152 ICE AKD GLACIERS. 



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 dif- 

 ferent 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 polyhedral granules, these granules, before any at- 

 tempt 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 latter do not produce the 

 comparatively firm coherence of the apparent heap of granules. 



The properties of ice here described are interesting fiom a 

 physical point of view, for they enable us to follow so clostly 

 the transition from a crystalline body to a granular one ; and 

 they give the causes of the alteration of its properties better 

 than in any other well-known example. Most natural substances 

 show no regular crystalline structure; our theoretical ideas refer 

 almost exclusively to crystallised and perfectly elastic bodies. 

 It is precisely in this relationship that the transition from fragile 

 and elastic crystalline ice into plastic granular ice is so very 

 instructive. 



