132 THE FOKMS OF WATEK IN 



occupy, in transverse bands, the flat surface of the gh - 

 cier. At Trelaporte, moreover, where the valley becomes 

 narrow, the bands are much sharpened, obtaining there 

 the character which they afterwards preserve through- 

 out the Mer de Glace. Other glaciers with cascades 

 also exhibit similar bands. 



§ 49. Sea Ice and Icebergs. 



338. We are now equipped intellectually for a cam- 

 paign into another territory. Water becomes heavier 

 and more difficult to freeze when salt is dissolved in it. 

 Sea water is therefore heavier than fresh, and the 

 Greenland Ocean requires to freeze it a temperature 

 31 degrees lower than fresh water. When concentrated 

 till its specific gravity reaches 1*1045, sea water re- 

 quires for its congelation a temperature 18£ degrees 

 lower than the ordinary freezing-point.* 



339. But even when the water is saturated with salt, 

 the crystallising force studiously rejects the salt, and 

 devotes itself to the congelation of the water alone. 

 Hence the ice of sea water, when melted, produces 

 fresh water. The only saline particles existing in such 

 ice are those entangled mechanically in its pores. They 

 have no part or lot in the structure of the crystal. 



340. This exclusiveness, if I may use the term, of the 

 water molecules ; this entire rejection of all foreign 



* Scoresby. 



