THE WORK OF SNOW AND ICE. 309 



crystals. The molecules are arranged on the hexagonal plan, and, as the 

 expansive power of freezing water shows, the arrangement is controlled 

 by a strong force. Once the definite crystaUine arrangement is estab- 

 lished, the molecules can be displaced only by relatively great force. 



Fig. 285. — An iceberg, west coast of Greenland. 



Snow crystals often continue to grow so long as they are in the atmos- 

 phere; but if they pass through an under-saturated stratum of air or 

 a stratum whose temperature is above 32° Fahr., they suffer from evap- 

 oration or melting. When they reach the ground, the processes of 

 growth and decadence continue, and the crystals grow or diminish 

 according to circumstances. 



A glacier is a colossal aggregation of crystals grown from snowflakes 

 to granules of much greater sizes. The microscopic study of new-fallen 

 snow reveals the mode of change from flakes to granules. The slender 

 points and angles of the former yield to melting and evaporation more 

 than the more massive central portions, and this change probably 

 illustrates a law of vital importance. It may often be seen that the 

 water melted from the periphery of a flake gathers about its center, 

 and if the temperature be right, it freezes there. This is a first step 

 toward the pronounced granulation of snow which has lain for some 

 time on the ground. If measured systematically from day to day, the 

 larger granules taken from beneath the surface of this coarse-grained 

 snow are found to be growing. In a series of experiments ^ to deter- 



* Carried out by C. J^. Peet and E. C. Perisho under the direction of one of the 

 authors. 



