136 ELEMENTARY SCIENCE 



Freezing. Let us take first the case of the freezing of 

 ice-cream by means of surrounding it with a mixture of 

 ice and salt. This seems a simple process, yet when we 

 stop to analyze what happens and why it happens, we find 

 that it is by no means as simple as it seems. We also find 

 that an understanding of this process helps us to under- 

 stand many other phenomena of nature. 



As we turn the handle of the freezer we note that the 

 mixture of salt and ice begins to melt. It melts more 

 rapidly than it would if we did not stir it somewhat by 

 our turning. Evidently, the stirring stimulates the action. 

 Similarly, we saw that stirring stimulates the absorption 

 of air by water. Now if, after some turning, we place our 

 hand on the metal cylinder which contains the cream, we 

 find that it is very cold, colder even than ice. Why is 

 this and how has it happened ? 



What has happened is that heat (a form of energy) has 

 been withdrawn from the contents of the freezer. Now 

 heat is a result of the motion of molecules. Therefore, 

 reduction of the heat of a substance means reduction of 

 the motion of its molecules. You have noted in the case 

 of water that increase of heat, if we start with ice, involves 

 the change of water from a solid to a liquid and finally to 

 a gas; in other words, change of the rate of molecular 

 motions in a substance involves, if carried far enough, 

 changes in the state of that substance. Now if we reverse 

 this process, subtracting heat instead of adding it, we 

 may rightly expect the changes in the state of a sub- 

 stance (liquid, solid, and gas) to occur in reverse order. 

 What has actually happened in the case of the cream in 

 the cylinder of the freezer? We started with it as a liquid; 

 we note that now it is being gradually transformed to a 



