SCIENTIFIC EESULTS 13 



snow freezes upon reachino; the frigid sen. But as increased solar 

 warming- raises the temperature, so it increases the rate of melting, 

 and the floe fintJly becomes thinner. 



The loss of salts during the warmer months of the year leaves the 

 polar cap mantled Avith fresh ice possessing a higher melting point 

 and therefore greater permanence. Old sea ice accordingly survives 

 much longer than the young, i)rovided both samples be of the same 

 volume, when ex])osed to similar melting conditions. Many people 

 have regarded with considerable skepticism the statement that sea 

 ice furnishes a supply of very good drinking water. Experienced ice 

 navigators, however, are well aware of the freshening of sea ice with 

 age, since the fresh-water pools on old pans have long been utilized 

 as a never-failing supply. The rule is to moor the ship with ice 

 anchors to a well-selected old floe and then run a hose to a near-bv 

 pool, where water has collected either from the weathered side of' 

 the ice itself or from the preceding winter's snow cover. It is im- 

 jiortant to pump only from pools located well in from the edge of 

 [he floe to avoid the possibility of contamination by salt spray.^ 



The physical behavior of salt water wdien subjected to freezing 

 temperatures is dilferent from that of fresh water under similar con- 

 ditions, because the former contracts right to the freezing point." 



The fact that salt-water ice is formed when the freezing mass is 

 at maximum density, causes it, therefore, to be slightly heavier than 

 fresh-water ice. The fact that it does not float lower is due mainly to 

 the considerable quantity of air which is in the water when it freezes. 

 The density of pure, fresh- water ice, according to Barnes (1928, p. 

 •25) is 0.91676, and that of sea ice formed from the water of the salin- 

 ity that normally comes under freezing conditions ranges from 0.857 

 to 0.924.1^ 



The relatively great range in the specific gravity of sea ice is due, 

 according to Malmgren (1928, p. 17), partly to large quantities of 

 air or water which the ice may absorb during the summer directly 

 from the atmosphere. The reason that water is not sucked up from 

 below into the spongelike vacuoles (caused by the escape of salts) is 

 because much of the lower layers of the sea ice in polar regions remain 

 unmelted even in summer and, therefore, waterproof. If, however, 

 sea ice drifts out of the Arctic into waters warmer than 0° C. (32° 

 F.), melting will begin below the water line, causing ingestion and 

 a drowning of the ice. Arctic pack ice, therefore, in the North 

 Atlantic floats lower than it did nearer its source. 



As a corollary to the above, if sea ice is exposed to temperatures 

 as low as -20° C. (-40° F.), nearly all the brine globules freeze 

 into salt crystals, causing the ice to swell to a maximum volume. 



The physical appearance of sea ice is quite different from that of 

 fresh-water ice; the former exhibiting an opaque slaty whiteness. 



° Pettfisson (]8S3, p. 304) coUected sea ice floating in the polar drift current nortliwest 

 of Spitsbergen which contained less than one-fourth the amount of chlorides found in the 

 unnking water of Stocicholm. 



'"As a matter of fact the saltiest water, i. e., pure ocean water, seldom reaches a 

 ireezing temperature, because it is usually protected by warm convectional currents. The 

 ireezing regions of the hydrosphere are confined mostly to the shallow waters of conti- 

 nental shelves and to epi-continental seas, where the salinity of the surface layers rarely 

 '"Sf'^pfls 33 0/00; so tliat freezing takes place at a temperature of —1.8° C. (28.8° F.). 



vVe performed the following experiment : A quart of sea water of salinity 34.3 0/00 

 was frozen into a cake of solid ice, with no opportunity for any of the salts to escape, 

 """ "Pspite its high specific gravity the block floated in a .iar of fresh water. The height 

 Hofi -f 1'^°'^ ^^^ noticeably lower than in the case of fresh-watei- ice, but the experiment 

 uennitely shows, nevertheless, that ordinary sea ice has reserve buoyancy. It is often 

 oDServed, for instance, floating off river mouths where the water is comparatively fresh. 



