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as those which our laboratory experiments reveal. It is
not, however, with the iridescences, however beautiful they
may be, that we have now to occupy our thoughts, but
with other effects which bear upon the two great standing
enigmas of meteorology—the color of the sky and the polar-
ization of its light.

It is possible, as stated, by duly regulating the quantity
of vapor, to make our precipitated particles grow from an
infinitesimal and altogether ultra-microscopic size to masses
of sensible magnitude ; and by means of these particles, in
a certain stage of their growth, we can produce a blue
Which shall rival, if it does not transcend, that of the deepest
and purest Italian sky. Let this point be in the first place
established. Associated with our experimental tube is a
barometer, the mercurial column of which now indicates
that the tube is exhausted. Into the tube is introduced a
quantity of the mixed air and nitrite-of-butyl vapor sufficient
to depress the mercurial column one-twentieth of an. inch;
that is to say, the air and vapor together exert a pressure
of one six-hundredth of an atmOSphere. I now add a quan-
tity of air and hydrochloric acid sufficient to depress the
mercury half an inch farther, and into this compound and
highly-attenuated atmosphere I discharge the beam of the
electric light. The effect is slow; but gradually within the
tube arises this splendid azure, which strengthens for a time,
reaches a maximum of depth and purity, and then, as the
particles grow larger, passes into whitish blue. This ex-
periment is representative, and it illustrates a general
principle. Various other colorless substances of the most
diverse properties, optical and chemical, might be employed
for this experiment. The incipient cloud in every case
would exhibit this superb blue ; thus proving to demonstra-
tion that particles of infinitesimal size, without any color of
their own, and irrespective of those optical properties ex-
hibited by the substance in a massive state, are competent
to produce the color of the sky.

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