Sec. 12. Artificial Sky.

But we have it in our power to imitate far more closely the natural conditions of this problem.  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 sending through this compound and highly attenuated atmosphere the beam of the electric light, within the tube arises gradually a 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 experiment is representative, and it illustrates a general principle.