170 



REFRIGERATION AND ITS' USES 



involves some of the principles just described. Finely broken 

 or crushed ice is mixed with common salt and the mixture is 

 then packed in the freezer around the can containing the 

 cream to be frozen. As we saw in Art. 196, such a mixture of 

 ice and salt has a melting point far below the melting point 

 of pure ice. Or, we may express the same truth in another 

 way: We may say that the freezing point of a saturated solu- 

 tion of salt and water is 7F., or 39 below the freezing 



point of pure water (Art. 

 196). If a mixture of one- 

 third salt and two-thirds 

 crushed ice is used in the 

 freezer, the temperature of 

 the mixture will tend to 

 fall to this point as the ice 

 melts. 



It is evident that as the 

 first portions of the ice 

 melt, the water formed will 

 dissolve some of the salt, 

 forming a saturated solu- 

 tion of salt water. We 

 also know that as the ice 



continues to melt it absorbs large quantities of heat. Most 

 of the heat absorbed by the ice, when melting, must be taken 

 from the freezer and from the can of cream. Both will, there- 

 fore, fall in temperature. Since the cream freezes at a tem- 

 perature much higher than the freezing point of the mixture, 

 it is evident that if a sufficient quantity of salt and ice is 

 used the cream can be completely frozen (Fig. 126). 



The only puzzling question in the process of freezing cream 

 is this: Why does the ice continue to melt when the tem- 

 perature falls below 32F., the ordinary melting point of ice? 

 The answer is: A mixture of salt and ice can exist as salt 

 and ice only at a temperature of 7F., or lower. If salt is 

 placed upon ice at a higher temperature, the ice melts and the 



FIG. 126. An ice cream freezer. 



