258 HELMHOLTZ 



This experiment shows that although changes in the 

 shape of the pieces of ice must take place in the course of 

 time in accordance with J. Thomson's explanation, by which 

 the more strongly compressed parts melt, and new ice is 

 formed at the places which are freed from pressure, these 

 changes must be extremely slow when the thickness of the 

 pieces of ice through which the heat is conducted is at all 

 considerable. Any marked change in shape by melting in 

 a medium the temperature of which is everywhere o, could 

 not occur without access of external heat, or from the un- 

 compressed ice and water; and with the small differences in 

 temperature which here come into play, and from the 

 badly conducting power of ice, it must be extremely slow. 



That on the other hand, especially in granular ice, the 

 formation of cracks, and the displacement of the surfaces 

 of those cracks, render such a change of form possible, is 

 shown by the above-described experiments on pressure; and 

 that in glacier ice changes of form thus occur, follows 

 from the banded structure, and the granular aggregation 

 which is manifest on melting, and also from the manner in 

 which the layers change their position when moved, and 

 so forth. Hence, I doubt not that Tyndall has discovered 

 the essential and principal cause of the motion of glaciers, 

 in referring it to the formation of cracks and to regelation. 



I would at the same time observe that a quantity of heat, 

 which is far from inconsiderable, must be produced by 

 friction in the larger glaciers. It may be easily shown by 

 calculation that when a mass of firn moves from the Col 

 du Geant to the source of the Arveyron, the heat due to the 

 mechanical work would be sufficient to melt a fourteenth 

 part of the mass. And as the friction must be greatest in 

 those places that are most compressed, it will at any rate 

 be sufficient to remove just those parts of the ice which 

 offer most resistance to motion. 



I will add, in conclusion, that the above-described granular 

 structure of ice is beautifully shown in polarised light. If 

 a small clear piece is pressed in the iron mould, so as to 

 form a disc of about five inches in thickness, this is suffi- 

 ciently transparent for investigation. Viewed in the polar- 

 ising apparatus, a great number of variously coloured small 



