PHYSICAL PROPERTIES OF SEA WATER 33 



is the amount of sea water enclosed in the ice. If the temperature of the 

 ice thus formed is lowered, part of the trapped sea water freezes, so 

 that the cells containing the liquid brine become smaller and the salt con- 

 centration in the enclosed brine becomes greater. Thus, sea ice consists 

 of crystals of pure ice separating numerous small cells containing brine, 

 the concentration of which depends upon the temperature of the ice. 

 If the ice is cooled to very low temperatures, solid salts may crystallize 

 out. 



If the temperature of such sea ice rises, the ice surrounding the brine- 

 filled cells melts and separated salt crystals dissolve. As the melting 

 goes on, the brine cells grow in size, and, when the temperature approaches 

 zero, the cells join, permitting the trapped sea water to trickle down. 

 Where the sea ice has been hummocked, all brine will flow dow^n, leaving 

 only the pure ice, which can be used as a source of potable water. In the 

 Arctic, part of the hummocks melt in summer, and the water that then 

 collects in pools on the ice floes is fit for drinking and cooking purposes. 

 Sea ice which has not been exposed to hummocking, on the other hand, 

 will become soggy and will disintegrate. 



The salinity of the ice depends upon the rapidity of freezing and 

 upon the temperature changes to which the ice has been subjected. At 

 very rapid freezing, brine and salt crystals may accumulate on ice sur- 

 faces, making the surface "wet" at temperatures of —30° to — 40°C 

 and greatly increasing the friction against sled runners and skis. 



Properties of Sea Ice. The properties of sea ice differ greatly 

 from those of fresh-water ice, since they depend upon the amount of 

 enclosed brine and upon the number of air bubbles left in the ice if all or 

 part of the brine has trickled down. 



At zero degrees the density of pure ice is 0.9168, but the density of 

 sea ice may be both above and below that of pure ice, depending upon its 

 content of brine and air bubbles. Values between 0.92 and 0.86 have 

 been reported. The specific heat of pure ice is about half that of pure 

 water. It depends upon the temperature of the ice, but varies within 

 narrow limits. The specific heat of sea ice depends on the temperature 

 and the salinity of the ice, because every lowering of the temperature 

 will involve freezing of some of the enclosed brine, and every raising 

 of the temperature will involve melting of ice surrounding the brine-filled 

 cells. The amounts of heat involved in these processes of freezing and 

 melting are so great that the specific heat is 6.7 at a temperature of —2°, 

 1.99 at -4°, 0.88 at -8°, and 0.60 at -16°. At very low temperatures, 

 when most of the salts have crystallized out, the specific heat approaches 

 that of pure ice. 



The latent heat of fusion of pure ice at atmospheric pressure is 79.67 

 g cal/g. No specific value of the heat of fusion can be assigned to sea 

 ice, because, owing to the presence of salts, melting takes place whenever 



