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2 2 2 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.

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