67 
nebular theory, can only result from attractive forces in a 
nebula at rest, and from its later condensation. The cooling 
which Mr. Spencer mistakes for the whole process, and cabs 
evolution, is only a secondary result of the condensation, or the 
heating process which directly results from attractive forces, 
and which must have gone before. Evolution is not simple 
cooling. Heating by attraction and pressure, and later coolino- 
of the central parts of each mass by transfer of motion towards 
the surface, are successive stages in the progressive develop- 
ment of cosmical change. r 
Y. Modern theories of Solar Heat, and the Dissipation of 
Eneigy, aie the next doctrines that I shall briefly examine. 
Two main views on the former have been lately proposed, 
the first is that of Mayer, accepted for a time by Sir W. 
Ihomson, but since abandoned. It assumes that the sun is 
hammered into a white heat by the continued impact of falling 
meteors. But this view belongs now to a past lunation of 
science. The present favourite is the doctrine developed by 
Helmholtz, adopted by Sir W. Thomson, and embodied by 
Mr. Spencer a,mong the latest improvements of his own system. 
He writes of it in these words : — 
Professor Helmholtz estimates that since the time when 
the matter of the solar system extended to the orbit of Nep- 
tune, there has been evolved 454 times the amount of heat 
which the sun has yet in store. He makes an approximate 
estimate of the rate at which the remainder is being diffused, 
showing that a diminution of his diameter by one ten-thou- 
sandfeh would produce heat at the present rate for two thousand 
years ; and that thus, at the present rate, his diameter would 
diminish one-twentieth in the next million years No uncer- 
tainty in the data, and consequent error in the inferred rate at 
which the sun expends his reserve of force, militates against 
the proposition that this reserve of force is being expended, and 
must in time be exhausted.” 
This same doctrine, of the ceaseless dissipation of the solar 
energy, and indeed of that of the whole universe, is also 
expounded by Professor Stewart in these words : — 
While you with the greatest ease transform work into heat, 
you can by no method in your power transform all the heat 
back into work. The process is not a reversible one. The 
consequence is that the mechanical energy of the universe is 
every day more and more changed into heat. Now, if this 
piocess goes on, and always in one direction, there can be no 
doubt about the issue. The mechanical energy of the universe 
will be more and more transformed into universally- diffused 
