Coal gas is made by heating bituminous coal in air-tight boxes or retorts. The heat breaks up the combinations of hydrogen and carbon in the coal, transforming them into other compounds, most of which are gaseous at ordinary temperatures. Among the new compounds thus made are tar, ammonia, and sulphureted hydrogen. The tar and ammonia are condensed and removed. The gas also undergoes purification and scrubbing; in the former process, the gas is forced in thin streams through pans filled with lime, oxide of iron, etc.; and in the latter, through bodies of liquid charged with certain chemicals.

Oil gas is made from petroleum in a similar way and from almost any kind of oil, grease, or fat.

Producer gas differs from the coal gas commonly used for lighting, in having much less combustible matter, and in having a large percentage of nitrogen. It is made by burning anthracite or bituminous coal in a closed furnace, with a supply of air too small for complete combustion. The average quality of producer gas contains from 10 to 15 per cent, of hydrogen, from 20 to 30 per cent, of carbon monoxide (CO), and from 40 to 60 per cent, of nitrogen. Producer gas burns with a dull reddish flame, and its heating value is about one-fourth that of good coal gas.

Water gas is made from anthracite coal and steam. The coal is placed in an air-tight cylinder, ignited, and raised to an incandescent heat by an air blast; the blast is then shut off, and dry steam is blown through the glowing fuel. The intense heat breaks up the steams into free oxygen and hydrogen, the oxygen combining with the hot carbon, forming CO, and the hydrogen passing along with it, but not combining. These are then led to a gas holder. The operations of blowing up and making gas alternate at intervals of about

3 minutes, until the fuel is exhausted. The fresh gas burns perfectly in heating burners, but when used for lighting purposes, is always enriched in carbon, by the vaporization of petroleum before it leaves the generator. The density of pure water gas is .4 that of air. It naturally has very little odor, but some impurities are allowed to remain to give the gas a perceptible odor.

Acetylene (chemical symbol, C2H2) is composed of 12 parts of carbon to 1 of hydrogen by weight, or 92.3 per cent, carbon and 7.7 per cent, hydrogen. It is the most brilliant illuminating gas known. Its density is about .91, and its weight at 32° F. is .073 lb. per cu. ft. It is without color, and has a strong odor, like garlic. It is poisonous to breathe, in about the same degree as ordinary gas. The heat developed by the combustion of 1 cu. ft. is theoretically 1,090 heat units. Acetylene is manufactured by mixing calcium carbide with water. The carbide is a mixture of coke and lime which had been fused in an electric furnace. It is reddish brown, or gray, in color, somewhat crystalline, and decomposes water like ordinary quicklime; the calcium takes oxygen from the water, forming oxide of calcium, or common quicklime, while the carbon combines with the hydrogen of the water, and forms the desired compound, acetylene. Considerable heat is given off during the operation.

Pure carbide of calcium will yield 5.4 cu. ft. of acetylene per pound; but it is hardly safe to reckon upon more than 4.5 cu. ft. from the commercial carbide. Special burners are required for acetylene illumination, provided with small air holes to supply more air to the flame than is obtained by ordinary burners. Acetylene will give a light of about 240 candlepower when burned at the rate of 5 cu. ft. per hour, while good coal gas only gives 16 candlepower at the same rate of combustion. Acetylene can be reduced to liquid form, at a temperature of 60°, by a pressure of about 600 lb. per sq. in., but is then unsuitable for use in buildings, owing to the danger of explosion. Acetylene corrodes silver and copper, forming explosive compounds, but does not affect brass, iron, lead, tin, or zinc. These facts should be borne in mind when constructing apparatus for its use.

Gasoline gas, or carbureted air, sometimes called air gas, is a mixture of gasoline vapor with air, and, when pure, is 80 rich in carbon that special burners must be employed for lighting purposes. The quantity required to produce 1,000 cu. ft. of gas of from 14 to 16 candlepower is about 4 1/2 gal. of the best grade, and more if the gasoline is of a lower grade. The specific gravity of gasoline is about .74 that of water. The temperature of a gasoline-gas machine should range between 40° to 80°. The highest grade of gasoline, that is, the grade that will evaporate most freely at ordinary temperatures, should be used for winter service. The generators should be located outside the building, in a sheltered place. An air pump is used to pump fresh air through the generator, and a mixing device is commonly employed for mixing air with the gas before it reaches the burners. The pump and mixer are usually located in the cellar of the building.