FREEZING OF LAKES. 145 



the Northern Ocean, during the summer months, 

 many lakes whose bottoms were frozen, and which 

 therefore formed so many basins of ice filled with 

 water, as is so frequently observed on the glaciers. 

 These were quite devoid of fish, while other lakes 

 in their immediate neighbourhood, which were 

 deeper, were well stocked with them. In the 

 latter, the bottom cannot be cooled below 0°, 

 because in that case the whole mass of water would 

 of necessity become solidified by degrees, which 

 would kill the fish. Just so we find that the great 

 deep lakes in the high latitudes of North America 

 are full of fish.* 



* The possibility of the existence of great masses of 

 water, which, resting in basins of ground frozen to more 

 than a hundred feet of depth, and covered during the 

 severest cold of winter with a coating of ice of at most ten 

 or tv elve feet in thickness, yet contain liquid water of and 

 above the temperature of 0°, depends upon the remarkable 

 property possessed by water, of attaining its greatest 

 density at the temperature of 4° C. (39°2 Fahr.), that is, 

 of expanding, as well by being cooled below as by being 

 heated above this temperature. The result of this pro- 

 perty is, that, during the summer months, the water at the 

 surface of one of these basins sinks as soon as it is warmed 

 up to 4°, until the whole mass of water has acquired this 

 temperature, and that it cannot then again be brought 

 up to the top by any subsequent change of temperature at 

 the surface. [While 



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