Smoke, a common term to signify the volatile products of the imperfect combustion of such organic substances as wood or coal. The smoke from burning wood is almost colourless, consisting principally of carbonic acid and water, whilst that produced by burning coal is generally laden with oily and tarry vapour and finely divided carbon (or soot). The different effects resulting from the use of these varieties of fuel is exemplified in the brightness of Paris as compared with the (too generally) gloomy atmosphere of London. In London the smoke nuisance is an old grievance; Evelyn the diarist wrote his Fumifugium in 1661; and in his diary explains the unpleasantness of the great fog of the winter of 1684 (when a fair was held on the Thames), and justly ascribes it to the 'fuliginous steam of the sea-coale.'
The smoke nuisance may justly claim pre-eminence for itself in Great Britain, where coal is generally cheap, and where its users either from ignorance or prejudice refuse to adopt means for its proper combustion. In bright warm windy weather the smoke so produced is carried away from towns and factories, and becomes lost to view by mixing with the air; but in cold calm weather with an atmosphere saturated with moisture (or what is a true white fog) the smoke is arrested, and mixing with it gradually accumulates, and forming as it were a vast aerial emulsion of water vapour, tarry matter, and soot, gives rise to the black or brown fogs now so common in the large cities. With a view to the prevention of such a disagreeable and dangerous condition of the atmosphere much has been attempted, more especially by the Society for the Abatement of the Smoke Nuisance (see its Memoirs), but so far without beneficial results.
In this article there falls to be explained, first, the causation of such dark-coloured fogs; secondly, what has been done and what may yet be effected in the direction of their prevention. (1) When a microscopic slide is passed quickly over a smoky flame, a thin, semi-transparent film is left on the glass, which when examined under a powerful microscope presents the appearance of numerous particles of amorphous carbon in a finely divided condition, each particle being surrounded by an areola or coating of oily or tarry matter. This explains why a black fog may be and will remain persistent, even while rain is falling, each particle of carbon being, so to speak, surrounded by a waterproof coating which repels moisture. Aitken has conclusively proved that the cause of the separation of liquid water from a moisture-laden atmosphere or white fog is due to particles of fine dust present in the air (see FOG, Vol. IV. p. 706). We should therefore expect that the introduction of solid particles of carbon into such an atmosphere would have the same effect, which doubtless would be the case did not each particle repel the vaporous particles of water in its immediate neighbourhood; at the same time their density is insufficient to cause them to fall through the turgid atmosphere as smuts.
In a smoky town when a breeze prevails the smoke in its horizontal passage through the air gradually parts with its greasy-coated carbon to anything which impedes its progress, such as buildings, trees, &c.; so much so that only a few miles away it loses its dolorous aspect and assumes the soft, dreamy haze so dear to the painter, but which after all is only an air emulsion of finely attenuated particles of grease.
The densest fog of this kind on being agitated will deposit all its carbon as smuts, and become changed into a white fog or mist; therefore changes of temperature causing convection currents, or electrical discharges, will disperse the densest fog with extraordinary rapidity. In like manner such a fog finding its way into a dwelling-house is exposed to currents of dry heated air, in which the condensed moisture reassumes its gaseous or invisible condition, while the other constituents are deposited as a grimy coating upon its walls, furniture, &c. Likewise in breathing such a fog-laden atmosphere the carbon and oily products are arrested by the air-passages, and become apparent in the expectorated secretion from the bronchial tubes. A comparatively small amount of solid carbon and oily vapour may thus bring about atmospheric conditions wholly at variance with all ideas of beauty, comfort, and cleanliness. Of course with perfect combustion only carbonic acid and water vapour are the products; both being colourless vapours. There are numerous mechanical stokers by which the green fuel is gradually carried from the furnace doors to the fire bridge, by which means the smoke and tarry vapours given off during the distillation stage are forced over the mass of incandescent fuel occupying the posterior position in the furnace. In some cases the fuel is also by mechanical means fed from the bottom of the grate bars, by which means the same end is gained. The only objection to the general use of such stokers is the expense of erection, and the cost of upkeep.
(2) Smoke Abatement.—It has been pointed out that wherever smoke is agitated—e.g. by atmospheric or electric currents—its carbon is quickly deposited by its particles agglomerating into masses too heavy to remain in suspension. This may be simply shown, as, when smoke is agitated by fanners or air-currents in a closed space, in a short time it loses its characteristic black colour, the carbon being deposited as smuts (see 'Condensed Carbon Particles in Smoke,' Jour. Soc. of Chem. Ind., vol. ix.).
Other plans by which smoke from furnaces is passed through water and washed have been tried with more or less success, and may come to be compulsorily adopted in factories, as is now the case with iron blast-furnaces, where the products—viz. carbon, hydrocarbons, ammonia, and even the carbonic and sulphurous acids present in all coal-smoke—are profitably utilised. From each ton of coal used in iron-smelting are obtained, of oil used for lucigens for burning, 64 lb.; pitch, 84 lb.; sulphate of ammonia, 20 lb.; the gross value about 3s. 6d., less charges. The net saving on each ton of coal may be taken at 2s., which is about one-fourth of the cost of the coal. By means of such appliances there is now no difficulty in preventing the emission of coloured smoke from any factory-chimney (Elliott's, Mond's, and other processes). But the real difficulty which meets all attempts at smoke abatement lies in the sentimental desire for the cheerful though smoky blaze of English house-fires. This causes, it has been estimated, in London alone the production of thousands of tons of black smuts per annum, which descend either on the city itself or in its immediate vicinity.
Long ago it was pointed out that to obtain the full advantage of the use of coal it should be carbonised—i.e. heated in closed retorts, when the whole of the volatile products, consisting of oil, tar, and gas (which latter is the immediate cause of the production of smuts), would be properly utilised, leaving a coke which would not only burn with a smokeless flame, but give out, weight for weight, a much larger amount of available heat than the uncarbonised coal. At the same time the inflammable gas (one of the products of this process) would either alone or mixed with the coke yield, when properly burned, a smokeless fuel for use in kitchens or fireplaces, thus doing away with this serious evil, as well as effecting an annual saving estimated by Macaulay at 45,000,000 tons of coal (representing a sum equal to £15,750,000) in Great Britain alone.
Many attempts have been made to effect this by constructing house-grates and cooking-ranges so that the raw coal is introduced from the bottom—i.e. at the fire-bars or grids, so that all gaseous products have to pass through an incandescent mass of carbon before reaching the chimney. But such attempts in general have failed, owing to the mechanical difficulties which have to be overcome and which would necessitate the entire reconstruction of the present house-heating arrangements.
In addition to the extreme discomfort and interruption to traffic which such fogs cause, it has hitherto been supposed that they seriously affect health (see FOG); but the general death-rate is not so much increased (see the publications of the Society for the Abatement of the Smoke Nuisance). Doubtless this may in part be owing to the disinfecting properties of carbon and sulphurous acids, in excess in the atmosphere at such periods.