=Types of Acetylene Generators.=--There are two general methods of generating acetylene for domestic illuminating and heating purposes: that of adding carbide to water, and that in which the water is mixed with carbide. The two types are illustrated in the diagrams shown in Figs. 209 and 210. The first method, that in which the carbide is dropped into water, is shown in Fig. 209. The tank _A_ is the generator and _B_ is the receiver or gas-holder. The tank _A_ holds a considerable quantity of water and is provided with a container _C_ for holding the supply of carbide. The tank _A_ is connected with the gas-holders by a pipe which extends above the water line in the tank _B_, where the gas is allowed to collect in the gas-holder _G_. A charge of carbide, sufficient to fill the holder with gas, is pushed into the tank _A_ by raising the lever _H_. Immediately the water begins to combine with the carbide and the bubbles of gas pass up through the water and are conducted into the tank _B_. The holder _G_ is lifted by the gas and its weight furnishes the pressure necessary to force the gas into the pipes, which conduct it to the burners. If this machine were provided with the proper mechanism to feed into the generator a supply of carbide whenever the gas in the holder is exhausted, the machine would represent the modern “carbide to water” generator. [Illustration: FIG. 209.--Diagram of a carbide-to-water acetylene-gas generator.] [Illustration: FIG. 210.--Diagram of a water-to-carbide acetylene-gas machine.] The “water to carbide” generator is shown diagrammatically in Fig. 210. As in the other figure, _A_ is the generator and _B_ is the gas-holder. A supply of carbide _S_ is placed in the generator and water from a tank _C_ is allowed to drip or spray onto the carbide. The gas collects in the gas-holder as before. This apparatus represents in principle the parts of a machine for generating acetylene by this process. The actual machines are arranged to perform the functions necessary to make the machines automatic in their action. Whatever the type of the machine, the object is to keep in the holders a sufficient amount of gas with which to supply the demand made on the plant. Machines representing each of the types described are to be obtained, but the greater number of those manufactured are of the “carbide to water” form. In the formative period of acetylene generators many accidents of serious consequence resulted from imperfect mechanism. Imperfections have been gradually eliminated until the machines which have survived are efficient in action and mechanically free from dangerous eccentricities.