August 26, 1904.] 



SCIENCE. 



259 



eipia, ' and, roughly speaking, about mid- 

 way between that epoch-making date and 

 the present moment. I 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 ponderable matter, scat- 

 tered in different combinations through 

 space, exhibiting most varied aspects under 

 the influence of chemical aifinity and tem- 

 perature, but through every metamorphosis 

 obedient to the laws of motion, always re- 

 taining its mass unchanged, and exercising 

 at all distances a force of attraction on 

 other material masses, according to a 

 simple law. To this ponderable matter he 

 would (in spite of Rumford) have prob- 

 ably added the so-called 'imponderable' 

 heat, then often ranked among the ele- 

 ments; together with the two 'electrical 

 fluids,' and the corpuscular emanations 

 supposed to constitute light. 



In the universe as thus conceived, the 

 most important forms of action between its 

 constituents was action at a distance; the 

 principle of the conservation of energy was, 

 in any general form, undreamed of; elec- 

 tricity and magnetism, though already the 

 subjects of important investigation, 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, des- 

 tined 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 me- 

 dium by which undulations could be con- 

 veyed. 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 scien- 

 tific world-picture of a new and prodigious 

 constituent— a constituent which has al- 

 tered, and is still altering, the whole bal- 

 ance (so to speak) of the composition. 

 Unending space, thinly strewn with suns 

 and satellites, made or in the making, sup- 

 plied sufficient material for the mechanism 

 of the heavens as conceived by Laplace. 

 Unending space filled with a continuous 

 medium was a very 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 vi- 

 brations 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 every one 

 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 telegraphy,t 

 intrinsically differ, not in kind, but in 

 magnitude alone. 



This, however, is not all, nor nearly all. 

 If we jmnp over the century which sep- 

 arates 1804 from 1904, and attempt to give 

 in outline the world-picture as it now pre- 

 sents itself to some leaders of contemporary 

 speculation, we shall find that in the in- 

 terval 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 of 

 energy, but by the more and more impor- 

 tant part which electricity and the ether 



* The hypothesis of an ether was, of course, not 

 new. But before Young and Fresnel it can not 

 be said to have been established. 



t First known through the theoretical work of 

 Maxwell and the experiments of Herz. 



