A large proportion of smoke consists of particles of pure unconsumed carbon, and this is accompanied in its passage up our chimneys by sulphurous acid, begotten by the sulphur which is contained in the coal to the amount of about eight pounds in every thousand; by sulphuretted hydrogen, by hydro-carbons, and by vapours of various kinds of oils, small quantities of ammonia, and other bodies not by any means contributing to a healthy condition of the atmosphere. A good deal of the heavier carbon is deposited along the walls of chimneys in the form of soot, together with a small percentage of sulphate of ammonia; this is as a consequence very generally used for manure. The remainder is poured out into the atmosphere, there to undergo fresh changes, and to become a fruitful cause of those thick black fogs with which town-dwellers are so familiar. Sulphuretted hydrogen (H_{2}S) is a gas well known to students of chemistry as a most powerful reagent, its most characteristic external property being the extremely offensive odour which it possesses, and which bears a strong resemblance to that of rotten eggs or decomposing fish. It tarnishes silver work and picture frames very rapidly. On combustion it changes to sulphurous acid (SO_{2}), and this in turn has the power of taking up from the air another atom of oxygen, forming sulphuric acid (SO_{3} + water), or, as we more familiarly know it, oil of vitriol. Yet the smoke itself, including as it does all the many impurities which exist in coal, is not only evil in itself, but is evil in its influences. Dr Siemens has said:--"It has been shown that the fine dust resulting from the imperfect combustion of coal was mainly instrumental in the formation of fog; each particle of solid matter attracting to itself aqueous vapour. These globules of fog were rendered particularly tenacious and disagreeable by the presence of tar vapour, another result of imperfect combustion of raw fuel, which might be turned to better account at the dyeworks. The hurtful influence of smoke upon public health, the great personal discomfort to which it gave rise, and the vast expense it indirectly caused through the destruction of our monuments, pictures, furniture, and apparel, were now being recognised."