FOG
To many nothing is more troublesome than a dense fog in a large town. It
paralyses traffic, it is dangerous to pedestrians, it encourages theft, it
chokes the asthmatic, and chills the weak-lunged.
In the country it is disagreeable enough; but never so intensely raw and
dense as in the city. On the sea, too, the fog is disagreeable and fraught
with danger. The fog-horn is heard, in its deep, sombre note, from the
lighthouse tower, when the strong artificial light is almost useless.
But a peculiar sense of stagnation possesses the dweller of the large
town, when enveloped in a dense fog. Sometimes during the day, through a
thinner portion, the sun will be dimly seen in copper hue, like the moon
under an eclipse. The smoke-impregnated mass assumes a peculiar "pea-soup"
colour.
Now, what is this fog? How is it formed? It has been ascertained that fogs
are dependent upon _dust_ for their formation. Without dust there could
be no fogs, there would be only dew on the grass and road. Instead of the
dust-impregnated air that irritates the housekeeper, there would be the
constant dripping of moisture on the walls, which would annoy her more.
Ocular demonstration can testify to this. If two closed glass receivers be
placed beside each other, the one containing ordinary air, and the other
filtered air (_i.e._ air deprived of its dust by being driven through
cotton wool), and if jets of steam be successively introduced into these,
a strange effect is noticed. In the vessel containing common air the steam
will be seen rising in a dense cloud; then a beautiful white foggy cloud
will be formed, so dense that it cannot be seen through. But in the vessel
containing the filtered air, the steam is not seen at all; there is not
the slightest appearance of cloudiness. In the one case, where there was
the ordinary atmospheric dust, fog at once appeared; in the other case,
where there was no dust in suspension, the air remained clear and
destitute of fog. Invisible dust, then, is necessary in the air for the
formation of fogs.
The reason of this is that a free-surface must exist for the condensation
of the vapour-particles. The fine particles of dust in the air act as
free-surfaces, on which the fog is formed. Where there is abundance of
dust in the air and little water-vapour present, there is an
over-proportion of dust-particles; and the fog-particles are, in
consequence, closely packed, but light in form and small in size, and take
the lighter appearance of fog. Accordingly, if the dust is increased in
the air, there is a proportionate increase of fog. Every fog-particle,
then, has embosomed in it an invisible dust-particle.
But whence comes the dust? From many sources. It is organic and inorganic.
So very fine is the inorganic dust in the atmosphere that, if the
two-thousandth part of a grain of fine iron be heated, and the dust be
driven off and carried into a glass receiver of filtered air, the
introduction of a jet of steam into that receiver would at once occasion
an appreciable cloudiness.
This is why fogs are so prevalent in large towns. Next the minute
brine-particles, driven into the air as fog forms above the ocean surface,
are the burnt sulphur-particles emanating from the chimneys in towns. The
brilliant flame, as well as the smoky flame, is a fog-producer. If gas is
burnt in filtered air, intense fog is produced when water-vapour is
introduced. Products of combustion from a clear fire and from a smoky one
produce equal fogging. The fogs that densely fill our large towns are
generally less bearable than those that veil the hills and overhang the
rivers.
It is the sulphur, however, from the consumed coals, which is the active
producer of the fogs of a large town. The burnt sulphur condenses in the
air to very fine particles, and the quantity of burnt sulphur is enormous.
No less than seven and a half millions of tons of coals are consumed in
London. Now, the average amount of sulphur in English coal is one and a
quarter per cent. That would give no less than 93,750 tons of sulphur
burned every year in London fires. Now, if we reckon that on an average
twice the quantity of coals is consumed there on a winter day that is
consumed on a summer day, no less than 347 tons of the products of
combustion (in extremely fine particles) are driven into the
superincumbent air of London every winter day. This is an enormous
quantity, quite sufficient to account for the density of the fogs in that
city.