Dr. Thomson adopted the atomic ideas of Mr. Dalton; be has applied them to the chemical system ; and given, in the journal which he edits, a statement of the weight of the molecules, in several substances, simple as well as compound. Like Berzelius and Wollaston, he disagrees with Dalton in putting the the weight of oxygen = 1.000.* Since Dr. Thomson regards water as composed of an atom of oxygen and one of hydrogen, it follows that in most substances, the weight he assigns to the atom, is twice the weight of the volume in a gaseous form, compared with the weight of the volume of oxygen in a gaseous form.
Dr. Wollaston, without expressly declaring himself a partisan or an opponent of the corpuscular hypothesis, looks upon what others have named atoms, or molecules, as relative weights, in which experience has shown that bodies most readily combine. He calls them chemical equivalents. Oxygen with him, is = 1.000; and water is composed of an equivalent of oxygen, and one of hydrogen. \Vollaston's numbers differ a little from tho48 of Thomson. The equivalents being to each other in fixed proportions, their mutual combinations may be calculated by the Rule of Three. And upon this principle, Dr. Wollaston has invented a very ingenious instrument to facilitate the cal culation. It is well known that Gunter's scale, employed in sliding-rules, is divided in such a manner, that the num bers 1, 2, 3, 4, &c. are arranged upon it so as to have their distances proportional to their logarithms. On a similar sliding-rule, covered with card, Dr. Wollaston marks his numbers on the slider ; but upon the two sides, in place of writing the numbers, he writes the names of the sub stances equivalent to them. 1 bus, whoever knows how to use the sliding-rule, knows also how to use this instru ment. Suppose, for example, any one wishes to decom pose muriate of soda, and to knew how much of the decomposing substances is required. He must draw the slider till 100 (or such other number as he may choose) corresponds to muriate of soda. Now, the names soda and muriatic acid, correspond to the number of parts of soda and of acid contained in 100 parts of the muriate. Opposite to the name sulphuric acid, stands the number of parts of this substance necessary to be used for decom posing 100 parts of the muriate of soda ; and, in like manner, the names nitrate of silver, sulphate of ammonia, &c. &c. are found opposite to the numbers indicating how many parts of them are necessary to he used in the same operation. The instrument we have just described, is a fine monument of its author's genius. Its utility is great and Dr. Wollaston has so judiciously chosen the bodies marked on it, that the greater part of those commonly employed in laboratories, are to be found in his list. Vet the scale admits no great extension; because there is not room for many names, of which, moreover, several must correspond to the same numbers; and, as the space gets filled with names, it will become more difficult to find the particular one• we are in quest of. Besides, when there happen to be two or more equivalents of the same sub stance, in a compound body, unless the number of them is known before hand, it will be difficult to discover the composition. Suppose we wish to find the composition of the sulphate of red oxyd of iron. Taking 100 parts, the
weight of its equivalent would be found = 129.5. The slider is drawn till 100 corresponds to the number of the sulphate; the name sulphuric acid then corresponds to 38.7, but in reality it contains 38.7 x 12=58; so that, to avoid being misled by the instrument, we ought to know that this salt contains the equivalent and half not only of sulphur, but also of sulphuric acid, and of oxygen in the oxyd. It is on this account, that Dr. Wollaston has omitted most part of such combinations. For some salts, contain ing water of crystallization, he has added to the name of the salt, the number of equivalents of water included in it, so that they arc indicated by the instrument.
To profit ,more extensively by this mode of calculation, Berzelius has formed an alphabetical list of all the substances whose composition is known, and annexed to it the formulae expressing the number of atoms contained in each substance, and the number expressing its weight, that of oxygen being supposed = 1.000. By means of these tables, and a common sliding-rule, such as may he found in the shop of any instrument-maker, all manner of compositions may be calculated. This plan has the advan tage over that of Wollaston, in so far as its utility is more general, all known substances being comprehended in it ; but the latter is more agreeable, as it presents all the equivalents at a single glance.
Sir II. Davy, in his paper, " On some of the Combina tions of oxymuriatic gas and oxygen," &c. Phil. Trans. 1811, observes, in speaking of Air. Dalton's opinions : " I shall enter no further at present into an examination of the opinions, results, x "' Inflammable bodies, acids, alkalis, Scc. must separate in uniform ratios." Sir Ilumphry appears then, whilst he admits that Dalton's results are deducible from his hypothesis, not to approve of the cor . pustular theory in itself, regarding it as too hypothetical. In volume first, part first, of his " Elements of Chemical Philosophy," Sir H. adopts the system of chemical pro portion, for most part as Mr. Dalton does; but he gives the name of proportion to what Dalton calls an atom, and Wollaston an equivalent.
In Germany, 1\1. Gilbert has carefully collected, in the Journal conducted by him, under the title of ?innalen der Physik, whatever has a reference to the subject of chemi cal proportions. A statement of the doctrine was given so early us the number for Dec. 1811, DI. Schweigger, too, in his Journalfiir Chcmie mid Physik, has, in like manner, adopted the system of chemical proportions, without, how ever, approving of the atomic theory, perhaps because it is too repugnant to the ideas entertained by the school, of which he seems to be a follower.
Among French chemists, Messrs. Gay Lussac and Thenard appear, by their latest writings, to have likewise begun to adopt the system of chemical proportions, though they have as yet published nothing specially on the subject.
Thus it would seem that the doctrine of determinate proportions in chemistry may be regarded as a settled truth, generally acknowledged among men of science, although the corpuscular theory, or the hypothesis con cerning the cause of these proportions, is adhered to by a smaller number.