150 Introduction to the Study of Science 



solid substance becomes liquid is known as the melting or fusion 

 point. And the quantity of heat needed to melt or fuse a 

 solid is known as the heat of fusion or sometimes as the latent 

 heat of fusion. 



56. Production and uses of low temperatures. Tempera- 

 tures below that given by melting ice have become necessary 

 for many purposes. Such low temperatures may be obtained 

 by a mixture of ice and common salt (about twenty-three per 

 cent of salt yields the lowest temperature). A still lower tem- 

 perature is secured by a mixture of ice with calcium chlorid in 

 crystals, or with magnesium chlorid. A typical use of such 

 mixtures with low freezing point is the making of ice cream 

 in the ordinary freezer. 



Ice cream freezing. A mixture of salt and cracked ice melts 

 more rapidly, that is, absorbs heat more rapidly and reduces 

 surrounding objects to a lower temperature, than does melting 

 ice alone. The particles of the salt act upon and facilitate the 

 separation of the particles of ice, while the particles of ice act 

 in the same manner and to the same end upon the particles 

 of the salt. All this requires heat energy, which must be sup- 

 plied largely by the surrounding objects. Thus salt and ice 

 change each other to a fluid condition, 

 with a resulting temperature of the mix- 

 ture much below the freezing point. 



The ice cream freezer (Fig. 41) is 

 usually made of material that conducts 

 STIRRING PAOOLE heat very poorly, such as wood or metal 

 with a heavy paper covering. The cream 

 FIG. 41. Diagram of container is made of good heat conducting 

 material so that the heat from the cream 

 mixture may pass freely into the salt and ice. As 'the tem- 

 perature of the salt and ice mixture is many degrees below 

 the freezing point of the cream, the heat continues to pass 

 in the same direction, from the cream to the colder mixture, 

 until the cream is frozen. 



ICE AND SALT 



