70 Chemistry. [Lecture 26. 



fluid. In proof I shall instance the slowness 

 with which ice and snow melt. When a thaw 

 comes on when the heat is far above the degree 

 of frost, though the ice is constantly surrounded 

 by air warmer than itself, and constantly receiv- 

 ing heat from it, yet it will be hours, sometimes 

 days, in dissolving. If nothing was necessary 

 to produce fluidity but mere sensible heat, we 

 might expect, that after it begins to melt a short 

 time would be sufficient to melt the whole; but 

 since it is so long in dissolving, and its heat is 

 not increased above the freezing point, nor the 

 water that runs from it above thirty-two or 

 thirty-three degrees, this obvious heat, or, in 

 more philosophical terms, this excess of caloric, 

 to which it is exposed must be absorbed, and 

 become latent. It is owing to this that ice 

 can be preserved in ice-houses, and that large 

 masses of ice and snow remain at the tops of 

 mountains whose heat is considerably above the 

 freezing point. 



If, on the other hand, we expose water to 

 freeze, and put a thermometer into it, at first 

 suppose it is twenty degrees warmer than the 

 cold air, it will lose a great many degrees during 

 the first five minutes, less the next, and so on ; 

 in half an hour (if the temperature of the air is 

 below frost) it will have arrived at that tempera- 

 ture, and we should expect that in two or three 

 minutes all of it would be frozen, if it depended 

 only vipon a diminution of sensible heat ; but 

 this is not the case, for we find at first a small 



