906 
first paper.') Now we will proceed with the task which we had 
set ourselves, and examine the Virial of attraction and of repulsion 
with a view to the drawing up of the equation of state in case of 
small volumes, especially at very low temperatures. 
(To be concluded). 
La Tour près Vevry, Autumn 1920. 
I gratefully express my thanks again to the van ’r Horr-fund, 
which has greatly facilitated the execution of this work. 
1) Though not all the objections, which some time ago Prof. LORENTZ was so 
kind as to communicate to me in a letter, may have been removed by this paper, 
yet | hope that some have been solved. Nobody can be more fully conscious of 
the great difficullies that are to be surmounted here, than the author himself. 
In the autumn of 1919 I had the privilege of having a discussion with Prof. 
EHRENFEST — to whom we owe the so important theory of the Adiabatic 
Invariants (1916), which theory was later so felicitously continued by BuRGERS 
(1917) and Krurkow (1919—1920) — on the contents of my first paper. He 
advanced, among others, the objection that not all the molecules on approach 
to other molecules would come in collision, after which they would again move 
away from them, but that some of them would remain for a time in the 
neighbourhood of them. This is perfectly true, but in case of gases we should 
then have to do with association, a case that was purposely left out of conside- 
ration by me as an unnecessary complication. But as we have to deal here not 
with gases, but chiefly with solid bodies, where the molecules only move to and 
fro between the neighbouring ones, this complication is, indeed, quite excluded. 
Besides already for a long time all the more recent theories of structure have 
rejected the idea of association in the solid, crystallized state, so that we are 
justified in leaving it quite out of consideration. But in my opinion there are greater 
difficulties, of an entirely different nature, to which I hope to return later. 
