1184 
Or will the entropy of the gas, which is monatomic from a thermo- 
dynamic point of view at very low temperatures, perhaps not be 
represented by (16), but have a value £AN log 2 smaller? This seems 
improbable, at least at first sight. A perfectly satisfactory answer to 
this question is probably to be expected only from a general theory 
of quanta. 
However, something can be said about the division by MN/ also 
without having recourse to the solid phase. Suppose we have the 
general theory of quanta. We come to the conclusion that for the 
determination of the thermodynamic probability we have not to 
reckon with infinitely small regions, but with such of a definite 
finite extent. This, however, holds only without reservation for 
systems the molecules of which are all different. Of a gas for which 
this is the case, we could not say that the entropy was proportional 
to the mass; it would much sooner contain a term k log N!. Now, 
N! 
however, the entropy of a mixture of different gases is / log — 
5 Ee Pen 
1 2 
greater than when the gases are equal, which can be thermody- 
namically derived for large values 7, etc, while it seems natural 
to consider it also as valid for small n’s, (7, ete. are of course the 
numbers of molecules for the different kinds, N is == Xu;). If now 
all n’s are =1, in other words, if the gas consists of nothing but 
different molecules, the entropy will be & log (N/) greater than for 
a gas consisting of nothing but equal molecules. For the latter we 
shall then have to subtract %/og(N/) from the original entropy 
expression. Such considerations have originally led me to the division 
by N/ and to the formula (46). 
Physics. — “On interference phenomena to be expected when Röntgen 
rays pass through a di-atomic gas.” By Prof. P. EnreENresr. 
(Communicated by Prof. H. A. Lorentz.) 
(Communicated in the meeting of February 27, 1915). 
As is known W. Friepricn ®) has ascertained that a beam of 
Röntgen rays passing through yellow wax and other amorphous solid 
substances gives interference rings on a photographic plate placed 
behind it. Liquid paraffin also gives a ring, which, however, does 
not represent a maximum of darkness, but an inflection point of 
1) W, Ertepricu, Kine neue Interferenzerscheining bei Röntgenstralen : Phys. 
@schir. 14, (1913), p. 317. 
