CLOUDS AND RIVERS, ICE AND GLACIERS. 



17 



diamond similarly placed is caused to glow like a star, 

 being afterwards gradually dissipated. And all this 

 while the air at the focus remains as cool as in any 

 other part of the room. 



47. To obtain the light-waves we employ a clear 

 solution of alum in water; to obtain the dark waves 

 we employ the solution of iodine above referred to. 

 But as before stated (32), the alum is not so perfect a 

 Biter as the iodine ; for it transmits a portion of the 

 obscure heat. 



48. Though the light-waves here prove their incom- 

 petence to ignite gun-cotton, they are able to burn up 

 black paper ; or, indeed, to explode the cotton when it 

 is blackened. The white cotton does not absorb the 

 light, and without absorption we have no heating. The 

 blackened cotton absorbs, is heated, and explodes. 



49. Instead of a solution of alum, we will employ for 

 our next experiment a cell of pure water, through which 

 the light passes without sensible absorption. At the 

 focus is placed a test-tube also containing water, the full 

 force of the light being concentrated upon it. The 

 water is not sensibly warmed by the concentrated 

 waves. We now remove the cell of water; no change 

 is visible in the beam, but the water contained in the 

 test-tube now boils. 



50. The light-waves being thus proved ineffectual, 

 and the full beam effectual, we may infer that it is the 

 dark waves that do the work of heating. But we clench 



