August i8, 1904] 



NA TURE 



369 



compare the different modes in which, at different epochs of 

 scientific development, this intellectual picture has been 

 drawn cannot fail to suggest questions of the deepest 

 interest. True, 1 am precluded from dealing with such of 

 these questions as are purely philosophical by the character 

 of this occasion ; and with such of them as are purely 

 scientific by mv own incompetence. But some there may 

 be sufficiently near the dividing line to induce the specialists 

 who rule by right on either side of it to view w^ith forgiving 

 eyes any trespasses into their legitimate domain which I 

 may be tempted, during the next few minutes, to commit. 



Let me, then, endeavour to compare the outlines of two 

 such pictures, of which the first may be taken to represent 

 the views prevalent towards the end of the eighteenth 

 century ; a little more than a hundred years from the publi- 

 cation' of Newton's " Principia," and, roughly speaking, 

 about midway between that epoch-making date and the 

 present moment. 1 suppose that if at that period the 

 average man of science had been asked to sketch his general 

 conception of the physical universe, he would probably have 

 said that it essentially consisted of various sorts of ponder- 

 able matter, scattered in different combinations through 

 sp.ice, exhibiting most varied aspects under the influence 

 of chemical affinity and temperature, but through every 

 metamorphosis obedient to the laws of motion, always re- 

 taining its mass unchanged, and exercising at all distances 

 .1 force of attraction on other material masses, according 

 to a simple law. To this ponderable matter he would (in 

 spite of Rumford) have probably added the so-called 

 •' imponderable " heat, then often ranked among the 

 elements; together with the two "electrical fluids," and 

 the corpuscular emanations supposed to constitute light. 



In the universe as thus conceived, the most important 

 form of action between its constituents was action at a 

 distance ; the principle of the conservation of energy was, 

 in any general form, undreamed of ; electricity and 

 magnetism, though already the subjects of important in- 

 vestigation, played no great part in the Whole of things ; 

 nor was a diffused ether required to complete the machinery 

 of the universe. 



Within a few months, however, of the date assigned for 

 these deliverances of our hypothetical physicist came an 

 addition to this general conception of the world, destined 

 profoundly to modify it. About a hundred years ago Young 

 opened, or re-opened, the great controversy which finally 

 established the undulatory theory of light, and with it a 

 belief in an interstellar medium by which undulations could 

 be conveyed. But this discovery involved much more than 

 the substitution of a theory of light which was consistent 

 with the facts for one which was not : since here was the 

 first authentic introduction ' into the scientific world-picture 

 of a new and prodigious constituent — a constituent which 

 has altered, and is still altering, the whole balance (so to 

 spi-akl of the composition. Unending space, thinly strewn 

 with '^uns and satellites, made or in the making, supplied 

 suflicient material for the mechanism of the heavens as con- 

 ceived by Laplace. Unending space filled with a continuous 

 medium was a verv different affair, and gave promise of 

 strange developments. It could not be supposed that the 

 ether, if its reality were once admitted, existed only to 

 convey through interstellar regions the vibrations which 

 happen to stimulate the optic nerve of man. Invented 

 originally to fulfil this function, to this it could never be 

 confined. .And accordingly, as evervone now knows, things 

 which, from the point of view of sense perception, are as 

 distinct as light and radiant heat, and things to which sense 

 perception makes no response, like the electric waves of 

 wireless telegraph\",' intrinsically differ, not in kind, but 

 in magnitude alone. 



This, however, is not all, nor nearly all. If we jump 

 over the century which separates 1S04 from 1904, and 

 attempt to give in outline the world-picture as it now pre- 

 vents itself to some leaders of contemporary speculation, we 

 shall find that in the interval it has been modified, not 

 merely by such far-reaching discoveries as the atomic and 

 molecular composition of ordinary matter, the kinetic theory 

 of gases, and the laws of the conservation and dissipation 



1 The hypothe 

 Vcungand Fre-ti 



r;innol Se said to have been es-abli^hed 

 ugh Ihe iheoretical work of Maxwell and the 



of energy, but by the more and more important part which, 

 electricity and the ether occupy in any representation of 

 ultimate physical reality. 



Electricity was no more to the natural philosophers in the 

 vear 1700 than the hidden cause of an insignificant pheno- 

 menon.' It was known, and had long been known, that 

 such things as amber and glass could be made to attract 

 light objects brought into their neighbourhood ; yet it was. 

 about fifty years before the effects of electricity were per- 

 ceived in the thunderstorm. It was about 100 years before 

 it was detected in the form of a current. It was about 120 

 years before it was connected with magnetism ; about 170 

 years before it was connected with light and ethereal 

 radiation. 



But to-day there are those who regard gross matter, the- 

 matter of everyday experience, as the mere appearance of 

 which electricity is the physical basis ; who think that the 

 elementary atom of the chemist, itself far beyond the limits 

 of direct perception, is but a connected system of monads- 

 or sub-atoms which are not electrified matter, but are 

 electricity itself; that these systems differ in the number of 

 monads which they contain, in their arrangement, and in 

 their motion relative to each other and to the ether ; that 

 on these differences, and on these differences alone, depend 

 the various qualities of what have hitherto been regarded 

 as indivisible and elementary atoms ; and that while in most 

 cases these atomic systems may maintain their equilibrium 

 for periods which, compared with such astronomical pro- 

 cesses as the cooling of a sun, may seem almost eternal, 

 thev are not less obedient to the law of change than the 

 everlasting heavens themselves. 



But if gross inatter be a grouping of atoms, and if atoms^ 

 be systems of electrical monads, what are these electrical 

 monads? It may be that, as Prof. Larmor has suggested, 

 they are but a modification of the universal ether, a modifi- 

 cation roughly comparable to a knot in a medium which 

 is inextensible, incompressible and continuous. But 

 whether this final unification be accepted or not, it is certain 

 that these monads cannot be considered apart from the 

 ether. It is on their interaction with the ether that their 

 qualities depend ; and without the ether an electric theory 

 of matter is impossible. 



Surely we have here a very extraordinary revolution. 

 Two centuries ago electricity seemed but a scientific toy. 

 It is now thought by many to constitute the reality of which 

 matter is but the sensible expression. It is but a century 

 ago that the title of an ether to a place among the con- 

 stituents of the universe was authentically established. It 

 seems possible now that it may be the stuff out of which 

 that universe is wholly built. Nor are the collateral in- 

 ferences associated with this view of the physical world 

 less surprising. It used, for example, to be thought that 

 mass was an original property of matter, neither capable 

 of explanation nor requiring it ; in its nature essentially 

 unchangeable, suffering neither augmentation nor diminu- 

 tion under the stress of any forces to which it could be 

 subjected ; unalterably attached to, or identified with, each 

 miaterial fragment, howsoever much that fragment might 

 vary in its appearance, its bulk, its chemical or its physical 

 condition. 



But if the new theories be accepted these views must he- 

 revised. Mass is not only explicable, it is actually explained. 

 So far from being an attribute of matter considered in itself, 

 it is due, as I have said, to the relation between the electrical 

 monads of which matter is composed and the ether in which 

 thev are bathed. So far from being unchangeable, it 

 changes, when moving at very high speeds, with every 

 change in its velocity. 



Perhaps, however, the most impressive alteration in our 

 picture of the universe required by these new theories is to 

 be sought in a different direction. We have all, I suppose, 

 been interested in the generally accepted views as to the 

 origin and development of suns with their dependent 

 planetary systems ; and the gradual dissipation of the energy 

 v-hich during this process of concentration has largely taken 

 the form of light and radiant heat. Follow out the theory 

 to its obvious conclusions, and it becomes plain that the- 

 stars now visibly incandescent are those in mid-journey 

 between the nebula; from which they sprang and the frozerL 



1 The modern history of eleclricity begins with Gilbert, but I have 

 throughout confined my observations to the post-Newtonian period. 



NO. 18 I 6, VOL. 70] 



