As the air in the carburetor takes up gasoline vapor, its specific gravity is increased until the air is saturated; and by adding the amount of atmospheric air necessary for complete combustion the weight is reduced to a definite amount which will be constant. The required mixture will, therefore, always weigh the same amount. The principle on which this mixer works is that described in physics as the principle of Archimedes: “that a body immersed in a fluid will lose in weight an amount equal to the liquid displaced.” In the application of the law, the gas in the mixer is the fluid, and the float--to be described--is the displacing body. [Illustration: FIG. 185.--Diagram illustrating the mixer of the Detroit cold-process system of gasoline lighting.] The mixer in Fig. 185, is shown cut across lengthwise. The outside casing is indicated by the heavy black lines. The gas which leaves the opening at the top--marked gas outlet--is a mixture of gasoline and air that may be used for exactly the same purpose and in the same manner as coal gas. It may be used in open-flame gas jets or in the mantle gas lamps for lighting purposes and also as fuel gas for domestic heating. The gas is distributed through the building in ordinary gas pipes which are installed as for any other kind of gas. In Fig. 183 the distributing pipes are indicated by the heavy lines. The valve in the air inlet, in the bottom of the mixer, controls the amount of air to be admitted. The entering gas from the carburetor being heavier than the desired mixture, will raise the float and in so doing will open the air valve and allow the air from the blower to enter. The float and valve are so adjusted that the desired mixture is attained when the balance beam is level. Any variation in the mixture will change its weight and the valve corrects the change whether it be too much or too little air. The openings at the bottom, marked _gas inlet_ and _air inlet_, are intended for the admission of the saturated vapor from the carburetor, and the atmospheric air, as required. The float which fills the greater part of the inner space is a light sheet-metal drum, that is tightly sealed and nicely balanced by a counterweight on the opposite end of the suspending bar. The counterweight is made adjustable by the device marked _movable adjusting weight_--in the drawing--which permits the quantity of entering gas to be slightly changed as the gasoline in the carburetor grows old. The adjustment of the counterweight to suit the gas given off from old gasoline in the carburetor, and the occasional rewinding, to elevate the blower weight, is practically all the attention this plant requires. It is a real gas plant which gives every service that may be obtained from coal gas. THE HOLLOW-WIRE SYSTEM OF GASOLINE LIGHTING AND HEATING