It is composed mainly of a compound of carbon and hydrogen, called light carbureted hydrogen. This often amounts to 90 per cent or more of the total volume. Consequently, the gas will develop more heat per cubic foot in burning than any other kind of gas except acetylene.

Natural gas is produced at the wells under great pressure, and in common practice, the pressure in the street mains and distributing pipes is allowed to be very much higher than is usual with manufactured gas.

203. Acetylene is a compound of carbon and hydrogen. Its chemical symbol is C2H1, and its composition is 12 parts of carbon to 1 of hydrogen by weight, or 92.3 per cent. carbon and 7.7 per cent. hydrogen. The proportion of carbon is extraordinary, and the compound appears to be overloaded. It is known to be unstable, and the gas is liable to decompose spontaneously and explosively, under the action of a violent shock or blow. There is, therefore, some danger in handling and using it.

Its density compared with that of the air is .91, and its weight at 32° F. is .073 pound per cubic foot. It is without perceptible color, and it has a strong odor like garlic. It is poisonous to breathe, in about the same degree as ordinary illuminating gas.

The heat which it is capable of developing by burning is theoretically 1,090 heat units per cubic foot.

Acetylene is manufactured by an indirect process, no direct process, suitable for common use, being at present known. The first step in the process is to form a compound of carbon with calcium. This is done by subjecting a mixture of coke and lime to the intense heat of an electric furnace. The product, which is called carbide of calcium, is a reddish-brown or gray material, opaque, somewhat crystalline, and it decomposes water like ordinary quicklime.

When it is desired to produce acetylene, the carbide of calcium is put into water. Both materials decompose. The calcium takes up oxygen from the water, forming oxide of calcium, which is common quicklime. 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 cubic feet of acetylene per pound; but the commercial material is impure, and gives in practice 4 1/4 to 4 3/4 cubic feet per pound, at atmospheric pressure.

204. Acetylene gas, when burned in ordinary Batswing burners, gives a dull, smoky flame, because the gas is not spread out sufficiently to secure from the air the oxygen required to burn the carbon properly.

To develop the full illuminating power of the gas, it is necessary to greatly enlarge the flame. This may be done by using a burner tip having the thinnest slit obtainable, and by giving the gas a heavy pressure-4 or 5 inches of water or more.

One valuable quality of acetylene is its ability to furnish lights of very small size, but of great brilliancy. With a properly made burner, a light about 1/8 inch in diameter can be made which will give the same illumination as an ordinary candle.

Carefully made tests show that acetylene will give a light of 240 candlepower, when burned at the rate of 5 cubic feet per hour; while good, ordinary illuminating gas will average about 16 candlepower at the same rate of consumption.

A flame giving a light of 20 candlepower will consume about 1/2 cubic foot of acetylene per hour; but to obtain this result, great care must be taken in the construction of the burner.

Acetylene can be reduced to liquid form, at a temperature of 60°, by a pressure of about 600 pounds per square inch; and it can then be stored in portable steel cylinders like other gases.

It corrodes silver and copper, and the compounds thus formed are explosive. It does not affect brass, iron, lead, tin, or zinc. These facts should be borne in mind when constructing apparatus for its use.

205. Gasoline gas or carbureted air, also called air gas, is a mixture of gasoline vapor with air. The pure vapor is so rich in carbon that, in order to burn it successfully for lighting purposes, it must be given a high pressure; and special burners must be employed, as for acetylene.

The pure gasoline vapor contains a much greater amount of carbon per cubic foot than ordinary illuminating gas; and in order to burn it in the same burners and at the same pressure, it must be diluted with air until the proportion of carbon equals that in ordinary coal gas.

The air furnishes a part of the oxygen required for combustion, but it also introduces a large proportion of nitrogen, which is an inert and useless material, being incombustible; and it operates to reduce the temperature of the flame and thus to diminish its brilliancy.

Gasoline is produced by distilling crude petroleum. Its specific gravity is about .74 that of water. It is really a mixture of a large number of hydrocarbon compounds, which differ slightly in their chemical proportions. All of them, however, will change from the liquid to the gaseous form, under ordinary atmospheric pressure, at a temperature ranging from 70° to 100°. If a tank containing liquid gasoline be opened to the air, the liquid will all pass away in the form of gas. The rapidity of the evaporation will depend upon the temperature, being very slow at 40°, quite rapid at 70°, and furious at 212°; and, if the liquid catches fire in any way, it will pass into gas with explosive violence. The burning liquid expands enormously and is very difficult to extinguish. Gasoline must be regarded as gas in a liquid form, and it should be clearly understood that it will resume the gaseous form whenever the opportunity is afforded.

It is generally regarded as a dangerous material to use or handle, but the danger arises from the recklessness or neglect of the persons using it. If the same care is taken to keep it shut up as is taken to keep coal gas confined, it is no more dangerous than the latter. A tank of gasoline should be treated as a reservoir of gas.

206. Gasoline is put upon the market in several grades. The highest grade, sometimes called winter gasoline, will evaporate at ordinary temperatures and leave nothing behind. The poorer grades contain more or less oil which will not evaporate without the aid of heat; this oil collects in the gas-generating apparatus, and must be removed from time to time.

The quantity of gasoline which is required to produce 1,000 cubic feet of gas, and which will give a light of 14 to 16 candles (when burning at the rate of 5 cubic feet per hour), is about 4 1/2 gallons of the best grade, but more is required, if the gasoline is of a lower grade.