ACTION OF DARK RADIATIONS. 207 



Though the light-waves here prove their incompetence 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. 



Instead of a solution of alum, we will employ for our next experi- 

 ment 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. 



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 clinch our inference by employing our opaque iodine 

 filter. Placing it on the path of the beam, the light is entirely 

 stopped, but the water boils exactly as it did when the full beam 

 fell upon it. 



And now with regard to the melting of ice. On the surface of a 

 flask containing a freezing; mixture we obtain a thick fur of hoar-frost. 

 Sending the beam through a water-cell its luminous waves are concen- 

 trated upon the surface of the flask. Not a spicula of the frost is dis- 

 solved. We now remove the water-cell, and in a moment a patch of 

 the frozen fur as large as half a crown is melted. Hence, inasmuch as 

 the full beam produces this effect, and the luminous part of the beam 

 does not produce it, we fix upon the dark portion the melting of the 

 frost. As before, we clinch this inference by concentrating the dark 

 waves alone upon the flask. The frost is dissipated exactly as it was 

 by the full beam. 



These effects are rendered strikingly visible by darkening with ink 

 the freezing mixture within the flask. When the hoar-frost is removed, 

 the blackness of the surface from which it had been melted comes out 

 in strong contrast with the adjacent snowy whiteness. When the 

 flask itself, instead of the freezing mixture, is blackened, the purely 

 luminous waves, being absorbed by the glass, warm it ; the glass 

 reacts upon the frost, and melts it. Hence the wisdom of darkening, 

 instead of the flask itself, the mixture within the flask. 



This experiment proves to demonstration that it is the dark waves 

 of the sun that melt the mountain snow and ice, and originate all the 

 rivers derived from glaciers. 



There are writers who seem to regard science as an aggregate of 

 facts, and hence doubt its efficacy as an exercise of the reasoning pow- 

 ers. But all that I have here taught you is the result of reason, taking 

 its stand, however, upon the sure basis of observation and experiment. 

 And this is the spirit in which our further studies are to be pursued. 



