i88oJ 
On Water and A ir . 
191 
behind that alum cell a sheet of paper, a sheet of 
glass, or a sheet of ice, sensitive as this substance 
is, and the rays that pass through the alum cell 
would be perfectly incompetent to warm the water or to 
melt the ice; so that these luminous rays are perfectly 
incompetent to heat transparent bodies. But if a body can 
absorb the luminous rays, then those rays can make them- 
selves manifest, or the action of the rays can make itself 
manifest. And hence it is that when I operated upon clean 
gun-cotton, the fibres of the gun-cotton, being transparent, 
did not at all absorb the luminous rays ; and when I con- 
verged upon it simply the luminous rays from our lamp 
those rays had no power to explode it. But when I allowed 
the total radiation from the lamp to fall upon it, then I 
exploded it. And also when I blackened the gun-cotton, 
those luminous rays were competent to explode it. You 
might suppose that rays that had passed through water or 
through ice had no power to heat anything afterwards. 
Fig. 22. 
You might imagine that in passing through cold water or 
ice the beam would be entirely robbed of its heat ; but I 
want to show you that that is a delusion. Here, for instance, 
I will take a piece of blackened gun-cotton as before, and I 
hope to be able to show you a very beautiful lens of ice. 
There it is — b (Fig. 22) — a lens of perfectly transparent ice. 
I intend to make use of that ice as a burning glass. The 
luminous rays shall pass from our camera, A, through the 
ice, and they shall be converged by the lens, b, and I expedl 
that at the focus of the lens we shall be able to explode the 
gun-cotton, c. Inasmuch as these are luminous rays, the 
gun-cotton is blackened in order to enable it to absorb them. 
We will, first of all, introduce a cell of water between the 
camera and the ice lens. You see that here we have a beau- 
tiful parallel beam, and Mr. Cottrell will interpose the ice 
