44 OTHER FACTS ABOUT HEAT 



are mixed they melt. The heat necessary for this comes in 

 part from the air and in part from the water in the test tube, 

 and the water in the tube becomes in consequence cold enough 

 to freeze. But the salt mixture does not freeze because its 

 freezing point is far below that of pure water. The use of 

 salt and ice in ice-cream freezers is a practical application of 

 this principle. The heat necessary for melting the mixture 

 of salt and ice is taken from the cream which thus becomes 

 cold enough to freeze. 



