4 Mr. W. Sutherland on the Fundamental 



are established can be followed in my different papers on- 

 Molecular Force and on a Kinetic Theory of Solids in the 

 Philosophical Magazine ; but for the convenience of chemical 

 readers, I will reproduce here briefly the essential steps of th« 

 reasoning, with the formulae necessary for thermochemical 

 applications. This will form the Introduction to Part L. 

 which will deal with inorganic compounds, Part II. relating 

 to organic. 



Introduction. 



The starting-point in the application of Dynamics to mole- 

 cular physics is the Virial equation of Clausius. If a number 

 of molecules (forming, say, a unit mass) are confined in a 

 volume v at pressure p, and if ^mY 2 is the kinetic energy of 

 translatory motion of any one, and <£(V) the force acting 

 between any two at distances r apart, then the Yirial equation 



where the single § denotes that the values of J mV 2 for all 

 the molecules are to be added together, while the double 

 symbol %% denotes first that all the values of r<j>(r) are to be 

 added for the forces between one particular molecule and all 

 the rest, and then that all such sums for all particular mole- 

 cules are to be added together. The best known attempt to 

 transform this equation to a form suitable for physical appli- 

 cations is that which resulted in the now famous equation of 

 van der Waals, namely, 



pv = Ue + Rd—. i :--, (2) 



the separate terms of which are to be interpreted as follows: — 

 6 is absolute temperature, R0 stands for fSi 771 ^ 2 ? an( ^ 

 R,#&/(v — b) stands for two thirds of that part of — %.%%'%r(p(r) 

 which results from the forces of repulsion that act during the 

 collisions of molecules, while — a/v stands for two thirds of 

 that part of — ^.^E2^0W resulting from the steady attrac- 

 tion of the molecules which produces the cohesion of liquids 

 and solids. The experiments of Amagat, and later of Ramsay 

 and Young, proved that the equation of van der Waals cannot 

 represent the facts of the vapours of compounds ; and from 

 these experiments I showed that the equation of van der Waals 

 applies, so far as we know at present, only to the gaseous 

 state of hydrogen, oxygen, nitrogen, and methane, and that a 

 different form represents the main facts of most compound 

 vapours. The point of most importance in this form for 

 present applications is that the part of — i>i%%r<j>{r) 



