AND ITS SELF-CONSERVATION. 197 



be regarded as either action or as reaction. And this is 

 in effect to regard each phase as both action and reac- 

 tion. So that the theory of the correlation of forces 

 seems already announced in germ in this law. Given cer- 

 tain conditions,, a definite quantity of heat will in its con- 

 sumption develop a corresponding definite quantity of 

 electricity. But just as well the case may be reversed and 

 a given definite quantity of electricity may be expended 

 and have for its product a corresponding definite quan- 

 tity of heat. And yet, in either case, the force expended 

 cannot even be conceived as existing otherwise than 

 as in action, and hence as meeting an equal quantity 

 of force in reaction. And in these cases the force in 

 reaction must evidently be molecular or atomic attrac- 

 tion. The percussion of world colliding with world must 

 develop sufficient heat to reduce both to the nebu- 

 lous state. And that compression is productive of heat 

 is sufficiently known from familiar facts, and is strikingly 

 illustrated from the estimated quantity of heat devel- 

 oped by the strain toward the center in case of all large 

 masses the heat of the sun being, as Helmholz suggests, 

 kept up by this means. 



In short this interrelation, let us repeat, is manifest 

 everywhere in nature, and the whole physical universe is 

 but its perpetual realization and illustration. 



But, allowing this to be the case, there appears 

 one grave difficulty, as we are assured by those who 

 insist that " science" must above all things be "exact." 

 And the grave difficulty is this: The energy of the 

 system may "run down" to a dead level, so that all 

 motion, all development, all change, all life, must for- 

 ever cease. 



