A Stiidy in the Natural History of Ice 309 



. cloudy, and the temperature of the air comparatively high, 

 the rate of melting as a whole may be so great that the lump 

 may melt away by the outside before the amount of radiation 

 available has been able to supply sufficient heat to the interior 

 to produce the intergranular fusion which permits complete 

 disintegration. 



In very cold winter weather, when the temperature of the 

 air is continuously below the melting temperature of ice, 

 exposure to the sun disarticulates the grains as perfectly as 

 it does in summer. But so long as the temperature of the 

 ice remains below the freezing temperature the grains hold 

 together. The appearance of the lump shows that it has been 

 in fact disarticulated. The moment its temperature reaches 

 o C disintegration takes place even in the dark, and the lump 

 becomes a heap of grains without there having been any 

 melting. Hugi's experiments in this direction are classical. 

 They were made during his Winterreise ins Eismeer in 

 January 1832, a date which marks one of the most important 

 epochs in the development of our knowledge of the natural 

 history of ice. 



Recapitulation. 



A glacier is a granular crystalline mass of ice, which, in 

 summer, is throughout at its melting temperature. In winter 

 it has the same temperature, with the exception of a super- 

 ficial shell of small thickness, which follows qualitatively the 

 atmospheric changes of temperature, subject to some retarda- 

 tion of epochs. It does not, however, begin to do so until the 

 atmospheric changes have proceeded so far that the surface of 

 the glacier is cooled below its melting temperature, and this 

 epoch is affected by the date when the glacier receives its 

 winter covering of snow. 



The granular surfaces are moistened with water derived 

 from the ice of the grain. This liquid contains the greater part 

 of the soluble impurities chiefly derived from the rock-debris 

 which is intimately mixed with the ice. In contact with it 

 the melting temperature of the ice is lowered, independently 

 of any alteration of pressure. But the supply of heat available 



