POLAR-ICE. — QUALITIES. 233 



knife, and polished them merely by the warmth 

 of the hand, supporting them during the opera- 

 tion in a woollen glove. I once procured a piece 

 of the purest ice, so large, that a lens of sixteen 

 inches diameter was obtained out of it ; unfortu- 

 nately, however, the sun became obscured before it 

 was completed, and never made its appearance again 

 for a fortnight, during which time, the air being 

 mild, the lens was spoiled. 



All young ice, such as bay-ice and light ice, 

 which fonn a considerable part of drift and packed 

 ice in general, is considered by Greenland sailors as 

 salt-water ice ; while fields, floes, bergs, and heavy 

 ice chiefly consist of fresh-water ice. Brash-ice 

 likewise affords fine specimens of the latter, which 

 when taken out of the sea, are always found crowd- 

 ed on the surface with sharp points and conchoidal 

 excavations. 



The most porous and opaque ice, and the most so- 

 lid and transparent, do not differ materially in their 

 density ; the highest specific gravity I have obser- 

 ved, (compared with fresh-water at a freezing tem- 

 perature) being 0.925, and the lowest 0.915. And it 

 is a little curious, that in several careful experiments 

 for ascertaining the specific gravity of ice, recently 

 made, the most transparent specimens have proved 

 the lightest, and the most opaque the heaviest*. The 



* I have made several experiments on the buoyancy of ice, 

 by cutting it into cubical or parallelopipedonal blocks, and 



