THE ICE AGE IX NORTH AMERICA. 



mold. This capacity of ice, when near the melting-point, 

 to undergo disintegration, and then to become suddenly re- 



mjr 



Fig. 4.— A, B, C, molds ; a, c, e, original forms of the ice ; b, d, f, the forms into which 

 they were molded. 



congealed, is probably that by which it simulates in its mo- 

 tion the properties of ordinary fluids, while at the same time 

 retaining other properties connecting it with the most brittle 

 of substances. 



It is thought, by Mr. Croll and others, that when heat 

 passes through a stratum of ice, as it is known to do, it involves 

 a process of transference from one particle of ice to another, 

 in which there are successive melting and freezing of the 

 particles in the progress of the heat, and that finally the mole- 

 cule of ice upon the opposite side, in becoming recongealea, 

 delivers up the unit of heat which had entered the stratum 

 from the other side. But, whatever be the explanation of 

 the process, the facts remain that ice behaves in many re- 

 spects like a fluid, and, on application of pressure, slowly ad- 

 justs itself to its bed or mold in obedience to the force ap- 

 plied, and, if time enough is given, moves wherever a fluid 

 would find its way. Ice is plastic under pressure and brittle 

 under tension. 



Snow is one form of ice, and, as every school-boy who 

 makes a snow-ball knows, can by a moderate degree of press- 

 ure be made into compact ice. The reason why snow is 

 white, and ice is blue, is that snow is pulverized, while in 

 ice the particles are brought into closer contact, and the 

 inclosed air is expelled, so that the real color of the substance 



