ANNIVERSARY ADDRESS. 29 



advances, on those in pure, and in other applied sciences. As a 

 guide in metallurgical processes, in the production of foods, and 

 in the manufacture of a vast series of inorganic and organic com- 

 pounds required by civilised communities, it has proved itself, 

 although of recent birth, a giant in its power of assistance, and 

 there is abundant evidence that in the near future its achievement 

 will transcendently eclipse that of the past. Not only so, but its 

 deeper side has enabled us to penetrate further into the knowledge 

 of the fabric out of which our world is built, for the contributions 

 of chemistry to physics have been very great. No general view 

 of molecular physics can be regarded as adequate, which does not 

 take account of molecular chemistry, or the whole series of physical 

 facts which have been revealed in the prosecution of chemical 

 research. The difficulties which face the investigator are of course 

 stupendous. Apart from those introduced in attempting anything 

 like a complete molecular theory of chemical reactions, in which 

 all energy changes and variation of physical properties, as related 

 to chemical constitution, shall be explained ; the difficulties inherent 

 in a more elementary theory, which attempts nothing further than 

 an explanation of the gaseous, liquid and solid states, and the 

 elastic properties of substances, are overwhelming, as anyone will 

 realize who critically reads, for example, any discussion of the 

 foundations of the kinetic theory of gases. Up to a certain point 

 the study is simple enough, but the moment the attempt to include 

 the observed facts, of departure from the ideal gaseous law, is 

 made, the intricate character of the problem really appears. 1 



1 The total kinetic energy of a system of molecules is represented by 

 the virial equation 



^\mv? = $pV + J22i?r 

 in which u denotes the mean of the squares of the velocities of the mole- 

 cules, m the mass of a molecule, p the pressure per unit of surface on the 

 envelope whose volume is V, E the attraction between two molecules 

 separated the distance r, and 2, as usual, the summation of the quantities 

 which it precedes, bein# double in the final term to indicate that the 

 summation is to include every pair of molecules that can be tiken in the 

 system. Now neglecting the attraction term and supposing the kinetic 

 energy to be proportional to the absolute temperature $ of the system, the 

 above equation becomes 



