Lesson xx.] FROST. 117 



observed, that, in Frost, the greater the cold the 

 greater the evaporation; and this, by the bye, fur- 

 nishes us with areason why there should generally 

 be more rain fall in February than in any other 

 month. 



The manner in which congelation is effected, is 

 now generally thus explained. When water is 

 exposed to <tn atmosphere colder than itself, it 

 parts with some of its free heat, and is reduced in 

 temperature : but no part of it begins to freeze 

 until the mass is reduced somewhat be!ow the 32d 

 degree of Fahrenheit's thermometer * : some of the 

 water then becomes solid, which, by changing its 

 state, sets at liberty a quantity of its combined 

 heat, which restores the fluid in contact with it 

 to the temperature of 32 degrees, or rather above 

 it. The congelation is, therefore, rather at a stand, 

 till this sensible heat is abstracted by the atmos- 

 phere, and the mass reduced somewhat below tire 

 32d degree. Another portion of water then con- 

 geals, and the process is again stopped by the 

 emission of heat. In this manner congelation 

 proceeds, and is performed at intervals which are 

 very observable when the phenomenon of freez- 

 ing is accurately attended to. 



Water which has been boiled freezes more rea- 

 dily than that which has not been boiled : it is also 

 observed, that a slight disturbance of the fiuid dis- 

 poses it to freeze more speedily. Water having 



* For a few remarks relative to this instrument, the Reader 

 is referred to the Appendix. 



it* 



