LAWS OF HEAT. WATEIt. 83 



water at the top, Rnmford made the upper fluid 

 boil without thawing the congealed cake below. 



Now, a law of water with respect to heat 

 operating in this manner, would have been very 

 inconvenient if it had prevailed in our lakes and 

 seas. They would all have had a bed of ice, 

 increasing with every occasion, till the whole was 

 frozen. We could have no bodies of water, 

 except such pools on the surfaces of these icy 

 reservoirs as the summer sun could thaw, to be 

 again frozen to the bottom with the first frosty, 

 night. The law of the regular contraction of 

 water by cold till it became ice, would, therefore, 

 be destructive of all the utility of our seas and 

 lakes. How is this inconvenience obviated ? 



It is obviated by a modification of the law 

 which takes place when the temperature ap- 

 proaches this limit. Water contracts by the 

 increase of cold, till we come near the freezing 

 temperature; but then, by a further increase of 

 cold, it contracts no more, but expands till the 

 point at which it becomes ice. It contracts in 

 cooling down to 40 degrees of Fahrenheit's ther- 

 mometer; in cooling further it expands, and 

 when cooled to 32 degrees, it freezes. Hence 

 the greatest density of the fluid is at 40 degrees, 

 and water of this temperature, or near it, will lie 

 at the bottom with cooler water or with ice float- 

 ing above it. However much the surface be 

 cooled, water colder than 40 cannot descend to 



