The chemical researches of the 18th cen tury were many and varied, but most of them, at least during the earlier decades, were essen tially qualitative in character. The fundamental importance of exact weight and measure came into recognition with extreme slowness. Throughout the greater part of the century one theory dominated chemical thought, the theory of phlogiston, proposed by Becher, but de veloped and completed by Stahl. The phe nomena of combustion had always attracted the attention of chemists, and the new theory was devised to explain them. Every combustible body was supposed to contain a peculiar non isolable substance, phlogiston; and when com bustion took place this substance was thought to be expelled. Thus lead, when heated in the air, undergoes a change analogous to that pro duced by combustion, and yields a calx, or, as we call it now, an oxide. This calx, combined with phlogiston, was thought to exist in the original metal, and to be freed from phlogiston when calcination occurred. In this speculation no account was taken of the weight of the several bodies, and the fact that the calx was heavier than the metal, that a gain, not a loss, was observed, seemed to offer no difficulty to the believers in the phl doctrine. To phlogiston a negative was ascribed, and by this device the real di culties of the prob lem were comfortably laid aside.
In 1774 Joseph Priestly, himself a believer in phlogiston, discovered oxygen; and so made, though unwittingly, the true interpretation of combustion possible. In 1766 Cavendish had discovered hydrogen; and in 1781 he proved that water was produced by the union of the two new gases. Cavendish also determined the composition of the atmosphere, and in these re searches the foundations of a new chemistry were laid. The two chief architects to build upon the foundation were a Frenchman, La voisier, and an Englishman, Dalton.
Lavoisier, by careful use of the balance as an instrument of research, proved. that matter was constant in weight and could neither be created nor destroyed. In any chemical change the weight of the substances engaged in the reaction remained unaltered. Studying combus tion he showed that it was merely combination with oxygen; and he pointed out that respira tion was a phenomenon of the same character. He also gave greater precision to the idea of an element, and announced the elementary na ture of the metals; and, in conjunction with other chemists, did much toward the estab lishment of a rational system of chemical nomenclature. Hitherto the names of com pounds had been arbitrary and often meaning less; now they were made to express with more or less accuracy the composition of the• substances described.
Lavoisier died in 1794, a victim of the French Revolution; and it was not until 1803 that next really great forward step in chemistry was taken by Dalton, who then first announced his famous atomic theory. To support this doc trine, which, being quantitative in form, had little in common with the atomistic speculations of the philosophers, Dalton established two laws, the laws of definite and multiple propor tions. That every chemical compound has a fixed and definite composition was recognized by Lavoisier and by other writers before him, but the fact was disputed by Berthollet, and it remained for Dalton to give its statement a pre cise form. Dalton then went further, and found that to every element a definite combin ing number could be assigned, and that when two elements united in more than one propor tion, even multiples of that number appeared.
Thus, taking the hydrogen weight as unity, the standard of comparison, oxygen always com bines with other elements in the proportion of eight parts or some simple multiple thereof, and so on through the entire list of elementary bodies. Each one has its own distinct com bining weight, and this was a condition which Dalton sought to explain. Fractions of the weights did not occur, fractional atoms could not exist, and the two thoughts were connected by Dalton. Chemical union, to his mind, be came a juxtaposition of atoms, whose relative weights were indicated by their combining num bers; and so the atomic conception was for the first time given a clear, quantitative expression. First, every element is composed of similar atoms which have constant weight. Secondly, chemical compounds are formed by the union of these atoms in simple numerical relations. Upon these fundamental statements the entire system' of chemical philosophy rests, so that for a hun dred years the history of chemistry has been the history of the atomic theory. All chemical calculations are based upon the atomic weights of the elements, and in all chemical formula; they are implied.
Since Dalton's time great labor has been expended upon the exact determination of atomic weights, and in the discovery and de scription of new elements and compounds. The general conclusions which have been established by this class of researches may be summarized as follows: Every chemical substance is either an element or a compound. The elements, which are not artificially separable into any simpler bodies, at least by no means yet dis covered, are comparatively few in number ; the compounds are innumerable. More than a hun dred thousand compounds are already known. A compound may be separated into its elements or built up from them; and its composition is absolutely invariable. In this respect com pounds differ from mechanical mixtures, in which any proportion may occur. Flour and sugar may be mixed together, but they still remain flour and sugar, each with its prop erties unchanged; no combination here takes place. In combination, as when gaseous hydrogen and gaseous oxygen unite to form liquid water, they do so only in one fixed pro portion and the characteristics of the original substances disappear. This fact of combination, the union of two or more bodies to form others which are widely different from them, is clear; but its mechanism is not yet understood. The elementary atoms of the compound are drawn and held together by some form of attraction, but its precise nature is unknown. The object of chemistry is to discover the laws which gov ern the union or the decomposition of sub stances, and to determine the limits within which such changes are possible. For the study of compounds, at least for the purpose of ascer taining their composition, two methods are em ployed. First, analysis, in which the com ponent parts of the compound are separated, and individually identified. Secondly, synthesis, in which the parts are forced to unite, and to form the compound which happens to be under investigation. Furthermore, analysis may be either qualitative or quantitative. In one case we merely ascertain what substances are pres ent, in the other we determine their exact quan tity. The elements now known, about 80 in number, are given in the following table, to gether with their atomic or combining weights and their symbols. The latter are abbrevia tions whose use will be explained presently.