It is well known that when a receiver filled with ordinary undried air
is exhausted, a cloudiness, due to the precipitation of the aqueous
vapour diffused in the air, is produced by the first few strokes of the
pump. It is, as might be expected, possible to produce clouds in this
way with the vapours of other liquids than water.
In the course of some experiments on the chemical action of light
on vapours which have been communicated to the Royal Society, I had
frequent occasion to observe the precipitation of such clouds; indeed,
several days at a time have been devoted solely to the generation and
examination of clouds formed by the sudden dilatation of mixed air and
vapours in the experimental tubes.
The clouds were generated in two ways: one mode consisted in opening
the passage between the filled experimental tube and the air-pump, and
then simply dilating the air by working the pump. In the other, the
experimental tube was connected with a vessel of suitable size, the
passage between which and the experimental tube could be closed by a
stopcock. This vessel was first exhausted; on turning the cock the air
rushed from the experimental tube into the vessel, the precipitation of
a cloud within the tube being a consequence of the transfer. Instead
of a special vessel, the cylinders of the air-pump itself were usually
employed for this purpose.
It was found possible, by shutting off the residue of air and vapour
after each act of precipitation, and again exhausting the cylinders of
the pump, to obtain with some substances, and without refilling the
experimental tube, fifteen or twenty clouds in succession.
The clouds thus precipitated differed from each other in luminous
energy, some shedding forth a mild white light, others flashing out
with sudden and surprising brilliancy. This difference of action is,
of course, to be referred to the different reflective energies of the
particles of the clouds, which were produced by substances of very
different refractive indices.
Different clouds, moreover, possess very different degrees of
stability: some melt away rapidly, while others linger for minutes
in the experimental tube, resting, as they slowly dissolve, upon its
bottom like a heap of snow. The particles of other clouds are trailed
through the experimental tube as if they were moving through a viscous
medium.
Nothing can exceed the splendour of the diffraction phenomena exhibited
by some of these clouds; the colours are best seen by looking along
the experimental tube from a point above it, the face being turned
towards the source of illumination. The differential motions introduced
by friction against the interior surface of the tube often cause the
colours to arrange themselves in distinct layers.
The difference in texture exhibited by different clouds caused me
to look a little more closely than I had previously done into the
mechanism of cloud-formation. A certain expansion is necessary to
bring down the cloud; the moment just before precipitation the cooling
air and vapour may be regarded as divided into a number of polyhedra,
the particles along the bounding surfaces of which move in opposite
directions when precipitation actually sets in. Every cloud-particle
has consumed a polyhedron of vapour in its formation; and it is
manifest that the size of the particle must depend, not only on the
size of the vapour polyhedron, but also on the relation of the density
of the vapour to that of its liquid. If the vapour were light, and the
liquid heavy, other things being equal, the cloud-particle would be
smaller than if the vapour were heavy and the liquid light. There would
evidently be more shrinkage in the one case than in the other; these
considerations were found valid throughout the experiments; the case of
toluol may be taken as representative of a great number of others. The
specific gravity of this liquid is 0.85, that of water being unity; the
specific gravity of its vapour is 3.26, that of aqueous vapour being
0.6. Now, as the size of the cloud-particle is directly proportional
to the specific gravity of the vapour, and inversely proportional to
the specific gravity of the liquid, an easy calculation proves that,
assuming the size of the vapour polyhedra in both cases to be the
same, the size of the particle of toluol cloud must be more than six
times that of the particle of aqueous cloud. It is probably impossible
to test this question with numerical accuracy; but the comparative
coarseness of the toluol cloud is strikingly manifest to the naked eye.
The case is, as I have said, representative.
In fact, aqueous vapour is without a parallel in these particulars; it
is not only the lightest of all vapours, in the common acceptation of
that term, but the lightest of all gases except hydrogen and ammonia.
To this circumstance the soft and tender beauty of the clouds of our
atmosphere is mainly to be ascribed.
The _sphericity_ of the cloud-particles may be immediately inferred
from their deportment under the luminous beams. The light which
they shed when spherical is _continuous_: but clouds may also be
precipitated in solid flakes; and then the incessant sparkling of the
cloud shows that its particles are _plates_, and not spheres. Some
portions of the same cloud may be composed of spherical particles,
others of flakes, the difference being at once manifested through the
_calmness_ of the one portion of the cloud, and the _uneasiness_ of the
other. The sparkling of such flakes reminded me of the plates of mica
in the River Rhone at its entrance into the Lake of Geneva when shone
upon by a strong sun.--_Proceedings of the Royal Society_, vol. xvii.
p. 317.
* * * * *
Clouds are so often referred to in these pages that I thought it might
be of interest to note the latest remarks on their formation.