90 FRAGMENTS OF SCIENCE 



To its almost total incompetence to emit white light, 

 aqueous vapor adds a similar incompetence to absorb white 

 light. It cannot, for example, absorb the luminous rays 

 of the sun, though it can absorb the non-luminous rays of 

 the earth. This incompetence of the vapor to absorb lu- 

 minous rays is shared by water and ice in fact, by all 

 really transparent substances. Their transparency is due 

 to their inability to absorb luminous rays. The molecules 

 of such substances are in dissonance with the luminous 

 waves; and hence such waves pass through transparent 

 bodies without disturbing the molecular rest. A purely 

 luminous beam, however intense may be its heat, is sen- 

 sibly incompetent to melt ice. We can, for example, con- 

 verge a powerful luminous beam upon a surface covered 

 with hoar frost, without melting a single spicula of the 

 crystals. How then, it may be asked, are the snows of 

 the Alps swept away by the sunshine of summer? I an- 

 swer, they are not swept away by sunshine at all, but by 

 rays which have no sunshine whatever in them. The lu- 

 minous rays of the sun fall upon the snow-fields and are 

 flashed in echoes from crystal to crystal, but they find 

 next to no lodgment within the crystals. They are hardly 

 at all absorbed, and hence they cannot produce fusion. 

 But a body of powerful dark rays is emitted by the sun; 

 and it is these that cause the glaciers to shrink and the 

 snows to disappear; it is they that fill the banks of the 

 Arve and Arveyron, and liberate from their frozen captiv- 

 ity the Ehone and the Ehine. 



Placing a concave silvered mirror behind the electric 

 light its rays are converged to a focus of dazzling bril- 

 liancy. Placing in the path of the rays, between the light 

 and the focus, a vessel of water, and introducing at the 



