258 Ice and its Natural History 



It is roughly triangular in shape. As the ice of the glacier is 

 subject to more or less disintegration, there are always ice- 

 bergs and small fragments of ice floating on its waters. The 

 portions projecting above the water are exposed in summer 

 often to a very powerful sun, and are loosened by the radiant 

 heat into their constituent grains. A lump lifted out is found 

 to consist of these disarticulated grains which rattle when 

 shaken, and in a strong sun fall to pieces. 



Size of Glacier Grains. In August 1895 I made an ex- 

 tended study of the structure of glacier-ice, principally from 

 the Aletsch glacier. The fragments of this glacier, which 

 float as icebergs in the Mergelin See, are exposed to the 

 powerful weathering influence of the summer sun, and are 

 comparatively easily dissected into their constituent grains. 

 A number of blocks were so dissected, in order to ascertain 

 the weight and size of the largest grains. The following 

 weights of single grains were determined: 700, 590, 450, 270, 

 255, 170, 155, and 100 grams. It was observed that blocks 

 of ice contained grains of all sizes, which fitted each other 

 so exactly that, in the fresh unweathered block, the whole 

 volume was rilled with ice. 



The following table gives a summary of the results of 

 dissecting seven blocks and weighing the grains. 



The figures in the table do not give an exact statistical 

 account of the blocks of ice. The smallest grains have most 

 frequently escaped being weighed, therefore the average size 

 of the grain comes out higher than the truth. On the other 

 hand all were melting rapidly, so that the grains when on 

 the balance weighed less than they did one hour before, and 

 much less than they did the day before. The figures in the 

 table give a general idea of the constitution or anatomy of 

 a block of ice taken from the lower part of a large glacier. 

 They are particularly interesting when we reflect that every 

 grain, even the largest, has grown, according to the rigid 

 laws of crystallomorphic development, from a single snow- 

 crystal which probably weighed no more than one or two 

 centigrammes. 



The action of the sun's rays on glacier-ice is twofold ; it 



