The combustion in the flue must be conducted with care, it should proceed at the lowest possible temperature, but this should not sink below a certain minimum. If the fire is too strong the greater portion of the carbon is burnt, which would otherwise be obtained as soot; the yield of soot is very small, and generally only ”dust soot” would be obtained, without any quantity of ”flying soot”. On the other hand, if too little air enters, so that the combustion takes place at too low a temperature, a large yield of soot is obtained, but it is of poor quality. Soot obtained by combustion at too low temperatures has not a pure black but a perceptibly brown colour; it has not the flocculent nature of the best product, a small weight of which occupies a very large volume, but is greasy and has a high specific gravity. The lower the temperature used in producing soot black, the further is the chemical change removed from that of ordinary combustion, and the nearer to that of dry distillation. Soot produced with an insufficient supply of air contains a considerable quantity of liquid and solid products of dry distillation, which give it a brown colour and the above-mentioned greasy nature. In many soot works the air supply is regulated in a most primitive manner: the workman places against the openings by which air is admitted to the burning materials a larger or smaller number of bricks according as the combustion appears to be too rapid or too slow. Unfortunately there is no clear sign to indicate the temperature most favourable for the production of the greatest quantity of soot, the appearance of the flame alone can be taken as a guide. If the flame appears pure white and very luminous and shows at its end no thick black smoke the combustion is very complete, and but a very small yield of soot is to be expected. On the other hand, if the flame continually threatens to go out the air supply is too small and a liberal amount of soot will be formed, but mixed with a large quantity of the products of dry distillation. As far as it can be described in words the flame should appear as follows: The colour should be a murky red, similar to that of the flame of a bad tallow candle; in shape the flame should be a long drawn out tongue, from the point of which a thick black smoke is clearly seen. The soot deposits in the long board flues in the form of flocks or dust. The properties of the soot which is deposited in the different parts of the long flue vary according to the distance from the place of combustion, near to which a good black soot is deposited, requiring, however, long grinding with oil or gum solution to form a good paint. At a greater distance the soft and very fine ”flying soot” is deposited; it is the most pure black and is regarded as the best quality. The soot deposited farther on is more and more brown, and has a more greasy nature the greater the distance from the place of combustion.