HEAT AND THE ATMOSPHERE 163 



low them. How can we explain this? This, like most 

 other phenomena of nature, cannot be fully explained 

 by just one thing. A number of things must be taken into 

 consideration. We witness the effect, but to explain it 

 fully, we must find a number of causes or "factors" in 

 the problem. One of the causes of the coldness of moun- 

 tain tops is the "thinness" of the surrounding air; its 

 capacity to absorb and retain heat is much less than that 

 of the denser air which lies below. Other "contributing" 

 causes will occur to you as we study our larger problem. 

 Similarly, the polar regions, even at sea-level, do not get 

 warm, although at certain seasons they received great 

 amounts of heat. Here the principal cause of coldness is 

 evidently different from that in the case of mountain tops; 

 it is that the heat received is so largely absorbed in the 

 process of melting ice and warming very cold water, that 

 the effects upon the temperature of the atmosphere are 

 relatively slight. 



Already you have learned that it is absolutely contrary 

 to the nature of heat to stay in the same place. You 

 learned this when you learned that heat is motion. Obvi- 

 ously, we cannot conceive of motion being at rest. To 

 be heat, it must continue to be motion; the motion of 

 molecules. 



But, you may say, what of a thermos bottle or a fire- 

 less cooker? Are not these devices by means of which we 

 confine heat and keep it in one place? Stop to consider. 

 A thermos bottle consists of an inner receptacle surrounded 

 (except for the opening) by a hollow metallic case. From 

 the hollowness of this metallic case, before it was sealed, 

 the air was withdrawn, thus producing a vacuum (see Fig. 

 59). Now a vacuum is a "non-conductor" of heat. 



