i88x.] The Source of Electric Energy. 383 
Non-conduCtors resemble opaques to light. Some of them 
absorb a portion of the eleCtric energy, others refuse it alto- 
gether. They are white to the electric ray. The opaque 
absorbents of electricity, however, have their direct analogy 
in phosphorescent substances. For the electricity which 
penetrates them does not become heat, but is subsequently 
emitted as electricity ; precisely as the light which pene- 
trates phosphorescents does not become heat, but is subse- 
quently emitted as light. 
Electricity and light hold to each other the following 
relations -Transparents to light are, as a rule, opaque to 
electricity. Transparents to electricity are opaque to light. 
EleCtric rays within a solid can no more escape from it into 
the opaque air than light rays can escape from the air into an 
opaque solid. The eleCtric rays may seem to differ from the 
light rays in that the former can be retained for a long time 
within a limited space, while light rays, when repelled, in- 
stantly disappear. But so do eleCtric rays when they have 
the same opportunity. Light has an unlimited expanse of 
transparent matter into which to disperse. Electricity would 
disperse with equal rapidity and completeness within an 
unlimited sphere of metal, for its rapidity of transmission is 
closely similar to that of light. The marked differences which 
we discover between these two modes of energy may entirely 
arise from the differences of condition under which we 
observe them. 
We may conceive of light under circumstances similar to 
those under which we usually observe electricity. For if 
we imagine a beam of light to enter a space surrounded by 
opaque surfaces, and closed so that the light cannot escape, 
we would have a case closely resembling that of a charged 
conductor surrounded by an insulating medium. Just how 
light would behave under such circumstances it is not easy 
to decide. We know, as will be shown further on, that it 
could not dart continually from side to side of the space. It 
might exist as a surface energy like electricity. It might 
slowly force itself into the surrounding surfaces, as electricity 
will force itself into insulating glass. Only this we can 
positively declare, that if a slight opening to the outer air 
were made any aCtive light rays would instantly dart through 
this transparent channel and escape from their confinement. 
But we also know that, in the case of a charged eleCtric 
conductor, if a slight opening be made through the opaque 
air, by bringing the extremity of a wire into contact with the 
conductor, the electric rays will instantly dart out by this 
conducting channel, and rush over the wire with the speed 
