AEROSTATICS. 235 



328. On the different quantities of heat, there- 

 fore, united to the substance which we call water, 

 depends its existence in the state of a solid, a li- 

 quid, or an elastic fluid. In passing through 

 these states in the order just mentioned, a great 

 increase of temperature is not only necessary, hut 

 a great quantity of heat must be absorbed, or from 

 being sensible, must become latent. 



This is true not only of water, but of mercury, oil, 

 and probably of all elastic fluids. It perhaps holds 

 of all bodies that are melted by heat, and it may be 

 true even of those fluids, which, like air, appear to 

 be permanently elastic. 



329. Since heat is absorbed when a solid passes 

 into a liquid state, whatever produces liquefac- 

 tion produces a diminution of sensible heat ; and 

 this, in the case of what are called Freezing Mix- 

 tures, can be employed to produce a cold of 90. 

 So, also, cold is produced by evaporation, or the 

 conversion of an incompressible into an elastic 

 fluid. 



A very beautiful process, in which the effect of eva- 

 poration to produce cold, is most distinctly shewn, 

 has been discovered by Professor LESLIE. It con- 

 sists in exposing water under a receiver, from 

 which the air is partially exhausted, to the action 

 of an acid which has a strong attraction for humidi- 

 ty. The water is quickly converted into ice. See 



Short 



