Oct. 6, 1887] 



NATURE 



533 



duction of electricity is always zero ; that you cannot pro- 

 duce positive electrification without an equal quantity of 

 negative also ; that what one body gains of electricity some 

 other body must lose. 



Now, whenever we perceive that a thing is produced in 

 precisely equal and opposite amounts, so that what one 

 body gains another loses, it is convenient and most simple 

 to consider the thing not as generated in the one body 

 and destroyed in the other, but as simply transferred. 

 Electricity in this respect behaves just like a substance. 

 This is what Franklin perceived. 



The second great law is that electricity always, under 

 all circumstance,flows in a closed circuit, the same quantity 

 crossing every section of that circuit, so that it is not 

 possible to exhaust it from one region of space and 

 condense it in another. 



Another way of expressing this fact is to say that no 

 charge resides in the interior of a hollow conductor. 



Another is to say that total induced charge is always 

 equal and opposite to inducing charges. 



[This is illustrated by the well-known experiment of 

 insulating and charging a parrot-cage with a sensitive 

 electroscope inside connected to its wires ; also by the 

 ice-pail experiment.] 



When we thus find that it is impossible to charge a body 

 absolutely with electricity, that though you can move it 

 from place to place it always and instantly refills the body 

 from which you take it, so that no portion of space can be 

 more or less filled with it than it already is, it is natural 

 to express the phenomenon by saying that electricity be- 

 haves itself like a perfectly incompressible substance or 

 fluid, of which all space is completely full. That is to 

 say, it behaves like a perfect and all-permeating liquid. 

 Understand, I by no means assert that electricity is such a 

 fluid or liquid ; I only assert the undoubted fact that it 

 behaves like one, i.e. it obeys the same laws. 



It may be advisable carefully to guard one's self against 

 becoming too strongly imbued with the notion that be- 

 cause electricity obeys the laws of a liquid therefore it is 

 one. One must always be keenly on the look-out for any 

 discrepancy between the behaviour of the two things, and 

 a single certain discrepancy will be sufficient to overthrow 

 the fancy that they may perhaps be really identical. Till 

 such a discrepancy turns up, however, we are justified in 

 pursuing the analogy — more than justified, we are im- 

 pelled. And if we resist the help of an analogy like this 

 there are only two courses open to us : either we must 

 become first-rate mathematicians, able to live wholly 

 among symbols and dispensing with pictorial images and 

 such adventitious aid ; or we must remain in hazy ignor- 

 ance of the stages which have been reached, and of the 

 present knowledge of electricity so far as it goes. I need 

 hardly say that by "modern views" 1 do not mean 

 ultimate views ; nor do I mean that I can give an account 

 of all the speculations and ideas floating in the minds of 

 some two or three of our most advanced thinkers. All 

 I attempt is to give an account of the stage which has 

 certainly been attained, and to ask you to take for granted 

 that the next quarter of a century will see as great advances 

 made upon these views as they are superior to the doctrines 

 inculcated by the ordinary run of text-books. 



Imagine now that we live immersed in an infinite ocean 

 of incompressible and inexpansible all-permeating perfect 

 liquid, like fish live in the sea, and how can we become 

 cognizant of its existence ? Not by its weight, for we can 

 remove it from no portion of space in order to try whether 

 it has weight. 



We can weigh air, truly, but that is simply because we 

 can compress it and rarefy it. An exhausting or con- 

 densing pump of some kind was needed before even air 

 could be weighed or its pressure estimated. 



But if air had been incompressible and inexpansible, 

 if it had been a vacuum-less perfect liquid, pumps would 

 have been useless for the purpose, and we should 



necessarily be completely ignorant of the weight and 

 pressure of the atmosphere. 



How then should we become cognizant of its existence ? 

 In four ways : — 



(i) By being able to pump it out of one elastic bag 

 into another [not out of one bucket into another : if 

 you lived at the bottom of the sea you would never 

 think about filling or emptying buckets — the idea would 

 be absurd ; but you could fill or empty elastic bags], 

 and by noticing the strain phenomena exhibited by the 

 bags and their tendency to burst when over full. [Water 

 (or air) was here pumped out of one elastic bag into 

 another, and the analogy with an electrical machine 

 charging two conductors oppositely was pointed out.] 



(2) By winds or currents ; by watching the effect of mov- 

 ing masses of the fluid as it flows along pipes or through 

 spongy bodies, and by the effects of its inertia and 

 momentum. [A hanging vane in a tube deflected by a 

 stream of water was here likened roughly to a galvano- 

 meter ; also the effect of suddenly stopping a stream of 

 water, as in a water ram, was mentioned as analogous to 

 self-induction.] 



(3) By making vortices and whirls in the fluid, and 

 by observing the mutual action of these vortices, their 

 attractions and repulsions. [Whirlwinds, sand-storms, 

 waterspouts, cyclones, whirlpools.] 



(4) By setting up undulations in the medium : i.e. by 

 the phenomena which in ordinary media excite in us 

 through our ears the sensation called " so7ind." 



In all these ways we have become acquainted with 

 electricity, and in no others that I am aware of They 

 correspond to the four great divisions of the subject which 

 I made above. But there are differences, very important 

 differences, between the behaviour of a material liquid 

 ocean such as we have contemplated and the behaviour of 

 electricity. First it is doubtful whether electricity by itself 

 and disconnected from matter has any inertia. It is by no 

 means certain that it has not : the experiments made by 

 Maxwell with a negative result need only prove either that 

 its speed of flow is very small, or that an electric current 

 consists of equal opposite streams of equal momentum. 

 The laws of electric flow in conductors are such as 

 indicate no inertia, and this fact would be conclusive were 

 it not that a recent brilliant paper by Prof Poynting 

 explains the reason of it completely otherwise, and 

 leaves the question of inertia quite open ; on the other 

 hand, the facts of magnetism seem definitely to require 

 inertia, or something~corresponding to it. Leaving this 

 therefore as an open question, there can be no doubt but 

 that when in connexion with insulating or dielectric 

 matter the combiitation most certainly possesses inertia. 



A more serious and certain difference between the 

 behaviour of electricity and that of an incompressible 

 fluid comes out in the fourth category— that concerned 

 with wave-motion. In an incompressible fluid the velocity 

 and length of waves would both be infinite, and none of 

 the phenomena connected with the gradual propagation 

 of waves through it could exist. Such a medium there- 

 fore would be incapable of sound vibrations in any 

 ordinary sense. On the other hand, it is quite certain 

 that the disturbances concerned in light radiation take 

 place at right angles to the direction of propagation— they 

 are tranverse disturbances — and such disturbances as 

 these no body with the entire properties of a fluid can 

 possibly transmit. Remember, however, that the medium 

 which transmits light is the ether and not sirnply electri- 

 city. We have nowhere asserted that electricity and the 

 ether are identical. If they are, we are bound to admit 

 that ether, though fluid in the sense of enabling masses 

 to move freely through it, has a certain amount of rigidity 

 for enormously rapid and minute oscillatory disturbances. 

 If they are not identical, we can more vaguely say that 

 ether contains electricity as a jelly contains water, but 

 that the rigidity concerned in the transverse vibrations 



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