POLAR ICE. — PROPERTIES. 237 



appearance of freezing is about the surface and 

 sides, in needles shooting along the surface, or 

 obliquely downward. The ice on the sides then 

 extends lower and lower, leaving a quantity of wa- 

 ter in the middle in the form of a cone, which 

 forms a receptacle for the air disengaged, as the 

 freezing process goes on. Hence the ice on the 

 sides, which is formed when the evolving air has 

 liberty to escape towards the centre, is found purely 

 diaphanous, while the last formed ice, in the shape 

 of a cone, having its base on the bottom of the ves- 

 sel and touching the surface with its apex, becomes 

 the receptacle for the disengaged air, and is neces- 

 sarily nebulous throughout. 



Ice, when rapidly dissolved, continues solid, as 

 long as any remains ; but when exposed to the air 

 at a temperature of only 2 or 3 degrees above 

 the freezing point, its solution is effected in a very 

 peculiar manner. Thus a large lump of fresh-wa- 

 ter ice, when acted on by such a process, if placed 

 in the plane of its formation, resolves itself into 

 considerable columns of a prismatic appearance. 

 These columns are situated in a perpendicular po- 

 sition, and are almost entirely detached, so that 

 when a blow is struck with an axe, the whole mass 

 frequently falls to pieces. In the land ice-bergs, 

 these columns are often of amazing magnitude, so 

 as, when separated, to form floating ice-bergs. 



