The qualities demanded of a good generator are: There must be no possibility of an explosive mixture in any of the parts; it must insure a cool generation of gas; it must be well-constructed and simple to operate; it should create no pressure above a few ounces; it should be provided with an indicator to show how low the charge of carbide has become in order that it may be recharged in due season, and it must use up the carbide completely. Because of the fact that the greater number of acetylene-gas machines of today are of the “carbide to water” type, in the description to follow that type of machine is used. They are generally made in two parts, one part containing the generating apparatus and the other acting as gasometer (gas-holder), but some machines are made in which one cell contains both the generator and gasometer. In Fig. 211 is shown a two-part, gravity-fed machine, in which all of the internal working parts are exposed to view. The tank (_a_), as in the diagram, is the generator and the tank (_b_) contains the gasometer marked _G_. Each tank possesses a number of appliances which are necessary to make the machine automatic in its action. The part _C_ of the generator contains the supply of carbide, broken into small pieces, a portion of which is dropped into the water whenever additional gas is required. The feed mechanism _F_ is controlled by the gasometer bell _G_, which is buoyed up by the gas it contains. When the supply of gas becomes low, the descending bell carries with it the end of the lever _F_, which is attached to the feed valve; this motion raises the feed valve and allows some of the carbide to fall into the water. The gas that is immediately generated passes into the gasometer through the pipe _P_, and as the bell is raised by the accumulating gas the valve _V_ is closed. The gas as it enters the gasometer passes through a hollow device _W_, that looks like an inverted T, the lower edge of which is tooth-shaped and extends below the surface of the water. The gas, in passing this irregular surface, is broken up and comes through the water in little bubbles, in order that it may be washed clean of dust. This device also prevents the return of the gas to the generator tank during the process of charging. [Illustration: FIG. 211.--Sectional view of the Colt acetylene-gas machine.] The gas escapes from the bell through the pipe _S_ to the filter _D_, where any dust that may have escaped the washing process is removed by a felt filter. It finally leaves the machine by the pipe _L_, at which point it enters the system through which it is conveyed to the different lighting fixtures.