We have omitted in this table the atomic weights of the alkalies and earths which are still placed among the elements, though Dalton must have already been acquainted with the great discovery of H. Davy upon the nature of the alkalies. The above figures give, however, a sufficiently good idea of the accuracy, or rather the inaccuracy, to which Dalton had attained in his own determinations, or in the discussion of those of others. At the same time they show us the exact sense in which we must regard these atomic weights. They are not, properly speaking, atomic weights in the sense which we now ascribe to the term; they are proportional numbers referred to unity, which represents the weight of hydrogen in hydrogen compounds. This may be seen from the following table, in which, for the sake of brevity, we have employed the symbols in use at the present day : Atomic Weights. Water contains 1 at. H, which weighs 1, and 1 at. O, which 8 14 Ammonia contains 1 at. H, which weighs 1, and 1 at. N, which weighs 5 6 Olefiant gas contains 1 at. H, which weighs 1, and 1 at. C, which weighs 5 Marsh gas contains 2 at. H, which weigh 2, and 1 at. C, which weighs 5 7 Carbon protoxide contains 1 at. C, which weighs 5, and 1 at. O, Carbonic acid contains 1 at. C, which weighs 5, and 2 at. O, which weigh 14 19 Protoxide of nitrogen contains 2 at. N, which weigh 10, and 1 at. O, which weighs 7. 17 • Atomic Weights. Binoxide of nitrogen contains 1 at. N, which weighs 5, and 1 at. Nitrous acid contains 2 at. N, which weigh 10, and 3 at. O, weigh 21 31 19 Nitric acid contains 1 at. N, which weighs 5, and 2 at. O, which weigh 14 We see that the atomic weights of oxygen, sulphur nitrogen, carbon, and phosphorus are deduced from the composition of their combinations with hydrogen, in which the existence is admitted of one atom of hydrogen combined with one atom of another body; and when there are two combinations with hydrogen, as is the case with carbon, the atomic weight is determined from that containing the least quantity of hydrogen. Thus the atomic weight of carbon is the quantity of carbon combined with 1 of hydrogen in olefiant gas. In marsh gas this quantity of carbon is combined with 2 of hydrogen. Such are the principles by which Dalton was guided in the determination of atomic weights, as they were conceived by him in 1808, and in the notation which was deduced from them. These principles are clearly demonstrated in the following table, which expresses the atomic constitution of the compounds mentioned above; the formulæ are analogous to those now in use:— 33 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 the preceding page, had to be modified in accordance with Gay-Lussac's great discovery. upon D 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 3 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 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. |