126 Application of the Kinetic Theory of Gases to Gravitation. 



that this term " potential " energy could still be used in a certain 

 sense, even after the theory of action at a distance had been 

 abandoned. We think it can be clearly shown that this is not 

 legitimate. For, by the rejection of the theory of " action at 

 a distance/' external matter or a medium (in a state of motion) 

 must be concerned in developing motion in matter; and 

 therefore it must be a case of kinetic energy, not " potential " 

 energy. Either (for example) the motion of approach of two 

 masses (or molecules) is developed (as supposed) without the 

 concurrence of external matter, or (secondly) this motion is 

 simply transferred to the masses from external matter. In 

 this latter case (which represents the case where the theory of 

 " action at a distance " is rejected) the energy exchanged can 

 only be the energy of motion (kinetic energy), not, therefore, 

 " potential " energy. It might, perhaps, be urged that even 

 when the theory of" action at a distance " is rejected, a raised 

 mass can still be said to have u potential " energy (due to its 

 position), because it can fall. This, however, may be proved 

 not to be legitimate. For, from the very fact that (by the re- 

 jection of " action at a distance ") the energy expended in 

 raising the mass was converted into kinetic energy, it cannot 

 have been converted into " potential " energy (i. e. an energy 

 which is not kinetic) as well. A double equivalent of energy 

 cannot be generated*. We think we have clearly shown, 

 therefore, that by the rejection of the theory of " action at a 

 distance," the idea of " potential " energy must (to be logically 

 consistent) be unreservedly abandoned. The rejection of 

 "potential" energy makes all energy of one character, viz. 

 energy of motion ; and then the great principle of the indestruc- 

 tibility of motion inevitably presents itself for acceptance. 

 With the theory of " action at a distance," the idea of " force " (in 

 the old sense of an action across space without the intervention 

 of matter) must be given up. Thus we have in the physical 

 world, only the two great fundamental conceptions of matter 

 and motion left ; or all physical phenomena come thus to be 

 correlated in one grand and fundamental aspect, viz. as con- 

 sisting in the various exchanges and phases of motion. 

 London, Jan. 11, 1878. 



Note.— We think it right to add that we make no claim to have 

 shown (as this had been already done by others) that the molecules 



* To say that a raised weight tending to approach the earth by the 

 action of the gravific medium, possessed "potential" energy because it 

 can approach the earth, would be like saying that a ship confined by a 

 cable and tending to approach a rock by the action of the wind, possessed 

 "potential" energy, because it can approach the rock (by the breaking of 

 the cable). The cases are evidently parallel. 



