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Se 
ADDRESS. 5 
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 space, exhibiting 
most varied aspects under the influence of chemical affinity and temperature, 
but through every metamorphosis obedient to the laws of motion, always 
retaining its mass unchanged, and exercising at all distances « 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 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 deliver- 
ances of our hypothetical physicist, came an addition to this general con- 
ception 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 speak) of the composition. Un- 
ending space, thinly strewn with suns and satellites, made or in the 
making, supplied 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 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, 
ag everyone 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,? intrinsically differ, not in kind, but in magnitude alone. 
This, however, is not all, nor nearly all. If we jump over the century 
! The hypothesis of an ether was, of course, not new. But before Young and 
Fresnel it cannot be said to have been established. 
? First known through the theoretical work of Maxwell and the experiments of 
Herz, 
