7 
spaces, has a natural limit, beyond which it cannot go, and to 
which it must approach more and more slowly as the change 
proceeds. 
III. — The Thermo-dynamic Theory. 
10. The doctrine of uniformity, in its extreme form, as held 
by Sir C. Lyell and many others, has found of late some strong 
opponents among our foremost analysts. Sir W. Thomson and 
Professor Tait would replace it by what may be called a 
Thermo-dynamic theory. They maintain that the solar energy 
is in process of constant dissipation, and that hypotheses 
assuming an average constancy of sun and storms for a million 
yeai’s “cannot possibly be true.” It is quite certain, Sir 
William thinks, that the solar system cannot have gone on as 
at present for a few million years, without the irrevocable loss, 
by dissipation, of a very considerable portion of the entire 
initial energy. He calculates, from Fourier's theory of the 
rate of conduction, and the specific heat of rocks at Edinburgh 
and Greenock, that the consolidation of the earth's crust 
cannot have taken place less than 20 nor more than 400 
millions of years ago; also that the general climate cannot 
have been sensibly affected by conducted heat from the centre, 
except within the first 10,000 years after the solidification, and 
that in 96 millions of years the thickness of the crust, through 
which a given amount of cooling would be experienced, would 
have increased fivefold. He admits that a wholly different 
view is maintainable, that internal heat is due to chemical 
combination, going on slowly everywhere at great unknown 
depths, and creeping onward gradually as the chemical 
affinities of each layer are saturated. But he thinks also that 
“ the less hypothetical view, that the earth is merely a warm, 
chemically inert body, cooling, is clearly to be preferred in 
the present state of science.” 
11. The objection may be urged, that the earth cannot well 
be supposed ever to have been a solid, uniformly heated, and 
7,000° warmer than the present heat of the surface, which is 
the hypothesis assumed. But Sir William replies that the 
solution may be easily modified, to meet the case of a liquid 
gradually becoming solid, at least when three fresh data have 
been supplied. And he argues further that the earth, “although 
once all melted, did in all probability become a solid at its 
melting temperature all through, or all through the outer 
layer which had been melted; and that not until it was thus 
completely solidified, or nearly so, did the crust begin to 
cool.” 
