126 ICE AND GLACIERS. 



a glacier current where it presses through a narrow rocky pass 

 into a wider valley. 



In the cases which we have described we see the change 

 in shape of the ice taking place before our eyes, whereby 

 the block of ice retains its coherence without breaking into 

 individual pieces. The brittle mass of ice seems rather to yield 

 like a piece of wax. 



A closer inspection of a clear cylinder of ice compressed 

 from clear pieces of ice, while the pressure is being applied, 

 shows us what takes place in the interior ; for we then see an 

 innumerable quantity of extremely fine radiating cracks shoot 

 through it like a turbid cloud, which mostly disappear, though 

 not completely, the moment the pressure is suspended. Such a 

 compressed block is distinctly more opaque immediately after 

 the experiment than it was before; and the turbidity arises, 

 as may easily be observed by means of a lens, from a great 

 number of whitish capillary lines crossing the interior of the 

 mass of what is otherwise clear. These lines are the optical 

 expression of extremely fine cracks l which interpenetrate the 

 mass of the ice. Hence we may conclude that the compressed 

 block is travelled by a great number of fine cracks and fissures 

 which render it pliable ; that its particles become a little dis- 

 persed, and are therefore withdrawn from pressure, and that 

 immediately afterwards the greater part of the fissures disappear, 

 owing to their sides freezing. Only in those places in which the 

 surfaces of the small displaced particles do not accurately fit to 

 each other some fissured spaces remain open, and are discovered 

 as white lines and surfaces by the reflection of the light. 



These cracks and laminae also become more perceptible when 



1 These cracks arc probably quite empty and free from air, for they are also 

 formed when perfectly clear and air-free pieces of ice are pressed in the form 

 which has been previously filled with water, and where, therefore, no air 

 could gain access to the pieces of ice. That such air-free crevices occur in 

 glacier ice has been already demonstrated by Tyndall. When the compressed 

 ice afterwards melts, these crevices fill up with water, no air being left. 

 They are then, however, far less visible, and the whole block is therefore 

 clearer. And just for this reason they could not originally have been filled 

 with water. 



