CHAPTER XI. JOHN DALTON ESTABLISHES THE ATOMIC THEORY. "Philosophy is the art of deciphering the mysteries of nature; and every theory which can explain the phenomena has the same evidence in its favour that it is possible the key of a cypher can have from its explaining that cypher."-HARTLEY. ET the historian, faithful to his trust, render all honour to such illustrious men, as Cullen, Black, Bergman, Wenzel, the kinsmen Higgins, Richter, and Proust, for excavating the foundations, and holding the plumb-line in the erection of an edifice that was to become, in the hands of John Dalton, a noble structure of magnificent proportions. These men were no common masons, but skilled designers, each of whom brought fitting patterns, and true carving power, to the architectural lines of the Temple of Chemistry, the adornment of which was so happily realised by the Grand Master-Dalton. In obtaining access through the outer approaches of all discoveries in art, science, or philosophy, there must necessarily be sappers and miners, the pioneers in the undertaking; and though several laboured right soldierly at the fortifications, William Higgins made the boldest effort to carry the citadel that contained the treasures of a new and fundamental doctrine in the physics of chemistry. That he did not entirely succeed was very much owing to his judgment being warped by phlogistic theories. It should be observed that whilst Cullen and Black stood in the relation of master and pupil and constant friends, the other, and equally renowned, coadjutors in the preliminary construction of the atomic theory acted independently of each other, and lived far apart, so that their labours were comparatively little known to each other, or to the world at large. Had the same freedom of intercourse existed a hundred years ago between nations and their representatives that now prevails, Dalton's discovery in 1803, admittedly based on his own unaided researches, would probably have been anticipated by ten or more years. Thus had the leading ideas in Mr Higgins' mind, pointing to the law of definite composition and multiple proportion, come to the knowledge of Lavoisier, the whole fabric of the atomic theory would have sprung forth as a happy generalisation worthy of this noble cultivator of the science. After recalling the import of the various essays emanating from Dalton's fertile mind during his first decennial period in Manchester-his clear conception of the nature of mixed elastic fluids, his eudiometrical observations, his inquiry into the tendency of elastic fluids to mutual diffusion, and his researches on the absorption of gases by water, through all of which may be traced an obvious and natural affiliation of thought -Dr Henry observes:-"To the same parentage we may now trace his first vision of the atomic constitution of matter. It is impossible to peruse the essay on the constitution of mixed gases, and especially to contemplate the plate of atomic symbols used by Dalton as late as 1835, by which it is illustrated (see appendix for plate), without perceiving that medita A Sketch of the Atomic Theory. 203 tion on the constitution of homogeneous and mixed elastic fluids had impressed his mind with a distinct picture of self-repellent particles or atoms. Thus, he affirms, homogeneous elastic fluids are constituted of particles that repel one another with a force decreasing directly as the distance of their centres from each other. Again it follows, too, that the distances of the centres of the particles, or, which is the same thing, the diameters of the spheres of influence of each particle, are inversely as the cube-root of the density of the fluid." But the plate which is reproduced in the appendix furnishes ocular demonstration that it was in contemplating the essential condition of elastic fluidity that he first distinctly pictured to himself the existence of atoms. As, however, the origin of this great conception is doubtless the most interesting circumstance in his life, I copy verbatim the following minute in my father's handwriting, dated 1830, February 13, of a conversation with Mr Dalton :-" Mr Dalton has been settled in Manchester thirty-six years. His volume on meteorology, printed, but not published, before he came here. At p. 132 et seq. of that volume, gives distinct anticipations of his views of the separate existence of aqueous vapour from atmospheric air. At that time the theory of chemical solution was almost universally received. These views were the first germs of his atomic theory, because he was necessarily led to consider the gases as constituted of independent atoms. Confirmed the account he before gave me of the origin of his speculations leading to the doctrine of simple multiples, and of the influence of Richter's table in exciting these views. Thus far, then, we can trace a natural filiation of thought, in unbroken sequence, from—(1.) The vigilant and persistent observation of meteorological phenomena, and specially of the variations of the atmosphere in weight, temperature, and moisture; to (2.) The theory of the relations of air and vapour, and of mixed gases; and finally, to the abstract conception of elastic fluidity, and of self-repulsive molecules or atoms. There remained, however, a wide space to be traversed, from this general physical conception of the existence of atoms to the experimental establishment of the relative weights of the ultimate particles of various chemical elements and compounds, announced by him two years afterwardsOctober 1803. Reference to a previous page (158) will show that Dalton, in one of his earliest chemical memoirs in 1802, had discovered, in the combinations of oxygen with nitrous gas, an undoubted example of multiple proportions; or to use his own words :-"These facts clearly point out the theory of the process; the elements of oxygen may combine with a certain portion of nitrous gas, or with twice that portion, but with no intermediate quantity." The steps by which he ascended from this first special example to the general law of multiple proportion seems pretty clearly indicated as resulting from the observations he made on the light carburetted hydrogen and olefiant gas. Dr Thomson of Glasgow, who spent a day or two with Dalton in Manchester, in August 1804, offers a clear narrative of the origin of the atomic theory in the following words :-" Mr Dalton informed me that the atomic theory first occurred to him during his investigations of olefiant gas and carburetted hydrogen A Sketch of the Atomic Theory. 205 gas, at that time imperfectly understood, and the constitution of which was first fully developed by Mr Dalton himself. It was obvious, from the experiments which he made upon them, that the constituents of both were carbon and hydrogen, and nothing else; he found, further, that if we reckon the carbon in each the same, then carburetted hydrogen contains exactly twice as much hydrogen as olefiant gas does. This determined him to state the ratios of these constituents in numbers, and to consider the olefiant gas a compound of one atom of carbon and one atom of hydrogen; and carburetted hydrogen of one atom of carbon and two atoms of hydrogen. The idea thus conceived was applied to carbonic oxide, water, ammonia, &c., and numbers representing the atomic weights of oxygen, azote, &c., deduced from the best analytical experiments which chemistry then possessed" ("History of Chemistry,” vol. ii., p. 291). In treating of carburetted hydrogen long afterwards, in 1810 ("New System," vol. i., p. 444), Dalton writes: "No correct notion of the constitution of the gas about to be described seems to have been formed till the atomic theory was introduced and applied in the investigation. It was in the summer of 1804 that I collected, at various times and in various places, the inflammable gas obtained from ponds." He had therefore been working at the analysis of this gas just previously to Dr Thomson's visit. Moreover, in his first table of atomic weights (see page 158), in which hydrogen being unity, carbon was estimated 4'3, olefiant gas is represented by 5'3—that is, C+H, and carburetted hydrogen from stagnant water by 6'3, or C+2H. This same table supplies other examples |