me minutes' exposure, however,
clouds begin to form, which grow in density and in beauty as the light
continues to act. In every experiments hitherto made with this
substance the column of cloud filling the experimental tube, was
divided into two distinct parts near the middle of the tube. In one
experiments a globe of cloud formed at the centre, from which, right
and left, issued an axis uniting the globe with two adjacent
cylinders. Both globe and cylinders were animated by a common motion
of rotation. As the action continued, paroxysms of motion were
manifested; the various parts of the cloud would rush through each
other with sudden violence. During these motions beautiful and
grotesque cloud-forms were developed. At some places the nebulous
mass would become ribbed so as to resemble the graining of wood; a
longitudinal motion would at times generate in it a series of curved,
transverse bands, the retarding influence of the sides the tube
causing an appearance resembling, on a small scale, the dirt-bands of
the Mer de Glace. In the anterior portion of the tube those sudden
commotion were most intense; here buds of cloud would sprout forth,
and grow in a few seconds into perfect flower-like forms. The cloud
of iodide of isopropyl had a character Of its own, and differed
materially from all others that I had seen. A gorgeous mauve colour
was observed in the last twelve inches of the tube; the vapour of
iodine was present, and it may have been the sky-blue scattered by the
precipitated particles which, mingling with the purple of the iodine,
produced the mauve. As in all other cases here adduced, the effects
were proved to be due to the light; they never occurred in darkness.

The forms assumed by some of those actinic clouds, as I propose to
call them, in consequence of rotations and other motions, due to
differences of temperature, are perfectly astounding. I content
myself here with a meagre description of one more of them.

The tube being filled with the sensitive mixture, the beam was sent
through it, the lens at the same time being so placed as to produce a
cone of very intense light. Two minutes elapsed before anything was
visible; but at the end of this time a faint bluish cloud appeared to
hang itself on the most concentrated portion of the beam.

Soon afterwards a second cloud was formed five inches farther down the
experimental tube. Both clouds were united by a slender cord of the
same bluis