1303 
waves only. A drawback of this method is that one cannot 
immediately see from the result how it depends on the fact 
whether the law of Hookr prevails or not; further that terms such 
as (6) occur which would disappear in another choice of co-ordinates. 
The relation between the changement of o and the compressibility 
Dersisk expresses by a coefficient «. By a consideration anaiogous to 
the one given above (after equation (7)) one can make out what 
will be the value of that number « when the law of Hooke 
holds. The scattered energy is found by DeBije to be proportional 
to 3a?-+-1, and therefore does not vanish for any value of a. 
Whether this result is exact or not would be best settled by appli- 
cation to this problem of the second method of point 3. The 
equations of motion however become already very complicated. 
In this connection one should compare the results obtained by 
Fincer’). Anyhow, one obtains already non-linear terms if 
Hooxkr’s law holds and therefore probably also scattering. 
The contradiction that in Desise’s final result the heat-conduction 
does not become o, if one takes « = 1, is for that reason but an 
apparent one. Even in that case namely the solid is not “ideal”. 
On the contrary one should say that such an ideal solid is a contra- 
diction in terms, as the elastic equations of motion can become in no way 
strictly linear. Nevertheless one may yet say that a real solid at a 
very low temperature approaches to an ideal solid. For one can 
always take the different heat-motions so small that in the equations 
of motion the terms of higher order are negligible. 
A remark, for the rest not in connection with the considerations 
we gave above, may yet follow. It is principally wrong to deduce the 
deviations of density in the elements of volume of a solid from the 
principle of BortzManN, in the same way as this has been done by 
Einstem for a gas. The correct way of course is with the aid 
of the normal vibrations. Now it is well-known how in the entirely 
comparable case of the radiation both ways give strongly different 
results. It has been tried at the time to explain the result of the 
first incorrect method with the aid of the light-quanta. The wrong 
point there, and this holds just as much for the solid, does not 
lie in the very principle of Borrzmann itself, but in the application. 
If namely the entropy of the whole is taken equal to the sum of 
the entropies of the parts, then this implies the independence of 
the probabilities of the conditions in those parts, which is in this 
GJ. Fuyeer, Wiener Sitzungsberichte, 103, 163 (1894). Also Conf. P, Durem, 
Recherches sur |’Elasticité. Paris 1916, 
