i88o.] 
On Water and Air . 
i8 9 
mon paper. The heat is so intense that I could have readily 
ignited a diamond. A very famous experiment of the cele- 
brated Florentine academicians was to ignite a diamond by 
means of a tremendous heat. Another way of converging 
the beam is by means of what is called a conjugate mirror. 
I will send a wide beam out through the aperture in our 
camera, and will then receive that beam upon another mirror. 
The second mirror will gather up the beam to a point or 
focus, and at that point we shall obtain all these tremendous 
effects to which I have been referring. I obtain in front of 
the lamp a fairly parallel beam, which, in the first instance, 
is not collected to a point or focus. But it is collected to a 
focus by the second mirror. [A focus having been produced, 
pieces of paper and sheet zinc were ignited therein, as in the 
former experiment.] 
I now want you to observe a further experiment. I have 
here a substance which, practically, does not interfere at all 
with the passage of the light which emanates from the 
Fig. 21. 
carbon-points. The light goes through it with the greatest 
ease. [A flat glass cell (c, c', Fig. 21 ), containing a solution 
of alum in water, was interposed in the track of the con- 
verging rays, in such a position that the focus fell beyond 
the cell.] I now place in the focus a piece of clean white 
gun-cotton. You see that the gun-cotton remains perfectly 
unignited, although it is a very inflammable substance. Now 
we will remove the cell containing the solution of alum, and 
you will at once notice what occurs. [The solution was 
removed, and the gun-cotton instantly exploded.) The so- 
lution of alum, being a perfectly transparent thing, does not 
