ICE AND GLACIERS. 135 



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 Ge"ant 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 suf- 

 ficient 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 transparent for investiga- 

 tion. 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 es- 

 sentially 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. 



In order to explain the continued coherence of the piece of ice 

 during its change of form, it is to be observed that in general the 

 cracks in the granular ice are only superficial, and do not extend 

 throughout its entire mass. This is directly seen during the pressing 

 of the ice. The crevices form and extend in different directions, like 

 cracks produced by a heated wire in a glass tube. Ice possesses a 

 certain degree of elasticity, as may be seen in a thin flexible plate. A 

 fissured block of ice of this kind will be able to undergo a displacement 

 at the two sides which form the crack, even when these continue to 

 adhere in the unfissured part of the block. If then part of the fissure 

 at first formed is closed by regelation, the fissure can extend in the 

 opposite direction without the continuity of the block being at any 

 time disturbed. It seems to me doubtful, too, whether in compressed 

 ice and in glacier ice, which apparently consists of interlaced poly- 

 hedral granules, these granules, before any attempt is made to separate 

 them, are completely detached from each other, and are not rather 

 connected by ice-bridges which readily give way ; and whether these 



