Address to the Royal Geological Society. 17 
ascertained physical laws, which, for their applicability to the 
matter in question, depend themselves upon hypothetical con- 
ditions or assumed data; the a priori validity of the mathematics — 
and the @ posteriovi validity of the laws sometimes, in a case of 
this kind, dazzle the eyes of the on-looker, and, by irradiation, 
spread on his mental retina over the conjoined assumed con- 
ditions, and conceal the doubtfulness of their validity. And this 
can happen even to the proposer of the conditions, who may have 
originally put them forward expressly as only hypothetical, and 
even as being an acknowledgedly imperfect representation of 
facts ; perhaps, indeed, he is more in danger of this than the 
on-looker. 
We shall find on examination that Sir W. Thomson himself 
originally intended his paper to be only a contribution to the 
investigation of the matter founded upon three conditions, which 
he assumed for convenience and simplification. The data assumed 
are, first, the uniform or approximately uniform temperature of 
the body of the earth when it had just solidified ; secondly, the 
conductivity and thermal capacity of the materials of the globe ; 
and thirdly, a certain low and constant temperature of the surface. 
He gives reasons for believing that the first assumption may be 
probable ; it is unquestionably quite possible, as far as we know 
at present. But he himself admits, or rather warns us, that the 
second is a pure assumption ; and the third must be an incorrect 
representation of the case. If one of the three factors which go to 
produce the result be purely and confessedly hypothetical, it 
matters little, as far as the immediate question is concerned 
whether the others be right or not. 
As to the jirst of the three conditions, Sir William believes that 
the materials of the earth nebula may have condensed somewhat 
regularly round a cold nucleus. The energy of the later part of 
the nebular, or meteoric rain would be much greater than that of 
the earlier, and cause a much higher temperature in the outer 
parts of the globe, but for the interior pressure which may have 
nearly equalized the temperature by raising it in the inner 
parts. 
We will not now presume to measure swords with Sir William, 
and to enter upon counter-speculation as to how the materials 
of the earth-nebula fell together; yet it would not be difficult to 
Scien, Proc. R.D,S., Vou, u., PT. 1 Cc 
