The contrast between town and country gardens makes suffi ciently apparent the detrimental effects of smoke upon vegetation. A deposit of soot, apart from the corrosive action of its acid constituents, not only impedes the transpiration of plants but also acts as a screen to sunlight. Acid rain is also directly harmful to plant life and affects adversely the soil, although soot is a valuable manure. The vitality of any type of vegetation usually suffers progressively as an industrial centre or large town is neared; crops diminish and reproductive powers are enfeebled. Evergreens become deciduous or may perish altogether, while many hardy perennials have to be replanted yearly. Some plants refuse altogether to grow in urban districts, others appear to be able to survive after a fashion almost anywhere.
There are grounds for regarding hopefully the prospects of industrial smoke abatement. The observation of scientific prin ciples in furnace design and air regulation coupled with the adop tion of continuous mechanical stoking have made possible the almost smokeless operation of modern large scale steam boilers, even when working on bituminous coals. Smokeless combustion is less easy to achieve in small hand-fired boiler furnaces, particu larly in the periods immediately following the opening of the fire-door and the introduction of fresh charges of fuel ; for the chilling effect of the cold fuel and the accompanying inrush of cold air, together with the increased resistance of the deeper fire bed, tend to the escape of unburned distillation products. Careful and systematic stoking at short intervals, however, by maintaining a fire of even depth with uniform air supply, can give surprisingly good results, and it is quite feasible in such a manner to improve greatly smoky plant. Courses of instruction in furnace manipula tion have been inaugurated in a number of large industrial towns, and the education of stokers is under consideration by a London Joint Committee on Smoke Abatement appointed in 1927.
Further, there is ample evidence to prove that coke is a con venient and efficient boiler fuel, capable of giving results not inferior to those obtainable with coal. Its bulkiness, however, necessitates the use of relatively large furnaces, and the substi tution of coke for coal in plant which is already hard-pressed to meet adequately the demands, would probably result in failure.
There is taking place, especially in small-scale furnaces, a gradual development of the use of gas and oil, which not only can be burned without thick smoke, but contain less sulphur than coal. The possibilities of pulverised fuel are also attracting attention; in this form solid fuel can be burned completely, but the sulphur products are of course not reduced, and unless special precautions are taken a large proportion of the ash may be carried into the atmosphere as a gritty dust. The question of the necessity for smoke production in exempted trades and special processes is a controversial one, but there are known methods of removing soot and dust particles from flue gases, for instance by washing out with water, by centrifugal action or by electrical precipitation, though these naturally entail a certain amount of expense. Gas firing is being successfully adopted in certain branches of the pottery industry and both gas and electrical furnaces are making some headway in formerly smoky metallurgical processes; while for power production the use of electricity is steadily developing.
There is little hope of burning bituminous coal smokelessly in domestic appliances and the only satisfactory solution in this field, apart from the wider adoption of central heating for large buildings, lies in its replacement by smokeless fuels. The possi bilities of such a course are governed mainly by economic con siderations. For certain purposes, such as cooking and intermittent heating gas, or in more limited circumstances, electricity, corn pares favourably with coal even in running cost ; but unfortunately at present prices the complete elimination of solid fuel is not feasible. Gas coke and anthracite suggest themselves as alterna tives to coal, but both are somewhat difficult to ignite and, although suitable for use in stoves or independent boilers, they are not always altogether convenient in ordinary grates. Further, anthracite is expensive, even after taking into account its high calorific value. Hence the widespread interest which has been aroused in the possibility of establishing on a commercial scale, low temperature carbonisation processes for the production from coal, at a competitive price, of ignitible solid smokeless fuels.