8 MARION EXPEDITION TO DA.VIS STRAIT AND BAFFIN BAY 



sea ice.* The former when found drifting in the ocean, is usually in 

 the form of icebergs -syhich from their relatively great mass and I 

 density, form especially dangerous obstacles in the paths of naviga- I 

 tion. Sea ice of course is the ice formed of salt water, and because 

 of its great horizontal proportions, covers a fairly wide expanse in 

 the northern hemisphere. 



The Properties of Sea Ice 



Sea ice differs from fresh-water ice because of the presence of salt 

 in the former and on account of this attribute great contrasting dif- 

 ferences occur between the physical j:)roperties of the two kinds of ice. 



We may learn much regarding the physical properties of sea ice ' 

 by observing what happens at the boundary between ice and water 

 when the water approaches freezing conditions. Water, accordinjr 

 to Johnstone (1923, p. 185). is composed of H^O molecules mixed 

 together in single, double, and triple combinations. The triplex 

 molecule, is the one closely associated with the formation of ice. 

 It is believed these triplex, so-called ice molecules, are present in 

 water in varying proportions, even above the freezing point, de- 

 pending upon the particular temperature of the liquid at the time. 

 They, therefore, lie suspended in the water, similar to salt in solu- 

 tion, until the saturation point is reached, which occurs at a tempera- 

 ture of —1.85° C. (28.7° F.). for common sea water (salinity 

 34 0/00) when the liquid is transformed to a solid state. The 

 subtraction of an amount of heat energy equivalent to the heat of 

 fusion of that given mass of sea water, often the addition of a ' 

 minute piece of ice itself, is sufficient to initiate actual freezing. 

 The first sign of a change of physical state under the microscope is a 

 cloud of small disklike particles which floculate and grow, finally 

 passing from an original colloidal state into true crystalline form. 

 If they are allowed to develop in quiet water, the crystals assume ' 

 most beautiful feathery designs, but more often they collect in a 

 network of fine spicules and elongated prisms, which extend down 

 and out as streamers and plates. If the water be exposed to a very 

 sudden chilling by the wintry atmosphere, such as often occurs within ' 

 the bounds of the polar ice cap, the colloidal stage of ice growth is 

 masked by the direct creation of the more familiar crystalline 

 structure. 



The processes of solidification of sea water differs in one important 

 particular from that of pure fresh water. The first crystals to ap- 

 pear in salt water are comparatively fresh because the pure H2O 

 molecules tend to separate and to congeal first. As freezing temper- ^ 

 atures continue, the process spreads to the unfrozen liquid lying 

 between the strings and plates of the initial ice crystals. This brine, 

 which has now become more or less imj^risoned, tends to sink by 

 virtue of its greater specific gravity, causing a sagging of the entire 

 pulpy mass, and much of the brine in this manner actually drains out. 

 Gradually, as the fabric attains a firmer structure, the remaining 

 salty liquid is completely entrapped and so prevented from escaping 

 out of the ice body. Therefore, we have finally a solid mass con- 

 sisting of innumerable pure ice crystals frozen together, between 



* Fresh-water ice formed In rivers and later discliarged downstream into the sea exhibits 

 a maximum field of dispersal in the Sil)erian oiling of the Arctic. Even liere. ho\vi'Vt'r. 

 river ice is of such comparatively limited extent, compared to tlie niauiiitiule of toe 

 regions under discussion, that no discussion of it is needed. 



