i6 4 



ELEMENTARY SCIENCE 



Hence, liquids placed in a thermos bottle change their 

 temperature only very slowly. If hot, their heat has no 

 easy path of escape save through the cork, which is itself 

 a poor conductor. If cold, the heat outside has similar 

 difficulty in entrance, and so the contained liquid stays cold. 

 Similarly, the fireless cooker depends upon the very slow 

 conduction of heat by the materials which surround the 

 central receptacle. Hence, well-heated food placed within 

 it continues to "cook," even in the absence of 

 fire. In neither case, however, is heat actually 

 compelled "to stay in the same place." The 

 rate of its migration is, of course, greatly re- 

 duced, but the motion continues, even though 

 largely confined. In fact, the "whole purpose 

 of a thermos bottle is not to stop the motion 

 of heat, but to preserve it, and it does this by 

 confining it as much as possible to a limited 

 space, thus preventing it from dissipating its 

 energy." 



* The S the7 Evidently, however, there can be no such 

 TOtke 0t the confinement of the sun's heat in the air or in 

 vacuum ne so ii or m the wa ter or in whatever it is 

 that absorbs it. After it has been first ab- 

 sorbed by air, or rock, or water, its journey here on earth 

 has but just begun. To be, it must keep moving, and so 

 it does keep moving about the earth, from one substance 

 to another and back again, and in its movements it has 

 profound effects upon all the life that inhabits the earth. 

 It can be transformed, and it can pass by radiation out 

 into space again, but it cannot be lost or destroyed or used 

 up. It is energy, and you have learned that energy is 

 indestructible. 



