Modes of Physical Force. 1 07 



action even still more prominently, for, as Sir Wm. 

 Thomson points out, this fact is readily accounted for 

 by supposing that the atmosphere is a dialectric across 

 which the earth and sun and moon act and react upon 

 each other inductively, and- that the necessary conse- 

 quence of this reaction is to charge the two surfaces of the 

 atmosphere to a certain depth with opposite electricity. 

 The facts, without question, lend themselves readily to 

 this view: and there is nothing at all forced in the view 

 itself. It is difficult also to avoid the conclusion that 

 what appears as atmospheric electricity at one time 

 appears as atmospheric heat at another, and vice versa, 

 the heat and the electricity being, in fact, inseparably 

 correlated. For, to mention only one among many 

 other reasons for so thinking a reason, too, to which 

 attention has not yet been directed sufficiently it is 

 surely a significant fact that the electricity and heat of 

 the air are inversely related to each other in the 

 summer and winter months, the potential being lower 

 and the temperature higher in the summer months, the 

 temperature being lower and the potential higher in the 

 winter months. And if these be the relations between 

 electricity and heat it is not very likely that the relations 

 between electricity and light will be different : for heat 

 and light are so closely related as to make it all but 

 certain that what may be said of the one may be said of 

 the other also. Nor is it a matter of indifference whether 

 it be so or not, for if it be so it is necessary to take a 

 very different view of light and heat to that usually 

 taken, and to regard them as originating, not in the sun 

 solely, but in an action in which the earth also is con- 

 cerned an action, it may be, of the nature of induction. 

 It is necessary, that is to say, to assimilate natural light 

 and heat to electric light and heat. There is, as it 



