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.
CHAPTER VI
THE NUMBERING OF THE DUST
If the shutters be all but closed in a room, when the sun is shining in, myriads of floating particles can be seen glistening in the stream of light. Their number seems inexhaustible. According to Milton, the follies of life are—
“Thick and numberless,
As the gay motes that people the sunbeams.”