A vapour was beginning to
pass out from the cave, at the highest part of the arch of entrance; a
phenomenon which, he was told, continued through the winter, and
announced or accompanied the departure of the ice: nevertheless, the
cold was so great that he could not remain in the glaciere more than
half an hour with any sort of comfort. The thermometer stood at 60 deg.
outside the cave, and fell to 10 deg.[178] when placed inside; but
thermometrical observations of that date were so vague as to be useless
for present purposes. The ice appeared to be harder than the ordinary
ice of rivers, less full of air-bubbles, and more difficult to melt.
M. Billerez enunciated a new theory to account for the phenomena
presented by the cave. He observed that the earth in the immediate
neighbourhood, and especially above the roof of the grotto, was full of
a nitrous or ammoniac salt, and he accordingly suggested that this salt
was disturbed by the heat of summer and mingled itself with the water
which penetrated by means of fissures to the grotto, and so the cave was
affected in the same way as the smaller vessel in the ordinary
preparation of artificial ice.
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