1857.] On the Conservation of Force. 445 



time, something happens whilst the time is passing which did 

 not happen before, and does not continue after : it is therefore 

 not metaphysical to expect an effect in every case, or to endea- 

 vour to discover its existence and determine its nature. So in 

 regard to the principle of the conservation of force ; I do not 

 think that to admit it, and its consequences, whatever they 

 may be, is to be metaphysical : on the contrary, if that word 

 have any application to physics, then I think that any hypo- 

 thesis, whether of heat, or electricity, or gravitation, or any 

 other form of force, which either willingly or unwillingly dis- 

 penses with the principle of conservation, is more liable to the 

 charge, than those which, by including it, become so far more 

 strict and precise. 



Supposing that the truth of the principle of the conservation 

 of force is assented to, I come to its uses. No hypothesis 

 should be admitted, nor any assertion of a fact credited, that 

 denies the principle. No view should be inconsistent or in- 

 compatible with it. Many of our hypotheses in the present 

 state of science may not comprehend it, and may be unable to 

 suggest its consequences ; but none should oppose or contra- 

 dict it. 



If the principle be admitted, we perceive at once, that a 

 theory or definition, though it may not contradict the principle, 

 cannot be accepted as sufficient or complete unless the former 

 be contained in it ; that however well or perfectly the definition 

 may include and represent the state of things commonly con- 

 sidered under it, that state or result is only partial, and must 

 not be accepted as exhausting the power or being the full 

 equivalent, and therefore cannot be considered as representing 

 its whole nature-, that, indeed, it may express only a very small 

 part of the whole, only a residual phenomenon, and hence give 

 us but little indication of the full natural truth. Allowing the 

 principle its force, we ought, in every hypothesis, either to 

 account for its consequences by saying what the changes are 

 when force of a given kind apparently disappears, as when ice 

 thaws, or else should leave space for the idea of the conversion. 

 If any hypothesis, more or less trustworthy on other accounts, 

 is insufficient in expressing it or incompatible with it, the place 

 of deficiency or opposition should be marked as the most im- 

 portant for examination; for there lies the hope of a discovery 



