ICE, WATER, AND STEAM 117 



surrounding air or other objects. If salt and ice are well 

 mixed, the ice will be melted rapidly by the salt, but the tem- 

 perature of the mixture will not remain at C. It will fall 

 to 17 C. or even slightly lower. The heat needed is taken 

 from the mixture itself, and the presence of the salt prevents 

 the water from freezing at the lower temperature, as it other- 

 wise would do. Such a freezing mixture of salt and ice is 

 often used to produce a temperature slightly below C., as in 

 freezing ice cream. 



There are a number of substances other than salt which 

 will produce the same result, and some of them will produce 

 even a lower temperature, but they are not commonly used, 

 because they cost much more than salt. 



