148 



Introduction to the Study of Science 



air within its compartments. The air cooled by contact with 

 the heat-absorbing ice must be allowed to fall into the food 

 compartments, where it forces the warm air away from the food 

 and upward into contact with the ice. Third, it should pro- 

 vide for the melted ice or waste water to 

 circulate over the interior surface of the 

 refrigerator, where it absorbs still more 

 heat from the air and food. Melted ice 

 has a temperature of 32 F. ; and if it is 

 drained from the refrigerator with a tem- 

 perature of 35 to 40 F., we know that it 

 is reducing the temperature of the air. 

 Finally, the efficiency of a refrigerator de- 

 pends upon the amount of heat that it 

 requires to convert ice from a solid into a 

 FIG. 40. Diagram of liquid. This point needs consideration, 

 since it is fundamental in all refrigeration, 

 compartments and cir- The freezing of water. Water freezes 



culation of the air. Qr congeals on l y after ft hag given up ft 



large amount of heat. Its temperature must reach the freez- 

 ing point, and then continued loss of heat causes it to freeze 

 without the sensible lowering of the temperature. If a pool of 

 water is perfectly still, its temperature may fall below 32 F., 

 but with the slightest agitation it begins to congeal and the 

 temperature jumps up to the freezing point. The amount 

 of heat that water loses in changing from liquid to solid form 

 without sensible reduction of temperature, is exactly what it 

 takes up in changing again from solid to liquid. 



Exercise : Heat used in melting ice. Into two one-quart tin 

 cans put equal weights of ice and of water at 32 F., and place these 

 in a larger vessel of boiling water. Stir contents of both cans con- 

 tinually with different thermometers. Remove the cans from hot 

 water as soon as that containing ice shows its temperature rising above 

 the freezing point. You find that melting the ice takes as much heat 

 as is required to raise the temperature of an equal weight of water 

 from 32 F. to about 150 F. or 160 F. 



