3 1 6 Of the P ropagatioii of Heat 



preservation of vegetable and animal life; and this could 

 only be done by contriving matters so as to prevent 

 this water from parting with its Heat to the cold 

 atmosphere. 



It has been shown, I believe I may venture to say 

 proved, in the most satisfactory manner, that liquids 

 part with their Heat only in consequence of their in- 

 ternal motions; and that the more rapid these motions 

 are, the more rapid is the communication of the Heat ; 

 that these motions are produced by the change in the 

 specific gravity of the liquid, occasioned by the change 

 of temperature ; and of course that they are more rapid, 

 as the specific gravity of the liquid is the more changed 

 by any given change of temperature. 



But it has been shown that the change in the specific 

 gravity of water is extremely small, which takes place in 

 any given change of temperature^ below the mean temper- 

 ature of the atmosphere^ and particularly when the tem- 

 perature of the water is very near the freezing point ; and 

 hence it follows that water must give off its Heat very 

 slowly when it is near freezing. 



But this is not all. There is a still more extraordi- 

 nary, and in its consequences more wonderful, circum- 

 stance which remains to be noticed. When water is 

 cooled to within eight or nine degrees of the freezing 

 point, it not only ceases to be farther condensed, but is 

 actually expanded by farther diminutions of its Heat ; 

 and this expansion goes on as the Heat is diminished, as 

 long as the water can be kept fluid ; and when it is 

 changed to ice it expands even still more, and the ice 

 floats on the surface of the uncongealed part of the 

 Fluid. 



Let us see how very powerfully this wonderful contriv- 



