Watts' “Dictionary of Chemistry”

Abstract

THE third of the four volumes of this excellent work has just appeared, and in value and interest this one does not stand behind the two previous volumes. Amongst the articles written by eminent specialists, one, the most important, is that contributed by Prof. J. J. Thomson, of Cambridge, on the theories of the molecular structure of bodies. It is from the interpretation of chemical phenomena, by the help of exact physical research, that we may most hopefully look for insight into the true explanation of these phenomena. And although the theory of the molecular constitution of matter now universally held has been adopted as regards chemical change ever since the publication of Dalton's new system of chemistry in 1808, the crucial proof of its necessity has only recently been given. Prof. Thomson briefly but clearly explains the historical development of this proof. The first attempt was made by Cauchy, founded on the dispersion which light experiences when it passes through transparent bodies. But this attempt was an incomplete one, and a less ambiguous proof was given by Osborne Reynolds in 1879, based upon the thermal effusion of gases. Lord Kelvin, Loschmidt, and others have gone still further, not only proving that matter possesses structure, but giving limits below which the “coarse-grainedness” of matter cannot lie. These conclusions are founded upon considerations of several distinct sets of phenomena, viz. surface-tension, the difference of potential which occurs when two metals are placed in metallic connection, the amount of polarization at the surface of an electrode and of an electrolyte, and the viscosity, the diffusion, and the conductivity for heat, of gases. The discussion of the methods by which the limit is reached in the case of surface tension is next clearly given, and the result arrived at that a thickness of 10-8 cm. must be comparable with the range of molecular action of the water molecules. The results of the well-known researches of Quincke on silver films and on capillary elevation, as summarized in a lecture delivered before the Chemical Society of London by Prof. Rücker in 1888, are then explained, and the limits of molecular action deduced from these experiments. Having given an idea of the coarse-grainedness of matter, Thomson proceeds to consider the various theories of that structure, and gives an account of the most important of these by Lord Kelvin and Lindemann. The evidence of molecular structure afforded by the spectra of bodies, that concerning the arrangement of the atoms in the molecule on the supposition that the atoms are vortex-rings, and the electrical theory of molecular structure, first brought forward by Helm-holtz in his Faraday Lecture, are all clearly discussed; and the author's own researches on the conduction of electricity by gases, which bear out the results of this latter theory, are adverted to. The whole article, which only extends over seven pages, forms an admirable exposition of a most important, if a somewhat difficult, subject, and shows what chemistry gains from the work of mathematical physicists.