ICE AND GLACIERS 259 



"bands and rings are seen in the interior; and by the ar- 

 rangement of their colours it is easy to recognise the limits 

 of the ice-granules, which, heaped on one another in irregu- 

 lar 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. 



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 de- 

 gree 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 dis- 

 turbed. 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 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 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 proper- 

 ties 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 relation- 

 ship that the transition from fragile and elastic crystalline 

 ice into plastic granular ice is so very instructive. 



