It will be noticed that the tank (_b_) is divided into two compartments, the upper portion containing the water in which the gasometer floats. The lower compartment is also partly filled with water which acts as a safety valve to prevent any escape of gas into the room in which the generator is located. The lower end of the pipes _P_ and _S_ are immersed in the water at the bottom chamber of the tank, from which the gas could escape in case too much is generated and finally exit through the vent pipe _U_ to the outside air. The float _A_ in the tank (_a_) is a safety device that prevents the introduction of carbide unless the tank contains a full supply of water. The float is a hollow metal cylinder connected by a rod to a hinged cup under the bottom opening of the carbide holder. When the water is withdrawn from the generator, the float falls and the cup shuts off the carbide outlet. [Illustration: FIG. 212.--Sectional view of a house equipped with acetylene lights and domestic heating apparatus.] The accumulation of lime, from the disintegrated carbide, requires occasional removal from the tank (_a_); the valve _K_ is provided for this purpose. The lever _S_ is used to stir up the lime which is deposited on the bottom of the tank, that it may be carried out with the discharged water. Machines of this kind that are safeguarded against leakage of gas or the possibility of accumulated pressure are practically free from danger in the use of acetylene. The accidental leakage of gas from defective pipes and fixtures produce only the element of risk that is assumed with the use of any other form of gas for illuminating purposes. Acetylene is distributed through the house in pipes in the same manner as for ordinary illuminating gas. The sizes of the pipes to suit the varying conditions of use are regulated by rules provided by the National Board of Fire Underwriters. These rules state definitely the sizes of pipes required for machines of different capacities. Rules of this kind and others that specify all matters relating to the use of acetylene may be obtained from any fire insurance agent. The general plan of piping is shown in Fig. 212. The generator _G_ is in this case a “water to carbide” machine and is shown connected to the kitchen range, as well as the pipe system which may be traced to the lamps in the different rooms, to the porch lights and to the boulevard lamp in front of the building. [Illustration: FIG. 213.--Acetylene gas burner.] [Illustration: FIG. 214.--Electric igniter for acetylene gas burners.] [Illustration: FIG. 215.--Electric igniter for acetylene gas burners.] The type of burner used in acetylene lamps is shown in Fig. 213. The gas issues from two openings to form the jet as it appears in the engraving. These burners are made in sizes to consume 1/4, 1/2, 3/4, and 1 foot per hour depending on the amount of light demanded.