1885.] The Relations of Mind and Matter. TISI 
lum of belief swung to its opposite side, and a general incredulity 
on the whole subject of ethers succeeded. In more recent times 
this incredulity is giving way, and our most skillful physicists are 
fully persuaded of the existence of one ether, the medium of light 
radiations. To those scientists the universe contains twò distinct 
substances, matter and ether, which differ radically in their con- 
ditions of existence. 
Students of the phenomena of light declare that there is no 
possible escape from this conclusion. Interstellar space is prob- 
ably occupied by ordinary matter in a state of extreme diffusion, 
and it might be conjectured that the rays of light could make 
their way through this substance from star to star. Science de- 
clares that this is impossible. Matter, under such circumstances, 
must be a very rare gas, and no gas can transmit transverse 
vibrations like those of light. Its degree of diffusion also would 
be far too great and too irregular. To transmit the rays of light 
some substance is needed that is very elastic; that, while exceed- 
ingly rare, is far more dense than diffused matter would be in 
interstellar space ; and that is in the condition of a rigid solid 
instead of a free gas. Yet it must be a solid so readily permeable 
as to present no resistance whatever to the movement of the 
spheres. As a ball will sink with little resistance through a thin 
jelly, without leaving a mark of its passage, so must a globe be 
able to plunge unopposed through the almost infinitely thin jelly 
of the ether. , : 
Ether has never been seen, felt, weighed. or measured. It is 
absolutely invisible and intangible. No vessel has pores so 
minute as to confine it. It has properties seemingly contradic- 
tory ; it must be excessively rare yet perfectly elastic ; its physi- 
cal state must resemble that of the. solid while its density is im- 
mensely less. We have no proof of its existence resembling our 
proofs of the existence of matter, yet we are forced to believe in 
it because physical science cannot possibly do without it. There 
are hundreds of phenomena which cannot be explained without 
it, but can readily be explained with it. 
Sir William Thomson, in his lecture before the Franklin Insti- 
tute during the 1884 meeting of the American Association for 
the Advancement of Science, said of this intangible necessity : 
“You may regard the existence of a luminiferous ether as a 
reality of science.” “One thing we are sure of, and that is the 
