84 FRAGMENTS OF SCIENCE. 



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

 ever intense may 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 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 Ehine. 



Placing a concave silvered mirror behind the elec- 

 tric 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 introduc- 

 ing 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 



