THE WORK OF SNOW AND ICE. 311 



retain molecules thrown from other granules, and that, other things 

 being equal, the retention of particles also depends on the curvature 

 of the surface, the less curved surface retaining more than the sharply 

 curved one. Under these laws, it is obvious that the larger granules 

 of smaller curvature will lose less and gain more, on the average, than 

 the smaller granules of greater curvature. It follows that the larger 

 granules will grow at the expense of the smaller. It is also to be noted 

 that, other things being equal, small granules melt more readily than 

 large ones, and that where the temperature is nicely adjusted between 

 melting and freezing, the smaller may lose while the larger gain. 



Another factor that enters into the process is that of pressure and 

 tension. The granules are compressed at the points of contact and put 

 under tension at points not in contact, and the pressure and tension 

 are, on the average, likely to be relatively greatest for the smallest 

 granules. Tension increases the tendency to evaporation and adds 

 its effects to curvature, and the capillary spaces adjoining the points of 

 contact probably favor condensation. Ice expands in crystaUizing 

 and pressure reduces the melting-point, while tension raises it. The 

 effect of this is sHght (p. 276), and it probably plays Httle part in glacial 

 action, but it is to be correlated with the much more important fact 

 that compression produces heat which may raise the temperature of the 

 ice to the melting-point, while tension may reduce the temperature to or 

 below freezing. There is therefore a tendency for the ice to melt at 

 the points of contact and compression, and for the water so produced 

 to refreeze at adjacent points where the surface is under tension. This 

 process becomes effective beneath a considerable body of snow", and 

 here the granules gradually lose the spheroidal form assumed in the early 

 stages of granulation and become irregular polyhedrons interlocked 

 into a more or less sohd mass. 



A third factor is also to be recognized, though its effectiveness is 

 unknown. Under severe wind pressure, air penetrates porous bodies 

 with appreciable facility. The "breathing'^ of soils and the curious 

 phenomena of '^ blowing -wells " and ^^blowing-caves" teach us of the 

 effective penetration and extrusion of the air under variations of baro- 

 metric pressure. In the snow-fields, and in the more granular portions 

 of glaciers near their heads, the porosity is doubtless sufficient to allow 

 of the appreciable penetration of the atmosphere. During a part of 

 the time, the probable effect is the condensation within the ice of moisture 



