Chapter I: History

Greek philosophy.

Although chemical actions must have been observed by man in the most remote times, and also utilized in such processes as the extraction of metals from their ores and in the arts of tanning and dyeing, there is no evidence to show that, beyond an unordered accumulation of facts, the early developments of these industries were attended by any real knowledge of the nature of the processes involved. All observations were the result of accident or chance, or possibly in some cases of experimental trial, but there is no record of a theory or even a general classification of the phenomena involved, although there is no doubt that the ancients had a fair knowledge of the properties and uses of the commoner substances. The origin of chemistry is intimately bound up with the arts which we have indicated; in this respect it is essentially an experimental science. A unifying principle of chemical and physical changes was provided by metaphysical conceptions of the structure of matter. We find the notion of "elements," or primary qualities, which confer upon all species of matter their distinctive qualities by appropriate combination, and also the doctrine that matter is composed of minute discrete particles, prevailing in the Greek schools. These "elements," however, had not the significance of the elements of to-day; they connoted physical appearances or qualities rather than chemical relations; and the atomic theory of the ancients is a speculation based upon metaphysical considerations, having, in its origin, nothing in common with the modern molecular theory, which was based upon experimentally observed properties of gases (see ELEMENT; MOLECULE).

Alchemy.

Although such hypotheses could contribute nothing directly to the development of a science which laid especial claim to experimental investigations, yet indirectly they stimulated inquiry into the nature of the "essence" with which the four "elements" were associated. This _quinta essentia_ had been speculated upon by the Greeks, some regarding it as immaterial or aethereal, and others as material; and a school of philosophers termed alchemists arose who attempted the isolation of this essence. The existence of a fundamental principle, unalterable and indestructible, prevailing alike through physical and chemical changes, was generally accepted. Any change which a substance may chance to undergo was simply due to the discarding or taking up of some proportion of the primary "elements" or qualities: of these coverings "water," "air," "earth" and "fire" were regarded as clinging most tenaciously to the essence, while "cold," "heat," "moistness" and "dryness" were more easily cast aside or assumed. Several origins have been suggested for the word alchemy, and there seems to have been some doubt as to the exact nature and import of the alchemical doctrines. According to M.P.E. Berthelot, "alchemy rested partly on the industrial processes of the ancient Egyptians, partly on the speculative theories of the Greek philosophers, and partly on the mystical reveries of the Gnostics and Alexandrians." The search for this essence subsequently resolved itself into the desire to effect the transmutation of metals, more especially the base metals, into silver and gold. It seems that this secondary principle became the dominant idea in alchemy, and in this sense the word is used in Byzantine literature of the 4th century; Suidas, writing in the 11th century, defines chemistry as the "preparation of silver and gold" (see ALCHEMY).

From the Alexandrians the science passed to the Arabs, who made discoveries and improved various methods of separating substances, and afterwards, from the 11th century, became seated in Europe, where the alchemical doctrines were assiduously studied until the 15th and 16th centuries. It is readily understood why men imbued with the authority of tradition should prosecute the search for a substance which would confer unlimited wealth upon the fortunate discoverer. Some alchemists honestly laboured to effect the transmutation and to discover the "philosopher's stone," and in many cases believed that they had achieved success, if we may rely upon writings assigned to them. The period, however, is one of literary forgeries; most of the MSS. are of uncertain date and authorship, and moreover are often so vague and mystical that they are of doubtful scientific value, beyond reflecting the tendencies of the age. The retaining of alchemists at various courts shows the high opinion which the doctrines had gained. It is really not extraordinary that Isaac Hollandus was able to indicate the method of the preparation of the "philosopher's stone" from "adamic" or "virgin" earth, and its action when medicinally employed; that in the writings assigned to Roger Bacon, Raimon Lull, Basil Valentine and others are to be found the exact quantities of it to be used in transmutation; and that George Ripley, in the 15th century, had grounds for regarding its action as similar to that of a ferment.

In the view of some alchemists, the ultimate principles of matter were Aristotle's four elements; the proximate constituents were a "sulphur" and a "mercury," the father and mother of the metals; gold was supposed to have attained to the perfection of its nature by passing in succession through the forms of lead, brass and silver; gold and silver were held to contain very pure red sulphur and white quicksilver, whereas in the other metals these materials were coarser and of a different colour. From an analogy instituted between the healthy human being and gold, the most perfect of the metals, silver, mercury, copper, iron, lead and tin, were regarded in the light of lepers that required to be healed.

Iatrochemistry.

Notwithstanding the false idea which prompted the researches of the alchemists, many advances were made in descriptive chemistry, the metals and their salts receiving much attention, and several of our important acids being discovered. Towards the 16th century the failure of the alchemists to achieve their cherished purpose, and the general increase of medical knowledge, caused attention to be given to the utilization of chemical preparations as medicines. As early as the 15th century the alchemist Basil Valentine had suggested this application, but the great exponent of this doctrine was Paracelsus, who set up a new definition: "The true use of chemistry is not to make gold but to prepare medicines." This relation of chemistry to medicine prevailed until the 17th century, and what in the history of chemistry is termed the iatrochemical period (see MEDICINE) was mainly fruitful in increasing the knowledge of compounds; the contributions to chemical theory are of little value, the most important controversies ranging over the nature of the "elements," which were generally akin to those of Aristotle, modified so as to be more in accord with current observations. At the same time, however, there were many who, opposed to the Paracelsian definition of chemistry, still laboured at the problem of the alchemists, while others gave much attention to the chemical industries. Metallurgical operations, such as smelting, roasting and refining, were scientifically investigated, and in some degree explained, by Georg Agricola and Carlo Biringuiccio; ceramics was studied by Bernard Palissy, who is also to be remembered as an early worker in agricultural chemistry, having made experiments on the effect of manures on soils and crops; while general technical chemistry was enriched by Johann Rudolf Glauber.[1]

Boyle.

The second half of the 17th century witnessed remarkable transitions and developments in all branches of natural science, and the facts accumulated by preceding generations during their generally unordered researches were replaced by a co-ordination of experiment and deduction. From the mazy and incoherent alchemical and iatrochemical doctrines, the former based on false conceptions of matter, the latter on erroneous views of life processes and physiology, a new science arose--the study of the composition of substances. The formulation of this definition of chemistry was due to Robert Boyle. In his _Sceptical Chemist_ (1662) he freely criticized the prevailing scientific views and methods, with the object of showing that true knowledge could only be gained by the logical application of the principles of experiment and deduction. Boyle's masterly exposition of this method is his most important contribution to scientific progress. At the same time he clarified the conception of elements and compounds, rejecting the older notions, the four elements of the "vulgar Peripateticks" and the three principles of the "vulgar Stagyrists," and defining an element as a substance incapable of decomposition, and a compound as composed of two or more elements. He explained chemical combination on the hypotheses that matter consisted of minute corpuscles, that by the coalescence of corpuscles of different substances distinctly new corpuscles of a compound were formed, and that each corpuscle had a certain affinity for other corpuscles.

Phlogistic theory.

Although Boyle practised the methods which he expounded, he was unable to gain general acceptance of his doctrine of elements; and, strangely enough, the theory which next dominated chemical thought was an alchemical invention, and lacked the lucidity and perspicuity of Boyle's views. This theory, named the phlogistic theory, was primarily based upon certain experiments on combustion and calcination, and in effect reduced the number of the alchemical principles, while setting up a new one, a principle of combustibility, named phlogiston (from [Greek: phlogistos], burnt). Much discussion had centred about fire or the "igneous principle." On the one hand, it had been held that when a substance was burned or calcined, it combined with an "air"; on the other hand, the operation was supposed to be attended by the destruction or loss of the igneous principle. Georg Ernst Stahl, following in some measure the views held by Johann Joachim Becher, as, for instance, that all combustibles contain a "sulphur" (which notion is itself of older date than Becher's _terra pinguis_), regarded all substances as capable of resolution into two components, the inflammable principle phlogiston, and another element--"water," "acid" or "earth." The violence or completeness of combustion was proportional to the amount of phlogiston present. Combustion meant the liberation of phlogiston. Metals on calcination gave calces from which the metals could be recovered by adding phlogiston, and experiment showed that this could generally be effected by the action of coal or carbon, which was therefore regarded as practically pure phlogiston; the other constituent being regarded as an acid. At the hands of Stahl and his school, the phlogistic theory, by exhibiting a fundamental similarity between all processes of combustion and by its remarkable flexibility, came to be a general theory of chemical action. The objections of the antiphlogistonists, such as the fact that calces weigh more than the original metals instead of less as the theory suggests, were answered by postulating that phlogiston was a principle of levity, or even completely ignored as an accident, the change of _qualities_ being regarded as the only matter of importance. It is remarkable that this theory should have gained the esteem of the notable chemists who flourished in the 18th century. Henry Cavendish, a careful and accurate experimenter, was a phlogistonist, as were J. Black, K. W. Scheele, A. S. Marggraf, J. Priestley and many others who might be mentioned.

Lavoisier.

Descriptive chemistry was now assuming considerable proportions; the experimental inquiries suggested by Boyle were being assiduously developed; and a wealth of observations was being accumulated, for the explanation of which the resources of the dominant theory were sorely taxed. To quote Antoine Laurent Lavoisier, "... chemists have turned phlogiston into a vague principle, ... which consequently adapts itself to all the explanations for which it may be required. Sometimes this principle has weight, and sometimes it has not; sometimes it is free fire and sometimes it is fire combined with the earthy element; sometimes it passes through the pores of vessels, sometimes these are impervious to it; it explains both causticity and non-causticity, transparency and opacity, colours and their absence; it is a veritable Proteus changing in form at each instant." Lavoisier may be justly regarded as the founder of modern or quantitative chemistry. First and foremost, he demanded that the balance must be used in all investigations into chemical changes. He established as fundamental that combustion and calcination were attended by an increase of weight, and concluded, as did Jean Rey and John Mayow in the 17th century, that the increase was due to the combination of the metal with the air. The problem could obviously be completely solved only when the composition of the air, and the parts played by its components, had been determined. At all times the air had received attention, especially since van Helmont made his far-reaching investigations on gases. Mayow had suggested the existence of two components, a _spiritus nitroaerus_ which supported combustion, and a _spiritus nitri acidi_ which extinguished fire; J. Priestley and K. W. Scheele, although they isolated oxygen, were fogged by the phlogistic tenets; and H. Cavendish, who had isolated the nitrogen of the atmosphere, had failed to decide conclusively what had really happened to the air which disappeared during combustion.

Lavoisier adequately recognized and acknowledged how much he owed to the researches of others; to himself is due the co-ordination of these researches, and the welding of his results into a doctrine to which the phlogistic theory ultimately succumbed. He burned phosphorus in air standing over mercury, and showed that (1) there was a limit to the amount of phosphorus which could be burned in the confined air, (2) that when no more phosphorus could be burned, one-fifth of the air had disappeared, (3) that the weight of the air lost was nearly equal to the difference in the weights of the white solid produced and the phosphorus burned, (4) that the density of the residual air was less than that of ordinary air. The same results were obtained with lead and tin; and a more elaborate repetition indubitably established their correctness. He also showed that on heating mercury calx alone an "air" was liberated which differed from other "airs," and was slightly heavier than ordinary air; moreover, the weight of the "air" set free from a given weight of the calx was equal to the weight taken up in forming the calx from mercury, and if the calx be heated with charcoal, the metal was recovered and a gas named "fixed air," the modern carbon dioxide, was formed. The former experiment had been performed by Scheele and Priestley, who had named the gas "phlogisticated air"; Lavoisier subsequently named it oxygen, regarding it as the "acid producer" ([Greek: oxys], sour). The theory advocated by Lavoisier came to displace the phlogistic conception; but at first its acceptance was slow. Chemical literature was full of the phlogistic modes of expression--oxygen was "dephlogisticated air," nitrogen "phlogisticated air," &c.--and this tended to retard its promotion. Yet really the transition from the one theory to the other was simple, it being only necessary to change the "addition or loss of phlogiston" into the "loss or addition of oxygen." By his insistence upon the use of the balance as a quantitative check upon the masses involved in all chemical reactions, Lavoisier was enabled to establish by his own investigations and the results achieved by others the principle now known as the "conservation of mass." Matter can neither be created nor destroyed; however a chemical system be changed, the weights before and after are equal.[2] To him is also due a rigorous examination of the nature of elements and compounds; he held the same views that were laid down by Boyle, and with the same prophetic foresight predicted that some of the elements which he himself accepted might be eventually found to be compounds.

Chemical Affinity.

It is unnecessary in this place to recapitulate the many results which had accumulated by the end of the 18th century, or to discuss the labours and theories of individual workers since these receive attention under biographical headings; in this article only the salient features in the history of our science can be treated. The beginning of the 19th century was attended by far-reaching discoveries in the nature of the composition of compounds. Investigations proceeded in two directions:--(1) the nature of chemical affinity, (2) the laws of chemical combination. The first question has not yet been solved, although it has been speculated upon from the earliest times. The alchemists explained chemical action by means of such phrases as "like attracts like," substances being said to combine when one "loved" the other, and the reverse when it "hated" it. Boyle rejected this terminology, which was only strictly applicable to intelligent beings; and he used the word "affinity" as had been previously done by Stahl and others. The modern sense of the word, viz. the force which holds chemically dissimilar substances together (and also _similar_ substances as is seen in di-, tri-, and poly-atomic molecules), was introduced by Hermann Boerhaave, and made more precise by Sir Isaac Newton. The laws of chemical combination were solved, in a measure, by John Dalton, and the solution expressed as Dalton's "atomic theory." Lavoisier appears to have assumed that the composition of every chemical compound was constant, and the same opinion was the basis of much experimental inquiry at the hands of Joseph Louis Proust during 1801 to 1809, who vigorously combated the doctrine of Claude Louis Berthollet (_Essai de statique chimique_, 1803), viz. that fixed proportions of elements and compounds combine only under exceptional conditions, the general rule being that the composition of a compound may vary continuously between certain limits.[3]

Dalton.

This controversy was unfinished when Dalton published the first part of his _New System of Chemical Philosophy_ in 1808, although the _per saltum_ theory was the most popular. Led thereto by speculations on gases, Dalton assumed that matter was composed of atoms, that in the elements the atoms were simple, and in compounds complex, being composed of elementary atoms. Dalton furthermore perceived that the same two elements or substances may combine in different proportions, and showed that these proportions had always a simple ratio to one another. This is the "law of multiple proportions." He laid down the following arbitrary rules for determining the number of atoms in a compound:--if only one compound of two elements exists, it is a binary compound and its atom is composed of one atom of each element; if two compounds exist one is binary (say A + B) and the other ternary (say A + 2B); if three, then one is binary and the others may be ternary (A + 2B, and 2A + B), and so on. More important is his deduction of equivalent weights, i.e. the relative weights of atoms. He took hydrogen, the lightest substance known, to be the standard. From analyses of water, which he regarded as composed of one atom of hydrogen and one of oxygen, he deduced the relative weight of the oxygen atom to be 6.5; from marsh gas and olefiant gas he deduced carbon = 5, there being one atom of carbon and two of hydrogen in the former and one atom of hydrogen to one of carbon in the latter; nitrogen had an equivalent of 5, and so on.[4]

The value of Dalton's generalizations can hardly be overestimated, notwithstanding the fact that in several cases they needed correction. The first step in this direction was effected by the co-ordination of Gay Lussac's observations on the combining volumes of gases. He discovered that gases always combined in volumes having simple ratios, and that the volume of the product had a simple ratio to the volumes of the reacting gases. For example, one volume of oxygen combined with two of hydrogen to form two volumes of steam, three volumes of hydrogen combined with one of nitrogen to give two volumes of ammonia, one volume of hydrogen combined with one of chlorine to give two volumes of hydrochloric acid. An immediate inference was that the Daltonian "atom" must have parts which enter into combination with parts of other atoms; in other words, there must exist two orders of particles, viz. (1) particles derived by limiting _mechanical_ subdivision, the modern _molecule_, and (2) particles derived from the first class by _chemical_ subdivision, i.e. particles which are incapable of existing alone, but may exist in combination. Additional evidence as to the structure of the molecule was discussed by Avogadro in 1811, and by Ampere in 1814. From the gas-laws of Boyle and J.A.C. Charles--viz. equal changes in temperature and pressure occasion equal changes in equal volumes of all gases and vapours--Avogadro deduced the law:--Under the same conditions of temperature and pressure, equal volumes of gases contain equal numbers of molecules; and he showed that the relative weights of the molecules are determined as the ratios of the weights of equal volumes, or densities. He established the existence of molecules and atoms as we have defined above, and stated that the number of atoms in the molecule is generally 2, but may be 4, 8, &c. We cannot tell whether his choice of the powers of 2 is accident or design.

Berzelius.

Notwithstanding Avogadro's perspicuous investigation, and a similar exposition of the atom and molecule by A. M. Ampere, the views therein expressed were ignored both by their own and the succeeding generation. In place of the relative molecular weights, attention was concentrated on relative atomic or equivalent weights. This may be due in some measure to the small number of gaseous and easily volatile substances then known, to the attention which the study of the organic compounds received, and especially to the energetic investigations of J. J. Berzelius, who, fired with enthusiasm by the original theory of Dalton and the law of multiple proportions, determined the equivalents of combining ratios of many elements in an enormous number of compounds.[5] He prosecuted his labours in this field for thirty years; as proof of his industry it may be mentioned that as early as 1818 he had determined the combining ratios of about two thousand simple and compound substances.

Chemical notation.

We may here notice the important chemical symbolism or notation
introduced by Berzelius, which greatly contributed to the definite and
convenient representation of chemical composition and the tracing of
chemical reactions. The denotation of elements by symbols had been
practised by the alchemists, and it is interesting to note that the
symbols allotted to the well-known elements are identical with the
astrological symbols of the sun and the other members of the solar
system. Gold, the most perfect metal, had the symbol of the Sun, [*];
silver, the semiperfect metal, had the symbol of the Moon, [*];
copper, iron and antimony, the imperfect metals of the gold class, had
the symbols of Venus [*], Mars [*], and the Earth [*]; tin and lead,
the imperfect metals of the silver class, had the symbols of Jupiter
[*], and Saturn [*]; while mercury, the imperfect metal of both the
gold and silver class, had the symbol of the planet, [*]. Torbern Olof
Bergman used an elaborate system in his _Opuscula physica et chemica_
(1783); the elements received symbols composed of circles, arcs of
circles, and lines, while certain class symbols, such as [*] for
metals, [*] for acids, [*] for alkalies, [*] for salts, [*] for
calces, &c., were used. Compounds were represented by copulating
simpler symbols, e.g. mercury calx was [*][*].[6] Bergman's symbolism
was obviously cumbrous, and the system used in 1782 by Lavoisier was
equally abstruse, since the forms gave no clue as to composition; for
instance water, oxygen, and nitric acid were [*], [*], and [*].

A partial clarification was suggested in 1787 by J.H. Hassenfratz and
Adet, who assigned to each element a symbol, and to each compound a
sign which should record the elements present and their relative
quantities. Straight lines and semicircles were utilized for the
non-metallic elements, carbon, nitrogen, phosphorus and sulphur (the
"simple acidifiable bases" of Lavoisier), and circles enclosing the
initial letters of their names for the metals. The "compound
acidifiable bases," i.e. the hypothetical radicals of acids, were
denoted by squares enclosing the initial letter of the base; an alkali
was denoted by a triangle, and the particular alkali by inserting the
initial letter. Compounds were denoted by joining the symbols of the
components, and by varying the manner of joining compounds of the same
elements were distinguished. The symbol \/ was used to denote a
liquid, and a vertical line to denote a gas. As an example of the
complexity of this system we may note the five oxides of nitrogen,
which were symbolized as

[*], [*], [*], [*], and [*],

the first three representing the gaseous oxides, and the last two the
liquid oxides.

A great advance was made by Dalton, who, besides introducing simpler
symbols, regarded the symbol as representing not only the element or
compound but also one atom of that element or compound; in other
words, his symbol denoted equivalent weights.[7] This system, which
permitted the correct representation of molecular composition, was
adopted by Berzelius in 1814, who, having replaced the geometric signs
of Dalton by the initial letter (or letters) of the Latin names of the
elements, represented a compound by placing a _plus_ sign between the
symbols of its components, and the number of atoms of each component
(except in the case of only one atom) by placing Arabic numerals
before the symbols; for example, copper oxide was Cu+O, sulphur
trioxide S+3O. If two compounds combined, the + signs of the free
compounds were discarded, and the number of atoms denoted by an Arabic
index placed after the elements, and from these modified symbols the
symbol of the new compound was derived in the same manner as simple
compounds were built up from their elements. Thus copper sulphate was
CuO + SO^3, potassium sulphate 2SO^3 + PoO^2 (the symbol Po for
potassium was subsequently discarded in favour of K from _kalium_). At
a later date Berzelius denoted an oxide by dots, equal in number to
the number of oxygen atoms present, placed over the element; this
notation survived longest in mineralogy. He also introduced barred
symbols, i.e. letters traversed by a horizontal bar, to denote the
double atom (or molecule). Although the system of Berzelius has been
modified and extended, its principles survive in the modern notation.

Extension of the atomic theory.

In the development of the atomic theory and the deduction of the atomic weights of elements and the formulae of compounds, Dalton's arbitrary rules failed to find complete acceptance. Berzelius objected to the hypothesis that if two elements form only one compound, then the atoms combine one and one; and although he agreed with the adoption of simple rules as a first attempt at representing a compound, he availed himself of other data in order to gain further information as to the structure of compounds. For example, at first he represented ferrous and ferric oxides by the formulae FeO2, FeO3, and by the analogy of zinc and other basic oxides he regarded these substances as constituted similarly to FeO2, and the acidic oxides alumina and chromium oxide as similar to FeO3. He found, however, that chromic acid, which he had represented as CrO6, neutralized a base containing 1/3 the quantity of oxygen. He inferred that chromic acid must contain only three atoms of oxygen, as did sulphuric acid SO3; consequently chromic oxide, which contains half the amount of oxygen, must be Cr2O3, and hence ferric oxide must be Fe2O3. The basic oxides must have the general formula MO. To these results he was aided by the law of isomorphism formulated by E. Mitscherlich in 1820; and he confirmed his conclusions by showing the agreement with the law of atomic heat formulated by Dulong and Petit in 1819.

While successfully investigating the solid elements and their compounds gravimetrically, Berzelius was guilty of several inconsistencies in his views on gases. He denied that gaseous atoms could have parts, although compound gases could. This attitude was due to his adherence to the "dualistic theory" of the structure of substances, which he deduced from electrochemical researches. From the behaviour of substances on electrolysis (q.v.) he assumed that all substances had two components, one bearing a negative charge, the other a positive charge. Combination was associated with the coalescence of these charges, and the nature of the resulting compound showed the nature of the residual electricity. For example, positive iron combined with negative oxygen to form positive ferrous oxide; positive sulphur combined with negative oxygen to form negative sulphuric acid; positive ferrous oxide combined with negative sulphuric acid to form neutral ferrous sulphate. Berzelius elevated this theory to an important position in the history of our science. He recognized that if an elementary atom had parts, his theory demanded that these parts should carry different electric charges when they entered into reaction, and the products of the reaction should vary according as a positive or negative atom entered into combination. For instance if the reaction 2H2 + O2 = H2O + H2O be true, the molecules of water should be different, for a negative oxygen atom would combine in one case, and a positive oxygen atom in the other. Hence the gaseous atoms of hydrogen and oxygen could not have parts. A second inconsistency was presented when he was compelled by the researches of Dumas to admit Avogadro's hypothesis; but here he would only accept it for the elementary gases, and denied it for other substances. It is to be noticed that J.B. Dumas did not adopt the best methods for emphasizing his discoveries. His terminology was vague and provoked caustic criticism from Berzelius; he assumed that all molecules contained two atoms, and consequently the atomic weights deduced from vapour density determinations of sulphur, mercury, arsenic, and phosphorus were quite different from those established by gravimetric and other methods.

Chemists gradually tired of the notion of atomic weights on account of the uncertainty which surrounded them; and the suggestion made by W.H. Wollaston as early as 1814 to deal only with "equivalents," i.e. the amount of an element which can combine with or replace unit weight of hydrogen, came into favour, being adopted by L. Gmelin in his famous text-book.

Atomic and molecular weights.

Simultaneously with this discussion of the atom and molecule, great controversy was ranging over the constitution of compounds, more particularly over the carbon or organic compounds. This subject is discussed in section IV., _Organic Chemistry_.The gradual accumulation of data referring to organic compounds brought in its train a revival of the discussion of atoms and molecules. A. Laurent and C.F. Gerhardt attempted a solution by investigating chemical reactions. They assumed the atom to be the smallest part of matter which can exist in combination, and the molecule to be the smallest part which can enter into a chemical reaction. Gerhardt found that reactions could be best followed if one assumed the molecular weight of an element or compound to be that weight which occupied the same volume as two unit weights of hydrogen, and this assumption led him to double the equivalents accepted by Gmelin, making H = 1, O = 16, and C = 12, thereby agreeing with Berzelius, and also to halve the values given by Berzelius to many metals. Laurent generally agreed, except when the theory compelled the adoption of formulae containing fractions of atoms; in such cases he regarded the molecular weight as the weight occupying a volume equal to four unit weights of hydrogen. The bases upon which Gerhardt and Laurent founded their views were not sufficiently well grounded to lead to the acceptance of their results; Gerhardt himself returned to Gmelin's equivalents in his _Lehrbuch der Chemie_ (1853) as they were in such general use.

In 1860 there prevailed such a confusion of hypotheses as to the atom and molecule that a conference was held at Karlsruhe to discuss the situation. At the conclusion of the sitting, Lothar Meyer obtained a paper written by Stanislas Cannizzaro in 1858 wherein was found the final link required for the determination of atomic weights. This link was the full extension of Avogadro's theory to all substances, Cannizzaro showing that chemical reactions in themselves would not suffice. He chose as his unit of reference the weight of an atom of hydrogen, i.e. the weight contained in a molecule of hydrochloric acid, thus differing from Avogadro who chose the weight of a hydrogen molecule. From a study of the free elements Cannizzaro showed that an element may have more than one molecular weight; for example, the molecular weight of sulphur varied with the temperature. And from the study of compounds he showed that each element occurred in a definite weight or in some multiple of this weight. He called this proportion the "atom," since it invariably enters compounds without division, and the weight of this atom is the atomic weight. This generalization was of great value inasmuch as it permitted the deduction of the atomic weight of a non-gasifiable element from a study of the densities of its gasifiable compounds.

Valency.

From the results obtained by Laurent and Gerhardt and their predecessors it immediately followed that, while an element could have but one atomic weight, it could have several equivalent weights. From a detailed study of organic compounds Gerhardt had promulgated a "theory of types" which represented a fusion of the older radical and type theories. This theory brought together, as it were, the most varied compounds, and stimulated inquiry into many fields. According to this theory, an element in a compound had a definite saturation capacity, an idea very old in itself, being framed in the law of multiple proportions. These saturation capacities were assiduously studied by Sir Edward Frankland, who from the investigation, not of simple inorganic compounds, but of the organo-metallic derivatives, determined the kernel of the theory of valency. Frankland showed that any particular element preferentially combined with a definite number (which might vary between certain limits) of other atoms; for example, some atoms always combined with one atom of oxygen, some with two, while with others two atoms entered into combination with one of oxygen. If an element or radical combined with one atom of hydrogen, it was termed monovalent; if with two (or with one atom of oxygen, which is equivalent to two atoms of hydrogen) it was divalent, and so on. The same views were expressed by Cannizzaro, and also by A.W. von Hofmann, who materially helped the acceptance of the doctrine by the lucid exposition in his _Introduction to Modern Chemistry_, 1865.

The recognition of the quadrivalency of carbon by A. Kekule was the forerunner of his celebrated benzene theory in particular, and of the universal application of structural formulae to the representation of the most complex organic compounds equally lucidly as the representation of the simplest salts. Alexander Butlerow named the "structure theory," and contributed much to the development of the subject. He defined structure "as the manner of the mutual linking of the atoms in the molecule," but denied that any such structure could give information as to the orientation of the atoms in space. He regarded the chemical properties of a substance as due to (1) the chemical atoms composing it, and (2) the structure, and he asserted that while different compounds might have the same components (isomerism), yet only one compound could have a particular structure. Identity in properties necessitated identity in structure.

While the principle of varying valency laid down by Frankland is still retained, Butlerow's view that structure had no spatial significance has been modified. The researches of L. Pasteur, J.A. Le Bel, J. Wislicenus, van't Hoff and others showed that substances having the same graphic formulae vary in properties and reactions, and consequently the formulae need modification in order to exhibit these differences. Such isomerism, named stereoisomerism (q.v.), has been assiduously developed during recent years; it prevails among many different classes of organic compounds and many examples have been found in inorganic chemistry.

Periodic law.

The theory of valency as a means of showing similarity of properties and relative composition became a dominant feature of chemical theory, the older hypotheses of types, radicals, &c. being more or less discarded. We have seen how its utilization in the "structure theory" permitted great clarification, and attempts were not wanting for the deduction of analogies or a periodicity between elements. Frankland had recognized the analogies existing between the chemical properties of nitrogen, phosphorus, arsenic and antimony, noting that they act as tri- or penta-valent. Carbon was joined with silicon, zirconium and titanium, while boron, being trivalent, was relegated to another group. A general classification of elements, however, was not realized by Frankland, nor even by Odling, who had also investigated the question from the valency standpoint. The solution came about by arranging the elements in the order of their atomic weights, tempering the arrangement with the results deduced from the theory of valencies and experimental observations. Many chemists contributed to the establishment of such a periodicity, the greatest advances being made by John Newlands in England, Lothar Meyer in Germany, and D.J. Mendeleeff in St Petersburg. For the development of this classification see ELEMENT.

Summary.

In the above sketch we have briefly treated the history of the main tendencies of our science from the earliest times to the establishment of the modern laws and principles. We have seen that the science took its origin in the arts practised by the Egyptians, and, having come under the influence of philosophers, it chose for its purpose the isolation of the _quinta essentia_, and subsequently the "art of making gold and silver." This spirit gave way to the physicians, who regarded "chemistry as the art of preparing medicines," a denotation which in turn succumbed to the arguments of Boyle, who regarded it as the "science of the composition of substances," a definition which adequately fits the science to-day. We have seen how his classification of substances into elements and compounds, and the definitions which he assigned to these species, have similarly been retained; and how Lavoisier established the law of the "conservation of mass," overthrew the prevailing phlogistic theory, and became the founder of modern chemistry by the overwhelming importance which he gave to the use of the balance. The development of the atomic theory and its concomitants--the laws of chemical combination and the notion of atoms and equivalents--at the hands of Dalton and Berzelius, the extension to the modern theory of the atom and molecule, and to atomic and molecular weights by Avogadro, Ampere, Dumas, Laurent, Gerhardt, Cannizzaro and others, have been noted. The structure of the molecule, which mainly followed investigations in organic compounds, Frankland's conception of valency, and finally the periodic law, have also been shown in their chronological order. The principles outlined above constitute the foundations of our science; and although it may happen that experiments may be made with which they appear to be not in complete agreement, yet in general they constitute a body of working hypotheses of inestimable value.

_Chemical Education._--It is remarkable that systematic instruction in the theory and practice of chemistry only received earnest attention in our academic institutions during the opening decades of the 19th century. Although for a long time lecturers and professors had been attached to universities, generally their duties had also included the study of physics, mineralogy and other subjects, with the result that chemistry received scanty encouragement. Of practical instruction there was none other than that to be gained in a few private laboratories and in the shops of apothecaries. The necessity for experimental demonstration and practical instruction, in addition to academic lectures, appears to have been urged by the French chemists L.N. Vauquelin, Gay Lussac, Thenard, and more especially by A.F. Fourcroy and G.F. Rouelle, while in England Humphry Davy expounded the same idea in the experimental demonstrations which gave his lectures their brilliant charm. But the real founder of systematic instruction in our science was Justus von Liebig, who, having accepted the professorship at Giessen in 1824, made his chemical laboratory and course of instruction the model of all others. He emphasized that the practical training should include (1) the qualitative and quantitative analysis of mixtures, (2) the preparation of substances according to established methods, (3) original research--a course which has been generally adopted. The pattern set by Liebig at Giessen was adopted by F. Wohler at Gottingen in 1836, by R.W. Bunsen at Marburg in 1840, and by O.L. Erdmann at Leipzig in 1843; and during the 'fifties and 'sixties many other laboratories were founded. A new era followed the erection of the laboratories at Bonn and Berlin according to the plans of A.W. von Hofmann in 1867, and of that at Leipzig, designed by Kolbe in 1868. We may also mention the famous laboratory at Munich designed by A. von Baeyer in 1875.

In Great Britain the first public laboratory appears to have been opened in 1817 by Thomas Thomson at Glasgow. But the first important step in providing means whereby students could systematically study chemistry was the foundation of the College of Chemistry in 1845. This institution was taken over by the Government in 1853, becoming the Royal College of Chemistry, and incorporated with the Royal School of Mines; in 1881 the names were changed to the Normal School of Science and Royal School of Mines, and again in 1890 to the Royal College of Science. In 1907 it was incorporated in the Imperial College of Science and Technology. Under A.W. von Hofmann, who designed the laboratories and accepted the professorship in 1845 at the instigation of Prince Albert, and under his successor (in 1864) Sir Edward Frankland, this institution became one of the most important centres of chemical instruction. Oxford and Cambridge sadly neglected the erection of convenient laboratories for many years, and consequently we find technical schools and other universities having a far better equipment and offering greater facilities. In the provinces Victoria University at Manchester exercised the greater impetus, numbering among its professors Sir W.H. Perkin and Sir Henry Roscoe.

In America public laboratory instruction was first instituted at Yale College during the professorship of Benjamin Silliman. To the great progress made in recent years F.W. Clarke, W. Gibbs, E.W. Morley, Ira Remsen, and T.W. Richards have especially contributed.

In France the subject was almost entirely neglected until late in the 19th century. The few laboratories existing in the opening decades were ill-fitted, and the exorbitant fees constituted a serious bar to general instruction, for these institutions received little government support. In 1869 A. Wurtz reported the existence of only _one_ efficient laboratory in France, namely the Ecole Normale Superieure, under the direction of H. Sainte Claire Deville. During recent years chemistry has become one of the most important subjects in the curriculum of technical schools and universities, and at the present time no general educational institution is complete until it has its full equipment of laboratories and lecture theatres.

_Chemical Literature_.--The growth of chemical literature since the
publication of Lavoisier's famous _Traite de chimie_ in 1789, and of
Berzelius' _Lehrbuch der Chemie_ in 1808-1818, has been enormous.
These two works, and especially the latter, were the models followed
by Thenard, Liebig, Strecker, Wohler and many others, including Thomas
Graham, upon whose _Elements of Chemistry_ was founded Otto's famous
_Lehrbuch der Chemie_, to which H. Kopp contributed the general
theoretical part, Kolbe the organic, and Buff and Zamminer the
physico-chemical. Organic chemistry was especially developed by the
publication of Gerhardt's _Traite de chimie organique_ in 1853-1856,
and of Kekule's _Lehrbuch der organischen Chemie_ in 1861-1882.
General theoretical and physical chemistry was treated with
conspicuous acumen by Lothar Meyer in his _Moderne Theorien_, by W.
Ostwald in his _Lehrbuch der allgem. Chemie_ (1884-1887), and by
Nernst in his _Theoretische Chemie_. In English, Roscoe and
Schorlemmer's _Treatise on Chemistry_ is a standard work; it records
a successful attempt to state the theories and facts of chemistry,
not in condensed epitomes, but in an easily read form. The _Traite de
chimie minerale_, edited by H. Moissan, and the _Handbuch der
anorganischen Chemie_, edited by Abegg, are of the same type. O.
Dammer's _Handbuch der anorganischen Chemie_ and F. Beilstein's
_Handbuch der organischen Chemie_ are invaluable works of reference.
Of the earlier encyclopaedias we may notice the famous _Handworterbuch
der reinen und angewandten Chemie_, edited by Liebig; Fremy's
_Encyclopedie de chimie_, Wurtz's _Dictionnaire de chimie pure et
appliquee_, Watts' _Dictionary of Chemistry_, and Ladenburg's
_Handworterbuch der Chemie_.

The number of periodicals devoted to chemistry has steadily increased
since the early part of the 19th century. In England the most
important is the _Journal of the Chemical Society of London_, first
published in 1848. Since 1871 abstracts of papers appearing in the
other journals have been printed. In 1904 a new departure was made in
issuing _Annual Reports_, containing resumes of the most important
researches of the year. The _Chemical News_, founded by Sir W. Crookes
in 1860, may also be noted. In America the chief periodical is the
_American Chemical Journal_, founded in 1879. Germany is provided with
a great number of magazines. The _Berichte der deutschen chemischen
Gesellschaft_, published by the Berlin Chemical Society, the
_Chemisches Centralblatt_, which is confined to abstracts of papers
appearing in other journals, the _Zeitschrift fur Chemie_, and
Liebig's _Annalen der Chemie_ are the most important of the general
magazines. Others devoted to special phases are the _Journal fur
praktische Chemie_, founded by Erdmann in 1834, the _Zeitschrift fur
anorganische Chemie_ and the _Zeitschrift fur physikalische Chemie_.
Mention may also be made of the invaluable _Jahresberichte_ and the
_Jahrbuch der Chemie_. In France, the most important journals are the
_Annales de chimie et de physique_, founded in 1789 with the title
_Annales de chimie_, and the _Comptes rendus_, published weekly by the
Academie francaise since 1835.