84 FKAGMENTS OF SCIENCE. 



be its heat, is sensibly incompetent to melt ice. We 

 can, for example, converge 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 sun- 

 shine 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 power- 

 ful dark rays is emitted 1 y 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 captivity the Rhone and 

 the Rhine. 



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 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 imme- 

 diately 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 transparent water of 

 the cell exerts no sensible absorption on the luminous 

 rays, still it withdraws something from the beam, which, 

 when permitted to act, is competent to melt the ice. 

 This something is the dark radiation of the electric 

 light. Again, I place a slab of pure ice in front of the 

 electric lamp ; send a luminous beam first through our 



