TRANSACTIONS OF SECTION A. 631 



an expression which has undoubtedly had a marked influence, not only on the 

 physics, but also on the metaphysics of our time. But whUe fully recognising 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 something 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 cannot, 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 

 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 a)i 

 absurdity. 



Let, for instance, a number of particles Pj, P^, &c., in succession, strike another 

 particle, Q. How can we in the translatory energy of the latter identify the parts 

 which Pj, P.J, &c., have contributed ? According to Professor 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^, Kg, &c., we ought to be able to say whether the energy of Pj has 

 ultimately gone into Rj or into R^, 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 considering energy as capable 

 of being 'labelled.' 



In the expressions we adopt to describe 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 definite 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, and not necessarily an actual displacement. We speak of 

 lines and tubes of force not only as a help to realise more clearly certain analytical 

 results, but as implying a physical theory which, at the same time, does not 

 commit us to anything. I do not find any fault with this, for it is a perfectly 

 legitimate and necessary 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 important fact that optical 

 and electrical actions are transmitted by means of 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 Fresnel hit on the idea of transverse vibrations, but there 

 is no theory of electricity in the sense in which there is an elastic solid theory of 



It 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 experimental 

 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 



» Phil. Mag., vol. xi. 1881, p. 36. 



