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glacial epoch which the researches of Agassiz and others had 
revealed. This profoundly thoughtful man was of opinion that, 
at a certain stage in the history of t the solar system, the sun’s 
radiation had suffered diminution, the glacial epoch being a 
consequence of this central chill. The celebrated French mathe- 
matician Poisson had another theory. Astronomers have 
shown that the solar system moves through space, and the 
temperature of space is a familiar conception with scientific 
men. It was considered probable by Poisson that our system, 
during its motion, had traversed portions of space of different 
temperatures ; and that during its passage through one of the 
colder regions of the universe, the glacial epoch occurred. 
Notions such as these were more or less current not many years 
ago, and I therefore thought it worth while to show how incom- 
plete they were. Suppose the temperature of our planet to be 
reduced, by the subsidence of solar heat, the cold of space, or 
any other cause, say one hundred degrees. Four-and-twenty 
hours of such a chill would bring down as snow nearly all the 
moisture of our atmosphere. But this would not produce a 
glacial epoch. Such an epoch would require the continuous 
generation of the material from which the ice of glaciers is 
derived. Mountain snow, the nutriment of glaciers, is derived 
from aqueous vapour raised mainly from the tropical ocean by 
the sun. The solar fire is as necessary a factor in the process 
as our Bunsen lamp in the experiment referred to a moment 
ago. Nothing is easier than to calculate the exact amount of 
heat expended by the sun in the production of a glacier. It 
would, as I have elsewhere shown,* raise a quantity of cast-iron 
five times the weight of the glacier not only to a white heat, 
but to its point of fusion. If, as I have urged elsewhere, instead 
of being filled with ice, the valleys of the Alps were filled with 
white-hot metal, of quintuple the mass of the present glaciers, 
it is the heat, and not the cold, that would arrest our attention 
and solicit our explanation. The process of glacier-making is 
obviously one of distillation, in which the fire of the sun which 
generates the vapour plays as essential a part as the cold of the 
mountains which condenses it.f 
It was their ascription to glacier action that first gave the 
parallel roads of Grlen Roy an interest in my eyes ; and in 1867, 
* “Heat a Mode of Motion,” fifth edition, chap. vi. ; Forms of Water, §§ 
55 and 56. 
t In Lyell’s excellent “ Principles of Geology,” the remark occurs that 
<l several writers have fallen into the strange error of supposing that the 
glacial period must have been one of higher mean temperature than usual.” 
The really strange error was the forgetfulness of the fact that, in the pro- 
duction of glaciers, heat played quite as important a part as cold. 
