It was natural to suppose that the prismatic structure which I found so
very general in the glacieres was the result of some cause or causes
coming into operation after the first formation of the ice. On this
point M. Thury's visit to the Glaciere of S. Georges in the spring of
1852 affords valuable information, for at that time the coating of ice
on the wall, evidently newly formed, did not present the _structure
areolaire_ which he had observed in his summer visit to the cave. He
suggests that, since ice is less coherent at a temperature of 32 deg.
F.--which is approximately the temperature of the ice-caves during
several months of the year--than when exposed to a greater degree of
cold, its molecules will then become free to assume a fresh system of
arrangement.[196] On the other hand, Professor Faraday has found that
ice formed under a temperature some degrees below the ordinary freezing
point has a well-marked crystalline structure.[197] M. Thury suggests
also, as a possibility, what I have found to be the case, by frequent
observations, that the prismatic ice has greater power of resisting heat
than ordinary ice; and on this supposition he accounts for the fact of
hollow stalactites being found in the Cavern of S.
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