62 FRAGMENTS 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 niay 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 winch 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 
