POLAR-ICE.^QUALITIES. 231 



tendency to produce fresh ice, is farther proved by 

 the concentration observed in a quantity exposed in 

 an open vessel to a low temperature, by the separa- 

 tion of the salt from tlie crystals of ice, in the pro- 

 gress of the freezing. Thus it is, that, in the cold- 

 est weather, when a ship exposed to a tempestuous 

 sea, is washed with repeated sprays, and thereby 

 covered with ice, that in diiferent places obstruct- 

 ing the efflux of the water overboard, a portion al- 

 ways remains unfrozen, and whicli, on being tasted, 

 is found to consist of salt water highly concentrated. 

 This arises from the freezing point of water falling 

 in a certain ratio, according to the degree of salt- 

 ness ; thus, thougli pure water, of specific gravity 

 1.0000, freeze with a temperature of 32°, water of 

 specific gravity 1.0263, containing about 5f oz. 

 (avoird.) of salt in every gallon of 231 cubic inches ; 

 that is, with the degree of saltness common to the 

 Greenland Sea, freezes at 28^°. Sea-water, concen- 

 trated by freezing, until it obtains the specific gra- 

 vity of 1.1045, requires a temperature of 13§° for its 

 congelation, having its freezing point reduced 18^° 

 below that of pure water ; and water saturated with 

 sea-salt remains liquid, at a temperature of zero. 



Thus, we are presented with a natural process 

 for extracting salt from the sea, or at least for great- 

 ly facilitating that process in a concentration of the 

 saline particles, by the agency of frost. 



