One of the most interesting features of these observations is the gradual alteration shown in the composition of the deposit from town to country, particularly in regard to carbonaceous matter and sulphates. In the heaviest deposits the proportion of carbonaceous and tarry matter is relatively high, usually amount ing to 20-30% of the total, but in the lightest deposits it may fall as low as 5–io%. On the other hand sulphates, which (as in city deposits account for only 5-15% of the total, may rise to 20-30% in country districts. The relation between the different constituents is shown in the following table derived from the Reports of the Atmospheric Pollution Committee for two selected stations, one, Malvern, a remarkably clean one, the other, Newcastle-on-Tyne, representing a heavy deposit.
Selective effects are to be expected in the deposition of atmos pheric impurities, for the larger particles naturally fall most quickly and are therefore deposited in the more immediate neigh bourhood of their source; but there are also present in smoke numerous particles so small that they become thoroughly mixed up with the air by eddies and may be carried many miles on the wind before ultimately reaching the ground or coming to rest on buildings or vegetation. Owens has devised an automatic instrument for filtering hourly through special discs of paper a measured volume of air, and so obtaining information in regard to the progressive changes in the quality of black suspended mat ter. In London the suspended impurity is at its minimum between midnight and 6 A.M., subsequently rising rapidly owing to the lighting of fires, to a maximum at 9 or io A.M., after which it gradually falls again; the summer charts are naturally less black than the winter ones. Sundays are on the whole the cleanest days, and certain characteristic differences between different week-days, varying from place to place, are exhibited. Owens has also ex amined microscopically the number and nature of the suspended atmospheric impurities, and has been able in some cases to recog nize salt crystals, mould cells, etc., in addition to soot particles and fused ash.
The duration of sunshine in such towns as Leeds, Sheffield and Manchester is in the winter months less than half that in outlying districts. In summer the deficiency is less marked but the inten sity is at all times impaired, particularly in respect of the ultra violet rays, which are now recognized as being so essential to health. A similar state of affairs holds in London but a steady
improvement has been in progress for many years, and while in the period 1881-1885 the City of London and Westminster re ceived only 20% of the sunshine registered at Kew, by 1916-1920 the corresponding proportion had risen to 53%.
Were it not for the action of the wind in spreading smoke, and its tendency to rise to the upper layers of the atmosphere, which normally are cooler than those down below, fog would be an everyday occurrence in thickly populated districts. Still weather, indeed, is in winter usually accompanied by more or less dense smoke fog, or haze, especially since on cold, clear nights radi ation to space chills the surface layers of air and is liable to pro duce in the atmosphere a "temperature inversion," or increase of temperature with height, which checks the upward drift of chim ney products. Smoke also contains hygroscopic particles which act as nuclei for the condensation of water vapour and tend to produce wet fogs in chilled air. As might be expected, not only the number of carbon particles but also the amounts of carbonic and sulphurous acid rapidly increase during fog.
The depressing effects of dirt and gloom perhaps scarcely need be insisted upon, but the exact relation between smoke and physi cal welfare is impossible to assess, owing to the large number of other contributory factors. Smoky fog is certainly the signal for an increase in the number of deaths from pulmonary and cardiac diseases, but since such fogs are associated with special types of weather they are not necessarily its sole cause. Cohen and Ruston, however, quote an interesting investigation by Ascher, who by a comparison with industrial and non-industrial areas was led to the conclusion that coal-dust, smoke and soot increase the death rate from acute lung diseases.
The sulphur acids in soot or rain attack and destroy building materials, particularly limestones, slates and zinc. Sulphuric acid reacts with the carbonates of limestone, turning them into much more soluble sulphates, which are dissolved out by rain, often to be precipitated later in disfiguring incrustations. A simultaneous increase of volume tends to cause the stone to flake away, dis integration being especially rapid if the sulphates are formed in cracks or flaws, for instance in stones possessing marked cleavage planes. Sandstones are not so directly affected as limestones, unless they contain a calcite binding material, the destruction of which may result in large masses being dislodged.