454 Mr. J. Croll on certain Hypothetical Elements 



vity. No one would for a moment think of denying the correct- 

 ness of this mode of reasoning in regard to heat, magnetism, 

 electricity, or any other form of force. Why, then, make an ex- 

 ception in the case of gravity ? But more than this, the very 

 idea of an exception is in itself absurd ; it is nothing less than 

 to make an exception in regard to a principle which we admit 

 holds universally true. 



To reconcile the common conceptions of gravity with that of 

 conservation, it has been said that the potential energy of gra- 

 vity does not simply consist in the tendency which bodies have 

 of approaching to each other, but consists also in the distance 

 through which that tendency is capable of continuing to act. 

 For instance, when two bodies approach each other under the 

 mutual influence of their gravity to one-half their former dis- 

 tance, their potential energies are diminished also to one-half 

 (although their tendency to approach is not diminished), because 

 the distance through which that tendency is now capable of act- 

 ing is but one-half of what it was formerly. 



The energy of a raised weight, for example, it is said is the 

 product of the gravitation-pull upon it, and the distance through 

 which this pull can act. 



f Force, or pull of gravitation 

 Energy, or work performed! _ J upon the weight x distance 



by descending weight J " passed over by descending 



[_ weight. 

 Or, 



Energy = a force x a length. 



It is certainly true that the amount of energy or work per- 

 formed is proportionate to the pull of gravitation x the distance 

 through which the weight descends. But I am unable to per- 

 ceive how this can meet Faraday's objection ; for it seems per- 

 fectly evident that the mere change of relative position cannot 

 constitute any form of force. Distance is a necessary condition 

 to the transformation of the potential energy of gravity into the 

 actual energy of vis viva, or of heat, &c. ; but distance itself does 

 not in any degree constitute this transformation. When bodies 

 arrive at contact, there can be then no further transformation of 

 potential energy into kinetic ; not because the potential energy 

 has been all consumed, but because the bodies are not in a con- 

 dition to allow of any further transference. The tendency to ap- 

 proach, though increased to infinity, would not help in the least 

 degree to produce any further transformation of potential energy 

 into kinetic; for the thing wanted is not more potential force, 

 but the necessary condition to transference. That which causes 

 bodies mutually to approach with velocity and thus produce 



