NATURAL HISTORY. 



5 S3 



considerable proportion^of packed 

 and drift ice in general ; where it 

 occurs in flat pieces commonly 

 covered with buovv, of various di- 

 mensions, but seldom exceeding 

 fifty yards in diameter. 



Fresh- Water Ice. 



Fresh-water ice, is distinguish- 

 ed by its black appearance when 

 floating in the sea, and its beau- 

 tiful green hue and transparency 

 when removed into the air. Large 

 pieces may occasionally be obtain- 

 ed, possessing a degree of purity 

 and transparency, equal to that of 

 the finest glass, or most beautiful 

 crystal; but generally, its trans- 

 parency is interrupted by nume- 

 rous small globular or pear-shaped 

 air-bubbles : these frequently form 

 continuous lines intersecting the 

 ice in a direction apparently per- 

 pendicular to its plane of forma- 

 tion. 



Fresh-water ice is fragile, but 

 hard ; the edges of a fractured 

 part are frequently so keen, as 

 to inflict a wound like glass. The 

 homogeneous and most transpa- 

 rent pieces, are capable of concen- 

 trating the rays of the sun, so as 

 to produce a considerable intensity 

 of iieat. With a lump of ice, of 

 by no means regular convexity, I 

 have frequently burnt wood, fired 

 gunpowder, melted lead, and lit 

 the sailors' pipes, to their great 

 astonishment ; all of whom, who 

 could procure the needful articles, 

 eagerly flocked around me, for 

 the satisfaction of smoking a 

 pipe ignited by such citvaordinary 

 means. Their astonishment was 

 increased, on observing, that the 

 ice remained firm and pellucid, 

 whilst the solar rays emerging 



therefrom were so hot, that the 

 hand could not be kept longer in 

 the focus, than for the space of 

 u few seconds. In the formation 

 of these lenses, I roughed them 

 with a small axe, which cut the 

 ice tolerably smooth ; 1 then scrap- 

 ed them with a knife, and polished 

 them merely by the warmth of the 

 hand, supporting them during the 

 operation in a woollen glove. 1 

 once procured a piece of the pvirest 

 ice, so large, that a lens of six- 

 teen inches diameter was obtained 

 out of it ; unfortunately, however, 

 the sun became obscured before it 

 was completed, and never made 

 its appearance again for a fort- 

 night, during which time, the air 

 being mild, the lens was spoiled. 

 The most dense kind of ice, 

 which is perfectly transparent, is 

 about one-tenth specifically lighter 

 than sea- water at a freezing tem- 

 perature. Plunged into pure water, 

 of temperature 32°, the proportion 

 floating above to that below the 

 surface, is as 1 to 15, and placed 

 in boiling fresh water, it barely 

 floats. Its specific gravity is about 



0.937. 



Fields, bergs, and other large 

 masses, chiefly consist of this kind 

 of ice. Brash-ice likewise affords 

 pieces of it, the surfaces of which 

 are always found crowded with 

 conchoidal excavations when taken 

 out of the sea. 



On the Formation of Ice on the Sea. 



Some naturalists have been at 

 considerable pains to endeavour to 

 explain the phenomena of the pro- 

 gressive formation of the ice in 

 high latitudes, and the derivation 

 of the supjdy, Avhich is annually 

 furnished, for replacing the great 



quantities 



