Carburetted air, made by varnishing air with gasoline, generally called gasoline gas, is very different from any of the gases mentioned. Carburetted air gas of standard quality contains 15 per cent of gasoline vapor to 85 per cent of air. A regulator or mixer for supplying gas having these proportions is shown, in section, in Fig. 133. It consists of a cast-iron case, in which is suspended a sheet-metal can B, filled with air and closely sealed. The balance-beam E, to which this is hung, is supported by the pin H, on agate bearings. Since the weight of the can B is exactly balanced by the ball on the beam E, movement of B can be caused only by a difference in the weight or density of the gas inside the chamber A and surrounding the can B. If the gas becomes too dense, B rises and opens valve C, thus admitting more air; and if it becomes too light, C closes and partially or wholly shuts off the air, as may be required. This gas is not a stable mixture, and great care must be taken, in piping it, to avoid traps and give positive inclination to all the pipe. It is easily condensed by change of temperature; and fixtures through which it is used, must be of a pattern that drain to the keys, so that they can be removed with a screw-driver to drain the arms. The gravity of the mixture varies with the grade of gasoline used. It may always be taken as the weight of air plus the gasoline carried with the air. Hence the greatest pressure is always at the lowest instead of the highest point; and instead of lighting a burner by holding the flame over it, we apply the match below. In general, other rules for piping gas supply, with the exceptions mentioned, and these following: the pipe should always be a size larger than for coal gas, except that 3/8-inch pipe may be run for two or three burners, and 3/8-inch openings are permissible.

To avoid having the pressure on the lower floor equal to the friction head of the whole system plus the weight of the gas, it is best to pipe the whole supply first to the top of the house. Then in Certain Proportions.

Fig. 133. Regulator or Mixer for Supplying Gas and Air Mixed.

Fig. 134. Pump for Forcing Carburetted Air through House Pipes..

Operated by Weight and Pulley. Pumps of this Kind are feed downward, and drip the main extremities into the initial main with a 3/8-inch connection. This permits circulation according to the temperature of the rooms; and, by giving just enough pressure at the pump to lift the gas easily to the top of the rising main, it feeds by gravity from that point. The least pressure possible is thus sufficient, and the pressure at each burner is constant.

Sometimes Run by City Water-Pressure.

Fig. 135. Carburetter in which Air is Charged with Illuminant by Passing over a Certain Number of Square Inches of Gasoline Surface, Depending on the Number of Burners..

Fig. 136. Carburetter with Pit Giving Access to Transfer Cocks, Pump Pipes, etc..

With the exception of one or two machines, the use of Argand or other special burners is necessary. The dough burner, oftenest used on gasoline, has an annular space below the tip, open at the top only. A thumb screw passes through the outer case, annular space, and inner wall, to the gas passage. When the carburetter is first filled, the gas is too rich, and the thumb-screws of the burners must be screwed out until the gas passing up can suck in more air from the annular space through the screw-hole. The gravity of the gasoline left in the carburetter grows constantly greater; car-burization takes place correspondingly slower; and the gas delivered is accordingly poorer. This is because gasoline of various gravities is found in every barrel, and the air takes up the lightest first.

The pump for these machines is a sheet-metal case on legs, with an inner drum, made like the drum of a wet gas-meter and sealed with water. The drum is generally operated by a weight, through the medium of pulleys, and a cord is wound on a spool attached to a shaft extending from the drum through a stuffing-box, all about as shown in Fig. 134. The pump is placed in the basement, where it will not freeze. In some makes, city water-pressure is made to run the pump by means of a water-wheel, gravity and impingement of the stream both acting to revolve the drum.

Carburetters are made of sheet metal, galvanized iron, or copper. One form is simply a strong tank with a float and telescope pipe operating through a stuffing-box. The float is hollow, and the air is introduced to the carburetter through it. The weight of the float submerges the holes through which the air enters the carburetter; and, as flotation takes place on the gasoline itself, the air is thus charged with it and ready for the burner. This type of machine will come nearer carburetting all the gasoline of a charge than any other. Its fault is that the gas is too rich and smoky.

Another type of carburetter passes the air over a given number of square inches of gasoline surface per burner, as indicated by the construction shown in Fig. 135. Still another reduces the necessary superficial area of gasoline by looping burlap in the case in such a way as to compel the air to pass through the burlap, which is charged with gasoline by capillary attraction. Charcoal filling has been used for the same purpose.