1178 
It should, however, be kept in view that the suppositions on the 
chemical forces used in this $, possibly do not sufficiently agree with 
reality; nor do they any longer appertain to pure classical mechanics: 
we have, namely, assumed that in case of a chemical binding the atoms 
must have a definite relative orientation, though we have not spoken 
at all of a rotation of the atoms. We might imagine other repre- 
sentations of the acting forces, but the one used seems to me the 
simplest and the most obvious. After what has been said at the 
conclusion of § 3, it will be clear that the application of (16) is 
probably not permissible for the entropy of the monatomic compo- 
nents, viz. in the case of one kind of atoms. 
The contents of this $ is then only interesting from a theoretical 
point of view, viz. to show how the same result as in $ 4 can be 
found in another way too. A third derivation of the entropy constant 
of the rotation for di-atomie molecules has been given by O. Stern’) 
by the aid of LANGEVIN's theory of paramagnetism. From this 
we see that this derivation only holds for the case that the two 
atoms do not perform the same function. The result agrees with 
ours. 
© § 6. General formula for the vapour pressure of a multi-atomic 
solid substance and the entropy of the vapour. 
We will now calculate the vapour pressure of arbitrary multi- 
atomic solid substances in an analogous way as we did in § 3 for 
monatomic ones. We then only assume (for simplicity’s sake) ?*) that 
the vapour is an ideal gas, i.e. a gas with independent molecules, 
whose energy, therefore, does not depend on the volume; the specific 
heat, however, may indeed vary with the temperature, if only 
classical mechanics remain of application. Hence the internal mole- 
cule movements need not exclusively consist of rotations and sine 
vibrations. For the solid substance we continue, of course, to con- 
sider the suppositions of $2 as valid, to which we may add, as in 
$ 4 that at every place in the erystal the molecule present there 
can only have one definite orientation. *) 
Thus we find for the probability that » of the .V-molecules belong 
to the vapour, the formula which is analogous with (10): 
1) O. Stern, Ann. d. Phys. 44, 497 (1914). 
*) It is shown in § 8 that the formula to be found for the entropy holds just 
as well for non-ideal gases and for liquids. 
5) The impossibility of another orientation must of course be understood so that 
a very great energy would be required for this. 
