On Water and Air. 
193 
1880.] 
great portion of these rays that are not capable of exciting 
vision. Bat you might infer from the experiment that we 
have made that there is something besides the light-giving 
rays that produced the effects that you have seen — the burn- 
ing of the paper and the exploding of the gun-cotton. 
Now we want to detach from the eledtric light this par- 
ticular portion of the emanation that is effective in producing 
those combustions and in boiling the water; and for this 
purpose we must discover a substance which shall cut off all 
the luminous rays, and allow to pass only those rays that 
are called invisible rays. Here I have such a substance. 
Mr. Cottrell has here a cell containing it. He will throw 
upon the screen the image of the carbon points from which 
our light is produced, and here is the substance to which I 
have referred. It is perfectly dark. If you look through U 
at the sun at noon-day you will find that this substance will 
cut off the rays of light. You will see that the opaque 
liquid contained in this cell will completely cut off every 
trace of the light from our little domestic sun. No trace of 
that light can pass through the liquid. Now what I want 
you to understand is that through this opaque . solution a 
certain portion of the radiation from our domestic sun will 
pass absolutely with perfedt freedom, and we shall be able 
to cause water to boil in the focus. When that has been 
done, I will interpose the alum cell in the path of the beam 
instead of the opaque solution, and you will see that when 
the alum cell is used the boiling immediately ceases. You 
will see that although all the light has been cut off by the 
solution, the water will still continue to boil. I said that 
we might have inferred from our former experiments that 
there was in the beam, besides the light, something which 
caused the water to boil ; but inferences ought never to be 
accepted in science if we have an opportunity of converting 
VOL. 11. (third series.) p 
Fig. 23. 
