230 The Size of the Ice-grain in Glaciers 



the largest, has grown, according to the rigid laws of crystal- 

 lomorphic development, from a single snow-crystal which 

 probably weighed no more than one or two centigrammes. 



In the Mergelin See, glacier-ice can be studied in a way 

 that is possible in no other place. The fragments of the 

 Aletsch Glacier which float in it are veritable icebergs, and 

 behave in the same way as their relatives in the Arctic or 

 Antarctic Ocean. In the middle of summer, however, they 

 are exposed to a much more powerful sun than either the 

 northern or the southern bergs. Consequently, the weathering 

 and disintegration, as well as the melting, proceed at a much 

 more rapid rate. 



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

 disarticulates the ice into its constituent grains, and it splits 

 the individual grain up into laminae perpendicular to the 

 principal axis of the crystal and bounded by the planes of 

 fusion discovered and described by Tyndall. These planes 

 are the distinguishing characteristic of the individual ice- 

 grain. 



Under the influence of radiant heat an ice-crystal begins 

 to melt at the surface which separates these laminae, and the 

 process of disintegration and decay is directed by their plane. 

 On the other hand, an ice-crystal, floating in water and losing 

 heat, generates ice laminae which are directed by the same 

 planes, which form the continuation of the corresponding 

 laminae of the parent crystal. This was well observed at the 

 end of August 1895. Every night a thin skin of ice was 

 formed at the shallow end of the lake, where the ice-blocks 

 are collected. As the grains in a block of glacier-ice are 

 distributed quite irregularly, the water line of a floating block 

 necessarily cuts a great number of grains, all of which are 

 oriented differently. The ice which was formed during the 

 night along this line was oriented crystallographically by the 

 grain with which it was in contact and from which it appeared 

 to spring in continuation of its crystalline laminae. This 

 produces a remarkable pattern of lines on the surface of the 

 lake-ice contiguous to a block of glacier-ice. 



Tyndall has described and figured the minute features of 



