An interesting criticism of the-findings in the above-mentioned paper was made to me by Professor Ahlniann. We stated that when the specific gravity.pf the firn exceeds about 0.82 it changes into ice. The transition point was fixed at the stage when the air spaces between the grains become sealed up and cease to be, communicating. How then, Professor Ahlmann asks, could further water impregnate the ice at 0.82 to give it a specific gravity of 0.91?
Unfortunately the last stages of the transition of “early” ice to “pure” ice were not fully investigated owing to lack of time. I intend to come back to this part of the investigations later. Therefore what follows, is only conjecture.
There is the possibility that under the influence of plastic deformation, air bubbles may wander and form bigger ones which are eliminated in crevasses, cracks or on the surface. Tyndall reported the emission of air with a loud hissing sound from cracks in the Mer de Glace.
There is also evidence that air bubbles become compressed. Wegener found that air bubbles in ice were under pressures of as much as 10 atmospheres. But a more likely explanation arises from the possibility that ice at a specific gravity of 0.91 only occurs when it has become impregnated with water before the air channels become sealed up. There is no evidence that ice entirely free from air is found anywhere except ‘at the surface of in crevasses. In other words, it is possible that if the firn is not fully impregnated with water before the air spaces cease to be communicating, it never becomes air-free until or unless it can discharge its air into the atmosphere at the surface or into crevasses. Clearly this is an interesting subject for further research.
