. We can generate in air artificial skies,
and prove their perfect identity with the natural one, as regards the
exhibition of a number of wholly unexpected phenomena. It has been
recently shown in a great number of instances by myself that waves of
ether issuing from a strong source, such as the sun or the electric
light, are competent to shake asunder the atoms of gaseous molecules.
The apparatus used to illustrate this consists of a glass tube about a
yard in length, and from 21/2 to 3 inches internal diameter. The gas or
vapour to be examined is introduced into this tube, and upon it the
condensed beam of the electric lamp is permitted to act. The vapour is
so chosen that one, at least, of its products of decomposition, as
soon as it is formed, shall be _precipitated_ to a kind of cloud. By
graduating the quantity of the vapour, this precipitation may be
rendered of any degree of fineness, forming particles distinguishable
by the naked eye, or particles which are probably far beyond the reach
of our highest microscopic powers. I have no reason to doubt that
particles may be thus obtained whose diameters constitute but a very
small fraction of the length of a wave of violet light.

Now, in all such cases when suitable vapours are employed in a
sufficiently attenuated state, no matter what the vapour may be, the
visible action commences with the formation of a _blue cloud_. Let me
guard myself at the outset against all misconception as to the use of
this term. The blue cloud here referred to is totally invisible in
ordinary daylight. To be seen, it requires to be surrounded by
darkness, _it only_ being illuminated by a powerful beam of light.
This cloud differs in many important particulars from the finest
ordinary clouds, and might justly have assigned to it an intermediate
position between these clouds and true cloudless vapour.

It is possible to make the particles of this _actinic cloud_ grow from
an infinitesimal and altogether ultra-microscopic size to particles of
sensible magnitude; and by means of these in a certain stage of their
growth, we produce a blue which rivals, if it does not transcend, that
of the deepest and purest Italian sky. Introducing into our tube a
quantity of mixed air and nitrite of butyl vapour sufficient to
depress the mercurial column of an air-pump one-twentieth of an inch,
adding a quantity of air and hydrochloric acid sufficient to depress
the mercury half an inch further, and sendi