January 26, 1893] 



NATURE 



293 



was sketched in a detailed Report communicated by me to the 

 Geological Society on April 25, 1888. My friend Prof. Lap- 

 worth has no scientific comrade who has more frankly and 

 practically acknowledged his great geological achievements than 

 Ihavedone. Arch. Geikie. 



January 23, 1893. 



The Identity of Energy. 

 T AM glad to see that in the introduction to his severely- 

 difficult memoir, published in the Philosophical Transactions for 

 1892, "On the Forces, Stresses, and Fluxes of Energy in the 

 Electromagnetic Field" (p. 427), Mr. Oliver Heaviside notices 

 and criticizes some ideas of mine, published in the Philosophical 

 Magazine (or ]\ine 1885 and other places, cmcerning energy. 



The statements I then made, and to which I still rigidly hold, 

 are (i) that energy has identity like matter, and not merely con- 

 servation ; (2) that whenever energy is transferred from one body 

 to another, it is also transformed from potential to kinetic, or vice 

 zrrsd. 



The basis of the first assertion is the fact that energy is always 

 passed on continuously through space, i.e. that its transfer ocfcurs 

 along a definite path, instead of merely appearing in one place 

 and disappearing in another. 



The law of conservation would be satisfied by disappearance 

 and equal reappearance ; the law of identity requires a continuous 

 act of transfer. The latter is true for matter, and I assert that 

 by thinking of a number of instances, it will be perceived true 

 for energy. In all mechanical instances, as of belts and shafting, 

 the transfer of energy is obvious ; it was not so obvious in electro- 

 magnetic actions, between dynamo and motor for instance, until 

 Prof Poynting clearly demonstrated that it was in accordance 

 with Maxwell's principles. 



Mr. Heaviside objects that we are not able to assert it for 

 gravitational energy. Well, that depends on what view we 

 take of gravitation ; but I submit that until something more is 

 certainly known about it, the safest plan is not to assert, but to 

 assume, that in this case also what is known in every other case 

 likewise occurs, and to trace the consequences of the hypothesis 

 in the hope that it may lead to some conclusion verifiable or 

 falsifiable by experiment. The reason I attach importance 

 to this doctrine of the identity or continuity of transfer of energy 

 is because it greatly simplifies the fundamental mechanical laws, 

 and emphasizes without risk of vagueness the denial of action at 

 a distance. 



If action at a distance (no matter how minute) can ever occur, 

 then indeed the continuous transfer of energy breaks down. But 

 observe that there is no necessity for the transfer to occur at a 

 finite velocity in order to avoid action at a distance, i.e. action 

 without a medium. By the thrust of an incompressible pole, 

 energy is transferred from butt to tip, just as really as if the com- | 

 pressed and recoiling layers could be demonstrated and its velo- 

 city measured. So likewise the pull of gravitation may be (and 

 pi'o tern. I believe is) transmitted by an incompressible (or nearly 

 incompressible) ether, so that the force is felt instantaneously 

 (or nearly instantaneously) at all distances where matter exists ; 

 but that by no means militates against a genuine act of transfer. 

 The conservation of matter makes experiments on gravitation 

 difficult ; if we could suddenly create or destroy a piece of 

 matter there might be some remote chance of determining the 

 rate at which its gravitative influence was felt. Especially if by 

 alternately generating and destroying it we could set up a series 

 of waves of perhaps measurable length. 



And although this is as yet impossible, many known facts 

 lead us to conclude that if gravitation has any velocity at all 

 short of infinite, it is at least immensely greater than the speed 

 of light. And seeing that the one phenomenon is concerned with 

 the transverse (electric) elasticity of ether, and the other with its 

 longitudinal elasticity, there is nothing surprising in that. 



By all means, however, as Mr. Heaviside urges, let gravita- 

 tion be included in general etherial equations whenever pos- 

 sible ; and it may perhaps be wise to assume some unknown 

 finite rate of propagation and trace its consequences with the 

 object of verifying or disproving them. 



So far as I understand, however, this is not unlike what 

 Helmholtz did, by his generalization of Maxwell's electro- 

 magnetic theory; with the result that the course of experiment 

 so far has been to justify the simple Maxwellian theory, and to 

 make the longitudmal ether thrust velocity practically infinite. 

 NO. 12 13, VOL. 47] 



And now for the second assertion, that whenever energy is 

 transferred from one body to another, it is also transformed, and 

 vice versd. This is to me not an opinion, but a demonstrated 

 theorem (as has been shown in the paper referred to) ; but it 

 must be understood in what sense I consistently use the word 

 body in this connection. I do not necessarily mean a visible lump 

 of matter. The molecules of a lump are to be regarded as 

 a diflerent " body " to the whole mass ; and again, the ether 

 everywhere embathing them is another distinct " body." 



But so long as a piece of matter is merely moving through 

 space with all the energy it may happen to contain, I do not 

 consider that a transfer at all. There is a transfer of energy in 

 one sense, viz. that of locomotion, but there is no transfer from 

 one body to another except when work is done at their point 

 of contact, and energy gained by one and lost by the other, 

 being transferred across their common boundary surface. In all 

 such cases of " activity " the energy transferred is necessarily in 

 the first instance transformed ; though by means of another trans- 

 fer it may very speedily be transformed back again ; and so 

 speedily sometimes is the re-transformation effected that the 

 intermediate condition has a tendency to get overlooked. In 

 wave-motion a transfer and transformation occurs during every 

 quarter period. 



Mr. Heaviside seems to think that the mere convection of energy 

 should he included as one kind of transfer ; but surely that is 

 scarcely convenient ? So long as energy retains its form and ad- 

 herence to one body, so long there is no true activity ; no work 

 is being done — the energy is simply stored. It may be stored in a 

 bent spring, or in a flying bullet, or in a revolving fly-wheel. It 

 is impossible to have kinetic energy at all without convection, 

 and a distinction must be drawn between the mere existence of 

 energy and the active and useful flux or transfer of the same. 



Mr. Heaviside further seems to consider circuital fluxes of 

 energy as strange and useless phenomena. But 1 see no 

 reason in this at all. The circulation of matter — for instance in 

 the inner circle of the Metropolitan railway — is, I suppose, 

 considered useful. The circulation of commodities is the essenceof 

 commerce. So does the circulation of energy constitute the 

 activity of the material universe. It is the act of transfer that 

 is beneficial (or the reverse) ; what becomes of a conservative 

 quantity is a minor matter. It must go somewhere, and may 

 very well, after a series of transfers, ultimately return to its 

 starting point. [Parenthetically I should like to preach here 

 against what I hold to be the pernicious doctrine of (at least 

 amateur) political economists, that because money locally spent 

 is not destroyed, but remains in the community, it does not much 

 matter how much transferring power is permitted or granted to 

 one individual, — as if the money itself were the useful com- 

 modity, and not the power of determining its direction of 

 transfer or non-transfer. The control of every transfer should 

 be jealously watched, for that is the greedily-desired power.] 



So long as circuital convection of energy goes on without 

 transfer — as, for instance, in the rim of a non-working fly- 

 wheel — so long the energy is merely stored ; but directly a belt is 

 fitted on with different tensions in its two halves, a portion of 

 the energy is tangentially tapped off, and transfer and activity 

 begin. The kinetic energy of the wheel is converted into strain 

 or stress energy of the belt, which then by simple locomotion 

 passes it on to something else. I perceive, however, that there 

 is a slight difficulty about this simple case of locomotive con- 

 veyance of stress energy by a really inelastic substance ; but 

 only because the details of any infinitely rapid process are 

 difficult to follow. I perceive moreover that in many cases it is 

 not worth while to attend to the alternate compressions and 

 motions which constitute a longitudinal pulse, and that the idea 

 of simple locomotion may be conveniently introduced to cover 

 the case of a stressed body moving ; but the convenience is I 

 think only attained by shutting our eyes to the essential pro- 

 cesses which in all actual matter must be occurring. 



I trust that Mr. Heaviside may find time to notice this letter, 

 and attack anything he disagrees with, in order that the whole 

 matter may become thoroughly clear. OLIVER LoDGE. 



A Proposed Handbook of the British Marine Fauna. 



I AM obliged to Prof. Thompson for his criticism of my 

 scheme, although only one of the points he raises is new to me 

 — as I think it will be to most zoologists — viz. that " there are 

 no nemalophores on the stem " in Antennularia. I thought A. 



