August 4, 1892J 



NATURE 



325 



back on the great changes involved in matters of primary im- 

 portance, such as the undulatory theory of light, the conserva- 

 tion of energy, and the second law of thermodynamics, I cannot 

 admit that there is much reason to be dissatisfied with the rate 

 at which new theories have been received. Those who experi- 

 ence a temporary check, owing to the fact thct public opinion is 

 not ripe for their ideas, are often amply rewarded after the 

 lapse of a few years. The disappointment which Joule may have 

 felt during the time his views met with adverse criticisms from 

 the official world of science was no doubt amply compensated by 

 the pleasure with which he watched the subsequent progress of re- 

 search in the new domain which his discoveries have opened out. 



The point is not one of academic interest only, for the fear of 

 repressing some important new discovery has a detrimental in- 

 fluence in another direction. The judgment of the scientific 

 world seems to me to be tending too much towards leniency to 

 apparently absurd theories, because there is a remote chance 

 that they may contain some germ of real value. A new truth 

 will not be found to suffer ultimately by adverse and even un- 

 reasonable criticism, while bad theories and bad reasoning, sup- 

 ported by the benevolent neutrality of those to whose judgment 

 the scientific world looks for guidance, are harmful in many 

 ways. They block the way to an independent advance and 

 encourage hasty and ill-considered generalizations. The con- 

 clusions I should draw from the considerations I have placed 

 before you are these : I believe that a reasonable censorship 

 exercised by our scientific societies is good and necessary ; that 

 those whose fate it is to be called on to express an opinion on 

 some work or theory should do so fearlessly according to their 

 best judgment. Their opinion may be warped by prejudice, 

 but I think it is better that they should incur the risk of being 

 ultimately found to be wrong than that they should help in 

 the propagation of bad reasoning. There is one matter, 

 however, on which all opinions must agree. Worse than bad 

 theory or logic is bad experimental work. Should we then not 

 rigorously preserve any influence or incentive which encourages 

 the beginner to avoid carelessness and to consider neither time 

 nor trouble to secure accuracy ? There is no doubt to my mind 

 that the prospect of admission to the Royal Society has been 

 most beneficial in this respect, and that the honourable ambi- 

 tion to see his paper published in the "Transactions" of that 

 Society has preserved many a student from the premature 

 publication of unfinished work. 



One of the principal obstacles to the rapid diffusion of a new 

 idea lies in the difficulty of finding suitable expression to convey 

 its essential point to other minds. Words may have to be 

 strained into a new sense, and scientific controversies con- 

 stantly resolve themselves into differences about the meaning 

 of words. On the other hand, a happy nomenclature has 

 sometimes been more powerful than rigorous logic in allowing a 

 new train of thought to be quickly and generally accepted. 



A good example is furnished by the history of the science of 

 energy. The principle of the conservation of energy has un- 

 doubtedly gained a more rapid and general acceptance than it 

 would otherwise have had by the introduction of the word 

 potential energy. A great theorem, which in itself seems to me 

 to be an intricate one, has been simplified by calling something 

 energy which, in the first place, is only a deficiency of kinetic 

 energy. The only record I can find on the history of the ex- 

 pression is given in Tait's " Thermodynamics," wherein the 

 term statical energy is ascribed to Lord Kelvin, and that of 

 potential energy to Rankine. It would be of interest to have a 

 more detailed account on the origin of an expression which has 

 undoubtedly had a marked influence not only on the physics, 

 but also on the metaphysics of our time. But while fully recog- 

 nizing the very great advantage we have derived from this term 

 "Potential Energy," we ought not, at the same time, 

 to lose sight of the fact that it implies some- 

 thing more than can be said to be proved. It 

 is easy to overstep the legitimate use of the word. 

 Thus, when Professor Lodge ^ attempts to prove that action at 

 a distance is not consistent with the doctrine of energy, he can- 

 not, in my opinion, justify his position except by assuming that 

 all energy is ultimately kinetic. That is a plausible but by no 

 means a necessary theory. Efforts have been made to look on 

 energy as on something which can be labelled and identified 

 through its various transformations. Thus we may feel a certain 

 bit of energy radiating from a coal-fire, and if our knowledge 

 was complete, we ought to be able to fix the time at which that 



' Phil. Mas. vol. xi. p. 36 (i38i). 



NO. I 188, VOL. 46] 



identical bit of energy left the sun and arrived on the surface of 

 the earth, setting up a chemical action in the leaves of the plant 

 from which the coal has been derived. If we push this view to 

 a logical conclusion, it seems to me that we must finally arrive 

 at an atomic conception of energy which some may consider an 

 absurdity. 



Let, for instance, a number of particles Pj, Pj, &c., in suc- 

 cession, strike another particle Q. How can we in the trans- 

 latory energy of the latter identify the parts which Pj, Po, &c., 

 have contributed ? According to Professor's Lodge's view, we 

 should be able to do so, for if the particle Q in its turn gives up 

 its energy to others, say Rj, R,, R3, &c., we ought to be able to 

 say whether the energy of Pj has ultimately gone into R, or 

 into R2, or is divided between them. It is only by imagining 

 that all energy is made up of a finite number of bits, which pass 

 from one body to another, that we can defend the idea of con- 

 sidering energy as capable of being "labelled." 



In the expressions we adopt to prescribe physical phenomena 

 we necessarily hover between two extremes. We either have 

 to choose a word which implies more than we can prove, or we 

 have to use vague and general terms which hide the essential 

 point, instead of bringing it out. The history of electrical 

 theories furnishes a good example. The terms positive and 

 negative electricity committed us to something definite ; we 

 could reckon about quantities of electricity, and form some defi- 

 nite notion of electrical currents as a motion of the two kinds of 

 electricity in opposite directions. Now we have changed all 

 that ; we speak of electric displacements, but safeguard ourselves 

 by saying that a displacement only means a vector quantity, an-d 

 not necessarily an actual displacement. We speak of lines and 

 tubes of force not only as a help to realize more clearly certain 

 analytical results, but as implying a physical theory to which, at 

 the same time, we do not wish to commit ourselves. I do not 

 find any fault with this, for it is a perfectly legitimate and neces- 

 sary process to state the known connection between physical 

 phenomena in some form which introduces the smallest number 

 of assumptions. But the great question " What is electricity ? " 

 is not touched by these general considerations. The brilliant 

 success with which Maxwell's investigations have been crowned 

 is apt to make us overrate the progress made in the solution of 

 that question. Maxwell and his followers have proved the im- 

 portant fact that optical and electrical actions are transmitted 

 through the same medium. We may be said to have arrived in 

 the subject of electricity at the stage in which optics was placed 

 before Young and F"resnel hit on the idea of transverse vibra- 

 tions, but there is no theory of electricity in the sense in which 

 there is an elastic solid theory of light. 



If the term electrical displacement was taken in its literal 

 sense, it would mean that the electric current consists of the 

 motion of the ether through the conductor. This is a plausible 

 hypothesis, and one respecting which we may obtain experi- 

 mental evidence. The experiments of Rayleigh and others 

 have shown that the velocity of light in an electrolyte, through 

 which an electric current is passing, is, within experimental 

 limits, the same with and against the current. This result shows 

 that if an electrical current means a motion of the ether the 

 velocity of the medium cannot exceed ten metres a second for a 

 current density of one ampere per square centimetre. This, 

 then, is the upper limit for a possible velocity of the medium ; 

 can we find a lower limit ? The answer to that question de- 

 pends on the interpretation of a well-known experiment of 

 Fizeau's, who found that the speed of light is increased if it 

 travels through water which moves in the same direction as the 

 light. If this experiment implies that the water carries the 

 ether with it, and if a motion of the ether means an electnc 

 current, we should be led to the conclusion that a current of 

 water should deflect a magnet in its neighbourhood. An ex- 

 periment made to that effect would almost certainly give a nega- 

 tive result, and would give us a lower limit for the velocity of 

 the medium corresponding to a given current. Such an experi- 

 ment, together with that of Rayleigh, would probably dispose 

 of the theory that an electric current is due to a translatory 

 velocity of the medium. This would be an important step, and 

 it would be worth while to arrive at a final settlement of the 

 question.' The whole question of the relation between the 



' Fizeau's results must either be due to the motion of matter through the 

 medium or to the fact tliat moving matter carries the ether with it. If it is 

 due to the former cause, and matter does not carry the ether with it, may^we 

 not consider that matter moving through the ether, that is a relative motion 

 of matter and ether, must produce effects oxvsX and opposite to those of ether 

 moving through matter? In that case the reasoning in the text would, 

 mutatis mutandis, hold good. 



