THE RELATIONS BETWEEN THE ATOMIC WEIGHTS OF THE at end THE ATOMIC THEORY. BOOK I. ATOMS. CHAPTER I. HISTORICAL INTRODUCTION-RICHTER-DALTON. THE hypothesis of atoms, put forward by the Greek philosophers, and revived in modern times by great thinkers, acquired a definite form at the beginning of this century. John Dalton was the first to apply it to the interpretation of the laws which he and Richter recognised as governing chemical combinations. Confirmed by the great discoveries of Gay-Lussac, Mitscherlich, Dulong and Petit, the hypothesis has assumed a definite form, connecting many various facts of a chemical and physical nature. Fundamentally it consists of modern ideas upon the constitution of matter. In common with correct ideas, it has grown with time, and nothing has as yet happened to stop its pro 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 opposition 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 baryta the relation between the sulphuric acid and the baryta is constant, simply because the acid and base must |