322 Professor J. Tyndall, [Jan. 23, 



the speaker observed a number of blocks of ice, which had been 

 placed loosely in a heap, frozen together at their places of con- 

 tact ; and he afterwards caused them to freeze together under 

 water as hot as the hand could bear. Facts like these suggested 

 the thought, that if a piece of ice — a straight prism, for example 

 —were placed in a bent mould and subjected to pressure it would 

 break, but that the force would also bring its ruptured surfaces into 

 contact, and thus the continuity of the mass might be re-established. 

 Experiment, as we have seen, completely confirmed this surmise : 

 the ice passed from a continuous straight bar to a continuous bent 

 one, the transition being effected, not by a viscous movement of the 

 particles, but through fracture and rcgelation. 



All the phenomena of motion, on which the idea of viscosity 

 has been based, are brought by such experiments as the above into 

 harmony with the demonstrable properties of ice. In virtue of this 

 property, the glacier accommodates itself to its bed while preserving 

 its general continuity, crevasses are closed up, and the broken ice 

 of a cascade, such as that of the Talefre, or the Ehone, is recom- 

 pacted to a solid continuous mass. But if the glacier accomplish 

 its movement in virtue of the incessant fracture and regelation of 

 its parts, such a process will be accompanied by a crackling noise, 

 corresponding in intensity to the nature of the motion, and which 

 would be absent if the motion were that of a viscous body. It is 

 well known that such noises are heard, from the rudest crashing 

 and quaking, up to the lowest decrepitation, and they thus receive 

 a satisfactory explanation.* 



* It is manifest that the continuity of the fractured ice cannot be com- 

 pletely and immediately restored after rupture. It is not the same surfaces that 

 are regelated, and hence the coincidence of the surfaces cannot be perfect. 

 They will enclose for a time capillary fissures, and thus the above theory 

 accounts satisfactorily for the known structure of glacier ice. I have 

 recently made the following experiments bearing upon this point. A piece of 

 ordinary ice was taken, and a cavity hollowed in it was filled with a strong 

 infusion of cochineal ; the ice was perfectly impervious to the liquid, which 

 remained in it for half an hour without penetrating it in the slightest degree. 

 A piece of the same ice was subjected to a gradually increasing pressure. 

 Flashes of light were seen to issue from it at intervals, indicating the rupture 

 of optical continuity, while a low, and in some instances, almost musical 

 crackling was, at the same time heard. Relieved from the pressure, the ice 

 remained continuous ; but a cavity being formed and the infusion placed within 

 it, the coloured liquid immediately diffused itself through the capillary 

 fissures, producing an appearance accurately resembling the drawings illustra- 

 tive of the infiltration experiments of M. Agassiz upon the glacier of the Aar. 

 This fissured structure, which is inconsistent with the idea of viscosity, is thus 

 shown to be the natural result of the pressures exerted upon the non-viscous and 

 brittle mass of the glacier. 



To account for a " bruit de crc'pitation " heard upon the Aar glacier, M. 

 Agassiz refers to an observation which might be made on a fine day in sum- 

 mer, and which would show the air within the glacier ice escaping from its 

 surface. M. Agassiz supposes the ice to be diathermanous ; that the sun- 

 beams therefore get through it and heat the air bubbles it encloses, which 



