Smoke
Except in towns, or in the immediate neighbourhood of smelting-furnaces and coke-ovens, or (though to a less degree) of brickworks and limekilns, Smoke can scarcely be regarded as hurtful to gardens and plantations. In and around large cities, particularly in the manufacturing districts of England and Scotland, the air is charged with soot, in either a fine or a coarse state of division. The soot consists chiefly of carbon, and along with it small quantities of various compounds (empyreumatic oils, &c.) formed in the combustion of the fuel. All are familiar with the black coating that settles on everything from an atmosphere polluted with Smoke. This coating upon leaves and twigs is injurious by clogging the stomata, or small openings through which air passes into the tissues; and it also hinders the work of the leaves by shutting out part of the light that they should receive to keep them in health, and to fit them to supply the plants with food. This source of danger can be overcome by keeping the plants clean by syringing, or by washing those that are so delicate as to require special care, and so valuable as to deserve the labour. Nor is the danger so frequent as might be imagined from the prevalence of a black, sooty deposit on plants, even those in greenhouses, since these deposits are far oftener composed of Fungi (Fumago, &c.) than of soot. Moreover, the presence of Carbonic Acid (the most abundant product of combustion) in the atmosphere is necessary to green plants, of which it is a most important food.
The real danger to plants in the vicinity of towns and smelting-works arises from the presence, in the gases formed during combustion, of poisonous products, of which Sulphurous Acid gas is by far the worst. Its presence is due to the existence of sulphur in the coal as an impurity. Hardly any coal is free from sulphur; and its compound, Iron Sulphide or Pyrites, may often be seen as a yellow, shining coat on the smooth sides of lumps of coal. The leaves of plants show traces of poisoning by Sulphurous Acid gas, when the proportion in the atmosphere does not exceed 1 in 1,000,000 parts, if exposed to this mixture for a considerable time; and Stockhardt found that Clover and grasses showed its effects when exposed twice, for two hours each time, to 1 part in 40,000, the leaves becoming brown at the tips, and the plants withering. But the experiments are usually carried on with plants in confined air, under bell glasses; and it must be remembered that, in the open air, plants are seldom exposed to teh continuous action of the gas, and that the danger is, therefore, considerably less than the amount of gas occasionally contained in the air would indicate. It has been found that the leaves of plants poisoned with Sulphurous Acid gas show, at first, translucent spots between the veins. These spots become dull green, and then brown, dry, and shrivelled. When water is copiously supplied to the roots, drops of water often stand along both sides of the larger veins of the leaves. Along the veins extend green borders, which form a network in the brown, dry leaves. The cells of the green parts retain abundance of fluid. This green network is an indication of the cause of harm; and chemical analysis of the leaves affords a proof of the presence of sulphur in excess, and confirms the indication given by the colour. The leaves of Conifers become dull green at the tip; then this changes into brown-red, sharply separated from the green parts below. But in these leaves the effect is very similar to that of frost, or other causes of injury; and the only certain indication is given by chemical analysis, proving the presence of excess of sulphur. Conifers are found to suffer most severely from the action of the gas, as their leaves are less readily renewed. Herbs, under similar exposure, suffer more than ordinary deciduous trees. Plants are injured less by exposure to the gas during darkness, and least of all during winter, when growth has ceased for a time. Limekilns are not so hurtful as smelting-furnaces, since the gas combines with the lime, and forms Sulphate of Lime, which prevents its escape in a hurtful form. Another injurious substance in the fumes from some works, is Hydrochloric Acid gas, which is emitted during the formation of various chemical products. The only thorough remedy is the stoppage of the emission of the gases; but the harm may be lessened by restricting these injurious processes to winter, or to the night-time if they must go on in summer.