This, together with the fact of
the porous nature of the rock in which most of the curious caves in the
world occur, which allows a considerable amount of moisture to collect
on all surfaces, and thereby induces a depression of temperature by
evaporation, may be held to explain the presence of a greater amount of
cold than might otherwise have been fairly reckoned upon in ice-caves.
The idea of cold produced by evaporation Pictet took up warmly,
believing that when promoted by rapid currents of air it would produce
ice in the summer months; and he thus explained what he understood to be
the phenomena of glacieres. But it will have been seen, from the account
of the caves I have visited, that the glacieres are more or less in a
state of thaw in the summer; and M. Thury's observations in the winter
prove conclusively that they are then in a state of utter frost, so that
the old belief with respect to the season at which the ice is formed may
be supposed to have been exploded. The facts recorded by Mr. Scrope[191]
would appear to depend upon the peculiar nature of rocks of volcanic
formation; and I am inclined to think there is very little in common
between such instances as he mentions and the large caves filled with
ice which are to be found in the primary or secondary limestone.
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