468 The Kinetic Theory of Gases. 
That is, it is not generally true. In the particular limiting 
case of an infinitely rare gas Maxwell’s law is true, at least 
to an infinitely near approximation, and in the same case the 
objections to A cease to be appreciable. For this reason 
Mr. Jeans is not open to the charge of inconsistency, because 
by virtue of his 37d he is in effect dealing in his paper only 
with the infinitely rare gas, at least so I understood him. 
If Mr. Jeans is right in his view that assumption A, and 
therefore Maxwell’s law, cannot be generally true in fact, 
then it necessarily follows that the orthodox theory of gases 
is a true theory only of the infinitely rare gas. Also the law 
of equipartition of energy, which is a corollary to Maxwell’s 
law, is not proved to hold in any case except that of the 
infinitely rare gas. 
S. H. Burpury. 
Tnrovuen the courtesy of the Editors I am able to adda 
note on Mr. Burbury’s letter. 
The issue of his letter is, I think, obscured by his not 
making any clear distinction between “ assumption A” (an 
assumption which may, rightly or wrongly, be made) and 
‘‘ absence of correlation ”’ (a result which may be proved). 
From Mr. Burbury’s point of view the latter follows from 
the former, but, given the latter, I do not think that it is 
necessarily a consequence of the former. The cause must 
produce the effect, but the effect may follow from any one 
of many causes. Anyhow, the two are not synonymous. 
From my point of view, as I have said throughout, I can- 
not regard ‘‘assumption A” as a genuine assumption at all. 
It is, therefore, from my point of view, futile to discuss 
whether “assumption A” is true or untrue, although I do 
emphatically disclaim having assumed it. What we may 
logically do, is to discuss whether ‘‘ assumption A ’—gqua 
assumption—is legitimate or illegitimate, and also whether 
“absence of correlation” (the closely related result) is—qua 
fact—true or untrue. This I hoped I had done in my original 
paper. The conclusion I reached was that the assumption 
was illegitimate, but that the fact was true. 
Leaving aside the difference between “assumption A” and 
‘“‘absence of correlation,” there is nothing inconsistent in 
holding simultaneously the view that an assumption is illegi- 
timate as an assumption but true in fact. For instance, 
Maxwell’s original proof of the law of distribution rested on 
the assumption which will be sufficiently indicated by the 
equation 
Plu, v, w) =f (u) f(r) f(w). 
