138 ELEMENTARY SCIENCE 



this heat. It cannot be lost, and certainly there is no 

 evidence that energy in the form of heat is passing into the 

 air around the freezer. What actually happens to it is 

 that it is absorbed by the liquefying salt and ice. The 

 heat energy which was present in the liquid which you 

 put into the cylinder is now present, in another form, in the 

 liquid which has formed outside the cylinder. 



You started with a liquid inside and with solids packed 

 around it. But you end this process of freezing ice-cream 

 with a solid inside and with a good deal of liquid outside. 

 The heat which was in the liquid cream has now gone into 

 the liquid water and salt, and as a result you have the 

 solid ice-cream, which was what you wanted. 



But what, you may ask, is the need of the salt? Why 

 is a mixture of salt and ice (a freezing-mixture) a better 

 heat extractor than pure ice ? This is because more heat 

 is absorbed in the melting of ice and salt together than is 

 absorbed in the melting of ice alone; there is the heat whiih 

 is absorbed by the melting ice plus the heat absorbed by 

 the melting (or dissolving) salt. 



That salt absorbs heat when it dissolves, you can prove 

 by a very simple experiment. Immerse the bulb of a 

 thermometer in a tumbler of water. When you are sure 

 that the mercury is stationary, add quickly a spoonful of 

 salt. How much does the mercury drop ? Of course the 

 mercury presently goes up again to its original mark be- 

 cause the water absorbs from the air the heat which it 

 gave up to the dissolving salt. But the heat absorbed by 

 the salt is still there, there to remain until the salt solidifies 

 again and gives it up. 



The same principle is involved in the fact that the freez- 

 ing-point of salt water is much lower than the freezing- 



