fee and its Natural History 257 



in the Polar Sea, we know that it will prove to be very far 

 from homogeneous. It may have a foundation of genuine 

 primary sea-ice ; but the ice forming the superstructure is 

 sure to consist of snow, frozen spray, and very likely frag- 

 ments of land-ice, all cemented together into a species of 

 conglomerate. We have seen that when this is exposed to 

 warmth it begins to melt at a temperature which may be 

 one or two degrees below the melting-point of pure ice ; and 

 the liquid furnished by the melting is salt water. The further 

 melting takes place in the ascending order of temperature ; 

 the salt ice of low melting-point disappearing first, and the 

 purer ice melting later. We thus see how ice can be cemented 

 by ice, just as metallic objects may be united by solder. In 

 both cases the binding material differs from the objects united, 

 chiefly in being more easily fusible. 



If we have a number of cubes of pure ice, which fit each 

 other exactly, and, if after being moistened with salt water 

 they are exposed to frost, they will solidify to a single block. 

 If this be exposed to the sun, the cementing salt ice will melt 

 first, and, when it ceases to bind, the constituent cubes of 

 pure ice will fall asunder, having themselves suffered prac- 

 tically no diminution due to melting. 



The Glacier Grains. Now this is precisely what happens 

 when a block of sound glacier-ice is exposed to the rays of 

 the sun for a short time ; and it is one of the most striking 

 and instructive experiments that can be made. Under the 

 influence of the sun's rays, the binding material melts first, 

 the continuity of the block is destroyed, the individual grains 

 become loose and rattle if the block be shaken, and finally 

 they fall into a heap. A block of glacier-ice is a geometrical 

 curiosity. It consists of a number of solid bodies of different 

 sizes and of quite irregular shapes, yet they fit into each other 

 as exactly and fill space as completely as could the cubes 

 above referred to. 



The granular constitution of glacier-ice can nowhere be 

 better studied than at the Mergelin See, the well-known lake 

 which exists so long as the Aletsch glacier maintains a water- 

 tight dam across the little side valley, which its waters occupy. 



B. 17 



