In using a new oil it is not easy at first to regulate the flow so that absolutely all is burned without any dropping off the burner; to prevent the loss of this portion, the lower end of the air regulator is conical, at the apex of the cone is a small tube and under this a vessel in which the unburnt oil is caught. In Fig. 32, S is the air regulator, T the vessel to catch the unburnt oil, L the pipe leading from each burner to the common pipe H, G the vessel in which a float regulates the flow from a larger reservoir, so that the liquid always remains at the same level. When tar oils are used, or thin mineral oils, the pipes which convey the oil to the burners may be made tolerably narrow, but when viscous oils or fish oils are burned, narrow tubes would offer too great resistance to the flow, so that it is always advisable to use fairly wide pipes. Viscous oils become considerably more fluid at higher temperatures, it is therefore well to place the reservoirs in the same room as the lamps; in winter the temperature of this room is kept fairly high by the burning of the lamps, and thus the oils remain fluid. FIG. 32. In recent years hydrocarbons of very low boiling point have been placed on the market at low prices; these produce a black of very good quality. On account of the low boiling point of these very inflammable liquids, particular care is necessary in burning them; they are very fluid and retain their fluidity at a low temperature. The reservoirs for them should be placed for safety outside the lamp room, and should be closed by an air-tight lid, with only one small opening through which air can enter. When these low boiling liquids are used, particular care must be taken to regulate the flow to the lamp, or large quantities will evaporate without being burnt. The materials used in the manufacture of soot black differ considerably in chemical constitution. Rosin, animal and vegetable fats, distilled oils and volatile hydrocarbons are used. Each of these substances gives different decomposition products when heated, to which proper regard must be had, since the quality of the black is dependent on them. The more difficultly volatile are the products of distillation, the higher must be the temperature at which the combustion is conducted, otherwise the soot will contain a considerable quantity of these products of distillation. In consideration of the great variety of materials used for making soot black, it is impossible to say exactly in what manner each should be treated: this must be left to the practical experience of the manufacturer. CHAPTER XLI. MANUFACTURE OF LAMP BLACK.