and indeed congealed to
ice, the second vessel being plunged in a mixture cold enough to
freeze the water. As a result of the process we obtain a mass of ice.
That ice has an origin very antithetical to its own character. Though
cold, it is the child of heat. If we removed the lamp, there would be
no steam, and if there were no steam there would be no ice. The mere
cold of the mixture surrounding the second vessel would not produce
ice. The cold must have the proper material to work upon; and this
material--aqueous vapour--is, as we here see, the direct product of
heat.

It is now, I suppose, fifteen or sixteen years since I found myself
conversing with an illustrious philosopher regarding that glacial
epoch which the researches of Agassiz and others had revealed. This
profoundly thoughtful man maintained the fixed opinion that, at a
certain stage in the history of the solar system, the sun's radiation
had suffered diminution, the glacial epoch being a consequence of this
solar chill. The celebrated French mathematician Poisson had another
theory. Astronomers have shown that the solar system moves through
space, and 'the temperature of space' is a familiar expression 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 everywhere not many
years ago, and I therefore thought it worth while to show how
incomplete 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 long-continued 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 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, [Footnote: 'Heat a Mode of Motion,' fifth ed