272 HEAT. 



them. Lead pipes filled with ice expand ; but if it is 

 often repeated, they are cracked into fissures. A strong 

 brass globe, the cavity of which was only one inch in di- 

 ameter, was used by the Florentine academicians for the 

 purpose of trying the expansive force of freezing water, 

 by which it was burst, although the force required was 

 calculated to be equal to fourteen tons. Experiments 

 were tried at Quebec, in one of which an iron plug, nearly 

 three pounds in weiglit, was thrown from a bomb-shell to 

 the distance of 415 feet ; and in another, the shell was 

 burst by the freezing of the water which it contained. 



This expansion has a most important influence in the 

 pulverization of soils. The water which exists through 

 all their minute portions, by conversion to frost, crowds 

 the particles asunder, and when thawing takes place, the 

 whole mass is more completely mellowed than could pos- 

 sibly be effected by the most perfect instrument. This 

 mellowing is, however, of only short duration, if the ground 

 has not been well drained to prevent its becoming again 

 packed hard by soaking with water. 



But this is not the most important result from the ex- 

 pansion of water. Much of the existing order of nature 

 and of civilized life depends upon, this property ; without 

 it the great mass of our lakes and rivers would become 

 converted into solid ice ; for, as soon as the surface became 

 covered, it would sink to the bottom, beyond the reach of 

 the summer's sun, and successive portions being thus add- 

 ed, the great body of all large rivers and lakes would 

 become permanently frozen. But instead of this disas- 

 trous consequence, the ice, by resting upon the surface, 

 Torms an effectual screen from the cold winds to the wa- 

 ter below. 



LATENT HEAT. 



If a vessel of snow, which has been cooled down to 

 several degrees below freezing by exposure to the severs 



