On Water and Air . 
[March, 
192 
lens in the path of that parallel beam. Those persons in 
front will observe the convergence of the rays by the ice 
lens. The lens brings the rays to a focus, and I will now 
remove our cell of water and there we have instantly the 
explosion of our gun-cotton. 
Before coming into the room, I thought that I would try 
and hasten the next experiment [that of making water boil 
by means of the beam from the electric lamp] by heating in 
a test glass the water that I am going to use before placing 
it in the focus; but I think that it will be better and fairer 
for the experiment if I begin with cold water, operating with 
that cold water as the sun operates with the cold water of 
the ocean. Mr. Cottrell will now give me a little cold water 
from the tap, and I will converge the beam so as to produce 
an intense focus in front of the lamp. I will now bring this 
cold water in a glass tube in front of that focus, and we will 
place across the beam the cell containing the solution of 
alum. The cold water may remain in the focus till dooms- 
day without the light producing any sensible heat in the 
water. The beam of light having passed through the cell of 
alum water, all the rays that are capable of heating water 
are completely taken away from the beam ; so that I might 
hold the water in a focus for any length of time without its 
even becoming warm. We will now take away the cell of 
alum and water, and we shall soon see that the water in 
the tube will be raised to ebullition. In order to make the 
experiment as conclusive as possible, it is worth while to 
begin with cold water. In a very short time, if you give me 
your patience, I trust to be able to boil the water in your 
presence. At the present time I see bubbles of steam being 
generated, and after a little while you will find it visibly 
boiling before you. I hear it beginning to boil already. I 
now feel the dancing of the water. [After a short interval.] 
Now I think that those in front can observe that the water 
is actually boiled by radiant heat. Mr. Cottrell will now 
introduce a cell containing alum water. The boiling instantly 
ceases. The light is no longer capable of boiling the water. 
We take the cell away, and the boiling recommences imme- 
diately. Now I will ask him to interpose a substance which 
will entirely cut away the light, and you will find that the 
ebullition of the water will not cease. In fadt, to speak 
particularly, the emanation from this artificial sun of ours, 
just like that from the natural sun, consists of two classes 
of rays, one capable of exciting vision, and the other, far 
more powerful, capable of exciting heat and not vision ; and 
here, in this case, the alum cell or the water cell cuts off a 
