=112. Pressure Lamps.= Pressure lamps (Figs. 54 and 55) have a strong tank which holds air and fuel, whether alcohol or gasoline. Air is pumped into the tank so that it presses on the fuel with force enough to push the fuel up the pipe leading from the bottom of the tank to the generator. The air cannot get into the pipe so long as there is fuel which is heavier than air in the tank, because the pipe which leads to the burner starts from the bottom of the tank. [Illustration: FIG. 54. Details of gasoline lamp.] [Illustration: FIG. 55. Pneumatic gasoline lamp.] The generator for changing the liquid fuel to gas is placed between the burners of the lamp, of which there are usually two. After the generator has been heated, the lighted lamps keep the generator hot. The gas being very light, continues to rise. It passes thru a place where it is mixed with air, and goes on into the burner, where it is ignited. If the lamp burns low, more air must be pumped into the tank to force up the gasoline or alcohol. When all the fluid has been burned, the lamp will go out, since, then, only the air which is under pressure in the tank will be coming into the burner. Extinguish the lamp by turning off the supply of fuel to the generator. To light these lamps, first heat the generator, as directed for the particular lamp in use, and then light the burners. Detailed directions cannot be given here, as they differ with different lamps. =113. Gasoline Lamps with Wicks.= There are some gasoline lamps made with wicks which help conduct the oil into the burner, where it is changed to gas by the heat from the lamp, mixed with air and burned in a mantle. The flame, from a mixture of alcohol or gasoline and air, is blue and gives off little light, but much heat. It is used with a mantle. =114. Alcohol Lamps with Wicks.= The wick of one type of alcohol lamp conducts the alcohol up thru a round tube which it completely fills. The tube prevents the fire from burning down into the bowl of the lamp. Alcohol makes a very hot and almost smokeless flame, even when little air is present. The mantle is put over the flame, and, when heated, gives a good light. Other ordinary fuels cannot be used on so simple a lamp because they would smoke the mantle. =115. Lighting Alcohol or Gasoline Lamps.= Heat the conducting pipe at the point where the fuel is to be changed to gas. (Directions for this come with each lamp, and they differ considerably.) After being heated sufficiently, the valve leading to the burner is opened and the burner lighted with a match or torch. Use clean gasoline for these lamps, unmixed with water or other substances. QUESTIONS FOR PART III 1. Are there any differences in the electric light globes on the market? If so, in what ways do they differ? How do these differences affect the lighting power of the globes? 2. What influence has the size and decoration of the room on the brilliancy of light from a given lamp?