THE FIRELESS COOKER AND THE REFRIGERATOR 31 



FIG. 19. A refrigerator. 



The refrigerator (Fig. 19), a much 

 older invention than the fireless cooker, 

 is likewise based upon the principle that 

 certain substances are poor conductors. 

 In the fireless cooker, we wish to prevent 

 the escape of heat from the vessel ; but 

 in the refrigerator we wish to prevent 

 its entrance. The power of a non- 

 conductor to keep out heat is tested 

 in a simple way. Put some ice-cold 

 water in a covered vessel, and an equal 

 quantity in an uncovered vessel. By 

 means of a thermometer notice the 



rapidity with which the water warms in the uncovered vessel^ 

 and the slowness with which the water warms in the covered 

 vessel. The heat in the air cannot pass readily through the 

 felt, and hence the water in the covered bottle is warmed much 

 more slowly than the water in the uncovered bottle. 



Wood is a poor conductor of heat, and hence refrigerators 



are usually made of thick wood ; in 

 addition, the space between the 

 walls is often filled with a non-con- 

 H ductor, such as mineral wool or saw- 

 dust. A refrigerator in order sat- 

 isfactorily to chill the food within it 

 must have its ice section placed at 

 the top. The air surrounding the 

 ice is chilled and falls; the less cool 

 air is pushed up and takes its place, 

 but becoming rapidly chilled, it in 

 turn falls (Fig. 20). In this way a 



F,c. 20 .- Circulation of air in """"ant circulation of cold air is 

 a refrigerator. maintained within the refrigerator 



