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." Can these, then, be counted? Yes, Dr. Aitken has numbered the dust of the air. I shall never forget my rapt astonishment the day I first numbered the dust in the lecture-room of the Royal Society of Edinburgh, with his instrument and under his direction. This wonderfully ingenious instrument was devised on this principle, that every fog-particle has entombed in it an invisible dust-particle. A definite small quantity of common air is diluted with a fixed large quantity of dustless air (_i.e._ air that has been filtered through cotton-wool). The mixture is allowed to be saturated with water-vapour. Then the few particles of dust seize the moisture, become visible in fine drops, fall on a divided plate, and are there counted by means of a magnifying glass. That is the secret!