LOWERING OF TEMPERATURE BY EVAPORATION. 831 



Such mixtures, one of which, consisting of ice and 

 common salt, has been used in a previous experiment, 

 are usually called ' freezing mixtures/ The fall in 

 temperature is the greater the more intimate the mix- 

 ture is ; for in that case the liquefaction and hence the 

 disappearance of heat take place more rapidly. The 

 ice should therefore be pounded to as small pieces as 

 possible (snow is better than ice), and the mixture con- 

 stantly and very briskly stirred. If a mixture of 

 about 900 gr of finely pounded ice and 300 gr of salt be well 

 stirred, the temperature soon falls to 21, and remains 

 at this point for some time. If some of the mixture be 

 placed in a tumbler, the outside becomes coated with a 

 layer of ice, like that which forms on window paries on 

 a cold day in the winter ; the cold glass condenses 

 upon its sides the vapour of water contained in the 

 air near it, and the water formed by the condensation 

 becomes frozen. 



Considerable quantities of heat become latent during 

 the evaporation of liquids, and if the quantity which 

 disappears in evaporating a liquid is not constantly re- 

 placed from any source of heat, evaporation becomes, 

 like liquefaction, a means of lowering temperature. 



It is from this cause that moist bodies are apt to 

 produce a sensation of cold ; they are constantly eva- 

 porating, and hence their temperature falls below that 

 of surrounding bodies. Liquids, whose boiling points 

 are below that of water (ether, alcohol, disulphide of 

 carbon) and therefore evaporate more rapidly, are 

 capable of producing considerable diminution of tem- 

 perature. If the evaporation of ether be accelerated by 

 passing a rapid current of air through the liquid so as 



