136 ICE AND GLACIERS. 



latter do not produce the comparatively firm coherence of the apparent 

 heap of granules. 



The properties of ice here described are interesting from a physical 

 point of view, for they enable us to follow so closely 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 struc- 

 ture; 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. 



