ICE AND GLACIEKS. 151 



is conducted is at all considerable. Any marked change in 

 shape by melting in a medium the temperature of which is 

 everywhere 0, could not occur without access of external heat, 

 or from the uncompressed 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 forma- 

 tion 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 struc- 

 ture, 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 mecha- 

 nical 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 sufficiently trans- 

 parent for investigation. Viewed in the polarising apparatus, a 

 great number of variously coloured small bands and rings are 

 seen in the interior ; and by the arrangement of their colours it 

 is easy to recognise the limits of the ice-granules, which, heaped 

 on one another in irregular order of their optical axes, constitute 

 the plate. The appearance is essentially the same when the 

 plate has just been taken out of the press, and the cracks appear 

 in it as whitish lines, as afterwards when these crevices have 

 been filled up in consequence of the ice beginning to melt. 



