The book was published in 1873. The Text-book of Heat was written during the same period, while “Matter and Motion,” “a small book on a great subject,” was published in 1876.
In 1873 and 1874 he was one of the examiners for the Natural Sciences Tripos, and in 1873 he was the first additional examiner for the Mathematical Tripos, in accordance with the scheme which he had done so much to promote in 1868.
Many of his shorter papers were written about the same time. The ninth edition of the Encyclopædia Britannica was being published, and Professor Baynes had enlisted his aid in the work. The articles “Atom,” “Attraction,” “Capillary Action,” “Constitution of Bodies,” “Diffusion,” “Ether,” “Faraday,” and others are by him.
He also wrote a number of papers for Nature. Some of these are reviews of books or accounts of scientific men, such as the notices of Faraday and Helmholtz, which appeared with their portraits; others again are original contributions to science. Among the latter many have reference to the molecular constitution of bodies. Two lectures—the first on “Molecules,” delivered before the British Association at Bradford in 1873; the second on the “Dynamical Evidence of the Molecular Constitution of Bodies,” delivered before the Chemical Society in 1875—were of special importance. The closing sentences of the first lecture have been often quoted. They run as follow:—
“In the heavens we discover by their light, and by their light alone, stars so distant from each other that no material thing can ever have passed from one to another; and yet this light, which is to us the sole evidence of the existence of these distant worlds, tells us also that each of them is built up of molecules of the same kinds as those which we find on earth. A molecule of hydrogen, for example, whether in Sirius or in Arcturus, executes its vibrations in precisely the same time.
“Each molecule therefore throughout the universe bears impressed upon it the stamp of a metric system, as distinctly as does the metre of the Archives at Paris, or the double royal cubit of the temple of Karnac.
“No theory of evolution can be formed to account for the similarity of molecules, for evolution necessarily implies continuous change, and the molecule is incapable of growth or decay, of generation or destruction.
“None of the processes of Nature, since the time when Nature began, have produced the slightest difference in the properties of any molecule. We are therefore unable to ascribe either the existence of the molecules or the identity of their properties to any of the causes which we call natural.
“On the other hand, the exact equality of each molecule to all others of the same kind gives it, as Sir John Herschel has well said, the essential character of a manufactured article, and precludes the idea of its being eternal and self-existent.
“Thus we have been led along a strictly scientific path, very near to the point at which Science must stop—not that Science is debarred from studying the internal mechanism of a molecule which she cannot take to pieces any more than from investigating an organism which she cannot put together. But in tracing back the history of matter, Science is arrested when she assures herself, on the one hand, that the molecule has been made, and, on the other, that it has not been made by any of the processes we call natural.