62 PR A OMENTS OF SCIENCE. 



that ray. The absorption depends on the synchronism 

 existing between the vibrations of the atoms from which 

 the rays, or more correctly the waves, issue, and those of 

 the atoms on which they impinge. 



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 lumi- 

 nous 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 sensi- 

 bly 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 answer, 

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

 rays which have no sunshine whatever in them. The 

 luminous 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 cap- 

 tivity the Rhone and the Rhine. 



Placing a concave silvered mirror behind the electric 

 light its rays are converged to a focus of dazzling brilliancy. 

 Placing in the path of the rays, between the light and the 

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

 piece of ice, the ice is not melted by the concentrated beam. 

 Matches, at the same place, are ignited, and wood is set 

 on fire. The powerful heat, then, of this luminous beam 

 is incompetent to melt the ice. On withdrawing the cell 

 of water, the ice immediately liquefies, and the water 

 trickles from it in drops. Reintroducing the cell of water, 

 the fusion is arrested, and the drops cease to fall. The 



