FLUIDITY. 41 



properly. If we attend,, for instance, to the manner in which 

 ice and snow melt, when exposed to the air of a warm room, 

 we can perceive, that however cold they may be at first, they 

 are soon heated up to their melting point, and begin at their 

 surface to be changed into water. Now if the complete change 

 of these bodies into water required only the farther addition of 

 a very small quantity of heat, a mass of them, though of consi- 

 derable size, ought all to be melted in a few minutes or se- 

 conds more, the heat continuing to be communicated from the 

 air around. But masses of ice and snow are well known to melt 

 with extreme slowness, especially if they be of a large size, as 

 are those collections of ice and wreaths of snow, that are formed 

 in some places during winter. These, after they begin to melt, 

 often require many weeks of warm weather, before they are 

 totally dissolved into water. The slow manner in which ice 

 melts in ice houses is also familiarly known to all. 



By examining what happens in these cases, it may easily be 

 perceived that a very great quantity of heat must enter the 

 melting ice, to form the water into which it is changed, and that 

 the length of time necessary for the collection of so much heat 

 from surrounding bodies, is the reason of the slowness with 

 which the ice is liquefied. When melting ice is suspended in 

 warm air, the entrance of heat into it is made sensible by a 

 stream of cold air descending constantly from the ice, which may 

 be perceived by the hand. It is, therefore, evident that the 

 melting ice receives heat very fast, but the only effect of this 

 heat is to change it into water, which is not in the least sensibly 

 warmer than the ice was before. A thermometer applied to the 

 drops or small streams of water as they come immediately from 

 the melting ice, will point to the same degree as when applied 

 to the ice itself. A great quantity of the heat, therefore, which 

 enters into the melting ice, has no other effect than that of 

 giving it fluidity. The heat appears to be absorbed or concealed 

 within the water, and cannot be detected by the thermometer. 



When ice is melted by means of warm water, this absorption 

 of heat is made exceedingly obvious. Thus on mixing a pound 

 of water at 1J2 with a pound of snow at 32, the snow is all 

 melted, and the mixture is two pounds of water of the tempera- 

 ture of 32V In being cooled down from '172 to 32, the hot 

 .water loses 140 degrees of heat, which convert the snow into 



