gress; but, in common with all fruitful ideas, it has been an instrument of progress even in the hands of its detractors. The latter are now few, and the hypothesis seems to make a firm stand against the regular opposi tion of some and the subtle attacks of others. In these pages we propose to discuss both its historical evolution and its present form, and we shall thus show the influence it has exercised upon the progress of science since the beginning of the century.

Dalton revived the hypothesis of atoms to explain the fact that in chemical combinations elements unite in fixed proportions, and in certain cases in multiple proportions. He admitted that these proportions represent the relative weights of indivisible particles of the bodies, which particles are brought into contact and grouped by the fact of combination. This led to the consideration of atomic weights, and the idea of representing the composition of bodies by symbols which indicate both the nature and the number of these particles and the proportion of the elements entering into combination. We have here two things which must not be confounded- facts and an hypothesis. We shall retain the hypothesis as long as it gives a faithful interpretation of facts, and enables us to group them, to connect them together, and to anticipate fresh ones -as long, in fact, as it proves fertile. An hypothesis thus formed rises to the rank of a theory. We shall endeavour to show, in demonstrating its origin, progress, and results, that this is the case with Dalton's conception.

I.

Simple bodies combine in definite proportions. This is one of the most firmly established truths of natural philosophy. It includes the two following facts:-Firstly, the relative weight of combining bodies is always fixed in every combination; secondly, the numbers which express these relations are interproportional for all kinds of combinations. We must clearly understand the meaning of these propositions.

Two simple bodies unite so as to form a given compound. As long as the compound lasts the relative. weights of the two elements will remain perfectly constant, whether the quantities acting upon each other have been great or small; the smallest particles, as well as the whole mass, will contain strictly proportional weights of these elements, which no physical circumstances, such as pressure or temperature, can modify. This is true for all kinds of combinations, the most simple as well as the most complicated. This fixity of the proportion in which bodies combine was acknowledged and admitted as a truth more than a century ago by some eminent chemists, and by all in the year 1806. Bergman was conscious of the truth, even if not logically convinced of it; in fact, the numerous quantitative analyses for which we are indebted to him would have been aimless or useless if he had been under the impression that the compounds he was analysing were formed in chance proportions. Lavoisier demonstrated in the clearest manner the fact of the constancy of the relations in which bodies combine. In every

oxide, in every acid, he said, the relation of oxygen to the metal is constant; and this relation should be exactly determined for every oxygen compound. He admits, moreover, that the difference between the acids of sulphur and the oxygen compounds of nitrogen is due to the power possessed by these simple bodies of uniting with oxygen in several proportions, each degree of oxidation corresponding to a fixed and constant relation between the weights of the two elements. The law' of fixity was thus distinctly admitted and clearly stated by Lavoisier; one step more, and he would have discovered the law of multiple proportions. He did not, however, make this decisive step. Even as regards the fixity of several proportions, though he was himself convinced of the fact, he was not successful in making it universally accepted. In the month of July 1799 his pupil Berthollet read at the Egyptian Institute, which was sitting at Cairo, a memoir entitled 'Researches upon the Laws of Affinity.' He there for the first time brought forward profound ideas upon the influence exercised by the physical condition, the cohesion, solubility, insolubility, and volatility of bodies upon the affinity and progress of chemical decompositions. Without denying the fixity of the composition of certain compounds, he attributed this fact to the chance influence of these physical conditions, which in some cases were constant, and would not allow that it partook of the character of a general law.

It is true, he said, that in sulphate of waryta the relation between the sulphuric acid and the baryta is constant, simply because the acid and base must

unite in this precise and fixed proportion to form a salt of absolute insolubility. Thus, here as in many other cases, constancy of composition is dependent upon a physical property, cohesion-in other words, the insolubility of the sulphate of baryta. But the rule is as follows: Chemical combinations take place in proportions which may vary within certain limits.

A salt formed by a soluble acid and a slightly soluble or insoluble base may be precipitated in an insoluble form, unvarying in composition, when the proportion of the base is exactly such as to cause the precipitation of the salt of this composition; but if the proportion of the base is increased the salt will still be precipitated, but its composition will be different, for it now consists of greater quantities of base for the same quantity of acid.

A metal, such as mercury, dissolved in nitric acid, will unite, in the process of oxidation, with quantities of oxygen varying between a maximum and a minimum. We cannot, therefore, maintain with Lavoisier that when a salt is formed by the action of an acid upon a metal, there is a constant relation between the quantity of the metal and the quantity of oxygen which the former takes from the acid in the process of oxidation.

These propositions of Berthollet were first opposed and successfully refuted by S. L. Proust. Having remarked, in 1799, that upon dissolving native carbonate of copper in an acid, and then precipitating the solution by an alkaline carbonate, he obtained a quantity of carbonate of copper equal to that of the native carbonate which had been dissolved, Proust drew from this fact

the conclusion that the composition of carbonate of copper is fixed and invariable, whether the salt has been formed in the depths of the earth or artificially by a chemical process. His subsequent researches enabled him to generalise this conclusion; and in speaking of these researches we must specially quote those upon the composition of the two oxides of tin, the sulphides of iron, and sulphide of antimony. In all these compounds the relation in weight between the two elements is constant; and if two simple bodies, by combining in different proportions, are able to form several compounds, as is the case with tin and oxygen, iron and sulphur, it is evident that in every degree of combination the relation in question is invariable.

Proust brought forward these facts, which he had. discovered in opposition to those upon which Berthollet took his stand, and showed that the latter allowed a different interpretation. Metallic solutions, where the metal enters into combination with variable quantities of oxygen; salts, which, when precipitated, may contain variable quantities of bases; or oxides of tin and lead, which have been obtained by the calcination of metals in contact with air, and which have fixed variable quantities of oxygen-in no case consist of, or constitute, definite chemical compounds, but are mixtures, in different proportions, of several compounds, all of which possess a fixed composition. The fixity of composition, indeed, seemed to Proust an essential attribute of combinations, a great law of nature-the pondus naturæ, justly recognised by Stahl.

This discussion, which is one of the most memorable