Under these circumstances, it is not out of place to consider through what channels the low temperature of the air surrounding the glacier may penetrate into the interior. The heavy cold air may of course sink from the surface into every large open space, such as the crevasses, large fissures, and _moulins_ or mill like holes to be described in a future article; it may also penetrate with the currents which ingulf themselves under the glacier, or it may enter through its terminal vault, or through the lateral openings between the walls of the valley and the ice. Indeed, if all the spaces in the mass of the glacier, not occupied by continuous ice, could be graphically represented, I believe it would be seen that cold air surrounds the glacier-ice itself in every direction, so that probably no masses of a greater thickness than that already known to be permeable to cold at the surface would escape this contact with the external temperature. If this be the case, it is evident that water may freeze in any part of the glacier. To substantiate this position, which, if sustained, would prove that the dilatation of the mass of the glacier is an essential element of its motion, I may allude to several other well-known facts. The loose snow of the upper regions is gradually transformed into compact ice. The experiments of Dr. Tyndall prove that this may be the result of pressure; but in the region of the _neve_ it is evidently owing to the transformation of the snow-flakes into ice by repeated melting and freezing, for it takes place in the uppermost layers of the snow, where pressure can have no such effect, as well as in its deeper beds. I take it for granted, also, that no one, familiar with the presence of the numerous ice-seams parallel to the layers of snow in these upper regions of the glacier, can doubt that they, as well as the _neve_, are the result of frost. But be this as it may, the difference between the porous ice of the upper region of the glacier and the compact blue ice of its lower track seems to me evidence direct that at times the whole mass must assume the rigidity imparted to it by a temperature inferior to the freezing-point. We know that at 32 deg. Fahrenheit, regelation renders the mass continuous, and that it becomes brittle only at a temperature below this. In other words, the ice can break up into a mass of disconnected fragments, such as the capillary fissures and the infiltration-experiments described in my "Systeme Glaciaire," show to exist, only when it is below 32 deg. Fahrenheit. If it be contended that ice at 32 deg. does break, and that therefore the whole mass of the glacier may break at that temperature, setting aside the contradiction to the facts of regelation which such an assumption involves, I would refer to Dr. Tyndall's experiments concerning the vacuous spots in the ice. [Illustration]