sturbing the molecular rest. A
purely luminous beam, however 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 captivity 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 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 cell of water and then through the ice. By means of
a lens an image of the slab is cast upon a white screen. The beam,
sifted by the water, has little power upon the ice. But observe what
occurs when the water is removed; we have here a star and there a
star, each star resembling a flower of six petals, and growing visibly
larger before our eyes. As the leaves enlarge, their edges become
serrated, but there is no deviation from the six-rayed type. We have
here, in fact, the crystallisation of the ic