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pable of being understood, a very beautiful effect which,
under favorable circumstances, might be observed in our
atmosphere. This experimental tube contains an inch of
the iodide-of-allyl vapor, the remaining 29 inches necessary
to fill the tube being air, which has bubbled through aque-
ous hydrochloric acid. Besides, therefore, the vapor of
iodide of allyl, we have those of water and of acid within
the tube. The light has been acting on the mixture for
some time, abeautiful incipient blue cloud being formed. As
before stated, the “incipient cloud ” is Wholly different in
texture and Optical properties from an ordinary cloud; but
it is possible to precipitate in the midst of the azure the
aqueous vapor so as to cause it to form in the tube a cloud
similar to the clouds of our atmosphere. An exhausted
vessel of about one-third of the capacity of the experi-
mental tube is connected with it, the passage uniting both
being closed by a stop-cock. On opening this cook the
mixed air and vapor rush from theiexperimental tube into
the empty vessel; and,'in consequence of the chilling due
to rarefaction, the vapor in the experimental tube is pre-
cipitated as a true cloud. What is the result ? Instantly
the centre-of the system of colored rings becomes bright,
and the whole series of colors corresponding to definite ra-
dial distances, complementary. While you continue to
look at the cloud, it gradually melts away as an atmos-
pheric cloud might do in the azure of heaven. And there
is our azure also remaining behind. The coarser cloud
seems drawn aside like~ a veil, the blue reappears, the first
ring-system, with its dark centre and correspondingly col-
ored circles, being restored.

Thus patiently you have accompanied me over a piece
of exceedingly difficult ground; and I think, as a prudent
guide, we ought to halt upon the eminence we have now
attained. W'e might go higher, but the bowlders begin
here to be very rough. At a future day we shall, I doubt

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