COSMIC PHILOSOPHY 



the difference of their temperatures — we imply 

 that action and reaction are always equal be- 

 tween the systems of molecules which compose 

 the two bodies. And the equality of action and 

 reaction between systems of atoms is taken for 

 granted in every proposition of chemistry ; as, 

 for instance, when we say that it will take four 

 molecules of any monatomic substance, like 

 hydrogen, to saturate a single molecule of any 

 tetratomic substance, like carbon. Now to as- 

 sert the equality of action and reaction, whether 

 between masses, molecules, or atoms, is to as- 

 sert that force is persistent. " The allegation 

 really amounts to this, that there cannot be an 

 isolated force, beginning and ending in nothing ; 

 but that any force manifested, implies an equal 

 antecedent force from which it is derived, and 

 against which it is a reaction. Further, that the 

 force so originating cannot disappear without 

 result ; but must expend itself in some other 

 manifestation of force, which, in being produced, 

 becomes its reaction ; and so on continually." * 

 Clearly, therefore, the assertion that force is per- 

 sistent is the fundamental axiom of physics : it 

 is the deepest truth which analytic science can 

 disclose. 



But now what warrant have we for this fun- 

 damental axiom ? How do we know that force 

 is persistent ? If force is not persistent, if a sin- 

 1 Spencer, First Principles, p. 1 88. 

 144 



