250 HELMHOLTZ 



though not completely, the moment the pressure is suspended 

 Such a compressed block is distinctly more opaque imme- 

 diately 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 



FIG. 117 



the interior of the mass of what is otherwise clear. These 

 lines are the optical expression of extremely fine cracks* 

 which interpenetrate the mass of the ice. Hence we may 

 conclude that the compressed block is traversed by a great 

 number of fine cracks and fissures which render it pliable; 

 that its particles become a little dispersed, 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 withdrawn from pressure, and that immediately afterwards 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 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 crevices 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 nar- 

 row fissures between them are full of water. A block of 

 ice thus formed of ice-granules adheres firmly together; but 



* 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, 

 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. 



