ICE AND GLACIERS. 133 



feebler it will become; so that with such feebla pressure the bridges 

 can only slowly increase, and therefore they will be readily broken 

 when we try to separate the pieces. 



It cannot, moreover, be doubted that in Faraday's experiments, in 

 which two perforated discs of ice were placed in contact on a hori- 

 zontal glass rod, so that gravity exerted no pressure, capillary attrac- 

 tion is sufficient to produce a pressure of some grammes between the 

 plates, and the preceding discussions show that such a pressure, if 

 adequate time be given, can form bridges between the plates. 



If, on the other hand, two of the above-described cylinders of ice 

 are powerfully pressed together by the hands, they adhere in a few 

 minutes so firmly, that they can only be detached by the exertion of a 

 considerable force, for which indeed that of the hands is sometimes 

 inadequate. 



In my experiments I found that the force and rapidity with which 

 the pieces of ice united were so entirely proportional to the pressure 

 that I cannot but assign this as the actual and sufficient cause of their 

 union. 



In Faraday's explanation, according to which regelation is due to 

 a contact action of ice and water, I find a theoretical difficulty. By 

 the water freezing, a considerable quantity of latent heat must be set 

 free, and it is not clear what becomes of this. 



Finally, if ice in its change into water passes through an inter- 

 mediate viscous condition, a mixture of ice and water which was kept 

 for days at a temperature of must ultimately assume this condition 

 in its entire mass, provided its temperature was uniform throughout ; 

 this however is 'never the case. 



As regards what is called the plasticity of ice, James Thomson 

 has given an explanation of it in which the formation of cracks in the 

 interior is not presupposed. No doubt when a mass of ice in different 

 parts of the interior is exposed to different pressures, a portion of the 

 more powerfully compressed ice will melt ; and the latent heat neces- 

 sary for this will be supplied by the ice which is less strongly com- 

 pressed, and by the water in contact with it. Thus ice would melt 

 at the compressed places, and water would freeze in those which 

 are not pressed : ice would thus be gradually transformed and yield 

 to pressure. It is also clear that, owing to the very small conduc- 

 tivity for heat which ice possesses, a process of this kind must be ex- 

 tremely slow, if the compressed and colder layers of ice, as in glaciers, 

 are at considerable distances from the less compressed ones, and from 

 the water which furnishes the heat for melting. 



