nstitute, in the opinion of our most eminent
authorities, the two great standing enigmas of meteorology. Indeed it
was the interest manifested in them by Sir John Herschel, in a letter
of singular speculative power, addressed to myself, that caused me to
enter upon the consideration of these questions so soon.

The apparatus with which I work consists, as already stated, of a
glass tube about a yard in length, and from 2.5 to 3 inches internal
diameter. The vapour to be examined is introduced into this tube in
the manner already described, and upon it the condensed beam of the
electric lamp is permitted to act, until the neutrality or the
activity of the substance has been declared.

It has hitherto been my aim to render the chemical action of light
upon vapours visible. For this purpose substances have been chosen,
one at least of whose products of decomposition under light shall have
a boiling-point so high, that as soon as the substance is formed it
shall be precipitated. 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 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 small
fraction of the length of a wave of violet light.

In all cases when the vapours of the liquids employed are sufficiently
attenuated, no matter what the liquid may be, the visible action
commences with the formation of a _blue cloud_. But here I must guard
myself against all misconception as to the use of this term. The
'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 blue cloud differs in
many important particulars from the finest ordinary clouds, and might
justly have assigned to it an intermediate position between such
clouds and true vapour. With this explanation, the term 'cloud,' or
'incipient cloud,' or 'actinic cloud,' as I propose to employ it,
cannot, I think, be misunderstood.

I had been endeavouring to decompose carbonic acid gas by light. A
faint bluish cloud, due it may be, or it may not be, to the residue of
some vapour previously employed, was formed in the experimental tube.
On looking across this cloud through a Nicol's prism, the line of
vision being horizontal, it was fou