On Water and Air . 
[March, 
190 - 
interfere with the luminous rays coming from our little sun. 
It must have been something else than the luminous rays 
which caused that gun-cotton to ignite. But now let us see 
whether these luminous rays do not possess heat. _ You will 
find that they do, for if I darken the gun-cotton with a black 
powder you will find that the luminous rays are quite com- 
petent to ignite it. [The cell of alum water was replaced m 
the track of the beam.] We will place our blackened gun- 
cotton in the focus, just as in the former case ; and you will 
see that, although this beam of light had no effeCt whatever 
upon the white gun-cotton, yet when the gun-cotton is 
blackened you have an explosion, although the light has 
passed through the solution of alum. » 
Now this leads me to say a few words for the purpose oi 
inquiring into the cause of these differences; and I think 
that if you follow me you will understand it perfectly well. 
I will tell you an experiment that I made yesterday. That 
is an exquisite plate of glass of the purest materials prepared 
for me by Mr. Chance, of Birmingham. It is perfectly 
white glass, altogether uncoloured, and you would imagine 
that it would be perfectly transparent to the radiation from 
our eleCtric light. Here is another plate [a plate of rock 
salt] not more transparent than the plate of glass, if looked 
at bv the eye. Yesterday we placed it in the path ot the 
beam. What occurred? First of all, the glass plate was 
placed in front of the lamp, and we examined the beam that 
had passed through it. We fou nd it enfeebled by its passage 
through the glass plate. We placed in the same position 
this other plate, which consists of rock salt, which is per- 
fectly transparent, and we also examined the beam that 
passed through the rock salt. We found the beam much 
less enfeebled than it was by this transparent glass plate. 
I then felt the rock salt by placing my hand upon it, and it 
was almost perfectly cold. I touched the glass plate, and I 
almost burnt my hand ; so that the glass had the power o 
intercepting the heat and lodging that heat in the body o 
the glass, which power was not possessed by the rock salt. 
In scientific language, we say that this glass is a better 
absorber of heat than the rock salt. The rock salt transmits 
the heat and does not absorb it : the glass absorbs the heat 
and does not transmit it. The effeCt of this difference is 
that a body that does not absorb heat can not be heated. 
A perfectly transparent body, when it is operated upon by 
the rays of pure light, can not be heated by those rays .of 
light. For instance, if I place my alum cell in front of the 
lamp and allow the heat to pass through it, I might place 
