518 THE LIFE OF JAMES D. FORBES. [CHAP. 



continually there is freezing of water and formation of 

 new ice round the ice which is pressed, while a portion 

 of the latter goes on melting. 



' This happens, for instance, wherf two pieces of ice are 

 pressed together, for thus they are firmly united to one 

 another by the water which freezes at their .surface of 

 contact, and united into one piece of ice. Under power- 

 ful pressure, which cools the ice more, this takes place 

 rapidly ; but even with very slight pressure it happens, 

 if one gives it time enough. Faraday, who discovered 

 this phenomenon, called it the regelation of ice. There has 

 been much dispute about the explanation of this ; I have 

 given you that which appears to me the most satisfactory. 



* This freezing together of two pieces of ice is easily 

 exhibited with pieces of any form, but they should not 

 be colder than zero ; they act best when they are beginning 

 to melt. One needs only to press them powerfully against 

 one another for a few instants and they unite. The 

 flatter are the two touching surfaces, the more firmly do 

 they unite, but very slight pressure suffices, if one only 

 leaves the two pieces of ice very long in contact. 



' This property of melting ice is made use of by boys 

 when they make snowballs and snow-men. It is known 

 that this succeeds only when the snow is beginning to 

 melt, or at least, when it is so little colder than zero that 

 it can easily be raised to that temperature by the warmth 

 of the hand. Very cold snow is a dry loose powder, 

 and does not stick together. 



1 What children making snowballs do on a small scale, 

 goes on on the most tremendous scale in glaciers. The 

 deeper portions of the originally loose and powdery snow 

 of the neve are pressed together by the overlying heaps 

 of snow, often many hundred feet deep, and under this 

 pressure agglomerate into denser and more solid layers. 

 The fresh fallen snow originally consists of tender micro- 

 scopic needles of ice. . . . Thus, whenever the sun plays 

 upon the upper layers of the snow field, water trickles 

 down and freezes again in the deeper layers where it 

 meets with colder snow ; the neve becomes first granular 



