Figxire 30. Disks of deep ice. 



abrasion of their edge as a result of a great number of collisions with other elements during their 

 rise to the surface of the water. A unit of such ice is completely transparent and is visible in 

 water only in the case of complete internal reflection; ordinarily it is about 4 to 8 mm in diameter 

 and 0. 1 mm or less thick. 



The nuclei of crystallization which form in the film of water enveloping bottom objects lead to 

 the formation of a variation of deep ice, bottom ice. 



It has been noted that the intensity of bottom ice formation is not the same on all objects: it 

 forms very well on metallic objects, less well on glass, and hardly forms on wood, for example. 



As they grow, pieces of bottom ice break loose from the bottom and float up to the surface, 

 often with soiled particles, rocks, and other objects frozen to them. * After the surface is covered 

 by the surface ice or deep ice which has floated to the surface, further formation of deep ice ceases 

 in fresh water. Actually, we have seen that the second fundamental condition necessary for ice 

 formation necessitates a great loss of heat by the water. This loss is hindered by the presence of 

 surface ice, and with further supercooling of the water, it ceases in favor of crystallization. 



Naturally, the most vigorous agitation is observed in the fast rapids of rivers where the 

 amount of deep ice is often four to five times greater than the amount of surface ice which forms 

 simultaneously. In the Angara, according to Altberg, deep ice is almost the exclusive formation. 



As Velikanov points out, the formation of deep ice does not cease for very long if there are 

 polynyas ,** in. which case ice formation always take place below these polynyas. Floating 



*In river rapids, bottom ice, slowly growing, sometimes reaches the very surface of the 

 water, creating characteristic ice formation called patrams. 



**Polynya- Any sizable sea water area, other than a lead, encompassed by ice. 



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