i8 



NATURE 



[September 4, igig 



sions, while materia! inertia depends on those dimen- 

 sions cubed, there must be a size when the two are 

 equal, i.e. when one might account for the other. 



Write the charge in terms of electrostatic potential 

 V. 



then 



3f'' 



the 



where c is i/V(/'K), the velocity of light. 



Put this expression for ni equal to -n a'p, 

 ordinary mass. 



Then the potential at which the two will be equal is 



Vi = 



/(?) 



Vhich, for density of water and for a sphere lo-" cm. 

 radius, is two volts — quite a reasonable electrolytic 

 value, such as is to be expected among atoms.' 



The moral of this elementary, but not very satis- 

 factory, argument is that not for bodies of atomic 

 size, but for something 100,000 times smaller in linear 

 dimensions, is it possible to explain inertia electro- 

 magnetically. But forty, or even twenty, years ago 

 one would have said : There are no bodies of this 

 size ; nothing can be smaller than an atom ! The 

 strange thing is that, as nearly everyone knows now, 

 bodies of this size have been discovered. They were 

 isolated by Sir J. J. Thomson in i8gg, having been 

 gradually led up to by Crookes's and many other 

 experirrients on cathode rays ; and they are shown 

 to be an apparently invisible unit or atom of elec- 

 tricity the inertia of which is wholly electric. 



The proof of this last statement I can onlv briefly 

 indicate. It is established by the effect of speed on 

 electric inertia. If an electric charge is moving with 

 something approaching the velocity of light, its inertia 

 increases without limit ; and the formula given about 

 1889 by Heaviside, Thomson, and others for electric 

 inertia as a function of speed is, in its verv simplest 

 form. 





+ higher powers). 



The velocity of light squared occurs in the denomina- 

 tor, so, before we can observe the increase, enormous 

 speeds are necessary. A cannon-ball, or even the 

 earth in its orbit, is hopelessly slow ; and we know 

 no artificial means of getting up such a speed as this 



" The argument i-; plausible, and, taken as an illu'^tration on ordinary 

 lines, will serve : .but considere 1 seriously it may be quite f tllacious, 

 although the main consequences which in the text are going to he drawn 

 arc con-ect. Few things are more surprising than the extraor.linarily large 

 charge held by or constituting an electron in proporiion to its size. The 

 charge is so large that ordinary arguments about electricity as it exists on 

 material spheres cannot be expected to apply. If they did, or in so far as 

 they do, the potential of an e!e. trci would not be two volts, but well over a 

 million volts ; and the density of the m hereal substance of which it is pre- 

 sumably composed (if its electric inertia is to be derived in any simple, 

 ordinary way from its bulk) would have to be nothine like that of water, 

 but of the order lo -, or a billion times the density of water. A thousand 

 tons, in fact, to the cubic milli 'letrc. 



We are here out of our depth among quantities on which a great deal of 

 work has to be done to reduce them to order. Vet it must no* b; supposed 

 that these figures are nonsensical. Thty require serious consideration ; and 

 that is all that can be said for them. I do not think there is any sense in 

 talking about the potential of an indivisible unit of charkie, but we can talk 

 ■ about the potential existing at the confines of an atom ;' and that i< a 

 reasonable magnitude, about 14 volts in the case of hydrogen, and not very 

 different for other elements. 



But on the other side of the subject everything points to the density of 

 ather bein^ exceedingly high, though rerhaps not so high as the above 

 estimate. It must at least be greatly denS' r thin platinum or lead, and 

 probablv immensely denser. 



A difficulty is often felt as to how ordinary matter like a planet can move 

 through such a medium without fnction. Density, however, does not involve 

 visco'ity ; the two are disconn-cted ; and resistance to motion would be 

 caused only by viscosity, of which the aether appears to have none. , 'I here 

 are many ways, more or less satisfactory, of picturing the perfectly free 

 motion of matter through ^n exceedingly subtantia! aether of space ; there 

 would be innumerable difficult'es in supposimr friction and consequent 

 generation of heat. It is quite certain that whatever the atther does it 

 does not dissipate energy. That imperfection belongs to the province of 

 ni^'Iecularly constituted matter.' 



NO. 2601, VOL. 104] 



last, VIZ. about nineteen miles a second. But for 

 tunately, radium does spontaneously what we cannot 

 do; It expels electrons with something less but not 

 very much less, than the speed of light; and Kauff. 

 mann's measure of the mass of these orojectiles thus 

 flymg at prodigious velocities, confirms the theorv 

 and removes any doubt as to the realitv of purelv 

 and wholly electric inertia for electrons. 



Furthermore, it was found that the verv same 

 electrons can be split off or detached from 'any or 

 every kmd of atom, that there is oniv one kind of 

 negative electron ; and though at first "there appeared 

 tobe many kinds of positively charged particles, the 

 evidence is tending to the discovery' of a single kind 

 of positive electron likewise; so' it is natural to 

 suppose that electrons are an essential irtgredient in 

 matter. And since they possess inertia, even those 

 which are clearly disembodied electric charges, it 

 becomes possible to surmise that in some sense,' or 

 in a certain grouping, they constitute the atom; that 

 they confer upon it the inertia with which we are 

 familiar; and that, in fact, electric inertia is the onlv 

 inertia that exists. 



Electric inertia began as the simulacrum of material 

 inertia; it has shown itself the very same thing, and 

 it seems likely to end by displacing every other kind 

 of inertia altogether. " ' . 



This is the electrical theory of matter. 



Assuming this theory for the present as a working 

 hypothesis, we may say that material inertia is 

 explained electromagnetically, i.e. is explained in 

 terms of the magnetic field which necessarily sur- 

 rounds and accompanies eivery charge in motion, 

 since a charge in motion constitutes a current. For 

 on this view a material body is but an aggregate of 

 such charges grouped according to some definite 

 pattern, positive and negative charges interlaced or 

 somehow intertwined, and so far apart in proportion 

 to their size that they do not interfere with each other 

 or cancel each other, nor apparently overlap or en- 

 croach on each other's field, to any measurable 

 extent. Is this possible? It is. For, comp.nring the 

 size of an electron with the size of an atom, we 

 perceive that they are relatively of the same order 

 as the size of a planet and the size of a solar svstem. 

 So it becomes possible to think of an atom as a sort 

 of solar system, with a positive nucleus or sun sur- 

 rounded by negative electrons revolving in regular 

 orbits round it. 



On this view, or, indeed, in any form of the elec- 

 trical theory of matter, the atom of matter consists 

 mainly of empty space ; in other words, it is exces- 

 sively porous, just as the solar system is mainly 

 empty space, and may be spoken of as excessively 

 porous, the actual material lumps being almost 

 infinitesimal in proporiion to the total bulk. .\ rapid 

 projectile or a ray of light passing through the solar 

 system would be unlikely to hit anything ; the chances 

 would be strongly against a collision. So also, if a 

 point be thrown through an atom, the chance of its 

 hitting anything is about i in lo.ooo. It might pass 

 through 10,000 atoms before striking. This experi- 

 ment has been tried by C. T. R. Wilson and others, 

 and that is, roughly speaking, the result. Sooner or 

 later a radium projectile meets with an obstacle and 

 is stopped, but it traverses a good number of atoms 

 on the average; it traverses quite a perceptible 

 distance even in a dense solid before it strikes a 

 nucleus. 



Matter accordingly seems to me — to us, I may say, 

 for in this most physicists are, I think, agreed — a 

 gossamer or milky-way structure, an impalpable acci- 



