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ploy oxygen, hydrogen, or nitrogen. With hydrogen curi-
ous effects are observed, due to the sinking of the clouds
through the extremely light gas in which they float. They
illustrate, but do not prove, the untenable notion of those
who say that the clouds of our own atmosphere could not
float if the cloud-particles were not little bladders instead
of full spheres. Before you is a tube filled with the nitrite-
of-amyl vapor, which has been carried into the tube by
hydrogen gas. On sending the beam through the tube a
delicate bluish-white cloud is precipitated. A few strokes
of the pump clear the tube of this cloud, but leave a resi-
due of vapor behind. Again, turning on the beam we have
a second cloud, more delicate than the first. This may be
done half a dozen times in succession. A residue of vapor
will still'linger in‘ the tube sufficient to yield a cloud of ex-
quisite delicacy, both as regards color and texture.

Besides the nitrite of amyl, a great number of other
substances might be employed, which, like the nitrite, have
been hitherto not known to be chemically susceptible to
light. This is, in fact, a representative case. One point
in addition I wish to illustrate, chiefly because the effect is
the same in kind as one of great importance in nature. Our
atmosphere contains carbonic-aCid gas, which furnishes food
to the vegetable world. But this food, as many of you
know, could not be consumed by plants and vegetables
without the intervention of the sun’s rays. As far as we
know, however, these rays are powerless upon the free car-
bonic acid of our atmosphere; the Sun can only decompose
the gas when it is absorbed by the leaves of plants. In the
leaves the carbonic acid is in close proximity with sub-
stances ready to take advantage of the loosening of the
molecules by the waves of light. Incipient disunion being
introduced by the solar rays, the carbon of the gas is seized
upon by the leaf and appropriated, while the oxygen is dis-
charged into the atmosphere.

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