In 1712, M. Billerez, Professor of Anatomy and Botany in the University of Besancon, communicated to the Academy[177] an account of a visit made by him to this cave in September 1711.  He found 3 feet of ice on the floor of the cave, in a state of incipient thaw, and three pyramids, from 15 to 20 feet high and 5 or 6 feet in diameter, which had been already considerably reduced in size by thaw.  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.  He had heard that some rivers in China freeze in summer from the same cause.[179]

In 1726, a further communication was made to the Academy by M. des Boz,[181] Royal Engineer, describing four visits which he had made to the grotto near Besancon at four different seasons of the year, viz., in May and November 1725, and in March and August 1726.  In all cases he found the air in the cave colder than the external air,[182] and its variations in temperature corresponded with the external variations, the cold being greater in winter than in summer.

M. des Boz ascribed the existence of ice in the cave to natural causes.  The opening being towards the north-east, and corresponding with a gorge in the hills opposite, running in the same direction, none but cold winds could reach the mouth of the grotto.  Moreover, the soil above was so thickly covered with trees and brushwood, that the rays of the sun could not reach the earth, much less the rock below.  Credible persons asserted that since some of the trees had been felled, there had not been so much ice in the cave.

In order to test the presence of salt, M. des Boz melted some of the ice, and evaporated the resulting water, but found no taste of salt in the matter which remained.[183] He denied the existence of the spring of water which previous accounts had mentioned, and believed that the water which formed the ice came solely from melted snow, and from the fissures of the rock.