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. 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. 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. Georges. At the commencement of the hot season, the atmospheric temperature of the glacieres rises gradually; and when it has almost reached 32 deg. F., the prismatic change takes place in the ice, extending to a limited depth below the surface. The central parts of the stalactites retain their ordinary structure, and are after a time exposed to a general temperature rather above than below the freezing point; and thus they come to melt, the water escaping either by accidental fissures between some of the prisms, or by the extremity of the stalactite, or by some part of the surface which has chanced to escape the prismatic arrangement, and has itself melted under increased temperature.
M. Hericart de Thury describes the peculiar structure of the ice which he found in the Glaciere of the Foire de Fondeurle. He found that the crystallised portions were very distinctly marked, displaying for the most part a six-sided arrangement; and in the interior of a hollow stalactite he found numerous needles of ice perfectly crystallised, the crystals being some triangular and some six-sided. He was unable to detect any perfect pyramid. I have already quoted Olafsen’s observations on the polygonal lining which he saw on the surface of the ice in the Surtshellir. The French Encyclopaedia  relates that M. Hassenfratz saw ice served up at table at Chambery which broke into hexagonal prisms; and when he was shown the ice-houses where it was stored, he found considerable blocks of ice containing hexahedral prisms terminated by corresponding pyramids.