FREEZING POINT. 63 



vinegar and the lime which the chalk contains, and a 

 new substance, called a salt, has been formed out of 

 both. So in the present case, we may suppose that 

 heat and the solid have combined to form a liquid, and 

 the property of heat to effect the senses and the ther- 

 mometer, has at the same time disappeared. Any liquid 

 may therefore be regarded as a compound of solid and 

 heat. The heat which thus disappears is called com- 

 bined, or latent heat. 



Mention some 140. FREEZING MIXTURES. - When Solids 



feezing mix- ta k e a liquid f orm by other means, as. for 



tures. How do ** ;. 



they produce example, when salt dissolves in water, the 

 temperature is generally much reduced. 

 Nitre, for example, reduces the temperature of water in 

 which it is dissolved from 15 to 18 degrees, and is there- 

 fore much used in the East, where it is abundant, for 

 cooling wines. Mixed nitre and sal-ammoniac have a 

 still greater effect. Sulphate of soda drenched with 

 strong muriatic acid, will reduce the temperature from 

 50 to zero. 



141. When two solids, on being mixed, 



Mention other . . 



freezing mix- become both liquid, still greater cold is 

 often roduced - Tnis is the case witn a 



duce greater mixture of snow with common salt, or with 

 chloride of calcium. By the former mix- 

 ture, used as shown in paragraph 121, ice cream is 

 frozen. By the latter mixture, a cold sufficient to freeze 

 mercury may readily be produced. For this purpose, 

 three parts of the salt are to be mixed with two of dry 

 snow. 



