14 Prof. Oliver Lodge on 



the body in which it was generated, it still exists in other 

 bodies in undiminished quantity, and so is capable of trans- 

 ference and retransference, by means of activity, and activity 

 alone, for ever. 



But how is such a wide-sweeping statement to be justified ? 

 Not by any hasty appeals to experience, not even by the 

 laborious researches of a Joule. Such experiments can indeed 

 examine the obviously weak places and can show that there 

 is no manifest flaw there, that all the apparent flaws dis- 

 appear on closer inspection, and, so far as can be seen, do 

 not exist. The law itself as a universal generalization must 

 be itself axiomatic, or else must be deduced from some other 

 and simpler axioms. 



I hold that it can be deduced from Newton's third law and 

 from the denial of action at a distance, thus : — Bodies can 

 only act on one another while in contact, hence if they move 

 they must move over the same distance * ; but their action con- 

 sists of a pair of equal opposite forces ; therefore the works they 

 do, or their activities, are equal and opposite ; therefore, by 

 definition (p. 13) , whatever energy the one loses the other gains. 

 In other words : in all cases of activity, energy is simply trans- 

 ferred from one body to another, without alteration in quantity. 



I claim also that the law of conservation thus established is 

 more precise than the ordinary law (which I confess always 

 seems to me rather vague, especially when such absurdities 

 as "possible" and "actual'' energy are put into its state- 

 ment, — a denier of the law could use no more deadly word 

 than " possible ") ; more precise and definite, I say, because it 

 is the law not only of conservation but of identity. I believe 

 that questions arising from this law of the identity of energy — 

 a study of the paths by which any given now-existing bit of 

 energy has reached its present locality, and of all that has 

 happened to it in the past — may prove in the future to be as 

 fruitful a region of enquiry as are studies in the history of any 

 given piece of matter, say the earth or the sun, or, to step 

 forward a little, the history of an individual mind, when we 

 realise some day that this, too, has a continuous, and perhaps 

 traceable, existence. 



However, Prof. MacGregor objects (p. 135) that my deduc- 

 tion of the conservation of energy proves it during transfer 



* Motion in the line of stress is here directly contemplated. As has 

 been already several times noticed, the possible dipping of bodies in con- 

 tact is not apparently allowed for, but in so far as they are smooth their 

 slip is ineffective, and if they are rough there is a tang-eutial stress to be 

 taken into account as well as a normal pressure; the friction-stress is 

 intermittent instead of steady, and molecular instead of molar, but other- 

 wise the above statement applies without complication, so far as it 

 ever applies to the immediate action of ordinary material bodies. 



