162 PROCTOR, ON LIGHT. 
is heat? A motion in the molecules of matter. So, at least, 
they are commonly defined. 
If heat is a motion in the molecules of matter, it cannot 
radiate except in the presence of matter. If radiation takes 
place with the velocity of light, we can only suppose it to be 
an undulation; if it takes place through inter-planetary 
space, we can only suppose it to be an undulation in lumini- 
ferous ether. What is heat? An undulation in luminifer- 
ous ether. 
Light is only known tous through the nerves, and through 
the chemical action it exerts upon matter. A sensation and 
a chemical force, both of which we attribute to motion in 
the molecules of matter. Light a motion in the molecules of 
matter ? 
You perceive how readily the definition of one has been 
made to fit the other. How indefinite is our idea of any 
force in the abstract. We know the forces only through their 
effects upon matter; and though we wish to comprehend 
the cause which produces this effect, we must be very cautious 
in adopting any definition, for by confining our ideas within 
a false boundary, we may blind ourselves to the reception of 
truths which would otherwise flow upon us. 7 
The time was—not long ago—when it was thought that 
light could be deprived of its heating power; when it was 
thought that light falling upon a black body was annihilated 
or absorbed. But the doctrine of the conservation of force 
has dispelled that of annihilation, and the theory of absorp- 
tion is no more tenable. Expose some charcoal to sunshine 
for a thousand years, it goes on absorbing with undiminished 
power; but set fire to the charcoal, and will you behold that 
that thousand years of glorious sunshine is concentrated into 
a few moments? No! For it is not there to come out. The 
sunshine which has poured into it is no more there than if 
you had poured water into a sieve. It has been coming out 
as fast as it went in. It fell upon the charcoal as light, but 
it left it as heat, or some other invisible modification of 
force. 
There is no such thing as annihilation, no such thing as 
absorption of light. Dark bodies only have the greater 
power of converting it into something else, a power probably 
the converse of that possessed by phosphorescent and fluo- 
rescent bodies, which convert some invisible rays into lumi- 
nous ones. 
The time was—not long ago—when light was believed to 
be reflected from the surfaces of bodies. And now it is only 
when we are on our guard that we bear in mind the thick- 
