimes represented by attaching the necessary number of hyphens to the symbol 
for the element. This enables the molecules of a substance to be represented by 
a kind of graphic formula. The symbol for hydrogen will have one hyphen ; oxygen, 
two ; nitrogen, three ; carbon, four ; etc. ; a bivalent oxygen atom may be repre- 
sented 0", —0—, 0=, 0<C, etc. The hyphens are usually attached so that the 
graphic formula occupies as little space as possible ; they are drawn in the most 
convenient direction. The atoms of a molecule are then supposed to be joined 
together by their valencies ; and this is represented diagrammatically by hyphens. 
The symbol for hydrogen chloride then becomes H— CI ; potassium iodide, K— I ; 
water, H— 0— H ; mercuric oxide, Hg=0 ; a molecule of hydrogen, H— H ; a 
molecule of oxygen, 0=0 ; carbon dioxide, 0=C=0 ; and 

H-N<« o<^:z^ «>c<^ 

Ammonia. Ferric oxide. Methane. 

Accordingly, the terms bonds or links are sometimes employed as well as valencies. 

Graphic formulae are also called structural or constitutional formulae. Structural 
formulcB primarily assume that the chemical properties of a substance are determined 
by the arrangement of the atoms in the molecules ; and if the molecules of two compounds 
of the same chemical composition have their atoms differently arranged, the properties 
of the two compounds will be different. Graphic formulae are sometimes very con- 
venient for representing the composition of compounds, but the student would err 


rather seriously if he supposed that the symbol given above for, say, methane 
represents the way the atoms are actually grouped in the molecule of methane. 
This would involve a leap far beyond our real knowledge, although the available 
evidence is in favour of the view that the atoms have a definite arrangement in the 
molecule, and, in some cases, the little knowledge we do possess can be better 
summarized by a graphic formula than in any other way. The graphic formula 
furnishes a clearer mental image of the curious way certain groups of atoms remain 
clustered together through a complex series of chemical changes than if the reaction 
were represented by ordinary symbols. The structural formula has a real and 
important signification ; it should symbolize the chemical character of the molecule. 
A graphic formula is thus a kind of dummy model illustrating the way a compound 
is formed, how it decomposes, and the relations between one compound and another. 
Indeed, chemists now investigate the position of a particular atom in a chain or 
ring of atoms, and find it to be at the side, in the middle, or in some other position 
relative to the remaining atoms. Without accepting C. F. Gerhardt's contention 
(1856) that lesformules chimiques nesont pas destinees a representerV arrangement des 
atomes,^ it must not be believed for one moment that the model simulates reality, 
since, for one thing, the formulae are built on a plane two-dimensional surface, 
whereas the molecule probably extends into three dimensions ; again, graphic 
formulae make the molecule appear as a fixed rigid structure, whereas there is some 
evidence indicating that the atoms within the molecule are in ceaseless rhythmic 
motion. The remarkable work which has been done by the aid of structural formulae 
will always justify their use in the past and present, whatever future generations 
may think of them. The wonderful development of organic chemistry, said J. U. 
Nef (1904), is a consequence of the simple valency concept. 

The doctrine of valency has furnished the chemist with a basis for calculation, 
and enabled him to deduce algebraically the existence of series of compounds previ- 
ously unknown. It has been said that the theory of valency has enabled the chemist 
to predict reactions of unknown compounds with other known compounds, and 
enabled him to found a mechanics of the atoms which in another direction is as 
wonderful as the mechanics of the astronomer which has enabled him to fix the 
position and path of an in\asible planet from its effect on the movements of one 
visible and known. Although the theoretical limitation seems valid in the majority 
of cases, yet there are several compounds whose existence appears contrary 
to the valency hypothesis — e.g. nitric oxide. However, where investigation is 
guided by a wrong theory, only those things which are sought are likely to be found, 
and the theoretical limitation may not have any real counterpart in nature. Hence, 
A. Gr. V. Harcourt 2 could say : 

A chemist who should depart from the general course, and set himself to prepare 
substances whose existence is not indicated by theory, would perhaps obtain results of more 
than usual interest. 

Maximum and active valency. — Most elements have more than one valency. 
Stannous oxide has a composition corresponding with SnO ; and stannic oxide, 
with Sn02. In the former case, the tin is said to be bivalent ; and in the latter, 
quadrivalent. There are thus two series of tin compounds — stannous and stannic. 
Similarly with copper, iron, etc. There are also two carbon oxides, carbon monoxide, 
CO, and carbon dioxide, CO2. If carbon monoxide could be written 0=C=C=0, 
and there is nothing in the analysis by weight which prevents this, all might be 
well ; but writing the formula in this manner would involve a contradiction of 
Avogadro's hypothesis, since the vapour density of carbon monoxide corresponds 
with the molecule CO, not C2O2. We cannot see the way clear to admit carbon 
monoxide as an exception to Avogadro's hypothesis, for that would introduce 
confusion into our system, and there would be no immediate prospect of restoring 
order. Some get over the difficulty by assuming that two of the free valencies 
in carbon monoxide mutually saturate one another, and write the graphic formula 


0=C: ; others assume that oxygen is quadrivalent, and write the graphic formula 

for carbon monoxide C=0 ; and for carbon dioxide, C<k, the two oxygen 

atoms are supposed to be doubly linked to one another and to the carbon atom. 
The question is therefore somewhat involved. The case of sulphur bivalent in 
hydrogen sulphide, H— S— H ; quadrivalent in sulphur dioxide 0=S=0 ; and 
sexivalent in sulphur trioxide 02=S=0, fits very well into this scheme. So do 
the series of compounds represented by ethane, C2Hg ; ethylene, C2H4 ; and acetylene, 
C2H2, which can be respectively represented by the graphic formulae : 

H^C-C^H Hv^jj^jj^H H— teC— H 

h/ \h H H 

Ethane (with single bonds) Ethylene (with double bonds) Acetylene (with triple bonds) 

provided it be assumed that the respective carbon atoms are joined by single, double, 
and triple bonds. It may be added that the circumstantial evidence advanced by 
organic chemistry strongly favours this assumption. 

Since chlorine or fluorine forms combinations with the metals far more generally 
than does hydrogen, it has been proposed to use chlorine or fluorine in place of 
hydrogen as the standard of valency. The hydrogen and fluorine valencies, however, 
are not always the same. . For instance : 

Hydrides . . LiH CaHg (BH3)2 CH4 PH3 SHg IH — 

Fluorides . . LiF CaFg BF3 CF4 PF5 SFg IF OsFg 

The maximum valency of the hydrides is thus attained with the tetrads ; but with 
fluorides, the maximum valency is reached with the octads. The preceding defini- 
tion of valency is troublesome if applied to azomide,HN3, although it works all right 
with ammonia, NH3. 

F. A. Kekule (1866) ^ argued that valency is a fundamental property of the 
atom which is just as constant and invariable as the atomic weight ; the equivalent 
weight of an element may vary, the valency cannot. E. Frankland (1852) showed 
that the elements of the nitrogen family are sometimes ter- and sometimes quinque- 
valent. A controversy whether valency is fixed or variable was carried on about 
1864 by F. A. Kekule, C. A. Wurtz, A. Naquet, H. Kolbe, and A. W. Williamson. 
The controversy, after all, turned out to be nothing more than ein Streit um ein 
Wort. If valency means maximum saturation capacity, this property is unchange- 
able, but if valency means that this maximum power is always exerted, and that 
every atom exerts a constant invariable valency, the doctrine est en desaccord 
Jlagrant avec les fails. The discussion was then diverted to atomic and molecular 
compounds (q.v.). Each element has a maximum valency towards certain other 
elements. When an element appears to have a lower valency than its maximum 
valency, the compound is said to be an unsaturated compound, in contrast with 
a saturated compound in which the atoms are exercising their maximum valency. 
In many unsaturated compounds, the valencies appear to diminish in pairs. The 
pairs of dormant or sleeping valencieSy crypto-valencies {k pv-n-ro^, hidden), or latent or ^05- 
sive valencies are supposed to be self -saturated. Hence W. Odling (1855) proposed to 
call elements with an odd number of bonds j)erissads (Trtpto-o-o's, odd), and those with 
an even number of bonds artiads (aprto?, even). It was also assumed that the sum 
of the valencies of the atoms forming a molecule is always an even number. 

As a matter of fact, the hypothesis of the self-saturation of the bonds in pairs 
breaks down completely. The idea probably arose from the application of an in- 
accurate hypothesis — started in 1864 by E. Erlenmcyer * — which is stated in some 
of the older books on chemistry in words like these : *' All chemical evidence shows 
that a body with unsatisfied bonds cannot exist by itself." All chemical evidence, 
as we shall see, shows nothing of the kind. Mercury and many other elements, 



when vaporized, give gases with one-atom molecules. The principle of self-satura- 
tion breaks down when applied to the nitrogen oxides, say nitric oxide, N'°0". 
The relative density (Avogadro's hypothesis) will not let us write N2O2, that is, 
0==N— N=0. We are therefore confronted with what appears to be an odd 
unsaturated valency in the molecule— N=0. Again, chlorine forms chlorine 
monoxide, CI2O, and chlorine peroxide, CIO2 ; indium forms the three chlorides, 
InCl, InCl2, InCls- The original form of the doctrine of valency is not tenable ; 
elements cannot be classed as invariably uni-, bi-, ter-, quadri-, . . . valent, nor 
as artiads and perissads, since some elements can have any of these valencies accord- 
ing to circumstances. Chlorine, nitrogen, ruthenium, and manganese can be cited 
as examples ; again, molybdenum forms a series of compounds with univalent 
chlorine or fluorine — M0CI2, M0CI3, M0CI4, M0CI5, and MoFg ; and vanadium forms 
VCI2, VCI3, VCI4, and VCI5. In view of facts like these, it is difficult to maintain 
the thesis that the apparent inconstancy of the valency of an element is due to 
the mutual saturation of pairs of valencies. Either a molecule can exist with free 
valencies, or Kekul6's maximum valency hypothesis breaks down when confronted 
with facts. 

A great many ingenious hypotheses, more or less satisfactory, have been sug- 
gested to explain the difficulties. At present we are compelled to frankly admit 
with W. Lessen (1880) and A. Claus (1881) that the active valency of an element 
is a variable habit of combination. An explanation of the meaning of valency is 
thus left open. C. A. Wurtz (1864) distinguished between what he called atomicite 
actuelle and atomicite virtiielle, and in order to distinguish between the greatest 
valency an element is known to exhibit, and the valency which actually prevails 
in a particular compound, the terms maximum or absolute valency and active 
or actual or free valency may be respectively employed. So far as we can see, the 
active valency of an element is dependent upon the properties of the atoms of the 
other elements with which it is combined as well as on the prevailing physical and 
chemical conditions to which the element is exposed. Thus sulphur is bivalent 
towards hydrogen, but it can be sexi valent with fluorine ; antimony, arsenic, and 
phosphorus are tervalent towards hydrogen, while phosphorus and antimony may 
be quinquevalent towards chlorine ; arsenic is tervalent towards chlorine — and 
there is some doubt if the pentachloride, ASCI5, has been made. 

Werner's Nomenclature. — With a complex series of salts, instead of representing 
the number of times the acidic radicle is contained in the molecule — e.g. CuCl, 
copper monochloride ; CUCI2, copper dichloride ; CuO, copper monoxide ; PtCl4, 
platinum tetrachloride, etc. — it is simpler, according to A. Werner,^ to represent 
compounds with the same valency by names ending in the same suffix or letter. 
Thus, if M represents an atom of a basic element, and X an atom of acidic univalent 

Table VII.— A. Werner's Nomenclature of Salts. 






Werner's name. 

Old name. 

MX 2 

MX 5 

















CuCl — cupraohloride 
CuCla— cuprochloride 
M0CI3 — molybdenichloride 
M0CI4 — molybdenechloride 
MoCl 5— moly bdanchloride 
MoFg^ — molybdonfluoride 
CI2O y — chlorinoxide 
OsO 4 — osmium enoxide 

copper monochloride 
copper dichloride 
molybdenum trichloride 
molybdenum tetrachloride 
molybdenmn pentafluoride 
molybdenum hexafluoride 
chlorine heptoxide 
osmium tetroxide 

The suffixes have been chosen to make them differ as little as possible from 
those already in existence. The only serious objection appears to arise with salts 

VOL. I. ^ 


like univalent and bivalent copper, mercury, etc., of the type CuCl, CuCl2 ; and 
HgCl, HgCl2, where cuprous becomes cupra-, and cupric, cupro- ; and mercurous 
becomes mercura-, and mercuric, mercuro-. 

The effect of external conditions on the valency of an element.— Active valency 
has been compared with friction in so far as it appears to be called into play by 
external causes which may vary from zero upwards, because the valency of an ele- 
ment is determined by the physical and chemical conditions under which the element 
is placed. For instance, 

(1) Temperature. — The valency of an element generally diminishes with rise of 
temperature, e.g. sulphur trioxide, SO3, when heated dissociates into sulphur dioxide, 
SO2, and oxygen ; and carbon dioxide, CO2, into carbon monoxide, CO, and oxygen. 
Copper oxide, CuO, at 1110° becomes cuprous oxide, CU2O ; and lead dioxide, 
Pb02, at 615° yields lead monoxide, PbO. 

(2) Pressure. — The valency of an element is often diminished with a decrease 
of pressure. Pressure usually facilitates chemical action. By heating bismuth with 
water at 280° under a pressure of 10,000 atm. the monoxide, BiO, is formed, but 
at higher temperatures and less pressure the sesquioxide, Bi203, is produced ; 
similarly antimony is said to form the monoxide, SbO, and aluminium the 
monoxide, AlO, under conditions where the sesquioxides would normally be 
produced. Carbon monoxide, CO, under a pressure of 600 atm. at 320° is partially 
converted into the dioxide, CO2, and free carbon. 

(3) Light or radiant energy. — Numerous physical and chemical changes are 
induced by exposure to light, and the reactions may be accompanied by changes 
in the valency of some of the elements concerned. Thus, by exposure to light 
ferric oxalate, Ee2"^(C204)3", is reduced to ferrous oxalate, Fe^^C204 — in symbols : 
Fe2(C204)3=2FeC2044-2C02 ; and an aqueous solution of mercuric chloride, 
HgCl2, is reduced to mercurous chloride, HgCl, under similar conditions : 4HgCl2 
+2H20=4HClH-02+4HgCl. Similar remarks, mutatis mutandis, apply to the 
effect of other forms of radiant energy. 

(4) Chemical reagents. — Changes in the valency of an element are usually induced 
by oxidizing or reducing agents. Thus, ferrous chloride, FeCl2, is oxidized to ferric 
chloride, FeCls, by the action of hypochlorous acid, HCIO ; the reaction is symbol- 
ized : 2Fe"Cl2+HCl+HC10=2Fe"^C]3+H20 ; and ferric chloride is reduced 
to ferrous chloride by the action of sulphur dioxide, 2Fe^"Cl3+S02+H20=2Fe"Cl2 
+2HCI+SO3. At the same time, it will be noticed, the sulphur dioxide, 0=S=0 

• ' 

is oxidized to sidphur trioxide, 0=S<^^ where quadrivalent sulphur probably 

becomes sexivalent. Hence, oxidation usually involves an increase in the valency 
of an element, and reduction a decrease. 


^ C, F. Gerhardt, Traiti de chimie organique, Paris, 1856. 

2 A. G. V. Harcourt, B. A. Bep., 36, 1875 ; J. U. Nef, Journ. Amer. Chem. Soc, 26. 1549, 
1904 ; F. A. Kekule, Liebig's Ann., 106. 129, 1858 ; S. Cannizzaro, Nuovo Cimento, 8. 71, 1858 ; 
A. S. Ck)uper, Compt. Bend., 46. 1157, 1858 ; Phil. Mag., (4), 16. 104, 1858. 

3 F. A. Kekule, Liebig's Ann., 104. 129, 1857 ; 106. 129, 1858 ; 117. 120, 1861 ; 137. 74, 
1866 ; Zeit. Chem., 7. 689, 1864 ; E. Frankland, Phil. Trans., 142. 417, 1852 ; Liebig's Ann., 85. 
329, 1853 ; C. A. Wurtz, Ann. Chim. Phya., (6), 43. 492, 1885 ; Compt. Bend., 43. 199, 1856 ; 
The Atomic Theory, London, 1880; Lecons de philosophie chimique, Paris, 1864; H. Kolbe, 
Liebig's Ann., 113. 293, 1860 ; 101. 257,' 1857 ; A. W. Williamson, Phil. Mag., (3), 37. 350, 
1850; Journ. Chem. Soc, 4. 350, 1852 ; W. Odling, ib., 7. 1, 1855; A. S. Couper, Compt. Bend., 
46. 1157, 1858 ; Phil. Mag., (4), 16. 104, 1858 ; A. Naqiiet, Zeit. Chem., 7. 679, 1864. 

4 E. Erlenmeyer, Zeit. Chem., 6. 65, 97, 609, 1863 ; 7. 1, 72, 628, 1864 ; W. Lossen, Lielig's 
Ann., 204. 336, 1880 ; Ber., 20. 3306, 1887 ; 14, 760, 1881 ; A. Glaus, Ber., 14. 432, 1881 ; A. 
Wurtz, LcQons de philosophie chimique, Paris, 1864, 

^ A. Werner, Neuere Anschauungen auf dem Gebiete der anorganischen Chemie, Braunschweig, 
13, 1905 ; London, 75, 1911 ; B. Brauner, Zeit. anorg. Chem., 32, 10, 1902. 


§ 14. The Polarity o! Valency 

The doctrine that the chemical forces by which the elements of bodies are held together 
or separated, are identical with the polar forces of electricity is now entirely established in 
the minds of the most profound and philosophical chemists of our time. — W. Whewell. 

An agent exhibits polarity when it is characterized not only by a numerical 
value, but also by a sign indicating the direction in which it will act. For example, 
during the electrolysis of binary compounds some elements always accumulate at 
one particular electrode ; the hydrogen, for instance, goes to the cathode, never to 
the anode ; and conversely, the oxygen goes to the anode, not to the cathode. It 
is therefore assumed that hydrogen carries a positive electrical charge, oxygen a 
negative charge ; otherwise expressed, oxygen has a negative polarity, hydrogen 
a positive polarity. 

In 1881, in a paper On the modern development of Faraday's conception of elec- 
tricity, H. von Helmholtz deduced from Faraday's work that during electrolysis 
the same quantity of either positive or negative electricity (96,540 coulombs) 
always accompanies each univalent atom, or each valency of a multivalent element, 
so that the same quantity of electricity passing through an electrolyte always sets 
free or transfers the same number of units of affinity (or valency) at each electrode. 
Otherwise expressed, an w-valent atom or radicle carries n unit charges of electricity. 
Electricity thus behaves as if it were divisible into definite elementary portions — 
positive or negative — which behave as if there were atoms of electricity. Following 
G. J. Stoney's proposal (1881), these unit or atomic changes of electricity are called 
electrons.^ It may therefore be said that valency is a polar phenomenon, each 
valency being associated with a positive or negative electron. The valency of a uni- 
valent hydrogen atom carrying a positive charge can therefore be called a positive 
valency, and each valency of a bivalent oxygen atom carrying two negative charges 
a negative valency. Each positive valency can be represented by a + or • sign 
attached to the symbol of the element, say H"" or H" ; and, in a similar manner, 
each negative valency represented by a — or ' sign, say or 0". These symbols 
properly interpreted represent observed facts. 

Again, during the electrolysis of certain compounds, some elements — arsenic, 
antimony, boron, bromine, carbon, iodine, nitrogen, phosphorus, selenium, silicon, 
sulphur, tellurium, etc. — act sometimes like hydrogen and sometimes like oxygen 
in that with some compounds a given element may accumulate at the positive pole 
and with other compounds at the negative pole. Otherwise expressed, the atoms 
of these elements sometimes carry positive and sometimes negative charges, so that 
in some compounds the atoms of these elements have positive valencies, and at other 
times negative valencies. R. Abegg (1904) called these elements with a dual nature 
amphoteric elements {aficfyL, both). Hence, a description of the valency of an element 
in a particular compound should indicate whether the active valency is positive or 
negative. In further illustration, the sulphur in hydrogen sulphide, H2S, has two 
negative valencies ; and in sulphur trioxide, SO3, the same element has six positive 
valencies, so that a change from sulphur with —2 valencies to sulphur with -}-6 
valencies involves a change of eight units of electricity— the algebraic difference 8, 
not the numerical difference 4 units. Similarly, in methane, CH4, the carbon atom 
has four negative valencies ; in carbon tetrafluoride, CF4, the carbon atom has four 
positive valencies ; so that the passage from the former to the latter again involves 
a change of eight valency units. To avoid confusion with valency as a number, the 
term polar number has been employed to represent the algebraic number of negative 
charges which are lost, or positive charges gained by an atom of an element m the 
formation of a given compound. The valency and polar number of nitrogen m 
ammonia are 3 and -3 respectively ; the valency of nitrogen in ammonium chloride 
is 5 and the polar number -3 {i.e. -4+1), as illustrated in the diagram. Fig. 4. 
In nitrous acid, HO.N : 0, with oxygen negative, the polar number is +3, and the 
valency 3 ; whereas in H-N=02, the polar number is still +3, but the valency 



is 5. In potassium permanganate, KO.MnOa, the polar number of manganese is 

+7, and the valency 7.^ 

D. I. Mendel^efE (lS71) assumed that the highest oxide (omitting the peroxide) 

gives the maximum valency of an element, 
and R. Abegg (1904) adopted practically 
the same suggestion for finding the maxi- 
mum positive valency of an element ; J. N. 
Friend (1908) suggested that the fluoride 
be employed for the same purpose. The 
hydrides usually give the numerical values 
of the negative valency of the non- 
metals. J. N. Friend (1908) has com- 
piled the following Table VIII showing the 

positive and negative values of some amphoteric elements with respect to their 

hydrides and fluorides : 

Fig. 2. — Polar Numbers of Nitrogen (—3). 

Table VIII. — Hydrides and Fluorides of Some Amphoteric Elements. 

Negative valency. 

Positive valency. 







Bromine . 
Nitrogen . 
Seleniima . 
Sulphm* . 








SH2 : 








It is interesting to note that the majority of the known amphoteric elements 
give 8 as the sum of the positive and negative valencies. K. Abegg (1904), indeed, 
assumed that all elements are amphoteric and possess 8 positive and negative 
valencies, but the observed facts with hydrogen, the alkali metals, and the inert gases 
do not favour this generalization. The positive valencies of the alkali metals appear 
to be so strong that they show little or no sign of their supposed negative valencies ; 
and the negative valencies of fluorine are so strong that they show little or no sign 
of positive valencies. E,. Abegg and G. Bodlander (1899) developed the hypothesis 
that elements have a different A^alency according as they are united with electro- 
positive or electronegative elements ; and that each element possesses the two 
kinds of valency — positive and negative. The usually accepted valencies of the 
non-metals are negative, and of the metals, positive ; R. Abegg and G. Bodlander 
called these the normal valencies of the elements ; and the secondary valencies of 
opposite polarity, active only under special conditions, were called contra- valencies. 
The normal valencies are supposed to be the stronger. The sum of the normal and 
contra-valencies, as indicated, is assumed to be 8, ranging over the different 
families of elements : 

Normal valencies 
Contravalencies . 
Polar number 


Na Mg Al 

+1 +2 +3 

—7 —6 —5 

-6 -4 -2 


Si P S CI 

-f4 —3 —2 — 1 

—4 -f5 +6 +7 

4-2 +4 +6 


Thus chlorine is univalent, polar number —1 in hydrogen chloride, HCl, where it 
is coupled with electropositive hydrogen ; but it has its maximum heptavalency, 
polar number +7, when it is united with electronegative oxygen in chlorine hept- 
oxide, CI2O7. In particular cases, neither all the normal nor all the contra-valencies 
may be active. The contra-valencies in a particular family of elements increase in 
activity as the atomic weights of the elements increase ; thus, in the halogen 
family, fluorine (atomic weight 19) does not form a compound with oxygen, while 
iodine (atomic weight 127) gives a stable oxide. All the normal valencies of an 
element are supposed to be equivalent,* but if one be saturated, the remainder are 
weakened. Consequently, the active valency of an element depends upon the 
electrochemical character of the associated atoms — arsenic pentafluoride, A8F5, 
for instance, is fairly stable (0. Euif and H. Graf, 1906), while arsenic pentachloride, 
AsCls, is so very unstable that it is doubtful if it really has been prepared (C. 
Baskerville and H. H. Bennett, 1902). The formation of the so-called molecular 
compounds by the union of two or more molecules is attributed to the presence of 
unsaturated, contra, or secondary valencies in at least one of the constituent atoms. 

There are some modifications of this theory of valency. Most are agreed about this 
interpretation of positive and negative valencies ; and the formation of double and associated 
compounds is supposed to be due to the exercise of residual, contra or secondary valencies. 
L. Spiegel (1902) assumed that elements possess secondary valencies which can be called 
forth only in pairs of equal and opposite sign, so that when not externally saturated they 
neutralize one another and impart no electro-chemical characters to the element. Spiegel 
called these extra-valencies, neutral affinities. S. Arrhenius (1904) made a similar assumption 
and called them electrical double valencies, and J. N. Friend (1908) used a similar hypothesis 
and called the sleeping valencies, residual or laient valencies. I. Langmuir (1916) assimied 
that the aggregation of molecules into liquid and solid masses is due to the exercise of the 
secondary valencies, and thus the cohesion of solids and liquids is due to the exercise of an 
attraction similar in kind to chemical affinity. The electron hypothesis will be described later. 


1 G. J. Stoney, Phil. Mag., (5), 11. 381, 1881 ; Proc. Dublin Soc., 3. 51, 1883; H. von Hebn- 
holtz, Journ. Chem. Soc., 39.' 277, 1881. 

2 R. Abegg, Zeit. anorg. Chem., 39. 330, 1904 ; R. Abegg and G. Bodlander, t6., 20. 453, 
1899 ; L. Spiegel, ib., 29. 365, 1902 ; D. I. Mendeleeff, Journ. Russian Phys. Chem. Soc., 1. 1, 1869 ; 
N. Morozoff, ib., 38. 481, 1906 ; J. N. Friend, Journ. Chem. Soc., 93. 260, 1908 ; W. Ramsay, ib., 
93. 778, 1908 ; 0. Ruff and H. Graf, Ber., 39. 67, 1906 ; S. Arrhenius, Theorien der Chemie, 
Leipzig, 1906 ; C. Baskerville and H. H. Bennett, Journ. Amer. Chem. Soc., 24. 1070, 1902 ; 
I. Langmuir, ib., 38. 1145, 2221, 1916 ; H. E. Armstrong, Phil. Mag., (5), 25. 21, 1888. 

§ 15. The Association of Atoms in Three Dimensions 

The arrangement of the atoms of a molecule in one plane is equally convenient in 
diagrams, and improbable as a natural fact.— A. G. Vernon Harcourt (1875). 

When our views are sufficiently extended as to enable us to reason with precision con- 
cerning the proportions of elemental atoms, we shall find the arithmetical relation will not 
be sufficient to explain their mutual action and we shall be obliged to acquire a geometrical 
conception of their relative arrangement in all three dimensions of solid extension. . . . 
When the number of particles (combined with one particle) exists in the proportion of 
4:1, stable equilibrium may take place if the four particles are situated at the angles of 
the four equilateral triangles composing a regular tetrahedron. ... It is perhaps too 
much to hope that the geometrical arrangement of primary particles will ever be perfectly 
known.— W. H. Wollaston (1808). 

In order to explain why the atoms of diatomic molecules travel about in pairs, 
it seems to be necessary to assume that the atoms exert an attraction on one another, 
and that the position of the atoms in space must be conditioned by the attractive 
forces. As Isaac Newton said in his OpticJcs (London, 1704) : 

How the particles which touch only in a few points can stick together and that so firmly 
as they do, without the assistance of something which causes them to be attracted or 
pressed towards one another, is very difficult to conceive. 


When two univalent atoms unite with one bivalent atom, it is natural to imagine 
two points of contact, and two directions in which the bivalent atom exerts its 
power of combination. This conception of direction appears to be almost necessary 
in the case of carbon with its four valencies, and organic chemists have founded 
upon this what is known as stereochemistry {a-rep^os, solid), or chemistry in three 
dimensions, or chemistry in space, on lines dimly foreshadowed by W. H. Wollaston 
in 1808, and A. M. Ampere in 1814. Since then, many chemists have thrown out 
hints of a tridimensional arrangement of the atoms in a molecule — L. Pasteur (1861), 
F. A. Kekule (1861), A. M. ButlerofE (1863), E. Paterno (1869), A. Gaudin (1873), etc. 
Thus, in his celebrated lecture, Recherches sur la dissymetrie inoleculaire des produits 
organiques (Paris, 1861), L. Pasteur asked : Are the atoms of (Z-tartaric acid grouped 
on the spiral of a helix winding to the right, or placed at the summits of an irregular 
tetrahedron, or disposed according to some other asymmetric grouping ? and 
replied : We cannot answer these questions. It was not until the appearance of 
J. H. van't Hoff's paper. On a system of atomic formulce in three dimension.'^, in 
Holland, September, 1874 ; ^ and J. A. le Bel's Stir la relations qui existent entre les 
formules atomique^, in France, November, 1874, that this idea was systematically 
developed as a working hypothesis in organic chemistry. After demonstrating the 
probability of the hypothesis that the carbon atom exerts its valencies in definite 
directions in tridimensional space, it appeared highly probable that other elements 
would be found to exhibit the same phenomenon, and thus arose a stereochemistry 

Fig. 3. — Diagrammatic representation of the Tetrahedron Theory of Quadrivalent Carbon with 
Single-, Double-, and Triple-linked Carbon Atoms. 

of nitrogen, sulphur, silicon, selenium, tin, etc. The relative directions of the four 
valencies of the carbon atom have been studied, and the attempt has been made to 
find the effect of the displacement of these directions upon the properties of the re- 
sulting compounds. It appears to be necessary to assume that the carbon atom is 
a material body with a certain shape and size, because K. Auwers (1890) has shown 
that in the case of two carbon atoms united by a double-bond, the linking forces 
probably act in such a way as to make an angle with each other and not a straight 
line joining the two points, because the existence of such forces acting from mere 
point-centres is highly improbable. Without making any suggestion as to the 
actual form of the tetrahedral arrangement of the valencies of the carbon atom — 
whether the attractive forces are concentrated at the apices (J. Wislicenus, 1888), 
or at the centres of the faces (A. Wunderlich, 1886) — organic chemists, following 
Wollaston's suggestion, find it convenient to represent graphically the four valencies 
of the carbon as acting in the direction of the line joining the centre with the 
apices of a regular tetrahedron. According to this hypothesis, the constitution of 
methane, CH4, will be that represented in the diagram. Fig. 5, where the circles 
represent the relative positions of the hydrogen atoms with respect to the central 
carbon atom ; similarly, for ethane, C2H6, with a pair of single-linked carbon atoms. 
Fig. 5, acetylene, C2H2, with a pair of triple-hnked carbon atoms. Fig. 5 ; and benzene, 
CqHq, with a chain of six carbon atoms alternately single- and double-linked so as to 
form a closed chain or ring. 

In the case of double- or triple-linked carbon atoms, are the lines assumed to 


be normally directed from the centre of the tetrahedron, bent with or without 
straining, or do the forces act rigidly in one fixed direction so that their com- 
ponents alone act in a direction parallel with the line joining the centres of the two 
tetrahedra ? If it be assumed, with A. Naumann (1890), that the two valencies 
joining a pair of double-linked carbon atoms in, say, ethylene, C2H4, are directed 
from the centre of a tetrahedron towards the apices, and if each of these forces be 
resolved in two directions according to the parallelogram of forces, the sum of the 
components of each of these forces acting in the direction of the line joining the 
centres of the two tetrahedra, is effective in holding the two carbon atoms together. 
If the force with two single-linked carbon atoms be taken as unity, the force holding 
a pair of double-linked atoms will be 0'577 X 2, and between a pair of triple-linked 
carbon atoms, 0'33x3. This is not in agreement with J. Thomsen's thermal data. 

A. von Baeyer, in a paper Ueher Polyacetyleneverhindungen (1885), showed that 
if the four valencies of carbon are directed from a centre towards the four corners 
of a regular tetrahedron, the lines must make an angle of 109° 28' with one another ; 
and he made the assumption that if the direction of the attraction be diverted, 
there will be a corresponding strain ; the greater the divergence, the greater the 
strain ; and the greater the strain, the less the stability of the resulting molecule. 
The negative heat of formation of acetylene with its two carbon atoms connected 
by a triple bond, and the great instability of the acetylene compounds, show that 
the three linldng bonds of the two acetylene carbons may be under some such strain ; 
otherwise it might be anticipated that a pair of triple-linked atoms would be more 
stable than a pair of double-linked atoms, and the latter in turn more stable than a 
pair of single-linked carbon atoms. J. Thomsen's study of the heats of formation 
of the hydrocarbons (1882) shows that the breaking up of a double-bond requires 
15*46 Cals. less thermal energy than a pair of single-bonds, and the breaking of a 
triple -bond requires 43 '92 Cals. less thermal energy than is needed for three single- 

Consequently, A. von Baeyer's strain theory of valency — Spannungstheorie — • 
assumes that the four valencies of the carbon atom normally act in the direction 
of the lines joining the centre with the apices of a regular tetrahedron making angles 
109° 28' with one another ; and if these directions be bent or diverted, the lines are 
strained as if they were elastic wires, so that the greater the divergence the greater 
the strain, and the less the stability of the molecule. It follows that if the carbon 
atoms all lie in one plane, the angles of divergence with ethylene and with tri-, 
tetra-, penta-, and hexamethylene, CnH-m, wiU ^^ 

CHg HgC^CHg 

^^^ ^^*^ II2C CHg n/\nTr HoC/ NcHo 

CHg HgC CH2 HgC CH2 HgC CH2 H2C CHg 

(C2H4), 54° 44' (C3H6) 24" 44' (C4H8), 9« 44' (CsHiq), 0** 44', (CgHig), —5' 16' 

Ethylene Trimethylene Tetramethylene Pentamethylene Hexamethylene 

and generally, for a ring compound of this type containing n carbon atoms in the 
ring, the angle of divergence will be 54° 44' less (w— 2) 90° -^-w. H. Sachse introduced 
further developments. 

This hypothesis explains how the members of the closed ring series increase in 
stability up to a maximum with pentamethylene, which should be more stable than 
all the other members of the series, for the higher members decrease in stability with 
increasing complexity ; the theory also explains how organic compounds with open 
chains have a greater tendency to form closed rings with five and six members than 
closed rings of greater or less complexity. F. Stohmann and C. Kleber's measure- 
ments (1892) of the energy required to break such rings and add two hydrogen 
atoms are in approximate agreement with this deduction ; so also is I. Traube's 
work (1899) on atomic volumes. There are, however, several series of compounds 
whose behaviour does not fit in quite so well with the hypothesis. For instance, 


it will be obvious that the strain theory itself cannot be a sufficient explanation of 
ring formation because it does not take the influence of chemical affinity into account 
— e.g. the influence of side-chains in facilitating the closing of the ring. H. N. 
Stokes (1900) applied a similar hypothesis to the phosphimic acids in which the 
phosphorus atoms form closed rings and the results were in general agreement with 
the hypothesis. 


1 J. H. van't Hoff, La chimie dans Vespace, Rotterdam, 1875 ; Bull. Soc. Chim., 24. 295, 338, 
1875; J. A. le Bel, ib., 22. 337, 1874; M. Berthelot, ib., 24. 338, 1875; F. W. Clarke, Amer. 
Chemist, 6. 81, 1875 ; L. Pasteur, Recherches sur la dissymetrie moUculaire des produits organiques 
naturels, Paris, 1861 ; Alembic Club Reprints, 14, 1897 ; F. A. Kekule, Liebig's Ann., 101. 200, 
1857 ; A. M. Butleroff, Zeit. Chem., 4. 549, 1861 ; Lehrbuch der organischen Chemie, Leipzig, 
1868 ; E. Patemo, Giorn. Scienze Nat. Palermo, 5, 1869 ; Gazz. Chim. Ital., 23. 35, 1893 ; A. 
Gaudin, L^ architecture du monde des atomes, Paris, 1873 ; K. Auwers, Die Entwicklung der Stereo- 
chemie, Heidelberg, 1890 ; A. Wunderlich, Configuration organischer Molekule, Leipzig, 1886 ; 
J. Wislicenus, Ber., 21. 581, 1888 ; A. Naumann, ib., 23. 477, 1890 ; A. von Baeyer, ih., 18. 2277, 
1885; A. G. V. Harcourt, B. A. Rep., 32. 1875 ; W. H. Wollaston, Phil. Trans., 98. 96, 1808 ; 
• F. Stohmann and C. Kleber, Journ. prakt. Chem., (2), 45. 475, 1892 ; I. Traube, Ueher den Raum 
der Atrnne, Stiittgart, 1899 ; N. N. Stokes, Bull. U.S. Geol. Sur., 167. 117, 1900 ; J. Thomsen, 
Thermochemische Untersuchungen, Leipzig, 1882; H. Sachse, Zeit. phys. Chem., 10. 203, 1892. 

§ 16. The Evolution of the Valency Concept 

The doctrine of valency is no mere speculation or hypothesis evolved by the brilliant 
fancy or imagination of one man ; it is the logical outcome of knowledge acquired step by 
step. The conception has been one of slow growth, for it gradually incorporated itself 
into science as the necessity arose for devising a suitable explanation for accumulated 
observations. — E. P. Venable (1899). 

While the mists which enveloped the concepts, molecule, atom, and equivalent, 
were being dispelled by illuminating rays of A. Avogadro's hypothesis, many theories 
to explain chemical composition were struggling for existence. In the resulting 
controversies — chiefly among J. J. Berzelius, J. B. A. Dumas, J. von Liebig, A. 
Laurent, and C. F. Gerhardt — the facts were interpreted by different hypotheses ; 
and, as A. Ladenburg (1886) has shown, this was far more favourable for progress 
than if a single theoretical opinion had come too prominently in front. The differ- 
ences of opinion quickened interest and experiment, and gave chemistry a very 
intimate knowledge of many classes of compounds, because the advocate of each 
hypothesis tried to support his own views by evidence which could be obtained 
only by a close study of the chemical characteristics of the compounds in dispute. 
It is difficult for chemists to appreciate the labour involved in clarifying the concepts 
which now appear so simple. As one writer has said : 

In the glamour of recent discoveries and the attractiveness of what is new and startling, 
the pioneer spade work of a bygone age is forgotten or undervalued, and A, Carrel adds : Almost 
every step in scientific progress which appears to be due to the efforts of one individual is, 
in reality, the result indirectly of the unknown scientific work of many others. 

The radicle or radical theories. — In his Traite elementaire de cJdmie (Paris, 
1793), A. L. Lavoisier supposed that chemical compounds were formed by the union 
of two bodies, and stated his belief that the composition of organic bodies depended 
upon the existence of complexes or radicles in union with oxygen. Adopting a 
suggestion of Guyton de Morveau (1787), Lavoisier called that portion of a compound 
which is combined with oxygen, la base or le radicle. In developing his celebrated 
dualistic polar hypothesis, J. J. Berzelius (1817) extended Lavoisier's idea. The 
dominant feature of Berzelius' hypothesis is that chemical compounds can all be 
resolved with two distinct parts electrically different. When J. B. A. Dumas and 
P. F. G. BouUay (1828) i announced their belief that ether, (C2H5)20, consisted of two 


parts— water, HgO, and a basic radicle, C2H4, which was called at the suggestion of 
J. J. Berzelius cetherine — the radicle etherine was thought to be always present in 
what are now called ethyl compounds. For instance, alcohol, C2H6O," would have 
been regarded as a binary compound, C2H4.H2O ; and ether, C4H10O, as 2C2H4.H2O. 
J. J. Berzelius at first opposed this hypothesis, but he afterwards incorporated the 
idea in his dualism. J. B. A. Dumas and P. F. G. Boullay's etherine hypothesis 
was not generally accepted because it did not adapt itself to the many new organic 
compounds soon afterwards discovered. The interesting feature about this 
hypothesis is that it represents an attempt to find a similarity in the structure of a 
series of chemical compounds which possess like fundamental properties by showing 
that they are all derived from one common primitive stock or type. 

In 1815, in his memoir Recherches sur V acide prussique, J. L.Gay Lussac2 announced 
the discovery of the radicle cyanogen, C2N2 (Kvavo?, blue ; y^wdw, I produce) ; 
and he showed that the group ON, or Cy, persists as a radicle through a whole series 
of chemical compounds. 

Cy.H Cy.Cl Cy.Br Cy.NHg CHaCO.Cy 

Cyanogen hydride. Cyanogen chloride. Cyanogen bromide. Cyanamide. Acetylcyanide. 

Again, in 1832, J. von Liebig and F. Wohler described a series of compounds of the 
radicle benzoyl, CqHsCO, of benzoic acid, in a memoir entitled Untersuchungen uber 
das Radikal der Benzoesdure, The benzoyl radicle persists in many chemical com- 
pounds — among others 


Benzoyl hydride. Benzoic acid. Benzoyl chloride. Benzamide. Ethyl benzoate 

The recognition of these two radicles — cyanogen and benzoyl — led to the development 
of what is now called the older radicle theory. On this hypothesis complex groups 
or radicles were supposed to exist unalterable in organic compounds, and to play 
the same role as elements do in inorganic compounds. According to J. von Liebig 

Cyanogen is a radicle (I) because it is a non-varying constituent in a series of compounds ; 
(2) because in these latter it can be replaced by other simple substances ; and (3) because 
in its compounds with a simple substance, the latter can be turned out and replaced by 
equivalents of other simple substances. 

Hence, while this hypothesis was in favour, organic chemistry was regarded by 
J. B. A. Dumas and J. von Liebig (1837) as the Chemistry of Compound Radicles. 
The purpose of organic chemistry was supposed to involve the investigation and 
isolation of radicles as the more intimate components of organic compounds. 
Cyanogen and benzoyl, said A. Ladenburg (1869), were the pillars of the radicle 
theory, and this hypothesis received further support from the work described in 
R. Bunsen's brilliant memoirs, Untersuchungen uber die Kakodylreihe (1839-43), in 
which it was shown that the so-called Cadet' s fuming liquid — obtained by A. A. F. 
Cadet in 1760 by distilling potassium acetate with arsenious oxide — contained the 
oxide of a radicle, with the empirical formula, As(CH3)2, and which he called kakodyl 
or cacodyl {KdKO)Sr]<i, ill-smelling). R. Bunsen succeeded in isolating the radicle 
itself, and also in preparing various salts — the chloride, bromide, fluoride, sulphide, 
etc. Modifications of the theory of radicles were discussed by J. J. Berzelius (1833) 
and J. von Liebig (1834-38), and the principle of radicles was generally accepted 
although it was not so much emphasized during the reign of the so-called type 

The type theories.— In 1834, J. B. A. Dumas 3 found that the hydrogen of many 
organic compounds could be replaced or substituted by chlorine in such a way 
that for every volume of chlorine introduced into a compound, an equal volume of 
hydrogen was lost ; and, shortly afterwards, J. B. Dumas found that when oxygen 
displaces hydrogen, half a volume of oxygen takes the place of one volume of hydro- 
gen. Otherwise expressed, while equal volumes of hydrogen and chlorine are 


equivalent, these elements possess only one-half the substituting value of the same 
volume of oxygen. A further study of substitution or metalepsis (/xcraXr^i/Ats, 
exchange), led J. B. A. Dumas, in his memoir, Sur k constitution de quelques corps 
organiques et sur la theorie des substitutions (1839), to the so-called substitution 
theory. J. B. A. Dumas discovered two important facts in his investigation of the 
action of chlorine on some organic compounds : (1) When a compound containing 
hydrogen is exposed to the dehydrogenating action of chlorine, bromine, or iodine, 
for each atom of hydrogen that it loses, it takes up an equivalent volume of chlorine, 
bromine, etc. (2) If a compound contains water, it loses the hydrogen without 
an equivalent substitution or replacement. The main assumption of the substitution 
theory hangs on the doctrine that the structure and character of organic compounds 
are not materially altered by the substitution of chlorine in place of hydrogen. 

A. Laurent in a paper Theorie des comhinaisons organiques (1836), and later, in 
his posthumous work, Methode de chimie (Paris, 1854), tried to reconcile the radicle 
theory with these new facts discovered by J. B. A. Dumas. When the substitution 
occurs equivalent by equivalent, the residual body exhibits certain analogies with 
the original substance, for the substitution occurs without disturbing the structural 
type — chlorine, for instance, may occupy the place left vacant by hydrogen. A. 
Laurent argued that all organic compounds have definite forms or nuclei — radicaux 
— and consist either of primary nuclei — radicaux fundamentaux — or of secondary or 
derived nuclei — radicaux derives — in which the hydrogen atoms have been replaced 
by others, or in which additional atoms have been taken up. This hypothesis was 
called the nucleus theory ; it included the idea of substitution, and was based on 
the radicle theory ; but it controverted the doctrine that radicles were unchange- 
able, for the atoms of a radicle can be replaced by others ; it gave the first hint of 
what is now known as " chemistry in space." The nucleus theory was specially 
favoured by L. Gmelin in his celebrated Handhuch der Chemie (Heidelberg, 1843 
et seq.), but it was not taken up by chemists generally. 

In 1839, J. B. A. Dumas prepared trichloroacetic acid, CCI3.COOH, in which 
three of the hydrogen atoms of acetic acid, CH3.COOH, are replaced by chlorine, 
and the resulting compound retains the chief characteristics of the parent acid. 
This led him, in his Memoire sur la hi des substitutions et la theorie des 
types (1840), to extend Laurent's nucleus theory to what is now known as the 
older theory of types, in which organic substances are supposed to be formed of 
particles which may be replaced or displaced, so to speak, without destroying the 
original substance. Compounds which have similar properties and a similar structure 
were classed as belonging to one chemical type — e.g. acetic acid and the chloroacetic 
acids. The relations between the members of a series of compounds belonging to 
one chemical type thus recall those assumed by A. Laurent to subsist between the 
original and the derived nuclei. J. B. A. Dumas also found it necessary to employ 
what he called the rnechanical type to classify compounds which are related in struc- 
ture but which manifest different chemical characteristics. Dumas rightly classed 
acetic acid and alcohol under the same mechanical type, which included a number 
of compounds which had little or no chemical relations with one another, though 
they may be regarded as belonging to one natural family because they may be 
derived by substitution one from the other — e.g. methane, CH4 ; formic acid, 
H.CO.OH ; carbon tetrachloride, CCI3.CI. J. B. A. Dumas' mechanical type 
resembled what H. V. Regnault (1838) had previously called the molecular type. 
If a substance changes without losing its mechanical type, it follows the law of 
substitution, but if it passes into another mechanical type, the law of substitution 
is not maintained during the reaction. By this statement, J. B. A. Dumas admits 
that his original idea of substitution is not always applicable, for an equivalent of 
hydrogen is not always evolved when another is introduced into the compound ; 
and a compound is not regarded as consisting of two parts, but is supposed to be a 
uniform whole with its component parts related in an analogous fashion to the worlds 
of a planetary system in which the atoms are held together by affinity instead of by 


gravitation. Just as the stability of a planetary system depends not on the intrinsic 
nature of the planetary units, but rather on their relative position with respect to 
one another and to the sun, so J. B. A. Dumas supposed that the chemical properties 
of a compound are primarily dependent on the arrangement and number of the 
constituent atoms, and in a less degree on their chemical nature. Dumas thus 
regarded the planetary molecule as the tyjpe of a series of compounds with a similar 
structure ; and therefore, he opposed a unitary theory of chemical coynfosition in 
place of J. J. Berzelias' dualism. Between 1838 and 1844, J. J. Berzelius vigorously 
fought a losing fight in his Jahresherichten against these encroachments on his 
dualistic views. C. F. Gerhardt having suggested that in compounds of an 
organic base with an inorganic acid, the organic portion of the compound, termed 
the copula, was supposed to unite by accouplement (copulation) with the inorganic 
acid. J. J. Berzelius tried to explain the substitution products obtained by J. B. A. 
Dumas by arbitrarily assuming that they were formed by the copulation or pairing 
of imaginary copulae ; he explained the formation of trichloroacetic acid, for example, 
by assuming it to be formed by the union of carbon chloride, C2CI6, with oxalic 
acid, H2C2O4 ; and he assigned different rational formula? to trichloroacetic acid 
and its parent — acetic acid. Berzelius' explanation broke down completely when 
L. H. F. Meslen (1842) showed that chloroacetic acid could be reconverted to the 
original acid by reduction with potassium amalgam. J. J. Berzelius obstinately 
opposed the theory of t5rpes with his last breath, but he fought practically alone. 
His one-time supporter, J. von Liebig, gave up the duaUstic hypothesis when it failed 
to explain the newer facts. 

In 1839, C. F. Gerhardt, in his memoir Sur la constitution des sels organiques a 
acides complexes et leur rapports avec les sels ammoniacaux, rejected the radicle 
theory and stated his belief that a compound must be regarded as a complex of atoms 
bound each to all, and all to each ; but he could not help admitting that certain 
groups of atoms do recur in chemical compounds. Accordingly, C. F. Gerhardt 
attempted to reconcile his h5rpothesis with observation, by what he called his 
theorie des residus — theory of residues — in which a group of atoms previously called 
a compound radical was termed le reste — a residue ; unlike radicles, C. F. Gerhardt's 
residues were not supposed to be present as such in a compound, for, said C. F. 
Gerhardt, je prends V expression de radical dans le sens de rapport, et non dans celui 
de corps isolahle ou isole. C. F. Gerhardt's molecule was une systeme unitaire — a 
simple edifice and not a double building ; all assumptions of a binary structure 
were excluded. He argued that the constitution of a compound can be deduced 
only from its modes of formation and decomposition, and that according to the theory 
of radicles several rational formulae and several radicles could be imagined in the 
case of one substance formed in different ways — e.g. barium sulphate formed by 
the reactions symbolized : (i) BaO+SOg ; (ii) BaOg+SOg ; and (iii) BaS-1-202. Con- 
sequently, a chemical type is nothing more than a general system of reactions. Acetic 
acid, water, and alcohol were classed in the same way because they undergo analogous 
reactions — say, when they are deoxidized to form aldehyde, hydrogen, and ethyl 
hydride, C2H5H, respectively. Gerhardt further supposed that when two substances 
react with one another, an element in the one combines with an element in the other 
to form one stable compound, and the residues also unite to form what he called 
corps copules and later corps conjuges, meaning copulated or conjugated compounds. 
Thus, the copula benzene, CgHg, unites with nitric acid, HNO3, to form water 
HoO, and the copulated compound nitrobenzene, CeHg.NOa. In modernized 
symbols : 

NO2) CeHs) _H ) ^CgHsi 
OH i H ! OH? NO2 f 

Consequently, in a reaction between two substances, each molecule is split into 
two parts, and the resulting residues unite in such a manner that a double exchange 
takes place, and Gerhardt said : J'appelle radicaux ou residus les elements de tout 


corps qui peuvent etre ainsi transportes dans un autre corps par Veffet d'une double 
decomposition, ou qui y ont ete introduits par une semhlahle reaction. It is not very- 
obvious why C. F. Gerhardt emphasized the distinction between his own type 
formulae and those of J. B. A. Dumas. The former clearly supposes substitution 
to be effected by replacing an element in a compound by an equivalent of another 
element, or by the residues (radicles) of the reacting substances, and this is but a 
restatement of the views of J. B. A. Dumas and A. Laurent. C. F. Gerhardt's 
conception of radicles, said C. Schorlemmer (1879), soon supplanted the older views, 
and its introduction into the theory of types led to the fusion of both theories. 

The discovery of the organic ammonias by C. A. Wurtz (1849) and A. W. von 
Hofmann (1850) * revealed the close relationship between the organic ammonia 
bases and ammonia itself, and the hypothesis that the former were derivatives of 
ammonia, NH3, produced by the substitution of hydrocarbon radicles in place of 
hydrogen atoms : 

NH3 (C2H5)NH2 (C2H6)2NH (C2H5)3N 

furnished the only satisfactory explanation of the constitution of these compounds. 
In this way, said C. A. Wurtz, the ammonia type was founded. Similarly, A. W. 
Williamson's Theory of Mtherijication (1850), dealing with the substitution of hydro- 
carbon radicles in place of the hydrogen atoms of water, established the water type. 
A. W. Williamson demonstrated the close relationship between 

Hjo ^2^5 \n CgHsi 



Water. Alcohol. Ether. 

In harmony with a prior suggestion made by A, Laurent in 1846, A. W. Williamson 
wrote : "I believe that throughout inorganic chemistry and for the beet known 
organic compounds, one single type will be sufficient — it is that of water represented 
as containing two atoms of. hydrogen to one of oxygen." Numerous nitrogen com- 
pounds were then referred to C. A. Wurtz and A. W. von Hofmann 's ammonia type, 
and many oxygen compounds were likewise referred to A. W. Williamson's water 
type as termes de comparaison. C. F. Gerhardt in his Traite de chimie organique 
(Paris, 1853-6) added hydrogen and (lydrogen chloride to the ammonia and water 
types, and he attempted to classify all organic compounds by reference to the four 
types : hydrogen, H2 ; hydrogen chloride, HCl ; water, H2O ; and ammonia, NH3. 
In 1857, F. A. Kekule, in an important memoir Veber die sogenannten gepaarten 
Verhindungen und die Theorie der 7nehratomigen Radicle, proposed to add methane, 
CH4, to the list of primitive or simple types, and to remove hydrogen chloride from 
the list because it is merely a special case of the hydrogen type. Thus arose the 
newer theory of types which now assumed the forms 

h} > |n 

H C 


Hydrogen type. Water type. Ammonia type. Methane type. 

A. Laurent had suggested in 1846 that alcohol and ether as well as inorganic acids 
and oxides could be regarded as derivatives of water. In 1848-9, T. S. Hunt 
published several papers in which he showed that the composition of many oxyge- 
nated compounds might be derived from water as a type, and he also referred the 
formul£e of hydrocarbons to hydrogen as a type ; but T. S. Hunt's work had little 
or no influence on the development of the theory of types since it was unknown to 
those who were working in Europe on this subject. 

A. W. Williamson introduced the idea of condensed types in 1850 ; dibasic acids 
like sulphuric acids and oxalic were regarded as derived from two molecules of water, 
and the acid radicle was supposed to replace one atom of hydrogen in each of the 
two molecules of water. W. Odling, in his paper On the Constitution of Acids 


and Salts (1855), developed the idea still further, and formulfie like these were 
obtained : 

h}0 type 

5)0, type 


C^HsOjo NO.Jo 

^202)0 . S02 ^ 

''Xt}o. ly. 

Acetic acid. Nitric acid. 

Oxalic acid. Sulphuric acid. 

Citric acid. Phosphoric acid. 

F. A. Kekule (1857) also extended the type theory to include mixed types 
supposed to be formed by the union of two simple or condensed types. For 
example, chlorosulphuric acid, (H0)C1S02, can be referred to a mixed hydrogen 
and water type ; and carbamic acid, HgN.COOH, was referred to the mixed 
ammonia and water types : 

h} so) h1- co^^ 

> H> h1« h}0 

Mixed type. Chlorosulphuric acid. Mixed type. Carbamic acid. 

The need for the introduction of condensed and mixed types showed the insufficiency 
of the type theory, for as the number and complexity of organic compounds increase, 
an indefinite number of types may be required. C. F. Gerhardt's type theory is 
now considered but an interesting phase in the evolution of systematic chemistry. 
The attempts to refer a large number of compounds to a limited number of types, 
and the consequent need for viewing individual compounds from many different 
points de vue, enabled chemists to see many analogies and contrasts previously 
hidden, and to realize dimly the remarkable relations the atoms of a compound 
bear each to each. It soon became evident that the theory of types represented 
an artificial arbitrary system of classification ; even C. F. Gerhardt (1856) admitted 
mes radicaux et mes types ne sont que des symholes, destines d concreter en quelquc sorte 
certains rapports de composition et de transformation, and H. W. Kolbe (1843 et seq.) 
seemed to get at the root of the matter when asked : *' Why are we to suppose 
that nature has restricted herself to forming all bodies on the models of these four 
types 1 Why on these models rather than on others ? The four types are nothing 
but a vain artifice." He answered that " the grouping of organic compounds into 
types verges on empty formalism, and is merely playing with formulae." He 
sought to replace the purely formal types by others which he considered to be 
related naturally with their derivatives. In a paper Ueber die chemische Konsti- 
tution und Natur der organischen Radikale (1851), H. W. Kolbe built up a newer radicle 
theory in which he eliminated those tenets which were not in harmony with fact ; 
he showed, as J. J. Berzelius supposed, that in organic compounds there are definite 
radicles which behave like the elements in inorganic compounds. The discovery 
of the organometallic compounds — typified by zinc ethyl, Zn(C2H5)2 — by E. Frank- 
land (1849), seems to exclude every doubt of the actual existence of compound 
radicles ; and H. W. Kolbe (1850) electrolyzed aqueous solutions of the salts of the 
fatty acids, and believed that he separated the constituent hydrocarbon radicles — 
as a matter of fact, he obtained products of the union of two radicles. Thus, with 
potassium acetate, CH3.CO.OK, he obtained gaseous carbon dioxide and ethane, 
(CH3)2 or C2H6 ; the potassium, simultaneously obtained, undergoes a secondary 
reaction with the solvent. The primary reaction in modern symbols is represented : 

iCHg'.iCO.OlK CO2 , CHg J. 

ICHgi-iCO.OlK CO2 CH3 "^ 

In conjunction with E. Frankland, H. W. Kolbe published a paper entitled Veber 
den natUrlichen Zusammenhang der organischen mit den anorganischen Verbindungen, 
die wissenschaftliche Grundlage zu einer naturgemassen Klassifikation der organischen 
chemischen Korper (1859), in which it was shown by numerous examples that organic 


compounds can be regarded as derivatives of inorganic compounds, and result 
from the latter — in some cases directly — by wonderfully simple substitution pro- 
cesses. Consequently, organic acids can be regarded as substitution derivatives of 
carbonic acid, and consequently, H. Kolbe argued that carbonic acid is a natural 
standard of reference for organic bodies because they are formed from this gas in 
the vegetable kingdom. He said : The carbonic acid type must therefore exist 
in the very nature of things, and it seems logical to refer all organic compounds 
to this type, since they are all in fact derived from it. 

For example — translating Kolbe's symbols into modem practice, and starting from 
carbonic acid, (H0)2C0- — ^when a jhydroxyl group, HO, is replaced by a hydrogen, H, atom, 
formic acid, H.CO.OH, is formed ; replacing OH by CHg furnishes acetic acid, CH3.CO.OH ; 
replacing two OH-groups by two H-atoms yields formaldehyde, H CO.H. If an OH-group 
be replaced by an H-atom, and an 0-atom by two H-atoms, methyl alcohol, CH3.OH, 
results ; and if an OH-group is replaced by CHg, and an O-atom by two H-atoms, ethyl 
alcohol, CH3.CH2.OH, is formed. 

H. Kolbe is here perhaps a little inconsistent, for C. A. Wurtz, in his Histoire des 
doctrines chimiques depuis Lavoisier jusqu'd nos jours (Paris, 1869), has pointed out 
that water and ammonia are agents as indispensable as carbonic acid in the pro- 
cesses of vegetable life. Kolbe's objections to C. F. Gerhardt's or F. A. Kekule's 
types also apply to his own carbonic acid type. 

The doctrine of valency. — In conformity with the general views of chemists 
early in the nineteenth century, J. L. Gay Lussac,^ in his Recherches sur Vacideprussique 
(1815), regarded salts as products of the union of an equivalent of an acid with an 
equivalent of the base, but T. Graham's important Researches on the Arseniates, 
Phosphates, and Modifications of Phosphoric Acid, published in 1833, showed that 
phosphorus pentoxide can unite with one, two, and three equivalents of water to 
form definite acids which can respectively unite with but one, two, and three 
equivalents of the base to form definite salts with characteristic properties. Five 
years later, J. von Liebig, in his memoir Ueber die Constitution der organischen 
Sduren (1838), found other acids to behave in a similar manner, and he employed 
the terms mono-, di-, tri-, and poly-basic acids to indicate the saturation" capacity 
of the acids for the bases. The idea of basicity was further extended to organic 
radicles ; and, in 1834, J. B. A. Dumas showed that an atom of hydrogen could 
be replaced by an atom of chlorine, but only by the equivalent of half an atom of 
oxygen, so that these quantities of chlorine and oxygen are equivalent to an atom 
of hydrogen. 

In his memorable paper On a New Series of Organic Compounds containing 
Metals, published in 1852, E. Frankland applied the idea of equivalency or satura- 
tion capacity to the elements. He showed that the power of the metals to combine 
with oxygen is reduced when the metal is copulated with compound radicles in such 
a way that, say, stannic ethyl oxide, (C2H5)2SnO, is to be regarded as stannic oxide, 
Sn02, in which one oxygen atom is replaced by two ethyl radicles ; and stannic 
ethide, Sn(C2H5)4, as stannic oxide with the two oxygen atoms replaced by four 
ethyl radicles. E. Frankland then remarked : 

When the formulae of inorganic chemical compoimds are considered, even a superficial 
observer is impressed with the general symmetry of their construction. The compounds 
of nitrogen, phosphorus, antimony, and arsenic especially exhibit the tendency of these 
elements to form, com/pounds containing three or five atoms of other elements ; and it is in these 
proportions that their affinities are best satisfied. . . . Without offering any hypothesis 
regarding the cause of this symnietrical grouping of atoms, it is sufficiently evident, from 
the examples just given, that such a tendency or law prevails, and that, no matter what the 
character of the uniting atoms may be, the combining power of the attracting element is 
always satisfied by the same number of these atoms. 

Thus Frankland led chemists to see that within certain limits the atoms of the 
elements possess definite saturation capacities ; and he proved that copulation 
is a consequence of the saturation capacity of the elements. In 1877, Frankland 
added that the hypothesis just outlined ' constitutes the basis of what has since 


been called the doctrine of atomicity or the equivalence of the elements." The far- 
reaching importance of the above quotation from Frankland was not realized until 
some years afterwards. A vague inkling of the operation of some such law among 
organic compounds was probably at the back of the minds of the founders of the 
different theories of types, for in representing chemical transformations as the result 
of substitutions of atoms or groups of atoms, the equivalency of the substituents 
must have been tacitly assumed ; but they were prevented from realizing the 
importance of the principle by laying too much stress on the position rather than on 
the nature of the atoms concerned. 

F. A. Kekule seems to have considered himself to have been the originator of 
the doctrine of the valency, or, as he termed it, the atmnicity of the elements. As a 
matter of fact, in 1854, two years after the publication of E. Frankland's paper, 
F. A. Kekule did obtain a clearer vision of the doctrine, and, in 1857, he explained 
the existence of primitive types — simple and mixed — by means of the valency of the 
constituent elements. Soon afterwards, A. S. Couper, independently of F. A. 
Kekule, published a paper, Sur une nouvelle theorie chimique (1858), in which he 
deduced constitutional formulae for many compounds from the valency of the 
elements, or rather, what he called affinity of degree of the elements as contrasted 
with the ordinary manifestations of chemical affinity, or, as he called it, elective 
affinity. A. S. Couper, for the first time, also represented the composition of com- 
pounds by joining the symbols of the elements or compound radicles by means of 
hyphens or linking bonds. In his Lehrhuch der organischen Chemie, oder der 
Kohlenstoff-Verhindungen (Stuttgart, 1859), F. A. Kekule symbolized the valency 
of an atom in graphic formulae by means of a diagram whose size represented the 
valency as illustrated in the following examples : 

Hydrogen chloride, HCl. Water, HgO. Sulphur dioxide SOg. Nitric acid, HNO3. 

There was no intention, of course, to convey any idea of the relative dimensions of 
the atoms. In 1865, A. C. Brown suggested a system in which the symbol of the 
element was surrounded by a circle, with a number of radiating Hues corresponding 
with the valency of the element. For instance, 

©KB) ®:® 

Hydrogen chloride, HCl. W^ater, HgO. Sulphur dioxide, SOg. Nitric acid, HNOg. 

The grouping was not meant to indicate the physibal but rather the chemical position 
of the atoms. E. Frankland adopted practically the same system in 1866, except 
that he omitted the circles round the symbols of the elements, and this method of 
pictorially representing the linking of the atoms of a molecule in definite order is 
virtually that employed by A. S. Couper, and it has persisted up to the present day. 
A. M. Butleroff (1861) followed up A. S. Couper's idea, and defined the structure of a 
chemical compound to be the mode in which the atoms are mutually linked together 
in the molecule. This does not afford any information of the position of the indi- 
vidual atoms in space. The chemical characteristics of a compound, said Butleroff, 
depend first upon the nature and relative quantity of its elementary constituents, 
and then on its chemical structure. 

F. A. Kekule (1857) classified the elements according to the replacing values of 
their atoms. Hydrogen, chlorine, potassium, etc., were called monobasic o-r mon- 
atomic elements ; oxygen and sulphur were dibasic or diatomic ; nitrogen, phos- 
phorus, and arsenic were tribasic or triatomic ; and carbon was classed as a tetra- 
basic or tetratomic element. There is an incongruity in the use of the terms 
mono-, di-, . . . atomic, since similar terms are employed to represent the number 


of atoms in a molecule ; and the confusion in the use of the terms mono-j 
bi-y . . . basic atoms with J. von Liebig's polybasic acids, led E. Erienmeyer (1860) 
to propose the terms ein-, zivei-^ drci-, and vier-iverthig which have come into use 
in Germany ; the equivalent uni-y bi-, ter-, and quadri-valent, used by L. Meyer, 
or mono-, di-, tri-, and tetra-valent, with W. Odling's alternative terms (1864) : 
monad, dyad, triad, and tetrad are now in use. J. Wislicenus used the terms 
monaffin, diaffin, triaffin, and tetraffin. In 1855, W. OdUng placed dashes beside 
the symbol of the atom or radicle to express what he called the replaceable, or 
representative, or substitution value of the atoms, and he recognized, as E. Frank- 
land did in 1852, that an atom can have more than one replaceable value. Various 
terms were used in place of valency during the clarification of the concept — e.g. 
saturation cajpacity, combining capacity, atom-fixing power, affinity units, affinity 
of degree, basicity, and atomicity. The two latter terms are objectionable. 
A. W. Hofmann (1865) considered that atomicity is a barbarous term ; and is best 
reserved to express the number of atoms in a molecule of an element ; the term 
basicity is also best retained to express the number of stages in which the replace- 
able hydrogen of an acid can be substituted by a metal. A. W. Hofmann did much 
to spread a knowledge of the doctrine of valency. He employed the term quanti- 
valence " to designate the particular atom-compensating power inherent in each of 
the elements," and added " this power must by no means be confounded with the 
specific intensity of the respective activities of the atoms." H. Wichelhaus ^ 
shortened A. W. Hofmann's quantivalence to valency (or valence) in 1868 : and 
H. Wichelhaus' term is now in general use. 

The doctrine of valency introduced by E. Frankland and amplified by F. A. 
Kekule soon stilled the controversies which had been waged between the advocates 
of the radicle and type theories. The nature of the problem was changed. Chemical 
formulae were no longer employed to represent types of double decomposition, 
but rather to show the relations which subsisted between the constituent atoms 
of a molecule. The doctrine of valency enabled chemists to see, as in a glass darkly, 
the intimate structure of the molecules by establishing the way in which the atoms 
are bound together. Consequently, neither the type nor the radicle theory could claim 
a victory, for the theory of composition based upon valency absorbed and assimilated 
them both ; it showed that chemists had really admitted a water type because 
there is a bivalent element oxygen ; an ammonia type because there is a tervalent 
element nitrogen ; and a methane type because there is a quadrivalent element 
carbon. As F. A. Kekule's mixed metaphor expressed it : " Both sides had been 
striving towards the same goal by different paths ; each side thereupon profited 
by the experience of the other, and with united forces sailed onward on the reunited 


1 A. Ladenburg, Vortrdge iiber die Entwicklungsgeschichte der Chemie, Braunschweig, 1869 ; 
A. L. Lavoisier, G. de Morveau, and A. F. de Fourcroy, Mithode de nomenclature, Paris, 1787 ; 
J. B. A. Dumas and P. F. G. Boullay, Ann. Chim. Phijs., (2), 37. 15, 1828 : J. J. Berzelins, 
Jahresh., 9. 286, 1830 ; 13. 190, 1834 ; Liebig's Ann., 3. 282, 1832. 

2 A. Ladenburg, Vortrdge iiber die Entwicklungsgeschichte der Chemie, Braunschweig, 1869 ; 
J. L. Gay Lussac, Ann. Chim. Phys., (1), 95. 136, 1815 ; F. Wohler and J. von Liebig, Liebig's 
Ann., 3. 249, 1832 : J. von Liebig, Liebig's Ann., 25. 3, 1837 ; 9. 1, 1834 ; 11. 10, 1834 ; 19. 270, 
1836 ; Pogg.Ann., 21. 533, 1831 ; Ann. Chim. Phys., (2), 37. 15, 1828; J. B. A. Dumas and J. von 
Liebig, C(mpt. Rend., 5. 567, 1837 ; R. Bunsen, Liebig's Ann., 24. 271, 1837 ; 31. 175, 1839 ; 
37. 1, 1841 ; 42. 14, 1842 ; 46. ], 1843 ; J. J. Berzelius, Pogg. Ann., 28. 626, 1833. 

3 J. B. A. Dumas, Ann. Chim. Phys., (2), 56. 113, 140, 1834 ; (2), 73. 73, 1840 ; J. B. A. Dumas 
and J. S. Stas, ib., (2), 73. 113, 1840; J. B. A. Dumas and E. M. Peligot, ib., (2), 74. 5, 1840; 
J. B. A.-Dumas, C&mpt. Rend.,Q. 699, 1838 ; 7. 474, 1838 ; 8. 609, 1839 ; 10. 149, 1840 ; A. Laurent, 
Ann. Chim. Phys., (2), 52. 275, 1833 ; (2), 59. 196, 1835 ; (2), 60. 220, 1835 ; (2), 61. 125, 1836 
(2), 63. 27, 42, 207, 377, 1836 ; (3), 18. 266, 1846 ; Compt. Rend., 10. 409, 1840 ; H. V. Regnault: 
Ann. Chim. Phys., (2), 59. 358, 1835 ; Liebig's Ann., 15. 60, 1835 ; 30. 139, 1839 ; J. von Liebig 
Liebig's Ann., 31. 119, 1839 ; 32. 72, 1839 ; 33. 301, 1840 ; 50. 295, 1844 ; C. F. Gerhardt, Ann 
Chim. Phys., (2), 72. 184, 1839; Precis de chimie organique, Paris, J 842; Journ. prakt. Chem 


(1), 27. 439, 1842 ; (1), 30. 1, 1843 ; H. F. Meslen, Ann. Chim. Phys., (3), 10. 233, 1842 ; E. 
Grimaux and C. Gerhardt, Charles Gerhardt, sa vie, son oeuvre, sa correspondence, Paris, 1900 ; 
C. Schorlemmer, Bise and Development of Organic Chemistry, London, 1894. 

* C. A. Wurtz, Compt. Berid., 28. 223, 1849 ; A. W. von Hofmann, Liebig's Ann., 74. 174, 
1850 ; A. W. Williamson, Jotirn. Chem. Soc., 4. 106, 229, 1852 ; H. Kolbe, ib., 7. Ill, 1855 ; 
W. Odling, ib.,7. 1, 1855; F. A. Kekule, Liebig's Ann., 104. 129, 1867; T. S. Hunt, Amer. Joum, 
Science, (2), 6. 170, 1848; (2), 7. 175, 1849; (2), 8. 89, 1849; A. Laurent, Ann. Chim. Phys., (3), 
17. 331, 1846; (3), 18. 266, 1846; H. Kolbe, Liebig's Ann., 45. 41, 1843; 54. 145, 1845; 69. 
258, 1849 ; 75. 211, 1850 ; 76. 1, 1850 ; 113. 293, 1860 ; Handworterbuch der Chemie, Braun- 
schweig, 6. 802, 1855 ; Veber die chemische Konstitution der organischen Kohlenwasser staff e, 
Braunschweig, 1869 ; H. Kolbe and E. Frankland, Liebig's Ann., 65. 288, 1848 ; H. Kolbe, ib., 
101. 257, 1857. 

s J. L. Gay Lussac, Ann. Chim. Phys., (1), 95. 136, 1815 ; T. Graham, Phil. Trans., 123. 253, 
1833 ; J. von Liebig, Liebig's Ann., 26. 113, 1838 ; E. Frankland, Phil. Trans., 142. 417, 1852 ; 
J. B. A. Dumas, Liebig's Ann., 32. 101, 1839 ; F. A. Kekul6, ib., 106. 129, 1858 ; 104. 133, 1857 ; 
Ber., 23. 1265, 1890 ; A. S. Couper, Compt. Bend., 46. 1157, 1858 ; Phil. Mag., (4), 16. 104, 1858 ; 
N. N. ButlerofE, Zeit. Chem,, 4. 549, 1861 ; E. Erlenmeyer, ib., 6. 65, 97, 609, 1863 ; 7. 1, 72, 628, 
1864 ; W. Odling, Journ. Chem. Soc, 7. 1, 1855 ; A. C. Brown, t6., 18. 230, 1865 ; Proc, Boy. Soc. 
Edin., 5. 429, 561, 1866. 

^ H. Wichelhaus, Liebig' s Ann. Suppl., 6. 257, 1868 ; A. W. von Hofmann, Introduction to 
Modem Chemistry, London, 169, 1865 ; L. Meyer, Die modernen Theorien der Chemie, Breslau, 67, 
1864 ; E. Erlenmeyer, Zeit. Chem., 3. 540, 1860 ; J. Wislicenus, Liebig's Ann., 128. 2, 1863. 

§ 17. Attempts to explain Valency 

The general test of truth is evidence. — J. M. C. Duhamel. 

The composition of all chemical compounds, says H. von Euler (1903), can be 
regarded as a function of a valency force — Valenzkraft — which is probably of an 
electric nature, and dependent on the temperature, pressure, and the nature of the 
solvent. Numerous attempts have been made to invent some peculiarities in the 
structure of the atoms which will explain that strange power manifesting itself 
as valency. Even Lucretius attributed the differences in the behaviour of his atoms 
to differences in their shape, size, and mode of motion. The subject has rather lent 
itself to hypotheses established by the absence of a knowledge of contradictory 
facts. A brief resume of the more striking forms of these hypotheses may act as a 
danger beacon. 

I. Differences in the valency of different elements have been explained by 
supposing that an atom of an n-valcnt element is compounded of n units, each of which 
is capable of attracting one other unit. A constant quantity of one element, said 
E. Erlenmeyer (1862), i never binds itself to more or to less than a constant quantity 
of another element-— this he called the law of constant affinivalencies. W. Odling 
(1855) called these attracting units suh-atoms ; G. Ensrud (1907), Kernen or nuclei ; 
L. Knorr (1894), Yalenzkorfer or valency bodies; E. Erlenmeyer (1867), affinivalencies ; 
A. W. von Hofmann (1865), minimum atom-binding quantities of an element ; and 
J. Wislicenus (1888), primitive atotns, which are located in certain parts of the atom 
and from which they exert their influence. W. Lossen, in an important paper Ueber 
die Vertheilung der Atome in der Molekul (1880), pointed out that this hypothesis 
cannot be sound, for if a constant mass of, say, carbon binds itself to a constant 
mass of oxygen in the molecule of carbon dioxide, CO2, the same mass of carbon is 
bound to half the same constant mass of oxygen in carbon monoxide, CO. Hence, 
the assumed constant mass must be variable. G. Ensrud (1907) supposed an atom 
to be compounded of an enveloping shell of a substance of small density with a 
nucleus of great density and eccentric shape. The envelopes of different atoms 
repel one another, the nuclei attract one another in the direction along which valency 
acts. An atom of an w-valent element has n nuclei. This hypothesis recalls J. F. 
Redtenbacher's Das Dynamidensystem (Mannheim, 1857). Some of these hypo- 
theses appear to have arisen by confusing the fractional parts of an atom with 
fractional parts of its weight, and assuming that the former are equal to the 

VOL. I. Q 


latter. There is nothing to show that if the atom were divided up into a number 
of attracting portions, each would be the same fractional part of the weight 
of the atom. The modern electron hypothesis of valency is one form of this 
hypothesis — vide Vol. III. 

II. Other hypotheses assume that valency is an attracting force localized at certain 
parts of the atom. The atoms are supposed to be joined together at these attracting 
points ; in other words, some parts of the atom are less active than others. This 
hypothesis has taken various forms. E. Erlenmeyer (1867) and A. Michaelis (1872) 
suggested that the attractive forces are not exerted uniformly in all directions as is the 
case with gravitation, but are specially strong in certain definite directions so that 
a straight line joining two atoms directly bound together expresses the direction 
of the mutually exerted force. A. Michaelis supposed an %-valent atom to have 
n such directions, and, if it is bound by n—x bonds, to have these mutual actions 
exerted in n—x such directions. A. C. Brown (1861-79) assumed that each atom 
possesses two kinds of attractive forces — positive and negative — and the point 
towards which these forces act was called a pole or active point. He made no as- 
sumption as to the nature of the attractive or repellent forces. An /i-valent 
element has n such positive and negative poles. When two atoms unite, the positive 
pole of the one attracts the negative pole of the other, and vice versa. When a 
bivalent atom combines with two univalent atoms, the forces emanating from the 
bivalent atoms will be divided between its two poles in some proportion depending 
on the forces of the two univalent atoms. In order to support the assumption that 
valency is due to centres of attraction localized on the atom, subsidiary hypotheses 
have to be invented. For instance, it has been assumed (i) that the atoms are bound 
to one another through the attraction of electric or magnetic charges localized on 
the atoms ; and also (ii) that the intensity of the attractive force is modified by the 
shape of the atom. 

(i) Electric charges localized on the atom. — The idea that the reacting units 
are polarized, and carry definite electric charges, each charge representing 
one valency, naturally grew from Davy's and Berzelius' electrochemical 
hypothesis, and Faraday's work. There are many modified forms of the 
hypothesis. For example, V. Meyer and E. Riecke (1888) assumed that the 
carbon atom is surrounded by an aethereal envelope which, in the case of iso- 
lated atoms, has a spherical shape like that supposed to be possessed by the 
atoms themselves. The atom in the core carries the specific affinities ; the 
sethereal envelope is the seat of the valencies. Each valency is determined 
by the presence of two opposite electrical poles — called double or di-poles — 
situated at the ends of a straight line which is small in comparison with the 
diameter of the sethereal shell. The four valencies of carbon are represented 
by four such di-poles each of which is able to move freely within the aethereal 
shell, and to turn freely about its middle point. The carbon atom attaches 
other atoms to its surface by the attractions of the di-poles. The modern form 
of the electric charge hypothesis will be discussed later. 

(ii) The shape of the atom. — J. H. van't Hoff, in his Ansichten iiher die 
organische Chemie (Braunschweig, 1881), showed that the attractive forces 
emanating from an atom will be uniform in all directions if the atom is spherical, 
but if the shape be not spherical the intensity of the force, at short distances, will 
be more concentrated in certain spots than in others. Thus, if the atom were 
shaped like a regular tetrahedron, it would behave as if it were quadrivalent, 
for the centres of the four bounding faces would represent maximal attractions. 
Given the number of maximal points on the atom, it would be possible to deduce 
the valency, and conversely. There will be as many maximal points as the 
figure has sides. If the faces are unequally distant from the centre, the 
maximal points may not all have the same value, so that, when the nature of 
the united atoms also determines the attracting power, the number of effective 
valencies of the attracting atom will be affected, and a change of valency will 


be observed on comparing combinations of an element with other different 
elements. J. Wislicenus (1888) has expressed a similar idea ; he said : 

It is not impossible that the carbon atom more or less resembles — perhaps very 
closely — the form of a regular tetrahedron ; and further, that the causes of 
those attractions which are exhibited by the so-called units of affinity or bonds 
are concentrated at the apices of this tetrahedral structure, so that where there 
is least matter there is most force. These attractions are possibly analogous 
to the electrical state of a metal tetrahedron charged with electricity. 

If the atoms be also in rapid vibratory motion, only the parts where the 
greatest attractions are exerted can retain their contacts, and therefore valency- 
will be reduced by a rise of temperature, for a rise of temperature probably 
augments the vibratory motions of the atoms. 

III. Another set of hypotheses has assumed that valency is due to the need for 
harmonizing the motions of the combining atoTns so as to form complexes whose parts 
move in stable equilibrium. One form of this hypothesis is indicated later on. 
According to L. Meyer (1884), the atoms in a molecule are not in a state of rest, 
but they move rotationally about a centre of equiUbrium ; the orbits of similar 
atoms in the molecules of the same substance are the same so that equivalent 
atoms have the same paths, but the orbits of different atoms are greater, the greater 
the valency of the atom. E. Molinari (1893) suggested a modification of this hypo- 
thesis in a paper entitled Motochemistry (moto, motion). The valency of an atom 
in a molecule is determined by the nature or energy of its oscillatory motion ; and 
he claims that the constitution of compounds is dependent upon the intramolecular 
movements rather than on the relative positions of the atoms in space. F. A. 
Kekule (1872) considered that valency is determined by the relative number of 
impacts which an atom receives from other atoms in unit time ; each of the uni- 
valent atoms in a diatomic molecule impinges once, while the bivalent atoms 
impinge twice in unit time. It is not very clear how this explains valency, and in 
1878, F. A. Kekule said that " the nature of the motion of atoms, unknown at present, 
may be imagined as an oscillatory one in such a way that the number of oscillations 
executed in unit time exactly represents the valency of the atoms." F. M. Fla- 
vitzky (1896), following N. N. Beketoff (1880),^ supposed that the atoms move 
in curves which lie in planes parallel to one another ; the atoms of different elements 
move in planes which are inclined at definite angles to one another ; the motion 
of the atoms of one element can be completely counteracted by the motions of the 
atoms of another element only when the two planes of motion are parallel ; other- 
wise, according to the size of the angle between the planes of motion, an atom of 
one element may require two, three, or more atoms of another element to balance it ; 
and only those components come into action which are parallel to the plane of motion 
of another atom. Accordingly, F. M. Flavitzky refers the valency of an element to 
the difference in the angles between the planes of the orbits of the different rotating 
atoms. J. H. van't Hoff, in his Die Lagerung der Atome im Raume (Braunschweig, 
1894), argued against the hypothesis which ascribed isomeric phenomena to the 
varied motions of the atoms because temperature presumably favours atomic 
motions, and yet the phenomena become less and less complex as the temperature 
rises, and constantly more complex as the temperature falls. 


1 E. Erlenmeyer, Liehig's Ann., 131. 124, 1864 ; Zeit. Chem., 6. 65, 97, 609, 1863 ; 7. 1, 72, 
628, 1864 ; W. Odling, Journ. Chem. Soc, 7. 1, 1855 ; A. C. Brown, On the Theory of Chemical 
Combination, Edinburgh, 1861 (1879) ; A. W. von Hofmann, Introduction to Modern Chemistry, 
London, 1865; G. Ensrud, Zeit. phys. Chem., 58. 257, 1907; L. Knorr, Liebig's Ann., 219, 
202, 1894; J. Wislicenus, Ber., 21. 681. 1888; W. Lessen, ib., 20. 3306, 1887: Liebig's 
Ann., 204, 336, 1880; A. Michaelis, Ber., 5. 411, 1872; Liebig's Ann., 315. 58, 1901 ; H. Davy, Phtl. 
Trans., 97. 1, 1807 ; J. J. Berzelius, Schweigger's Journ., 6. 119, 1812 ; Essai sur la thiorie des 
proportions chimiques et sur V influence chimique et electricite, Paris, 1819 ; M. Faraday, Phil. 
Trans., 124. 77, 1834 ; V. Meyer and E. Riecke, Ber., 21. 946, 1888. 


2 N. N. Beketoff, Ber., 13. 2404, 1880; F. M. Flavitzky, Zeit. anorg. Chem., 19. 201, 1896; 
E. Molinari, Joum. prakt Chem., (2), 48. 113, 1893 ; L. Meyer, Die modernen Theorien der Chemie 
und ihre Bedeutung fiir die chemische Mechanik, Breslau, 1884 ; London, 1888 ; F. P. Venable, 
Joum. Amer. Chem. Soc., 21. 192, 220, 1899 ; F. M. Flavitzky, Zeit. anorg. Chem., 11. 264, 1896. 

§ 18. Atomic, Molecular, and Specific Volumes 

Modem developments in crystallography indicate with ever increasing distinctness 
that the chemical atom even when its individuality is shrouded by combination with other 
different atoms, exhibits characteristics which are essentially its own, and which are 
discernible in the compounds into which it enters. — W. J. Pope and W. Barlow (1907). 

So far as the balance can indicate, the weight, and by inference the mass, of 
an atom remains uniformly constant during all chemical changes ; but the evidence 
is less clear with respect to the volume or space occupied by the atoms of an element 
when it enters into chemical combination. A. le Royer and J. B. A. Dumas i opened 
up the subject in 1821 with an attempt to determine the equivalent volumes of 
the elements by dividing their atomic weights by their respective specific gravities ; 
the quotients were called atomic volumes. 

The atomic volume of an element is obtained by dividing the atomic weight by its 
specific gravity ; similarly the molecular volume represents the moleciilar weight divided 
by the specific gravity. Consequently, the atomic volume represents the space occupied 
by the aggregates of atoms, including the interstitial spaces, whose weights are proportional 
to the atomic weight ; otherwise expressed, the volume occupied by a quantity of the 
element proportional to the atomic weight. The term equivalent volume was used before 
the concept of the atom had been clarified by Avogadro's hypothesis. At the suggestion 
of J. J. Berzelius, H. SchrSder employed the term tnolecular volume in place of equivalent 
volume ; and H. Kopp's term specific volume had the same cormotation. It has 
been urged that the term specific volume is objectionable because the specific gravity of 
a body is the weight of unit volume, and the term specific volume by analogy suggests the 
volume of unit weight. The terms atomic volume and molecular volume here employed 
are defined by the ratio 

Atomic weight . , . , Molecular weight ,, , 

-^ r^ .^ = Atomic volume ; -5 r^ ?- — =Molecular volume. 

Specific gravity Specific gravity 

Consequently, if the atomic or molecular weight be expressed in grams, the atomic or 
moleciilar volume respectively denotes the number of cubic centimetres occupied by a 
gram-atom or gram-molecule. 

It follows from Avogadro'shypothesisthatallgaseshave the same molecular volume. 
If the centres of gravity of the molecules of liquids were situated at the same average 
distance apart — as they probably are with gases — a given volume of different 
liquid would contain the same number of molecules ; and the molecular weights of 
different liquids would be proportional to the specific gravities — as is also probably 
the case with gases. Similar remarks apply to solids. With liquids and solids, 
however, the molecules must be located at different distances apart because the 
molecular weights of different liquids and solids are not proportional to their specific 
gravities. The molecular volumes of liquids and sohds do not exhibit the same 
uniformity as those of gases. This might have been predicted from the fact 
that while the coefficients of thermal expansion and the compressibilities of the 
different gases are approximately the same, each solid and each liquid has its own 
characteristic constant. 

The molecular volume of gases can be compared at an arbitrarily defined standard 
temperature and pressure ; but since liquids are obviously not in the same molecular 
condition, they are therefore not under comparable conditions at any one arbitrarily 
defined temperature. Consequently, H. Schroder^ suggested that liquids would be 
more nearly in the same comparable state at the temperatures at which their 
vapour pressures are the same — e.g. at their boihng points under a standard pressure. 
In the case of solids, the effect of temperature is not so marked as with hquids, and 
in the first approximation, the specific gravity is taken at a convenient atmosphere 
temperature — say 0°, 4°, 15°, etc. A. Horstmann, W. Lessen, and A. Bartoli 


contend that (i) while the so-called atomic volume refers not only to the space filled 
by the atom, but also to the space in which the atom oscillates, it is not likely, 
a priori, that the molecules will be in the same state at 1°, the boiHng point of butane, 
as they are at 317°, the boiling point of octadecane ; (ii) relations similar to those estab- 
lished at the boiHng temperature are likewise manifest at, say, the arbitrary tem- 
perature 0° ; and (iii) the boiling point cannot be a strictly comparable state because 
it is affected by pressure to a different extent in the case of different liquids. 
G. Tschermak, F. Krafft, and G. le Bas take the melting point as a comparable 
state. In a valid corresponding state, the pressure, temperature, and volume should 
be expressed in terms of their critical values, and T. E. Thorpe has emphasized the 
fact that C. M. Guldberg has shown that the ratio of the critical temperature Tc 
to the absolute boiling point Tj approximates to a constant. Consequently, the boiling 
temperatures are approximately equal fractions of the critical temperatures. Con- 
sequently, properties like the molecular volume which change but slowly with 
temperature, are comparable at the ordinary boiling points. The results by the 
different methods do not show any very decisive evidence in favour of any one 
method, since relations which are revealed by the one may be obscured by the 
other. I. Traube emphasized the disturbing effects of molecular association and 
claimed that this can be eliminated by determining the molecidar volume in dilute 
solution. The idea was applied many years previously by L. Playfair and J. P. 
Joule, who argued that " solution in water is the obvious means of destroying the 
cohesion of a body without at the same time altering its chemical properties." 

From their observations on atomic volumes A. le Koyer and J. B. A. Dumas tried 
to show that the atomic volumes are multiples of one and the same number and 
thus form an arithmetical series, but more extended investigations proved this 
tentative hypothesis was not in accordance with fact. At this period, the chemical 
combination of gases in volumetric proportions was attracting much attention, and 
attempts were made to show that solids likewise unite in definite volumetric pro- 
portions. For example, W. Herapath ^ tried to prove that the atomic volume of 
oxygen in a metal oxide bears a simple numerical relation to that of the metal with 
which it is combined. The same problem was attacked by C. J. B. Karsten (1832) 
and by P. F. G. BouUay (1840). Here again, more accurate observations falsified 
the hypothesis. F. Ammermiiller (1840) concluded from his observations that the 
molecular volumes of compounds containing the same elements in different propor- 
tions are either the same, or else stand to one another in rational proportions. J. F. 
Persoz (1839) showed that equivalent amounts of many compounds of analogous 
composition have the same molecular volume, and he tried unsuccessfully to 
establish A. le Royer and J. B. A. Dumas' arithmetical rule. 

H. Kopp's first publication, Ueber die Voraushestimmung des specifischen Gexvichts 
einiger Klassen chemischer Verhindungen, appeared in 1839 and his last publication 
on the subject was made in 1889. The earlier papers are mainly occupied in collect- 
ing material and in finding the best conditions for comparing the data. In 1844 
H. Kopp tentatively concluded : 

(1) Equal differences in composition correspond with equal differences in specific 
volume. (2) Equivalent amounts of oxygen and hydrogen in liquid compounds occupy 
nearly the same volume. (3) The specific volume of a compound is equal to the sum 
of the specific volumes of its components. The same element almost invariably preserves 
the same specific volume. Isomeric compoimds have the same specific volumes which 
stand to one another in the same relation as the molecular weights of the compounds. 
Variations in the chemical constitution of isomeric compounds are without effect on their 
specific volume. (4) Comparisons of specific volumes of liquids are only valid at tempera- 
tures at which the vapour pressures of the liquids are equal. 

H. Kopp considered that these conclusions did not rest on a very firm experimental 
basis, and he therefore made accurate determinations of the physical constants 
required for testing them rigorously. The results of this work enabled him 
to take a general survey of the subject in his memoir, Beitrdge zur Stijchiometrie 


der physikalischen Eigenschaften chemischer Verhindungen, 1855. His main 
conclusions were : 

(1) The selection of the temperature of equal vapour pressure as a basis of comparison 
seems to be warranted by the fact that regularities are thereby made evident which 
otherwise are not apparent. (2) Differences of specific volume are proportional to differ- 
ences in chemical composition. (3) Isomeric liquids of the same chemical type have equal 
specific volumes. (4) The substitution of hydrogen for an equivalent amount of oxygen 
only slightly affects the specific volume. (5) One atom of carbon can replace two atoms 
of hydrogen without altering the specific volume. 

The molecular volumes of the members of a homologous series of liquids which 
difier in composition by CH2 increase nearly 22 units for each increment of CH2. 
Thus, the molecular volume of formic acid, H.COOH, is 41'3 ; of acetic acid, 
CH3.COOH, 63-6 ; and of propionic acid, C2H5.COOH, 856. Hence, the mole- 
cular volume of the group CH2 is 22. Further, the replacement of one atom of 
carbon by 2 atoms of hydrogen in a compound usually makes no marked change 
in the molecular volume, and hence it is inferred that the atomic volume of carbon 
is nearly equal to the molecular volume of H2. Since the molecular volume of 
CH2 is 22, it follows that the atomic volume of carbon is 11. The difference, 
22 — 11=11, thus represents the molecular volume of H2, and the atomic volume of 
hydrogen is 5'5. Again, the molecular volume of water is 18"8 ; deduct 11, the 
value of H2, and the atomic volume of oxygen 7*8 remains. The molecular volumes of 
a large number of compounds can be calculated from the data so obtained, and 
compared with those obtained by actual experiment. The results for many carbon 
compounds are quite satisfactory. Thus, with alcohol, C2H5OH, the molecular 
volume will be (2 Xll)+(6 x5-5)+7-8=62-8. The observed value is 62-2. Hence, 
if a compound contains ni atoms of atomic volume Aj ; 7^2 atoms of atomic 
volume A2I . . . , the 

Molecular volume, 'y=%i^2-|-W2^2"i" • • • 

H. Kopp here over-emphasized the additive character of this property, but he 
did point out that the specific volume of a liquid is determined not only by its com- 
position but also by its constitution, for he found that the relative position of the 
oxygen atom in a molecule affected the specific volume. The atomic volumes of the 
oxygen atoms in carbonylic and hydroxy lie oxygen are respectively 122 and 7 "8. 
The idea will be clear by comparing methyl alcohol, CH3OH, with formaldehyde, 
H.COH, and with formic acid, H.COOH— 

H>C<OH ^-^<l 0=^<0H 

Methyl alcohol Formaldehyde Formic aoid 

(Hydroxylic oxygen). (Carbonylic oxygen). (Hydroxylic and carbonylic oxygen). 

By applying similar methods to those described above, it is found that the atomic 
volume of carbonylic oxygen is 12*2. The molecular volume of methyl alcohol is 
accordingly 4x5-5+ll+7-8=40-8 ; of formaldehyde, 2x5-5+ll+12-2=:34-2 ; 
and of formic acid, 12-2+7'8+2x5*5+ll=42-0. Consequently, it is inferred that 
one and the same atom may have different atomic volumes according to 
the conditions under which it is placed. In further illustration, sexivalent 
sulphur has an atomic volume 120 ; quadrivalent sulphur, 22*6 ; and bivalent 
sulphur, 28*3. Nitrogen in ammonia and related compounds has an atomic volume 
23 ; in cyanogen compounds, 28 ; and in nitroxyl compounds, 33. Hence, the 
molecular volume can sometimes he used (l) for estimating the molecular weight of a 
liquid from its specific gravity and cojwposition ; and (2) it may reveal peculiarities 
in the constitution of the molecule. For instance, it may be used to show whether 
carbonyhc or hydroxylic oxygen is present. 

Examples.- — ^(1) The observed molecular volume of acetic acid, C2H4O2, is 63'7. The 
only molecular voliime compatible with this is 64, deduced on the assumption that the 
compound contains one hydroxylic oxygen atom (7-8), and one carbonylic oxygen (12-2). 


The formula for acetic acid is therefore written CHj— CO — OH. (2) The density of 
phosgene, COClg, at its boiling point, is 1-415. What is the atomic volume of chlorine, on 
the assumption that the atomic volume of oxygen is 12-2; and of carbon U-0 ? Ansr. 
99/1-415 = 12-2 + ll'0 + 2a; ; x, the required atomic volume, is therefore 23-4. 

A large number of solid and liquid compounds — over a thousand — have been 
examined. The pioneer work was done by H. Kopp and extended by many other 
workers.4 With solids the data which have been accumulated are even more difficult 
to deal with, since the disturbing factors seem to be even more perplexing than is 
the case with hquids. Although many additive regularities have been detected 
ranging over a limited number of compounds, yet, almost every investigation has 
emphasized the constitutive nature of this property, and narrowed the range of 
the simple additive rule. Even the increment CH2 in a homologous series, when 
determined at the boiling points, is not additive, for its effect becomes greater as 
the series is ascended, but, as A. Horstmann showed, the effect is not so marked 
when the comparison is made at equal temperatures, or, as F. Krafft has shown, at 
the melting points. H. Kopp thought that isomerides of similar structure had the 
same molecular volume, but P. Dobriner and R. Gartenmeister have shown that 
the effect is related with the boiling points, for the lower boiling isomer has the 
larger molecular volume ; J. C. Brown, A. Zander, T. E. Thorpe, and W. Stadel 
showed that isomers with an *so-structure also have the larger molecular volume. 
F. Neubeck showed that the molecular volume of the benzene derivatives is modified 
according as the groups occupy the ortho-, para-, or meta-position. P. Walden and 
T.Liebisch found that the race mic isomer of stereo-isomerides has the smaller mole- 
cular volume, and I. Traube that the trans-isomer has the greater molecular volume. 
H. L. Buff also showed that the atomic volume of an element varies according to 
its degree of saturation, and that an unsaturated carbon atom has a larger atomic 
volume than a saturated one. Hence, argued H. L. Buff, the atomic volume of 
an element decreases as saturation proceeds. This was confirmed by R. Schiff and 
W. Lossen, who found that on passing from a saturated carbon atom to one with the 
double ethylene linkage, the molecular volume decreases about 8'5 units, and on 
passing from the double ethylene to the triple acetylene linkage, the molecular volume 
decreases about 6*5 units. In a homologous series of ethylene linkages, the effect 
produced by each is rather less than the preceding one. A. Horstmann found that 
" unsaturated compounds with closed chain formulae have considerably smaller 
molecular volumes than those with open chain formulae and multiple hnkages of 
the atoms." R. Willstatter showed that the contraction in the molecular volume 
which accompanies the conversion of a normal chain hydrocarbon to the ring or 
cycloid structure is larger than is caused in passing from a saturated to an un- 
saturated compound. Consequently, molecular volumes are dependent upon 
differences in the structure of the compound as well as on the nature of the atoms 
in the molecule. 

The difference between the molecular volumes of the MO oxides and the atomic 
volume of M gives fairly constant values for the atomic volume of oxygen, but in 
other cases very different values are obtained. Thus, the oxygen in cupric oxide 
has an atomic volume 5"1, and in cuprous oxide 10*5. B. Brauner and J. I. Watts 
have investigated the atomic volumes of the oxides, and found the results are in 
accord with the periodic law, and conclude : 

(1) In strong bases the oxygen has a negative value. (2) In the oxides of heavy metals 
and metalloids the volume of the oxygen is positive. (3) The earth metals unite with 
oxygen without any appreciable change of volume, and thus form a connecting link between 
acids and bases. (4) The higher the specific volume of the element in the oxide, the less 
positive or more negative is the specific volume of the oxygen. (5) The more negative the 
value of the oxygen, the greater is the afifinity of the metal for the oxygen. 

L. Play fair and J. P. Joule ^ noted that the molecular volumes of certain highly 
hydrated salts— e.^. sodium decaquocarbonate, and the alkali dodecaquophosphates 
and dodecaquoarsenates — are exactly equal to that of the water, considered as ice, 


which they respectively contain, so that the molecules of the salt proper seem to exist 
in the interstitial spaces of the water since they exert no apparent influence on the 
bulk. The relation does not hold with salts less highly hydrated — e.g. borax, 
sodium pyrophosphate, and aluminium sulphate — where the molecular volume is 
the joint effect of the water considered as ice, and of the salt. R. Schiff also showed 
that the members of certain classes of hydrated salts have practically the same 
molecular volume — e.g. the alums have a molecular volume of about 277 ; the 
double sulphates of the type M2'M"(S04)2.6H20 have a common molecular volume 
of about 207 ; and the vitriols of the tjrpe M"S04.7H20, isomorphous or not, have 
the same molecular volume 146. T. E. Thorpe and J. I. Watts have further shown 
that the volumes occupied by the several molecules of water in polyhydrates vary 
with the degree of hydration, for the molecular volumes of hydrated salts are not 
usually equal to the sum of the molecular volumes of the anhydrous salt and of the 
water (18"8). With the magnesium sulphates, for example, 

MgSOi pliis ... 1 2 5 6 7 HgO 

Molecular volume . 45-3 55-6 67-0 112-4 130-8 1464 

The first molecule of water, the constitutional water or the water of halhydration of 
T. Graham, here occupies a less volume than the remaining molecules. The second 
molecule of water raised the molecular volume 11 "4 ; the next three molecules of 
water raise the molecular volume an average of 11*8 ; the sixth molecule raises the 
constant ]8*4, and the seventh, 15'6. T. E. Thorpe and J. I. Watts obtained 
analogous results with the series of sulphates MSO4.WH2O, when n varied from to 6. 
This is in harmony with H. Kopp's general conclusion that the water molecules of 
a hydrated salt contribute in different degrees to the total molecular volume, for in 
salts containing a small number of water molecules (1 to 3), he found the average 
molecular volume of the water is 12*4 ; in others containing a larger proportion 
(2 to 7), the average molecular volume is 13"4 ; and in a third class, with the largest 
proportion of water molecules (3 to 10), the average molecular volume is 15'3. 
F. W. Clarke compared similarly the molecular volumes of a series of chlorides 
MCI2.WH2O, when n varied from 2 to 6 ; and for a series of hydrated oxides — 
B2O3.3H2O ; I2O5.H2O ; K2O.H2O ; CuO.HgO ; SrO.H20 ; BaO.HgO ; AI2O3.3H2O ; 
Mn203.H20 ; Fe203.H20. In the former, the molecular volume of the water 
varied from 12*5 to 15*0, and in the latter from 7*4 to 19-4. F. W. Clarke's 
results emphasize the difference between water of crystallization and water of con- 
stitution in that the chemical differences implied by these expressions are connected 
with the relative magnitudes of the spaces occupied by chemically comparable 
quantities of the hydrated salts. The contraction which occurs in the dilution of 
sulphuric acid with water is indicated in Fig. 27, Cap. X. 

The atomic volume of an element obtained by dividing atomic weight by its 
specific gravity is not the same as the atomic volume deduced by H. Kopp from the 
molecular volumes when the element is in combination. The two values are not 
usually the same. For instance, 

H. Kopp, atomic volume 
Calculated from element 

In 1831, T. Thomson compiled a table of atomic volumes of the metals, and noted 
a correspondence in the atomic volume of the elements most nearly related with 
one another. When the atomic volumes are plotted against the atomic weights, 
L. Meyer 6 showed in 1869 that a periodic curve is obtained like Fig. 4 in Cap. VI, 
where (1) the waves increase in amplitude as the atomic weights increase ; (2) the 
elements of similar chemical properties occupy corresponding positions on the 
waves ; (3) the more volatile and easily fusible elements occur on the crests or rising 
portions of the curve, and the elements which fuse with difficulty are in the troughs 
or on the descending portions of the curve. The curve was found by W. Borchers 
to be more regular and the relations between the elements clearer if the equivalent 




















volume — atomic weight -f- maximum valency — be employed in place of the atomic 

The molecular volume of an element varies with the conditions under which the 
molecules are placed. The atom is presumably always in oscillatory periodic 
motion, and this motion gives rise to volume ; consequently, the molecular volume 
is a relative measure of the space inhabited by the molecule ; it represents the 
smallest space which the molecule requires for itself under the existing conditions. 
Similar remarks apply to the atomic volume so that each atom can be regarded 
as a material nucleus surrounded by an envelope, shell, or space — called the 
sphere of action or sphere of influence into which no other atom or mole- 
cule can penetrate. The sphere of influence is thus regarded as the effective 
boundary surface of an atom. This is what is sometimes called the vibratory or 
oscillatory volume of an atom, that is, the space within which the material nucleus 
performs its oscillations. Such a space would have the quasi-rigidity characteristic 
of a material nucleus rapidly revolving about a mean position. There is, however, 
no need to make any assumptions as to the nature of the internal character of the 
atomic nucleus with its encircling shell ; it is not even necessary to assume that the 
complex is spherical. Under ordinary circumstances the complex can be regarded 
as the atom itself, since the so-called sphere of influence is the actual boundary by 
which we know and measure the behaviour of the atom. This is the concept of the 
atomic volume as pictured by D. I. Mendeleeff (1889), 0. E. Meyer (1899), T. W. 
Richards (1901), etc. 

In J. D. van der Waals' equation {p-\-av~^){v—h)=RT, the term b represents 
the volume occupied by the substance, i.e. the molecular volume at absolute zero, 
since at this temperature v becomes equal to b and represents the volume occupied by 
the substance of the atoms — ^the atom nucleus as it may be called — but it is said 
to be four times the actual volume of the molecule. It is not practicable to 
compare the values of b for different substances because of the lack of data ; but 
from the theory of corresponding states, it may be shown that the critical volume Vg is 
three times the value of 6, or Vc=Sb ; and the so-called critical coe£&cient, Tdpc, 
or, the ratio of the critical temperature and critical pressure, is related to b by 
the expression Tclpc=^hlR, where R is the gas constant ^v/273. 

F. Exner ^ showed that, according to R. Clausius and 0. F. Mossotti, (jit^— l)/(jLt2+2) 
is equal to the ratio of the volume actually occupied by matter to the apparent 
volume of the substance, when /ju represents the refractive index for waves of infinite 
wave length, and it is found that fi^ is equal to the dielectric constant. Consequently, 
as P. A. Guye has shown, the product of (/x^— 1)/(jlc2+2) with the molecular volume 
will be a measure of the space filled by matter in a gram-molecule of a substance. 
Consequently, the magnitude 6 of J. D. van der Waals' equation, the critical volume, 
and the critical coefficient may be represented as functions of the molecular refrac- 
tion. I. Traube has shown that b is between 3'5 and 4 times as large as the molecular 
refraction, MR, and P. A. Guye found therelation MR=18TclPc' From I. Traube's 
result, it follows that atomic refraction can be employed as a measure of the material 
nucleus of an atom composed of a material nucleus and an encircling shell or 
sphere of influence. The sphere of influence represents a kind of shell about the 
atom nucleus, and it is presumably that portion of the atom which is permeable to 
light, and constitutes a dielectric medium which enables electromagnetic radiations 
to be transmitted through a body at a speed which is characteristic of the particular 

T. W. Richards 8 has shown that while it is assumed that the molecules of 
a gas are particles moving independently at some distance apart, it is doubtful 
if there are such interstitial spaces in liquids and solids. The impermeability of 
glass to oxygen, nitrogen, and water for long periods does not lend support to the 
view that there are empty spaces between the molecules ; and he limits the per- 
meability which has been observed in rigid compact solids, to such substances as 
can enter into the chemical structure of the soUds themselves. It is therefore 



inferred that in solids and liquids the atoms formed of material nuclei with enveloping 
shell are in close contact with one another at the boundary of their envelopes. 
Such atoms are considered to be compressible and elastic ; they can contract and 
expand, or vibrate among themselves even when their surfaces are closely packed 
together ; and they are quite capable of sustaining and transmitting the vibratory 
motions called heat. E. Griineisen's observations show that the compressibility 
of a number of metals — copper, silver, aluminium, iron, and platinum — loses only 
7 per cent, in cooling from the ordinary temperature down to that of liquid air, and 
by extrapolation very little more diminution will occur in passing down to the 
temperature of absolute zero, so that it is probable that metals are as compressible 
at absolute zero as they are at ordinary temperatures. The value of 6 in J. D. van 
der Waals' equation is fairly constant over a wide range of pressure, but it suddenly 
begins to diminish when very high pressures have been attained. Again, for carbon 
dioxide, the value of h increases as the temperature rises, thus, if a=000874, and 
v=unity at 0° and 760 mm., then 6=51 c.c. per gram-molecule at 0°, and 64 c.c. 
at 100°. For hydrogen likewise, h is 13'8 c.c. at 0° and 15' 1 c.c. at 100°. On the 
other hand, H. K. Onnes has shown that the apparent volume h of helium atoms is 
smaller at high than at low temperatures ; at 0°, h is 12 c.c, and at 100°, 10*4 c.c. 
per 4 grms. of helium. This is not what would be anticipated, and T. W. Richards 





















..J / 






J J 













/J/bmic u/e/^hts — >- 
Fig. 4. — Compressibility and Atomic Volume Curves of the Elements. 

makes the tentative suggestion that " the greater velocity of the colliding atoms 
at the higher temperature has a greater compressing effect so that at high tem- 
peratures the atoms seem to occupy less space than at lower temperatures." 

T. W. Richards has shown that the compressibilities of the elements — i.e. the 
relative contractions in volume per megabar (0'987 atm.) per sq. cm. — are closely 
related to their atomic volumes, for the structure of the two curves is very similar as 
indicated in Fig. 4, where the atomic volume curve is dotted, and the compressibility 
curve is continuous. The greater the densities of the elements the less their 
compressibility. The elements with the larger atomic volumes are the more com- 
pressible and the more easily melted and volatilized. Consequently, the com- 
pressibilities of different substances are not only dependent on the magnitude of 
the applied pressure, but also on the internal pressure produced by the mutual 
cohesive attraction between the particles. In gases, the cohesive pressure is small, 
and accordingly the compressibility is large ; in solids and liquids the cohesive 
pressure is large, and the compressibility is small. T. W. Richards' theory of 
compressible atoms thus reveals the existence of internal cohesive and affinity 
pressures holding the atoms and molecules together. 

Cohesive pressure exerted by the cohesive forces which hind the molecules together. — 
Cohesion manifests itself in various ways ; the most obvious is the mechanical 
resistance which a body offers to the separation of one part of a substance from 
another, and it appears more or less modified in such properties as ductility, 


malleability, tenacity, hardness, surface tension, volatility, etc. The density of a 
given substance is a manifestation of an internal pressure — the greater the density, 
the greater the internal pressure. When a rise of temperature produces a marked 
effect on the volume, it may be assumed that the internal pressure is less than when 
a rise of temperature produces only a slight effect on the volume. Substances in 
which the particles are held together by a high cohesive attraction are usually 
difficult to volatilize ; they have a small atomic volume ; a relatively large density ; 
a high surface tension ; high latent heat of evaporation ; and are least compressible. 
Conversely, if the atomic volume be relatively large owing to a small cohesive 
attraction, these substances will be most volatile and have the greatest com- 
pressibility. In illustration, T. W. Kichards found that the three isomeric xylenes 
agree well with these deductions : 

o-Xylene. m-Xylene. p-Xylene. 

Density (20°) . . . 0-8811 0*8658 0-8611 

Boiling point . . . 144-0° 139*0° 136*2° 

Compressibility (20°) . . 60 X 10-« 63*5 X 10-« 66*2 X 10-« 

Surface tension (20°) . . 3*09 2*96 2*92 mgrm. per mm. 

T. W. Richards also compared some properties of two isomeric butyric esters, and 
found in each case : 

Specific Compressi- comnrSmtv Coefficient of Boiling Surface ^^^^l 

gravity. bility. 'p?Satm expansion. point. tension. ^^n. 

butyrate 0*8785 76-9xlO-« 13-6 X 10-« 0-001247 120*8° 24-58 34-7 
Ethyl iso- 

butyrate 08710 90-8 X 10-« 15-0x10"^ 0*001294 109*8° 23*30 33*9 

The denser substance has the less compressibility, the less decrease in com- 
pressibility with an increase of pressure, the less coefficient of thermal expansion, 
the higher boiling point, the greater surface tension, and the greater heat of 
vaporization. All this is in accord with the assumption that a great cohesion 
produces an internal pressure which is effective in reducing the molecular volume. 
Hence it follows that not only is the atomic volume dependent upon the nature 
and location of the different atoms in the molecule, but also on the cohesive attraction 
of one molecule for another. 

If the atomic volume be related with the cohesive pressure, and if the valency of an 
element in a compound be related with the atomic volume, it might be anticipated that 
th9 cohesion will be a function of the valencies of the combined elements. W. Sutherland 
f oimd that the valencies of the elements in simple substances like sodium chloride influenced 
the cohesion, but he was unable to establish a relationship for more complex bodies. A. P. 
Mathews found empirically that the cohesion factor a of J. D. van der Waals' equation is 
related with Uv, the number of valencies, and the molecular weight M by the expression 
a=0-3l25xlO^^MI!v ; and from the known relations of a to the critical constants T^, v,, 
and pc, it foUows that Mp^j:v=0-00^3Tc^ ; and also that Mi:v=4:-3xlO-^{VcTc)', 
A. P. Mathews uses these expressions for computing the valencies of chlorine, oxygen, 
sulphur, nitrogen, phosphorus, and the elements of the argon family. 

Affinity fressure, produced by the chemical affinity or mutual attraction of adjacent 
atoms. — The atomic volume of an element depends on the nature of the associated 
atoms. In the middle of the eighteenth centur}% R. Kirwan sought to measure 
the force of the attraction between the atoms in a chemical compound from the 
diminution in volume which attended the union of two substances ; while H. Davy 
and others have alluded to the increase in density of the product of the union of 
two substances with a powerful affinity for one another. Thus, W. Miiller-Erzbach 
(1881) said that in similarly constituted solids, those are the most stable which are 
formed with the greatest contraction— e.^r. when lead replaces silver ; potassium, 
sodium ; or, when chlorine replaces bromine or iodine, contraction occurs, and the 
products of the replacement are the more stable. According to F. Ephraim and 
P. Wagner, the molecular volume of a stable compound is smaller than the sum of 



the volumes of its decomposition products, as shown by the schonites studied by 
A. E. H. Tutton. The double alkali magnesium sulphates have smaller molecular 
volumes than the copper or manganese salts, although the atomic volume of the 
metal is greater with the first than with the other two. The stability of the salts 
is more strictly parallel to their molecular volumes than to the atomic volumes of 
the free metals. The percentage contraction on the atomic volume rather than the 
actual contraction suffered by any particular atom is thus the important criterion 
of the stability of a compound. 

D. I. Mendeleeff has shown that the greater the affinity of the elements for one 
another, the less the atomic volume of the resulting compound. Thus the contrac- 
tion which occurs during the formation of potassium or sodium oxide is greater than 
in the formation of stannic oxide. As V. Braun expressed it, the specific gravity of 
solid chemical compounds is high in proportion to the intensity of the affinity which 
unites their components, and G. S. Johnson inferred that the affinity of iodine in 
potassium tri-iodide is small because combination occurs without contraction. 
W. Miiller-Erzbach and I. Traube have emphasized the same idea. Other things 
being equal, elements with the greatest densities have the least chemical 
affinity and sufiei least change in their atomic volumes when they enter 
into combination — these elements will be found distributed about the troughs 
of the curve, Fig. 4. Conversely, elements with the smallest densities are 
usually most energetic chemically and suffer the greatest contraction in 
their atomic volumes when they form chemical compounds — these elements 
will be found distributed about the peaks or crests of the curve. Fig. 4. 

The fact that the less the density, or the greater the compressibility of an 
element, the greater its contraction on combination, is best illustrated by 
comparing the contractions occurring during the formation of similar compounds 
of the elements having widely different compressibilities but similar affinities. 
Strontium and lead are not very different in cohesive pressure as shown by the 
closeness of their boiling points. When a gram-atom of strontium unites with a 
gram-molecule of chlorine, there is a contraction of 32*6 c.c, and lead gives under 
similar conditions a contraction of 20' 1 c.c. If affinity causes contraction this is 
just what would be anticipated because the affinity of strontium for chlorine is 
greater than that of lead ; this is confirmed by the respective heats of formation 
772 and 346 kilojoules per gram-molecule. The compressibilities of the elements 
of the alkah metals and the contractions which occur, in c.c. per gram-molecule, 
when the corresponding chloride is formed, are as follows : 

Table IX. 



(C.c. per gram-molecule). 

Lithium .... 
Sodium .... 
Potassium .... 
Rubidium .... 
Caesium .... 



Not all examples will give such unequivocal evidence of the effect of chemical 
affinity in determining the atomic volumes of liquids and solids, because the effects 
of chemical affinity will be modified or even overshadowed by the effects of cohesion. 
Both must always be present, and it may be difficult to discriminate between the 
two effects. Similarly, the heat of formation Q of a compound runs parallel with 
the free energy, and may be regarded as proportional to the work done by the 
affinity pressure between two elements. Similarly, when two elements unite, the 
contraction A is evidence of the affinity uniting the elements. The contraction A 



is the difference between the molecular volume of the compound, and the sum of 
the atomic volumes of the component elements in a free state. The quotient of 
the volume contraction. A, by the heat of formation, Q, will give a measure of 
the average compressibility. In Table X, T. W. Richards showed that the com- 
pressibility effect with the alkali halides must be the same in each member of the 
series, and the values of A/Q for the different salts should fall in the same order 
of magnitude as the compressibilities of the free alkali metals if the hypothesis 
relating affinity pressure to compressibihty be correct. This is actually the 


Table X.-— Compressibilities and Affiiuity Pressures of the Alkali Halides. 


of metal. 

Sum of 





Heat of 



LiCl . 
NaCl . 
KCl . 

8-8x10 « 





16-8 j 383 
21-5 399 
32-2 427 


LiBr . 
NaBr . 
KBr . 




130 334 
150 j 359 
26-3 398 


Lil . 
Nal . 




5-3 1 257 

8-0 ' 289 

16-9 1 335 




In the light of T. W. Richards' hypothesis, we can also see that for H. Kopp's 
rule to be vaHd the internal pressures of all compounds at their boiUng points should 
be the same — subject to small variations due to differences in molecular complexity. 
The intense intermolecular pressures under which the molecules exist modify the 
boiling points, the surface tensions, the viscosities, etc. The effects produced by 
cohesive and affinity pressures on atoms with an elastic compressible envelope, 
as postulated by T. W. Richards, show that the volume occupied by an atom in the 
free state cannot be the same as in the combined state, and that the volume of an 
atom in combination will vary with the nature and orientation of the other atoms 
with which it is combined. 

H. Schroder 9 worked on the subject of atomic volumes simultaneously with 
H. Kopp ; he accepted F. Ammermiiller's conclusion that equal volumes of the two 
oxides of copper contain the same amounts of copper, and multiple amounts of oxygen, 
and assumed that in the two compounds with the atomic volumes : CuaO =24*36 ; 
and CuO =12-35 or Cu2O2=24*70, the quantities of copper are the same, and that 
the volume of the copper is in each case the same. The volume occupied by the 
oxygen in cuprous oxide then stands to that in cupric oxide as 1 : 2. H. Schroder 
concluded with H. Kopp that the molecular volume of a compound is the sum of 
the volumes of the component atoms. The former considered that the atomic 
volume of a given element under similar structural conditions throughout all its 
compounds is variable — the latter assumed that under these conditions the atomic 
volume of an element is constant. The different atomic volumes which an element 
can assume in different compounds were regarded by H. Schroder to be simple 
multiples of a certain unit volume which he called the stere. The stere is not the 
same for all elements, but it varies within comparatively narrow Umits. When two 
elements are combined, one of them assumes the unit volume of the other, so that 
the stere of one element dominates the volume of the compound, and the molecular 
volume of the compound may be represented as a simple multiple of the stere of 
one of the contained elements. For example, the stere of silver is 5-14, and the 

Stere value. 

Molecular volume. 
Calculated. Observed, 

. 5-14x6 = 



. 5-14x5 = 



. 5-14x6 = 



. 5-14x8 = 




atomic value is twice this, namely, 10'28, so that metallic silver occupies 
2 steres. 

Silver oxide 
Silver chloride 
Silver bromide 
SQver iodide 

Hence, the atomic volume of oxygen is J(6— 2)=2 silver steres ; of chlorine, 5—2=3 
silver steres ; of bromine, 6—2=4 silver steres ; and of iodine, 8—2=6 silver steres. 
The general conclusion is that the volumes of equivalents of difEerent elements 
are approximately equal, or stand in some simple relation with one another. This 
naturally raises the question whether there is any connection between the valency 
of the atoms and its effect on the molecular volume. In a general way, an increase 
in the valency of an atom is attended by an increase in the molecular volume, 
although, as W. Stadel has shown, the molecular volume is influenced by all the 
atoms in the molecule. G. le Bas i° compared the molecular volumes of eighteen 
hydrocarbons of the paraffin series at their melting points, and found that the 
quotient, obtained by dividing the molecular volume by the total number of valencies 
of the carbon and hydrogen atoms present, is a constant — very nearly 2' 97. In 
illustration, the molecular volume of dodecane, C12H26, is 2199 ; there are 12x4 
carbon valencies, and 26 hydrogen valencies, or a total of 74 valencies ; consequently 
219'9-h74=2'971. The constant 2*97 thus represents one unit of valency in these 

I. Traube ^i has investigated molecular volumes from a novel point of view. 
He takes the specific gravity at ordinary temperatures — usually about 15° — and 
he also allows for the association of the substance. I. Traube defines the molecular 
solution volume, V^i of a substance in water by the relation 

Molecular solution volume, V^=i -— ^- . — — — 

Ug JJw 

where M denotes the molecular weight of the dissolved substance, Dg and Dy, are the 
specific gravities respectively of the solution and of water, and A denotes the number 
of grams of water in which a gram-molecule of the substance is dissolved. If v 
denotes the ordinary molecular volume, defined by MjD, the difference v~Vm 
denotes the contraction which occurs in the process of solution, and is called the 
molecular contraction, and it is found that if ionization and association effects are 
eliminated, the molecular contraction has the constant value v— F^=135 c.c. per 

If ionization occurs, the number of ions which in their action are equivalent to non- 
ionized molecules must be taken into consideration. If a denotes the degree of ionization 
of a solute decomposed into n ions, then, in place of 13 '5 c.c, the molecular contraction 
= 13-5{l-t-(w — l)a}. If association occurs, the association factor must be considered. 
The association factor is a number which represents how many times the molecular weight 
of a substance is greater than corresponds with the simple gaseous molecule. In place of 
13*5 c.c. the molecular contraction is 13-5/)8 c.c. per gram -molecule. 

The molecular solution volume can be calculated from the volume constants 
of the constituent elements according to H. Kopp's additive rule, and the intro- 
duction of a correction factor. I. Traube found that this correction factor is a 
constant 12*4 c.c, so that if n^, ^2, . . . denotes the respective number of atoms of 
atomic volume Ai, A2, • . • ; and 2/71^=^2^1+^2^2+ • • • 

Molecular solution volume, V^^=EnA-\-\2'^ 
The term SnA also includes a correction term for multiple bonds, etc. 

By empirical calculation from the observed molecular volumes, I. Traube has computed 
the solution volume constants in c.c. per gram-atom for different radicles. He obtains : 
C, 9-9; H, 31; F, 65; CI, 13-2; Br, 17-7; I, 21-4; CN, 132; Na, 31; N"i, 16; 


Nv 10-7; P"i, 17; pv 28-5; double bonds, -17; triple bonds, -3-4; Hydroxylic 
oxygen (OH), 2-3 ; Hydrosulphylic sulphur (HS), 15-6. Oxygen atoms united to carbon 
by a double bond, 5-5 ; sulphur atoms united to carbon by a double bond, 15-5; oxygen 
atoms in a carbonyl group, or imited to a carbon atom with a hydroxyl group attached 
to it, 0-4. The observed density of ether, (C2H5)20, is 0-7201 at 15° ; the molecular 
weight is 74. Compare the observed and calculated molecular volume. The observed 
value is 74/0-7201 = 102-7 ; the calculated value is (4 x9-9)+(10 x3-l) + 5-5+26'9=102-0. 

If as before v denotes the molecular volume, and if there is no ionization, 
V— F^=13'5/y4, then v=I!nV+12-4:-\-lS-5ip, where the association factor j3 is 
usually nearly unity — e.g. with phosphorus trichloride, and carbon and sihcon tetra- 
chlorides, j8=unity ; for benzene, j3=ri8 ; for toluene, 1-08, etc. — but with water 
^=3'06 ; formic acid, 18 ; acetic acid, 1'56 ; methvl alcohol, 1'79 ; ethyl alcohol, 
1-67, etc. When j8 is unity Vrn=EnA-\-\2-^ ; and' 

Molecular volume, v=SnA-\-2b'^ 

meaning, according to I. Traube, that " in the formation of any molecule from its 
atoms there is always a dilation ; the molecular dilation is the same or nearly the 
same for all substances ; it is independent of the chemical nature of the substance 
and can be only slightly modified by constitution ; and at 15°, the molecular solution 
volume in aqueous solution is 12*4 c.c. per gram-molecule, and the molecular volume 
25'9 c.c. per gram-molecule." Given the molecular volume it is possible to calculate 
the association factor which may or may not agree with that deduced by other 

I. Traube regards En A as the sum of the spaces occupied by the matter of the 
atoms of a molecule. While the internal or nuclear volume of a molecule is the 
space actually filled by the mass of the atom, the external volume is the nuclear 
volume increased by the volume of a shell of combined sether. The external atomic 
volume corresponds with the magnitude h of J. D. van der Waals' equation, and 
is 3*5 to 4 times as large as the internal or nuclear volume. The difference Vrn—EnA 
gives what I. Traube calls the molecular CO- volume— symbolized Cor. The 
co-volume is a magnitude dependent on the temperature ; for 15°, the molecular 
co-volume is 259 c.c, and at B°, the molecular co-volume is Cot.„ (1+0*003670), 



Molecular co- volume, Co„=:24-5(l +0-003670) 

very nearly. There is a close formal analogy between the temperature effect of the 
CO- volume and the volume of gas. Since, for every newly formed gram-molecule 
there is an expansion equal to the co-volume, and for every molecule which dis- 
appears there is a corresponding contraction, I. Traube concluded : " In a reaction 
between homogeneous liquids, the co-volumes of the initial and final products of 
the reaction stand in a simple rational ratio " — this is J. A. C. Charles' law applied 
to liquids. Since also I. Traube assumed that the molecular volume is the sum of 
the true molecular volume and the co-volume, Avogadro's rule apphed to Hquids 
becomes " with the same conditions of temperature and pressure, the co-volumes, 
or the volumes in which the molecules move, are all equally great." 

I. Traube's method can be employed for calculating the molecular volume, and also 
the molecular weight M of an unknown substance of known specific gravity D. In 
this case, since v=M/D, the chemical formula which gives the closest value to 


is the desired chemical formula. Many examples will be found in H. Biltz's Die 
Praxis der Molekulargewichtsbestimmung (Berlin, 1898 ; Easton, Pa., 1899). 

The observed specific gravity of tetrachloroethane is 1-6258 (15°), the empirical formula 
by analysis is CHCL. Hence, if the formvda CHClj obtains, the ratio M/D = 516 and 
2nA=3d% or M/D -ZnA = 12-2 ; if the formula be C2H2CI,, M/Z) -27*7^ =103-2-78-8 


=24-4; and if the formula be C3H3CI8, M/D-Zw^ =164-8- 118-2=36-6. Here then 
24*4 approximates closest to the theoretical co-volume 26'9, and the formula is accordingly 

According to I. Traube, the atomic CO-volume is the difference between the 
internal and external atomic volumes ; and it represents the volume of the com- 
bined aether. I. Traube further postulates that the atomic co- volume is occupied by the 
valency electrons, i.e. the electrons which endow the atom with valency ; for, unHke 
the molecular co-volume, the atomic co-volume varies in size and is proportional to 
the nuclear volume and the valency of the atom. I. Traube employed molecular 
refraction as a measure of the nuclear volumes of the atoms in a compound and found 
that the molecular refractive power, MR, of a saturated compound is proportional 
to the total number of valencies, n, of the component atoms. The value of the 
quotient MRjn for a large number of compounds deviates but little from the mean 
0"787. I. Traube calls 0*787 the refraction stere — in illustration, the molecular 
refraction of alcohol, C2H5OH, is 12-71, and n is 8+5+2+1=16 ; and 12-71-M6 
=0'794:. The nuclear volumes of the atoms in a molecule are therefore proportional 
to the valencies of the atoms. 

W. C. Roberts-Austen 12 suggested that the remarkable influence of traces of 
elements on masses of metals is proportional to the atomic volumes of the con- 
taminant. He showed that the metals or metalloids near the troughs of L. Meyer's 
periodic curve. Fig. 4, Cap. VI, do not diminish the tensile strength of gold ; 
and that the metals which render gold fragile occupy high positions on the curve. 
Hence he argues : 

There is some relation between the influence exerted by the metallic and other im- 
purities and either their atomic weights or their atomic volumes. It seems hardly probable 
that it is due to atomic weight, because copper, with an atomic weight of 63-2, has nearly 
the same infl\ience on the tenacity of pure gold as rhodium, with an atonlic weight of 104, 
or as aluminium, the atomic weight of which is 27-0. It will be evident from the following 
table, which embodies the results of the author's experiments, that metals which diminish 
the tenacity and extensibility of gold have high atomic volumes, while those which increase 
those properties have either the same atomic volume as gold, or a lower one. Fiu-ther, 
silver has the same atomic volume as gold, 10 "2, and its presence in small quantities has 
very little influence, one way or the other, on the tenacity or extensibility of gold. 

It is suggested that the atoms with a small atomic volume can fill up interstitial 
spaces which would otherwise remain void and this without disturbing the dis- 
position of the other atoms, while atoms with a large atomic volume act prejudicially 
by driving the atoms further asunder. The following experiment by E. Warburg 
and F. Tegetmeier illustrates a porosity in solids which will permit the passage of 
elements with a small atomic volume, but strain off those with a larger atomic 

A cell with a glass partition with sodium amalgam about the anode and mercury about 
the cathode was heated to between 100° and 200°- — when the glass became slightly con- 
ducting. In about 30 hrs. an appreciable quantity of sodium had passed from the glass 
into the mercury. The glass remained transparent, for the sodium lost by the glass was 
replaced by that from the mercury amalgam. W. C. Roberts -Austen showed that in the 
electrolysis of the glass, the passage of the sodium follows the ordinary law of electrolysis. 
If lithium amalgam be used, the glass becomes opaque, and then lithium acciunulates in 
the merciuy. The glass loses no potassium, but 7 '8 per cent, of sodium, and gains 4*3 
per cent, of lithiiim. With potassium amalgam, the potassium does not replace the sodium 
lost by the glass. It is suggested that the lithium atoms with an atomic volume 15*98 can 
replace sodium atoms with an atomic volume 16*04, while potassium atoms with an atomic 
volume 24 are too large to take the place of the smaller sodium atoms. The glass diaphragm 
has thus been said to act as a mechanical sifter for the potassium atoms. 

The simple relation between atomic volumes and tenacity is no doubt modified 
when compounds are formed. F. Osmond also showed that elements with a smaller 
atomic volume than iron retard the transformation of j3 to a iron, while elements 
with a larger atomic volume than iron either have no influence upon the transition 
temperature or else raise that temperature. 

Several investigators have traced the influence of the atomic volume of a metal 


on the mechanical properties ; for example, A. Wertheim and H. Tomlinson have 
shown that there is a relation between the atomic volume and elasticity ; W. Suther- 
land, between the atomic volume and rigidity ; R. A. Fessenden, between atomic 
volume and cohesion ; and H. Crompton, between the latent heat of fusion and the 
molecular volume of a compound. H. Crompton also showed that the molecular 
heat of fusion L ; the absolute fusion temperatures T ; and the valencies n 
of the elements, are so related that LjTn is a constant ; and that