3 1 8 Of the Propagation of Heat 



this I answer, that there are two causes which conspire to 

 prevent the immediate formation of ice at the surface of 

 the water : First, the specific gravity of the particle of 

 water at the surface being increased at the same moment 

 when it parts with Heat, it begins to descend as soon as 

 it begins to be cooled, and before the air has had time to 

 rob it of all its Heat, it escapes and gets out of its 

 reach; and, secondly, air being a bad conductor of Heat, 

 it cannot receive and transmit or transport it with suffi- 

 cient celerity to cool the surface of water so suddenly as 

 to embarrass the motions of the particles of that liquid 

 in the operation of giving it off. 



But to return to our lake. As soon as the water in 

 cooling has arrived at the temperature of about 40, as 

 at that temperature it ceases to be farther condensed, its 

 internal motion ceases, and those of its particles which 

 happen to be at its surface remain there; and, after being 

 cooled down to the freezing point, they give off their la- 

 tent Heat, and ice begins to be formed. 



As soon as the surface of the water is covered with ice, 

 the communication of Heat from the water to the at- 

 mosphere is rendered extremely slow and difficult ; for 

 ice being a bad conductor of Heat forms a very warm cov- 

 ering to the water, and moreover it prevents the water 

 from being agitated by the wind. Farther, as the tem- 

 perature of the ice at its lower surface is always very 

 nearly the same as that of the particles of liquid water 

 with which it is in contact (the warmer particles of this 

 Fluid, in consequence of their greater specific gravity, 

 taking their places below), the communication of Heat 

 between the water and the ice is necessarily very slow on 

 that account. 



As soon as the upper surface of the ice is covered 



