HISTORY.] CHEMISTRY 465 and iron with sulphur, in two definite proportions respec tively. In a controversy with Berthollet that extended over some years he successfully demonstrated that all com pounds are definite, and contain only certain determinate proportions of their constituents. In August 180-1, Dalton (1766-1844) communicated to Dr Thomson his theory of the atomic composition of bodies. Dalton s ideas had been vaguely foreshadowed by W. Higgins of Pembroke College, Oxford, in a publication entitled A Comparative View of the Phlogistic and Antiphlogistic Theories, where we read that " In volatile vitriolic acid a single ultimate particle of sulphur is intimately united only to a single particle of dephlogisticated air ; and in perfect vitriolic acid every single particle of sulphur is united to two of dephlogisticated air, being the quantity necessary to saturation. As two cubic inches of light inflammable air require but one of dephlogisticated air to condense them, we must suppose that they contain equal number of divisions, and that the difference of their specific gravity depends chiefly on the size of their ulti mate particles ; or we must suppose that the ultimate particles of light inflammable air require two or three or more of dephlo gisticated air to saturate them. If this latter were the case, we might produce water in an intermediate state, as well as the vitriolic or the nitrous acid, which appears to be impossible ; for in whatever proportion we mix our acids, or under whatever circum stance we combine them, the result is invariably the same." (2d d., 1791.) The atomic theory first suggested itself to Dalton during his investigations on marsh-gas, or light carburetted hydro gen, and olefiant gas. He calculated that if the weight of carbon in each of these compounds were reckoned to be the same, then marsh-gas contained exactly twice the weight of hydrogen present in olefiant gas. He further observed that the quantity of oxygen in carbonic acid gas was twice as much as in carbonic oxide gas. These and similar facts he conceived might be explained by assuming the ultimate particles of matter to be incapable of further division, or, in other words, atoms, possessing definite weights, the ratios of which could be denoted by numbers, the weight of an atom of hydrogen being taken as unity. The combination of these atoms with one another would account then for the definite proportions in which the elements united. For clearness the atoms were represented by symbols, thus : Relative Weights. O Oxygen 6 5 Hydrogen 1 Carbon 5 <D Azote 5 Binary compounds with their weights were denoted thus : OO Water 7 5 O Olefiant gas 6 00 Ammonia 6 O Carbonic oxide 1 1 -5 and ternary compounds in a similar manner : O^O Carbonic acid gas 18 Ether 11 O Carburetted hydrogen gas 7 The weight of the smallest particle of a compound was therefore, according to Dalton s theory, to be obtained by adding together the weights of its constituent atoms ; the laws discovered by Wenzel and Richter were thus in cluded within the law of atomic weights. The new theory was promulgated in Dr Thomson s System of Chemistry, 3d edit,, 1807. In the same year Thomson showed that the amount of oxalic acid required to form strontium binoxalate was twice as great as that in the case of the oxalate, supposing the weight of strontium in each salt to be the same ; and Dr "Wollaston proved that three potassium oxalates were obtainable, to form which a definite amount of potash needed weights of oxalic acid in the proportion of 1 : 2 : 4. In 1808, the year of the publication of Dalton s New System of CJiemical Philosophy, Gay-Lussac made known to the world the laws of the combinations of gases by volume to which his attention had been directed by the discovery which he and A. Yon Humboldt had made, that a definite volume of oxygen combined with exactly twice its bulk of hydrogen. He pointed out that there is a simple relation between the volumes of two gases which unite together, and also between their collective volume in the uncombined and in the combined condition. Thus, three volumes of hydrogen combine with one of nitrogen to form two volumes of ammonia ; one volume of chlorine with one of hydrogen produces two volumes of hydrochloric acid gas ; and two volumes of nitrogen and one of oxygen give two volumes of protoxide of nitrogen. The law of definite proportions was shown to hold good with respect to the volumes as well as to the weights of combining bodies. In 1811 Avogadro, remarking that equal variations of temperature and pressure produce in all gases and vapours the same changes of volume, enunciated the hypothesis that equal volumes of any gas or vapour contain the same number of atoms, and the same doctrine was brought forward in 1814 by Ampere. On this hypothesis the density of equal volumes of gases w r as shown to represent the relative weights of their atoms ; and thus, as it had been discovered that gases frequently do not unite volume for volume, a distinction came to be drawn between atomic weights and equivalents. If we say atom for volume, wrote Berzelius, we find in Gay-Lussac s discovery one of the most direct arguments in favour of Dalton s hypothesis. Berzelius (1779-1848), from considerations based on the law of combination by volume, accounted the atoms of elements distinct from their equivalents. Thus two volumes of hydrogen were recognized as the equivalents of one volume of oxygen, the relative weights of equal volumes of the two gases being those of their atoms. Berzelius adopted 100 parts of oxygen as his standard of atomic weight, the atomic weight of hydrogen was there fore 6 24, its equivalent, 12 48. He considered that the atoms of aluminium, arsenic, antimony, bromine, chlorine, fluorine, hydrogen, iodine, nitrogen, and some other elements had a weight equal to o^y half that of their equivalents, which latter were double and inseparable atoms. In place of the symbols used by Dalton to denote the proportions ii* which the elements combine by weight, he employed a notation in which letters were used to indicate the names of the elements. He introduced also an abridged notation, in which the equivalents or double atoms were represented by drawing a bar through the symbol of the element. A dot being used to signify an atom of oxygen, the formula of water was written H- ; and HGt denoted hydrochloric acid, which was viewed as consisting of a double atom of hydrogen united to a double atom of chlorine, an hypothesis which left unexplained the fact that the combination of the so-called double atoms of hydrogen and chlorine resulted always in the formation of two molecules, instead of one, of hydrochloric acid gas. Berzelius constructed a table of atomic weights and equivalents, which the discovery by Dulong and Petit in 1819 of the connection between the specific heats and the atomic weights of the elements, and that of the law of iso morphism by Mitscherlich in 1820, enabled him to modify and improve. The equivalent notation of Berzelius was adopted by Gay-Lussac, and displaced in time that founded upon the atomic weights ; but it was not generally applied with strictness to all compounds, molecular and net equivalent formulae being employed in some cases. It had this objection, that it masked the relative atom-fixing
V - 59