194 
On Water and Air . 
[March 
those inferences into fadts. Perhaps it is worth while to 
make an experiment. By means of this larger lens— I (Fig. 
23 ) — x w m converge upon the beam emanating from the 
camera L the dust of the room. You see the beam passing 
through the air of the room. If we bring the lens forward 
a little from the light, you have a conical beam, and here 
you have the point of convergence of the beam at P. I will 
just mark that point of convergence. I have here an 
instrument (held in the hand at p) which will enable me to 
test whether or not there is any heat in that beam. Mr. 
Cottrell has associated this instrument with the magnetic 
needle which you see suspended there before you ; and if I 
breathe for a moment on this instrument, you will see the 
effedt which the warmth of my breath produces upon the 
needle. It is an instrument used for the purpose of convert- 
ing heat into electricity, and causing the magnetic needle to 
move. The warmth of my breath will be sufficient to pro- 
duce a large deflection of the needle. I breathe, and there 
you see the needle moves away, and, as I have said, a large 
deflection is produced. 
Well, now, here we have our beam of light converged in 
the dust of the room. I will place upon this side of the 
instrument [the thermo-eleCtric pile p] this cap, and on the 
other face I will place a little reflector. It will require a 
little time before the effeCt of the waste heat that I com- 
municated by my breath disappears. We will now bring 
the needle to rest. We will cause the pile to get cool by 
presenting it to a cool part of the room, and the needle will 
gradually come back to zero. If I present the pile towards 
the piece of ice, perhaps it will be still better. It will cause 
the needle to come back still more rapidly towards zero. If 
I place this instrument in the path of this beam of light, 
you will get a deflection of the red end of the needle towards 
me. Now I want you to see that if we place this opaque 
solution (contained in a cell, c,) in the path of the beam and 
entirely cut off the light, still the heat will pass through, and 
fall upon the pile, and you see that effedt produced which is 
indicated by the movement of the needle. 
Now I want to colled! these heat rays, and show you that 
they are capable of boiling water. We have here our focus 
(f, Fig. 21), and I will boil the water as before. Mr. Cot- 
trell will go to the other lamp and cause its light to fall upon 
the boiling water, for I want you to see the experiment. 
[The tube of water which had been used in the previous 
experiments was again caused to boil in the eledtric beam. 
The cell (c, c', Fig. 21) containing the opaque solution was 
