had found to regulate the absorption of gases by water, he proposes a theory in explanation of it, according to which he contends that gases, such as oxygen, nitrogen, carbonic acid, &c., when in aqueous solution, are mechanically mixed with water, not chemically combined with it-a view which has not been adopted by other chemists. Gases so mixed with water,' says he, 'retain their 'elasticity or repulsive power among their own particles, just the 'same in the water as out of it, the intervening water having no other 'influence, in this respect, than a mere vacuum! He goes on to compare his gas dissolved in water to a pile of shot-a particle of gas pressing on the surface of water is analogous to a single 'shot pressing upon the summit of a square pile of them;' and on the opposite page he has inserted an engraving of a pyramidal pile of balls left unshaded, with a dark ball surmounting the apex. This is entitled, View of a square pile of shot, &c. The lower globes are to represent particles of water; the top globe represents a particle of air resting on particles of water.' Further on are two other engravings, the one of a Horizontal view of air in water,' the other a Profile view of air in water,' in which dots and crosses are taken to represent particles of air, with spaces of water between them. We have specially referred to these engravings, as affording additional illustrations of the hold which a belief in the atomic constitution of matter had taken of Dalton's mind, and the use which he made of it in discussing purely physical problems (or, at least, what he considered such), before he had occasion to apply it to chemical questions at all. At the close of the essay comes the acknowledgment of the difficulty which attends a hypothesis of mechanical absorption. If the mingling of gases with liquids, on which they do not act chemically, be but a mechanical action, like the mingling of indifferent gases with each other, how happens it that water dissolves its own bulk of one gas, such as carbonic acid, and only three per cent. of its volume of another, such as oxygen? We should expect, if the mechanical view were true, that all gases should be equally soluble in water; for if water act as a vacuum would do, it must act in the same way on every gas. Dalton saw the difficulty, and devised a hypothesis to overcome it. We give his own words :- Why does water not admit its 'bulk of every gas alike? This question I have duly considered, ' and though I am not yet able to satisfy myself completely, I am nearly persuaded that the circumstance depends upon the weight and number of the ultimate particles of the several gases: those whose particles are lightest and single being 'least absorbable, and the others more, according as they increase in weight and complexity." To this there is a foot-note Subsequent experience renders this conjecture less probable.' And the text is followed by a passage which we print in italics - An inquiry into the relative weights of the ultimate particles of bodies is a subject, as far as I know, entirely new; I have been 'prosecuting this inquiry with remarkable success.' On the succeeding page is a Table of the relative weights of the ultimate particles of gaseous and other bodies.' This was the first table of atomic weights, and every one of them was wrong, with the exception of hydrogen, which was assumed as unity. We extract four of the numbers: Hydrogen Carburetted hydrogen from stagnant water 1 5.5 6.3 5.3 Such, then, were the steps by which Dalton was conducted to the discovery of the laws of combining proportions. He was testing, by experiment, the truth of a hypothesis as to the cause of the specific solubility of gases in water, which proved in the end to be quite untenable; but, like Columbus, who missed an El Dorado, but found an America, he discovered something better. From what Dr. Thomson tells us, he was struck by observing that the quantity of hydrogen in fire-damp is exactly twice that in heavy carburetted hydrogen, the quantity of carbon being the same in both. His constant reference of the properties of masses to those of their smallest molecules led him at once to connect these proportions, in which the carbon and hydrogen occurred, with the relative weights of their ultimate particles. We may suppose him to have reasoned somewhat thus-Hydrogen and carbon are made up of particles which have different weights, 'the carbon atoms being all six times heavier than the hydrogen ones; but if hydrogen and carbon have atoms differing in relative weights; oxygen, nitrogen, and every other elementary sub'stance will have atoms differing in relative weight also; and 'these may be ascertained by finding the relative weights according to which the masses made up of them combine with each other.' To Dalton's mind, filled, as it were, already with the conception of everything consisting of atoms, it was only necessary to introduce the additional idea of these atoms differing in relative weight, and all the laws of combining proportion rose at once into view. He was gifted with a bold, self-reliant, farglancing, generalizing spirit, and the researches he had long been prosecuting had doubtless strengthened greatly that faith in the uniformity of nature's laws which we all inherit as an essential part of our mental constitution. We may believe that, without an effort, and almost instinctively, he would infer that if hydrogen followed a law of multiple proportion in its higher combinations with carbon, a similar relation would be found to hold in every case where the same elements united to form more than one compound. The detection of the other laws of combining proportion must have been immediate; but this has been so fully illustrated already, that we need not enter on the subject again. It must never be forgotten that Dalton's atomic views gave him the same advantage in detecting the laws of chemical combination which they afford us in apprehending and expounding them. In confirmation of the view we have taken of the development of the atomic hypothesis, we would refer to Dalton's contributions to the first six volumes of the Manchester Memoirs,' which, gone through consecutively, will conduct every reader, we believe, to the conclusion we have arrived at. It is confirmed by Dalton's reference to the carburetted hydrogens already considered, and by the way in which Dr. Thomson introduces the earliest published account of the atomic theory, not while discussing chemical affinity or the laws of combination, but quite abruptly under the head of the density of the gases. Dalton himself always connected his later chemical with his earlier physical discoveries. When he published the second edition of his 'Meteorological Essays,' in 1834, forty-one years after the publication of the first, he said, in reference to the few alterations it contained—I have been the more anxious to preserve the first 'edition unchanged, as I apprehend it contains the germs of 'most of the ideas which I have since expanded more at large in different essays, and which have been considered discoveries of 'some importance.' We wind up this long discussion with a single remark. Dalton's views of chemical combination, including both the facts and the hypothesis which expressed and explained them, are generally known as his Atomic Theory.' To Dalton himself the evidence in support of the existence of ultimate indivisible particles appears to have seemed so conclusive, that he considered the doctrine of atoms in the light of an induction from the data furnished by observation and experiment; and this without reference to any other than purely physical questions. We cannot, indeed, sufficiently reiterate that he was an atomist before he was a chemist. In his lips, therefore, the name 'Atomic Theory' was consistent, and had a clear meaning. It was John Dalton's atomic theory of chemical combining proportions; his theory of atoms connected with his discoveries in chemistry, so as at once to account for, and to expound them. To those, however, who cannot by any process of generalization establish to their own satisfaction, or to that of others, the actual existence of atoms, (and it includes almost every one who thinks on the subject at all,) and for whom the doctrine of atoms is only a questionable, and, we may say, an indifferent hypothesis, Dalton's view is 'an atomic hypothesis of combining proportion.' It matters comparatively little, however, whether we say atomic theory or atomic hypothesis, provided we keep perfectly distinct what is matter of assumption concerning atoms, from what is matter of fact concerning laws of combining proportion. The only chemist who has adopted Dalton's views is Dr. Thomson, who affirms that unless we adopt the hypothesis with which Dalton set out-namely, that the ultimate particles of 'bodies are atoms incapable of further division, and that chemical ' combination consists in the union of these atoms with each other'we lose all the new light which the atomic theory throws upon chemistry.' Dalton's other contemporaries-Davy, Wollaston, and Berzelius-on the other hand, protested against confounding the question of atoms with that of combining proportions, and declined to employ the word atom. Davy substituted the term proportion; Wollaston, that of equivalent the best of all the titles by which the combining weight of a body can be indicated. Notwithstanding this, it is notorious that the word atom is universally employed; the phrase equivalent comparatively seldom. Some of Dalton's less discriminating admirers have built much upon this, as showing that even the opponents of an atomic view of matter are obliged to use its phraseology. This is true so far as the word atom is concerned; but in the language of a chemist of the present day, that term has no other meaning than the phrase equivalent; to which it is preferred only, we believe, because it contains half as many syllables, and is more easily pronounced. Liebig has justly observed that the use of the word atom is like that of the term element. The latter does not signify a body that cannot be, but only one that has not been decomposed; atom, not a particle which cannot be, but only one which, up to a certain point, has not been divided. Hence the chemist has no scruple in applying the term atom to a group of molecules considered as a whole, although he is quite certain that this compound whole may be, and often is, divided. He speaks, for example, of an atom of water, of carbonic acid, of sugar, and the like. The announcement of the atomic theory to the chemists of Europe was like a lighted torch passed round among lamps, trimmed and filled with oil, and ready to be kindled. Some heard with incredulity, like Davy; others with gladness, like Thomson; none, probably, without astonishment, that the humble teacher of mathematics, had extracted more meaning out of his imperfect and even inaccurate analyses than they, even Berzelius and Wollaston, out of their scrupulously exact ones. It was so, however. In Spain, France, Germany, Sweden, and elsewhere, many were seeking to discover the laws regulating chemical combination, every one of them probably acquainted with a wider range of chemical phenomena, and a better analyst than Dalton; but he beat them all. So true is it, what Thomas Carlyle says, that 'the eye sees what it brings the power to see." No great discovery, perhaps, was ever welcomed so heartily and immediately as the announcement in the atomic theory of the laws of combining proportion. The chemists looked over the analyses recorded for other purposes in their laboratory books, and found on every page ample confirmation of Dalton's discoveries. Davy, Thomson, Wollaston, but above all, Berzelius, furnished every day better proofs than Dalton himself could show, that in every essential point his views were as just as they were beautiful and original. The question of Dalton's exact merit was at one time a good deal discussed, and is certain to be made matter of discussion again, as soon as a complete memoir of him is published. The sketch we have given of the path by which the atomic theory was reached enables us, we think, to set at rest the question of the rival claims of Higgins and others. In deciding the question of merit in reference to any scientific discovery, three points require in every case to be considered. The first, The question of time-Who earliest made the discovery? The second, The question of desert-Who had the greatest merit in making it? The third,-The question of practical effect-Who aroused the world by his discovery, and made it tell upon the progress progress of science? The last is, if not the only, at least the main point in the popular estimation of the merits of discoverers. It is the peculiar office of a journal such as ours to see that the two former receive at the hands of all equal consideration. The question of time admits of no dispute. The law of constant proportion had been recognised by Bergman and Proust,not to mention others, before Dalton's time, nor did he ever claim its discovery. The law of reciprocal proportion was made out completely by Wenzel and Richter, in 1777. The law of multiple proportion was recognised clearly and fully by Higgins, in 1789. The law of compound proportion was discovered by Dalton, in This is the state of matters so far as time is concerned, and leaves no choice in the adjudication of merit in regard to the question of priority of discovery. Justice admits of no 1803. |