There are two types of machines. In one the water drips on the carbide; in the other, the more common type, the carbide is dropped into the water. As soon as the carbide touches the water, it gives off acetylene gas. The gas is caught in and fills a bell above the water. As it fills the bell, it raises it, and when the bell reaches a certain height, it trips a lever to the door which lets in the carbide and closes it. When the gas is used, the bell goes down and, passing the lever, opens the door to let in a small amount of carbide. Improvements have been made in the plants and in installing them until there is less danger from explosions than formerly. Great care should be taken in operating them to avoid accidents. Since the gas is highly explosive, fire, lighted lamps and cigars must be kept away from the vicinity of all acetylene plants. Only one person should take the care of the plant, the others should understand how. =280. Directions for Operating Acetylene Plant.= 1) Charge by daylight--remove all residuum, and fill with fresh water before adding any carbide. 2) Follow exact directions for the machine used in the order directed. =281. Cautions to Be Observed in Using Acetylene Gas.= 1) Do not apply a light to any opening that is not equipped with a regular acetylene burner tip. 2) See that any workman repairing a generator first removes carbide and drains all water out, and disconnects it from piping and removes it to the open air, where he then fills all compartments with water to force out gas before using soldering irons. 3) An open light should never be permitted nearer than ten feet from the generator. The generator should never be nearer than fifteen to twenty feet from furnace or stove. Do not hunt for gas leaks with a flame or light. [Illustration: FIG. 157. Pressure tank for gas.] 4) Do not use any artificial light except electric light when cleaning or repairing generator, or carry a lighted pipe or other fire about it, even when empty. 5) If water in any chamber should freeze, do not attempt to thaw it with anything but hot water. 6) Keep the motor oiled. Oil once in six months. =282. Compressed Gases and Oils.= Gases, such as Blau gas, Pintsch gas, and prestolite gas which is compressed acetylene gas, are compressed in strong tanks and sold for use in lighting and light housekeeping. Gasoline and alcohol also are occasionally stored in very strong tanks under enough pressure to make them flow thru very small pipes to the point where they are wanted for use. These are frequently used for lighting isolated public buildings, such as rural schoolhouses. As the gas or oil is used, the pressure diminishes. There is usually a pump attached to the tank to pump in air in order to keep up the pressure. The pump is similar to a bicycle pump (Fig. 157). QUESTIONS FOR PART X 1. What is the difference between the treadle and a motor-power machine?