CHAPTER XIX. 



CORRELATION OF FORCES AND CONSERVATION 

 OF ENERGY. 



doctrine of correlation of forces, now so gen- 

 erally known and accepted, will require but brief 

 reference in this place. And so much the less need it be 

 here dwelt upon, as the whole course of our argument 

 thus far has been chiefly a statement of that doctrine in 

 its more universal form. All possible phases of force 

 have been shown to be necessarily interrelated, and even 

 interfused, so that in reality the exclusive occupancy of 

 a given field by one single phase of force would be 

 wholly impossible. Force, to be force at all, must be 

 complex. At every point where force is, there it is as 

 active; and in its activity it must involve strains and 

 counter-strains productive of constant changes in the 

 intensity of molecular energy, which changes become 

 manifest as heat, or chemical action, or electric (or mag- 

 netic) polarization; or rather in all these it becomes 

 manifest simultaneously in varying degree. Not merely 

 percussion, but also compression both due to attrac- 

 tion give rise to heat, which is itself a mode of repul- 

 sion. And in general we find everywhere complete 

 confirmation of the third law of motion that ' ' to every 

 action there is always an equal and contrary reaction." 



But this law, in thus presenting the two-fold nature 

 of force, also declares implicitly that either phase may 



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