222^ FRAGMENTS OF SCIENCE. 



the stars, and even the nebulae, are composed : the principle, 

 namely, that a body which is competent to emit any ray, 

 whether of heat or light, is competent in the same degree 

 to absorb that ray. The absorption depends on the syn- 

 chronism which exists between the vibrations of the atoms 

 from which the rays, or more correctly the waves, issue, and 

 those of the atoms against which they impinge. 



To its incompetence to emit white light, aqueous vapor 

 adds 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 in- 

 competence of aqueous vapor to absorb luminous 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 substances without dis- 

 turbing the molecular rest. A purely luminous beam, how- 

 ever intense may be its heat, is sensibly incompetent to 

 melt the smallest particle of ice. We can, for example, 

 converge a powerful luminous beam upon a surface covered 

 with hoar-frost without melting a single spicula of the ice- 

 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 solar 

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

 nous 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 ab- 

 sorbed, and hence they cannot produce fusion. But a body 

 of powerful dark rays is emitted by the sun, and it is these 

 rays that cause the glaciers to shrink and the snows to dis- 

 appear ; it is they that fill the banks of the Arve and Ar- 

 veyron, and liberate from their frozen captivity the Rhone 

 and the Rhine. 



