OF THE GASEOUS AND LIQUID STATES. 409 



the condensed state the properties of the substance depend on the mutual 

 action of molecules when engaged in close encounter, and this is determined 

 by the particular constitution of the encountering molecules. We cannot there- 

 fore extend the dynamical theory from the rarer to the denser state of sub- 

 stances without at the same time obtaining some definite conception of the 

 nature of the action between molecules when they are so closely packed that 

 each molecule is at every instant so near to several others that forces of great 

 intensity are acting between them. 



The experimental data for the study of the mutual action of molecules 

 are principally of two kinds. In the first place we have the experiments of 

 Regnault and others on the relation between the density, temperature, and 

 pressure of various gases. The field of research has been recently greatly 

 enlarged by Dr Andrews in his exploration of the properties of carbonic acid 

 at very high pressures. Experiments of this kind, combined with experiments 

 on specific heat, on the latent heat of expansion, or on the thermometric effect 

 on gases passing through porous plugs, furnish us with the complete theory 

 of the substance, so far as pure thermodynamics can carry us. 



For the further study of molecular action we require experiments on the 

 rate of diffusion. There are three kinds of diffusion that of matter, that of 

 visible motion, and that of heat. The inter-diffusion of gases of different kinds, 

 and the viscosity and thermal conductivity of a gaseous medium, pure or mixed, 

 enable us to estimate the amount of deviation which each molecule experiences 

 on account of its encounter with other molecules. 



M. Van der Waals, in entering on this very difficult inquiry, has shewn 

 his appreciation of its importance in the present state of science ; many of his 

 investigations are conducted in an extremely original and clear manner ; and 

 he is continually throwing out new and suggestive ideas ; so that there can be 

 no doubt that his name will soon be among the foremost in molecular science. 



He does not, however, seem to be equally familiar, as yet, with all parts 

 of the subject, so that in some places, where he has borrowed results from 

 Clausius and others, he has applied them in a manner which appears to me 

 erroneous. 



He begins with the very remarkable theorem of Clausius, that in stationary 

 motion the mean kinetic energy of the system is equal to the mean virial. 

 As in this country the importance of this theorem seems hardly to be appre- 

 ciated, it may be as well to explain it a little more fully. 



VOL. II. 52 



