[ 297 ] 
XXVII. /Vie Mixing of Gases. — Remarks on Mr. BurbmVs 
repent Papers. By Prof. W. McF. Orr, M.A.* 
I BEG to make some remarks on the two papers by 
Mr. Burbury on the Diffusion o£ Gases which have 
recently appeared in this Magazine f. 
Mr. Burbury appears to dispute the propositions : (1) that 
when two gases mix by diffusion the process is an irreversible 
one ; (2) that every irreversible process is attended by a loss 
o£ available energy. 
As regards the former proposition, Mr. Burbury, in the 
first paper, after discussing some points in the kinetic theory 
of gases, suggests % that the diffusion would be reversible,, 
provided the system were completely isolated, a condition 
which it is impossible to realize in nature, but admits that it 
is irreversible in any experiments that can practically be 
made. On this view, the point at issue may be said by one 
unfamiliar with the kinetic theory of gases to be an abstract 
question affecting a system under conditions so unnatural 
that, whichever way it is decided, it nevertheless allows the 
natural process to be, as a matter of fact, irreversible. 
Mr. Burbury does not discuss the latter of the two 
propositions directly, but turns his attention instead to an 
equivalent theorem relating to increase of entropy, and 
merely for the particular case in hand, i. <?., the diffusion of 
gases. On this point he urges § that there is no gain of 
entropy in any one collision between molecules, and that 
therefore no increase can result from all the collisions taken 
together. It may, I think, be fairly replied, again from a 
standpoint outside the kinetic theory, that it is quite un- 
necessary to consider such questions as this and others 
which he raises. The usual definition of entropy makes no 
reference to single molecules but deals with matter in bulk ; 
the Kinetic Theory of Gases is an interesting annexe of 
Thermodynamics, but the latter in no way depends on the 
former ; if the molecular theory gives results which contradict 
our experience, necessarily limited to that of gross matter, 
then, as happens from time to time with other branches of 
science, it requires revision, as regards logical development, 
or premisses, or both. Moreover, a being who could have 
experience of individual molecules would, in all probability, 
not understand the notions of pressure, temperature, heat,. 
* Communicated by the Author. 
f Phil. Mag. July and Sept. 1907. 
t L. c. p. 125. 
§ L. c. p. 124. 
