ICE AND GLACIERS 257 



As regards what is called the plasticity of ice, James 

 Thomson has given an explanation of it in which the form- 

 ation of cracks in the interior is not presupposed. No doubt 

 when a mass of ice in different parts of the interior is ex- 

 posed to different pressures, a portion of the more power- 

 fully compressed ice will melt ; and the latent heat necessary 

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

 compressed, 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 conductivity for heat which ice 

 possesses, a process of this kind must be extremely 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. 



To test this hypothesis, I placed in a cylindrical vessel, 

 between two discs of ice of three inches in diameter, a 

 smaller cylindrical piece of an inch in diameter. On the 

 uppermost disc I placed a wooden disc, and this I loaded 

 with a weight of twenty pounds. The section of the narrow 

 piece was thus exposed to a pressure of more than an 

 atmosphere. The whole vessel was packed between pieces 

 of ice, and left for five days in a room the temperature of 

 which was a few degrees above the freezing-point. Under 

 these circumstances the ice in the vessel, which was exposed 

 to the pressure of the weight, should melt, and it might be 

 expected that the narrow cylinder on which the pressure 

 was most powerful should have been most melted. Some 

 water was indeed formed in the vessel, but mostly at the 

 expense of the larger discs at the top and bottom, which 

 being nearest the outside mixture of ice and water, could 

 acquire heat through the sides of the vessel. A small welt, 

 too, of ice, was formed round the surface of contact of the 

 narrower with the lower broad piece, which showed that 

 the water, which had been formed in consequence of the 

 pressure, had again frozen in places in which the pressure 

 ceased. Yet under these circumstances there was no ap- 

 preciable alteration in the shape of the middle piece which 

 was most compressed. 



KC VOL. xxx i 



