CHAPTER XII.

LATER LIFE OF DALTON.

ALTHOUGH the atomic theory must ever be considered as Dalton's great discovery, we find that he obtained it at the end of a series of investigations of themselves sufficient to have made him a conspicuous character. These earlier labours had great influence in advancing physical knowledge, and first brought him into repute. We find him so early as 1804 lecturing in the Royal Institution, when his theory was scarcely known but to himself, and when it was not expected to form part of a lecture. Fame was now beginning to hover round him, and he was not insensible of the change. But to persons of his habits, fame is less welcome in person than by letter, as we may say. They are less fitted for receiving the compliments of the world addressed to them in their presence, than for receiving those more impressive results,-great respect and deference; and, above all, the pleasure of seeing the influence they exercise upon society. Dalton was evidently conscious of the position to which he was entitled in the scientific world, although equally conscious that he was out of place in those brilliant assemblies in which scientific men in capitals occasionally mingle. He seems to write as if it were rather curious, although true, that he, too, was one of them, that the hope and struggle of his life to attain a position in science had been realized, but to feel at the same time that his life was not there, but as before, in his laboratory.

It not unfrequently happened, after this period, that he was engaged in controversy; and we find him there acting with great ease and deliberation, always without fear. Like most scientific men, he was destined to modify or to contradict

some of his earlier conclusions, having found that he formed laws too hastily. At page 9 of his "New System," he says, "Sometime ago it occurred to me as probable, that water and mercury, notwithstanding their apparent diversity, actually expand by the same law, and that the quantity of expansion is as the square of the temperature from their respective freezing points. Water very nearly accords with this law according to the present scale of temperature, and the little deviation observable is exactly of the sort that ought to exist, from the known error of the equal division of the mercurial scale. By prosecuting this inquiry, I found that the mercurial and water scales divided according to the principles just mentioned, would perfectly accord as far as they were comparable; and that the law will probably extend to all other pure liquids; but not to heterogeneous compounds, as liquid solutions of salts."

In Vol. II.* he says, "The great deviation of the scales between the temperatures of freezing water and freezing mercury is sufficient to show, as Dulong and Petit have observed, that their coincidence is only partial :" and so acknowledges his error.

In the first volume † he also said, that "the force of steam from pure liquids, as water, ether, &c., constitutes a geometrical progression to increments of temperature in arithmetical progression." Also, "that the expansion of permanent elastic fluids is in geometrical progression to equal increments of temperature." Again,‡ "that the force of steam in contact with water, increases accurately in geometrical progression to equal increments of temperature, provided these increments are measured by a thermometer of water or mercury." This seemed an ingenious group of laws, and although he gave them up, yielding to the results of Dulong, he published them unaltered in the edition or reprint of 1842. But that

津 Page 289. † Page 13. Page 11.

was his custom, even when his opinion changed; it was more with a view of obtaining copies of his own works, than with the view of continuing any law proved to be incorrect, as appears from the preface to his Meteorology.

In his "New System," Part 1st, he treats of heat and the constitution of bodies; in Part 2nd, which was published in 1810, he treats of the chemical elements. In these volumes it is remarkable how thoroughly every idea has been revolved in his own mind, and become his own, before he has ventured to write it. Every chapter shows a strict independent thought, but, on the other hand, the book is wanting in the results of others, and could never consequently be a complete system. He endeavours to construct the whole science himself, more than could be accomplished by any man. The book is written in a more attractive manner than systems of chemistry now assume; and there is a constant discussion of questions which give an insight into the state of knowledge of the time and the tendency of chemistry. Still the arrangement was not well adapted for the young student, although the work was a great fund of thought for the advanced man of science. Even now few will be able to read it without advantage.

Part 2nd is principally taken up in determining the composition of bodies and their atomic weights. In the appendix he enters into discussion with Gay Lussac. He there says, Gay Lussac's "opinion is founded upon a hypothesis that all elastic fluids combine in equal measures, or in measures that have some simple relation one to another, as 1 to 2, 1 to 3, 2 to 3, &c.; in fact, his notion of measures is analogous to mine of atoms; and if it could be proved that all elastic fluids have the same number of atoms in the same volume, or numbers that are as 1, 2, 3, &c., the two hypotheses would be the same, except that mine is universal, and his applies only to elastic fluids. Gay Lussac could not see that a similar hypothesis had been entertained by me and abandoned as untenable." In this he refers to the following in p. 188 of

Part 1st. "At the time I formed the theory of mixed gases I had a confused idea, as many have, I suppose, at this time, that the particles of elastic fluids are all of the same size; that a given volume of oxygenous gas contains just as many particles as the same volume of hydrogenous." But he arrived at the conclusion, "That every species of pure elastic fluid has the particles globular, and all of a size; but that no two species agree in the size of their particles, the pressure and temperature being the same." Then he concludes, in Part 2nd, "The truth is, I believe, that gases do not unite in equal or exact measures in any one instance; when they appear to do so, it is owing to the inaccuracy of our experiments. In no case, perhaps, is there a nearer approach to mathematical exactness, than in that of 1 measure of oxygen to 2 of hydrogen; but here the most exact experiments I have ever made, gave 1.97 hydrogen to 1 oxygen."

This discussion brings out prominently some of the points of Dalton's character. He objected to the idea of bulk being taken as a combining proportion; it was his great object to show the importance of weight and the completeness with which it answered every purpose. He conceived the combination by bulk as accidental, evidently because he had not examined the relations of the subject with sufficient care, and probably with some aversion, as his own discovery seemed in question. He is strict in the examination of the analyses of others, and seeks mathematical precision, when he could so very easily, in his own researches, overleap a few per cent. Yet no one would have been, on theoretical grounds, so likely to arrive at Gay Lussac's law as Dalton himself, as he paid most attention to the bulk of atoms, giving the relative diameters of particles of the different gases after giving the relative specific and atomic gravity. He did not always obtain an analysis so correct as that mentioned above, he continually, and to the last, insisted on the atomic weight of oxygen being 7; and he gives the specific gravity as 14 times that

of hydrogen, and as he gave the specific gravity of oxygen at 1.127, he was compelled to raise very high that of hydrogen, which he made .0805. We should have supposed Dalton to be, of all men, the most fitted for seeing exactly the position which Gay Lussac's law would take, and for extending our view of it; but it was not so. His mind had probably become too much engrossed with his own views of the case; and the belief that the whole subject had been attained, came in at last to shorten his vision. It is not in science only that men show that they are not the best fitted for seeing their own position, and without this knowledge a false step is not easily prevented.

His memoirs, after this period, were generally on less important subjects. We see in them the same quality of originality, the same inclination to strike at the root of a subject, but it is done with less power; the result is rather an idea which he leaves to be worked out. Some of them are hurried, some are careless, some are unfinished. Had it lain within the scope of this memoir, I might have shown many sentences full of latent beauty, which have since budded and blossomed.

Of these the following is not the least remarkable, although it had gone farther than a mere idea; it was long put into practice by him. As far as I know, therefore, he is to be considered as the originator of analysis by volume, which has long been practised to a great extent in the manufactories of Lancashire. In this respect chemists are still behind Dalton, and have not yet got into the habit of using all those advantages which his works have offered, although the actual knowledge on the subject has advanced far beyond the period of Dalton's latest years.

In one of these memoirs on the analysis of spring and mineral waters, we find that he gives directions for the centigrade method of testing, and Mr. John Gough Watson, his pupil, informs me that he used it constantly. He says,*

* Mem. Phil. Soc., Vol. III., new series, p. 59, 1814.