440 



NATURE 



\_Sept. 20, 1877 



of actual inrition," as a disiiiigitiihing title, cannot mean 'any- 

 thing ehe than this -that the other E., potential E., is E. of 

 ahoHt-to- supervene mntinn, or that it does not perform work 

 e«ce(it throiiirh the resulting E. of motion. We do not say that 

 these rioctors intended what we have mentioned, bat their words 

 unquestionably go to convey that impression ; and what makes 

 this so particularly mischievous is that poor Publius is already so 

 susceptible to that impression, being prepared for it by the titles 

 "potential" and "actual" E. 



1 shou'd not be at all surprised if some would try to argue 

 that the phrase "potintial E." need not be taken to mean more 

 than sir..pl rhis, viz., that the E. so called e.\ists in possibility 

 nlatwe/v to the body or system that may be in question, that its 

 potentiality merely implies that it is absent from and acquirable 

 by that body or system, and not that it is altogether out of 

 actual existence. 



I. Now even supposing this to be true, though I have never 

 seen any evirlence of it, and even if we should grant this to be a 

 right usage when the body ax system is followed through the 

 history of its changes, it is a wrong usage when, as in a book or 

 cha' ter on E., Energy is the subject, when it is the conserved E. 

 itself which is to be followed through its migrations. Why 

 should this grand conserved E. be stigmatised as merely potential 

 when it di>es not happen to be in a certain mass? Relatively to 

 that mass it may be some times potential, but relatively to itself 

 it IS, as we shall see, alvla^s actual. 



But we cannot concede that the potentiality of this mode of E. 

 implies merely the above. I believe it is usually intended to 

 mean much more ; and, at any rate — whatever iho.se who use the 

 word may intend — it logically involves much more ; and this is 

 what poor PuUius is chiefly concerned with. Now if we con- 

 sider the words of Clerk Maxwell quoted above, we shall see 

 that in the case of a separate unconfected system, such a state- 

 ment coming from him cannot possibly mean that the said E. is 

 in actual existence outside of the system, and is waiting there 

 until the system takes possession of it. If it is not in the system 

 there is nowhere else where it can be ; therefoie it is not in 

 actual physical existence at all, although connected with existence 

 by someincon;eivable paraph\.sical link. The acquiring of it is a 

 kind of creation of it. Curiously enough Stewart and Tait' 

 speak of the " creation " and "annihilation" of both types 

 of E. 



2 Here, then, comes our second complaint. E. is " the power 

 of performing work ;" therefore potential E., which is intended 

 to be the power of acquiring E., is the power of acquiring the 

 power of doing work. E. is already a potentiality ; therefore 

 potential E. is a potentiality which, itself, exists only in poten- 

 tiality. There is here a double remotion from tangibility, which 

 may be gratifying to the metaphysicians, who rejoice the more 

 the harder the nuts you give them to crack ; but poor Publius 

 finds bonnes boticlics of this sort rather trying to his molars. 



3. Potent'al E., in the present sense, being, as we have seen, 

 undeniably out of actual physical existence, poor P. does not 

 feel that he has giined much when he learns that the sum of the 

 actual and potential E.s of the universe is a constant quantity — 

 for this is the form in which the grand principle of the conser- 

 vation of E. is usually, or at least frequently, presented to him 

 by the doctors. A rigid physicist, who himself believes in 

 nothing but the physical, teaches poor P. something which 

 compels him to stand with one toot on the land of physics and 

 the other in the sea of metaphysics, in order to reach it all. 

 This teacher forces poor P. to recognise the metaphysical, while 

 he scorns to do so himself. The combination of the two 

 characters of conductor and of pure finger-post, in the same 

 person puzzles Publius a good deal. Sometimes, when poor 

 Publius thi'nks that he has grasped the principle in the above 

 form, it seems to him to turn out only a truism, after all ; and 

 indeed no less a man than Sir John Herschel sympathised with 

 him in this idea.^ I am not sure that they are right ; they seem 

 to overlook that this potential E., though undeniably out of 

 physical existence, is by some mysterious paraphysical operation, 

 recoverable in its former quantity. However, P.'and Sir John 

 are right, so far, that the doctors will sometimes inadvertently 

 allow themselves to present a physical principle of E., which is 

 very far from self-evident, in a form which has all the appear- 

 ance of a logical truism; e.g. when we are told that "the E. 

 exerted is equal to the work performed." P. sajs I could have 



- " Familiar Lectures," p. 463. See Rauliiiie's answer to Herschel, Fhil, 

 Ma^ , February, 1867. 



told you that from the definition of E, , wliich is "the power of 

 performing work." 



4. Potential E being that which is not had in actual possession 

 by the body (or system) in question, how can that body be " a 

 store of potential E. ? " ' How can the body contain that which 

 is not in it ? The d tctors should explain this. However, I am 

 glad to find that my cousin Barney was not so wrong, after all, 

 when he complained that Ireland was swarming with absentee 

 landlords. 



5. But to pass now from a posteriori objections to the phrase 

 "potential E." in the present sense. This potential E. is so 

 called to distini;uish it from actual E. so called, and yet it is just 

 as immediately and directly efficient in performing its work as 

 actual E. itself, and, therefore, as truly actual as any E. can be. 

 When a certain quantity of potential E. is followed by its 

 equivalent actual E., what is the actual E. of the body but the 

 direct work of the potential E. done against the inertia of the 

 body ? It is from the doctors themselves, of course, that I learn 

 this. And yet it is very curious to observe how often they shrink 

 from directly stating this, and how ingeniously they will avoid it 

 (one doctor actually denies it). They will sometimes tell you 

 that the potential E. is " transjormcd " into the subsequent 

 actual E. and f. v. Sometimes, when they feel that this evasive 

 euphemism is unsuitable to their immediate purpose, they will 

 use what I, with the utmost deference as well as difference, hold 

 to be the proper word, viz., "transfer" ; but having made this 

 concession they refuse to proceed further, and shirk telling us 

 from what or to what the transference is made (more of this 

 presently). As we have said the kinetic E. of the moving body 

 is the direct work of the equivalent potential E. that preceded 

 it ; and if the work be, as it is, actual, the E. must be so too ; 

 as long as we remain in the realm of physics. 



But more than this ; the potential E. of a mass, as it is ex- 

 pressed, can do other direct work than that of producing actual 

 E. in the mass concerned. Take the case of a clock weight, 

 which is so olten adduced, though never, as far as I know, for 

 the purpose of illustrating its own proper lesson. When wound 

 up it has, as we are told, potential E. ; but in its descent, while 

 working the clock, it never acquires more than the indefinitely 

 small quantity of actual E. v/hich is due to its excessively slow 

 motion ; and this actual E. is doing; no work during the descent, 

 since the velocity of descent is uniform. The only work that 

 this actual E. performs is to produce an infinitesimal amount of 

 heat at the instant of the weight's reaching the lowest point of 

 its descent ; that is to say, when the clock has stopped. This 

 is only one instance of a whole class of cases in which, as it is 

 expressed, a mass does work by means of its potential E. which 

 exists only in possibility, without ever having any actual E. 

 which it can apply to that work ! Moreover there are cases 

 outside of molar physics in which it is not yet known, for certain, 

 whether the E. present is conventionally actual or potential ; and 

 yet, in either case, the work is done immediately and directly ; 

 and therefuie the E. is truly actual whether conventionally so or 

 not. Therefore " potential E.," in the present sen.se, is a wrong 

 title for this or any mode of E., and this being so, " actual E.," 

 as the distinguishing title of the other E., is wrong too; since 

 both are actual. 



6. There are a very few of our doctors who use the name 

 "potential E." with another reference solely, and who, as it 

 would appear, designedly abstain from giving it the meaning of 

 " E. existing in posse" probably on account of some ol the in- 

 conveniences we have mentioned ; and yet they will use epithets 

 which at least tend somewhat in the same direction. They speak 

 of it as being " E. of repose " " (meaning of course reposing E.), 

 as being " of a quiet nature," i* "dormant,"* "quiescent,"* 

 "tranquil,"" and " passive "^ (!), in opposition to the other 

 type of E., which they coirespondin|,ly call "active"* and 

 " living." " Now poor Publius is strongly inclined to think that 

 if he had spoken thus they would have said that he had not yet 

 got hold oi the precise scientific meaning of E. It seems to him, 

 though he trembles to say it, that although in popular usage the 

 phrases, "quiet," " dormant, "&c., and " active energy " may do 

 very well, and convey a correct meaning, viz., that intended by the 



' Thomson and Tait (\'at. Phil. p. 178) virtually say this, but with 

 them pot. E. does^not mean E. existing in possibility. So they are all right 

 in doing so. 



= Baliour Stewart, " Cons, of E.," pp. 27, 143. 3 Of. cit., p. 23. 



^ Stewart and Tait, " Uns. Univ.," p. 109; also Tail's ' Glasgow Lect." 



5 Op. cit., p. III. ^ J?o , p. 147. ? Tait, "Glasgow Lecture. 



8 " Un.s. Univ.," p. ni ;iTait, " Glasg. Lect," and Tyndall, " Heat," 

 2nd edition, p. 140. ^ Stewart, " Cons, of E.," p. 27. 



