22 Prof. Oliver Lodge on 



of particles, all forming part of what we usually speak of as 

 a single body*. 



To take one of the examples of Prof. MacGregor, viz. a 

 loaded air-gun with its muzzle plugged, so that its bullet or 

 wad is not allowed to be shot. The compressed air has 

 potential energy ; on its release its energy is transferred to 

 the moving wad, which instantaneously hands it on to the air 

 near the muzzle, compressing it, and thus retransforming 

 itself into the potential form. 



If it be objected that the energy never at any instant all 

 existed in the kinetic form, the answer is no, but it neces- 

 sarily passed through that form; it could only be by the 

 intervention of motion that the energy of the air at one end 

 was transferred to the air at the other end. The wad itself is 

 unnecessary and may be dematerialized; the only thing moving 

 may be the gas atoms themselves, but the atoms which are 

 rushing are not the same as those under strain ; the moving 

 atoms are precisely those which have escaped from strain. As 

 everybody knows, where the rush of gas is greatest in a con- 

 stricted pipe there the pressure is least. Air rushing from 

 one reservoir into another illustrates the law precisely, though 

 I choose it as a case at first sight favourable to Prof. 

 MacGregor's contention ; in the narrow part of the pipe air is 

 flowing against a higher pressure in front of it, it is com- 

 pressing the air ahead of it by reason of its own momentum ; 

 the energy of each molecule is availably kinetic to precisely 

 the extent to which it ceases to be potential, and any residual 

 potential energy which does not become momentarily kinetic, 

 and kinetic in the available sense of a windy rush, has not 

 been transferred at all, but remains simply stored, as it would 

 have been had nothing been released and no activity been 

 manifested. 



If the two reservoirs (or two Leyden jars) are for instance 

 equal in capacity, and if one had been originally empty, 

 then one quarter the original energy remains still potential 

 in the originally full vessel, another quarter has been trans- 

 ferred by the rush of gas into the originally empty vessel, and 

 two quarters have been dissipated, so to speak, converted into 

 heat by friction. If there is no friction, then this half of the 

 original energy remains alternating for ever from the kinetic 

 to the potential form, like the bob of a pendulum, and transfer- 

 ring itself at every half swing from the one vessel to the other. 



Lest this last statement should permit misconception, it 

 may be necessary to state explicitly that the energy is only 

 hazily attributed to the vessel — the energy belongs of course 



* This sentence is true in one sense, but it is not a final or complete 

 statement, and in a later paper a modification will be introduced. 



