1908] I :o?i Ice and its Natural History. 261 



studied than at the Mergeliu See,' the well-known lake which exists 

 so long as the xlletsch glacier maintains a watertight dam across the 

 little side valley, which its waters occupy. It is roughly triangular in 

 shape. As the ice of the glacier is subject to more or less disintegra- 

 tion, there are always icebergs 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 extended 

 study of the structure of glacier ice, principally from the Aletsch 

 glacier. The fragments of this glacier, which float as icebergs in the 

 iVlergelin 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 : 7U0, 590, 450, 270, 255, 

 170, 155, and lUO grams. It was observed that blocks of ice contained 

 grains of all sizes, which fitted each other so exactly that, in the fresh 

 un weathered block, the whole volume was tilled with ice. 



The following table (p. 20) gives a summary of the results of 

 dissecting seven blocks and weighing the grains. 



Tiie tigures in the table do not give an exact statistical account 

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

 escaped being w^eighed, therefore the average size of the grain comes 

 out higher than the truth. On the other hand all were melting 

 rapidly, so that the grams when on the balance weighed less than 

 they did one hour before, and much less than they did the day before. 

 The tigures 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 tne 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 ra}s on glacier ice is twofold ; it disarticu- 

 lates the ice into its constituent grains, and it splits the inctiviaual 

 grain into lamina perpendicular to the principal axis of the crystal 

 and bounded by the planes of fusion aescribed by Tyndall. Iflese 

 planes are the distinguishing characteristic of ttie individual ice-giam. 



Under the intiuencc of laaiant heat an ice-crystal begins to melt 

 at the contiguous surfaces of these laminai, and the process of dis- 

 integration 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 loim the continuation 

 of the corresponding iaminse ot the parent crystal. This was well 

 observed at the end of August 1895, Every night a thin skm of ice 



