CHAPTER II. LAW OF VOLUMES. GAY-LUSSAC-AVOGADRO AND AMPÈRE-BERZELIUS. I. THE atomic weights established by Dalton were really proportional numbers; they represented the proportion in which bodies combine, expressed by the relative weights of their ultimate particles. The atoms of simple bodies are equivalent to each other. We may, therefore, consider the terms atomic weights, proportional numbers, and equivalents as at this time synonymous. We owe the last term to Wollaston; H. Davy preferred the expression proportional numbers.' The atomic constitution of bodies follows very naturally from the ideas of Dalton. In binary compounds atoms unite in the ratio of 1 to 1, and in multiple compounds formed by two given elements in the ratio of 1 to 1, 1 to 2,1 to 3, 2 to 3, &c. This simple conception, which is clearly demonstrated in the table upon the preceding page, had to be modified in accordance with Gay-Lussac's great discovery. The relations between the combining volumes of gases are very simple, and the volume of the compound formed bears, moreover, a very simple ratio to the sum of the volumes of the combining gases. This proposition embraces a great number of facts, which present no exceptions and which together constitute a great law of nature, the law, namely, of GayLussac. Suitably interpreted, it has become one of the foundations of chemical science. The following are the facts; the interpretation will be developed presently :2 vol. of hydrogen unite with 1 vol. of oxygen to form 2 vol. of aqueous vapour.1 2 vol. of nitrogen unite with 1 vol. of oxygen to form 2 vol. of nitrogen protoxide. 1 vol. of nitrogen unites with 1 vol. of oxygen to form 2 vol. of nitrogen dioxide. 1 vol. of nitrogen unites with 2 vol. of oxygen to form 2 vol, of nitrogen peroxide. 1 vol. of chlorine unites with 1 vol. of hydrogen to form 2 vol. of hydrochloric acid gas. 2 vol. of chlorine unite with 1 vol. of oxygen to form 2 vol. of .hypochlorous anhydride. 1 vol. of nitrogen unites with vol. of hydrogen to form 2 vol. of ammonia. 2 vol. of carbon protoxide unite with 2 vol. of chlorine to form 2 vol. of phosgene gas. 2 vol. of ethylene unite with 2 vol. of chlorine to form 2 vol. of vapour of ethylene chloride. Thus it appears that very simple relations exist not only between the volumes of gases entering into combination, but also between these volumes and the volume occupied by the gas or vapour of the com 1 The volumetric composition of water was discovered in 1805 by Gay-Lussac and Humboldt. This observation formed the starting point of Gay-Lussac's discoveries. pound body. It should be remarked, moreover, that, as far as we know at present, the volumes of the combining gases are always reduced to 2 vol. after combination.1 Bearing this fact in mind, we may return to our historical account. Gay-Lussac rendered unexpected assistance to the ideas of Dalton. The fixed relations which are admitted between the weights of elements entering into combination, the simple relations which exist between the weights of a given element in the multiple combinations of that element, are again encountered when the combining volumes of gases are considered. Connecting these two orders of facts, and following up the interpretation which Dalton gave of the former, may we not conclude that the relative weights of the gaseous volumes entering into combination exactly represent the relative weights of the atoms-in other words, that there exists a simple relation between the specific gravities of elementary gases and their atomic weights? Gay-Lussac perceived this simple relation, and Berzelius defined it a few years afterwards; but Dalton refused to accept it, ignoring and repudiating the solid support which the great French chemist gave to his ideas. In fact, the relation which exists between the densities of gases and their atomic weights is not so simple as we should at first sight be led to expect, and as for a long time it was thought to be. It is a difficulty which will soon be apparent, and This applies particularly to the first seven cases, in which the volumetric relations are as simple as possible, and cannot be reduced. The two last cases will be discussed presently. which has only quite recently been overcome, after sixty years of investigation and labour. Nevertheless the theoretical conception which embraces the two orders of phenomena in question, and which establishes a link between fixed and multiple chemical proportions and the law which regulates the combinations of gaseous volumes, was accurately formulated in 1813 by the Italian chemist Amedeo Avogadro. Starting from the discoveries of Gay-Lussac, Avogadro arrived at the conclusion that there exists a simple relation between the volumes of gases and the number of elementary or compound molecules which they contain. The most simple and at the same time the most probable hypothesis which can be brought forward upon this point is, he says, to admit that all gases contain in a given volume the same number of integral molecules.1 These molecules must, therefore, be equidistant from each other in different gases, and placed at distances which, in relation to the dimensions of the molecules, shall be exactly sufficient to neutralise their mutual attraction. This hypothesis, according to Avogadro, is the only one which gives a satisfactory explanation of the fact of the simplicity of relations between the volumes of gases entering into combination. The following result of this hypothesis is important if it is true that equal volumes of gases contain the same number of molecules, the relative weightsthat is to say, the densities of equal volumes-ought to represent the relative weights of the molecules. Thus the molecular weights of hydrogen, oxygen, and nitro 'Journal de Physiq e, vol. xxxiii. p. 58. gen will be expressed by the ratio of their densities -i.e. 1, 15, 13.1 But in considering the molecular weights of compound bodies we encounter a difficulty, which arises from the difference of contraction experienced by gases in the act of combination. Supposing water to be formed by the union of two volumes of hydrogen and one volume of oxygen contracted to one volume, it is clear that the weight of this single volume, compared with that of one volume of hydrogen, would be 17 (15+2); 2 or again, supposing one volume of ammonia were formed by the contraction of three volumes of hydrogen and one volume of nitrogen, the weight of this volume of ammonia must be 16. Now, experiment proves that the densities of aqueous vapour and of ammonia are half the above numbers —namely, 8.5 and 8 —a result which agrees with the fact that two volumes of hydrogen and one volume of oxygen are condensed into two volumes of aqueous vapour, and, on the other hand, that three volumes of hydrogen and one volume of nitrogen are condensed into two volumes of ammonia. Since one volume of aqueous vapour contains only one volume of hydrogen and volume of oxygen, a molecule of water can only be formed of one molecule of hydrogen and molecule of oxygen; and, for the same reason, one molecule of ammonia must be formed of 1 molecule of hydrogen and molecule of nitrogen, and a molecule of hydrochloric acid gas of molecule of hydrogen and molecule of chlorine. It follows that the matter contained in the unit 1 The correct numbers are 1, 16, 14. |