ICE AND GLACIERS. 143 



vices then fill with water, and such ice then consists of a 

 quantity of minute granules from the size of a pin's head 

 to that of a pea, which are closely pushed into one another 

 at the edges and projections, and in part have coalesced, 

 while the narrow fissures between them are full of water. 

 A block of ice thus formed of ice-granules adheres firmly 

 together ; but if particles be detached from its corners 

 they are seen to consist of these angular granules. Gla- 

 cier ice, when it begins to melt, is seen to possess the 

 same structure, except that the pieces of which it consists 

 are mostly larger than in artificial ice, attaining the size 

 of a pigeon's egg. 



Glacier ice and compressed ice are thus seen to be sub- 

 stances of a granular structure, in opposition to regularly 

 crystallised ice, such as is formed on the surface of still 

 water. "VVe here meet with the same differences as be- 

 tween calcareous spar and marble, both of which consist 

 of carbonate of lime ; but while the former is in large, 

 regular crystals, the latter is made up of irregularly 

 agglomerated crystalline grains. In calcareous spar, as 

 well as in crystallised ice, the cracks produced by inserting 

 the point of a knife extend through the mass, while in 

 granular ice a crack which arises in one of the bodies 

 where it must yield does not necessarily spread beyond 

 the limits of the granule. 



Ice which has been compressed from snow, and has 

 thus from the outset consisted of innumerable very fine 

 crystalline needles, is seen to be particularly plastic. 

 Yet in appearance it materially differs from glacier ice, 

 for it is very opaque, owing to the great quantity of air 

 which was originally enclosed in the flaky mass of snow, 

 and which remains there as extremely minute bubbles. 

 It can be made clearer by pressing a cylinder of such ice 

 between wooden boards ; the air-bubbles appear then on 

 the top of the cylinder as a light foam. If the discs are 



