142 ICE AXD GLACIERS. 



from clear pieces of ice, wliile 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 dis- 

 appear, 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 ' which inter- 

 penetrate the mass of the ice. Hence we may conclude 

 that the compressed block is traversed by a great num- 

 ber of fine cracks and fissures, which render it pliable ; 

 that its particles become a little dispersed, and are there- 

 fore withdrawn from pressure, and that immediately after- 

 wards 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 sm-faces by the reflection of 

 the light. 



These cracks and laminae also become more perceptible 

 when the ice — which, as I before mentioned, is below zero 

 immediately after pressure has been applied — is again 

 raised to this temperature and begins to melt. The cre- 



• These cracks are 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, 

 DO 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. 



