196. Until-recent years only one kind of gas was used for illuminating and heating purposes, and that was obtained by the distillation of bituminous coal. The demand for gas for heating purposes, however, became so great, that new processes were invented, and other varieties of gas have been introduced, so that now all forms of gaseous fuel are called by the general name of gas.

The varieties of gas now commonly used are as follows: Coal gas, oil gas, water gas, producer gas, natural gas, gasoline gas (or carbureted air), and acetylene.

197. Coal gas is made by heating bituminous coal in air-tight boxes or retorts. The heat breaks up the combinations of hydrogen and carbon which exist in the natural coal, and transforms them into other compounds, most of which are gaseous at ordinary temperatures. Among the new compounds thus made are tar, ammonia, and sul-pliuretecl hydrogen. The tar condenses in the apparatus, and is pumped out. The ammonia is formed by the union of hydrogen with nitrogen, and has an offensive odor. Great care is taken to condense this mixture and remove it before the gas leaves the apparatus.

The worst impurity, however, is formed by the union of hydrogen with the sulphur contained in the coal. This is called sulphureted hydrogen. It is one of the vilest smelling substances known, and is very poisonous to breathe.

The odor of ordinary coal gas is due mainly to small traces of ammonia which remain in it. These impurities are removed by compelling the gas to flow in thin streams through pans filled with lime, oxide of iron, or other chemicals; or by causing it to bubble through bodies of liquid which have been charged with suitable chemicals. The former process is called purification, and the latter scrubbing. The chemicals absorb the various impurities, while the gas undergoes no change, except cleansing.

198. Oil gas is made from petroleum in a similar way, or from almost any variety of animal or vegetable oil, grease, or fat. Even oily refuse and city garbage have been used successfully for the production of gas.

Good illuminating gas can also be made from wood, peat, sawdust, in fact, almost any kind of combustible material, by substantially the same process.

199. Producer gas differs from the coal gas commonly used for lighting purposes, in having much less combustible matter, and in having a large percentage of nitrogen. The average quality of producer gas contains 10 to 15 per cent. of hydrogen, and 20 to 30 per cent. of carbon monoxide. These constitute the combustible part of the gas, nitrogen forming about 40 to 60 per cent. of the total volume. This gas burns with a dull reddish flame; its value for heating purposes is about one-fourth that of an equal volume of good coal gas.

Producer gas is made by burning coal, either bituminous or anthracite, in a closed furnace with a supply of air which is purposely made too small to permit of perfect combustion. The air is usually supplied by a steam jet blower, and the amount is regulated so that it is barely sufficient to convert the carbon in the fuel into carbon monoxide, CO. The nitrogen in the air remains unchanged, and passes off into the gas pipes with the CO, as an inert, useless accompaniment, merely swelling the total volume of the product.

200. Water gas is a mixture of hydrogen and carbon monoxide, with only a very small percentage of nitrogen. It is made from anthracite coal and steam. The coal, in lumps from 2 to 3 inches in diameter, is placed in an airtight cylinder lined with firebrick. It is ignited, and blown up to a bright incandescent heat by means of an air blast; then the blast is shut off, and a current of dry steam is blown through the mass of glowing fuel. The great heat causes the steam to break up into free oxygen and hydrogen.

The oxygen combines with the hot carbon, forming CO, and the hydrogen passes along with it, but without combining. These are then led off through suitable pipes to a gas holder. As soon as the incandescent fuel becomes a little dull or cooled, the steam is shut off, and the fire is again blown up bright with the air blast. The operations of blowing up and making gas are worked alternately at intervals of about five minutes, until the fuel is exhausted.

The fresh gas, as thus made, contains less carbon than good coal gas, and, consequently, will not burn with as bright a flame. It burns perfectly in heating burners, but when it is to be used for lighting purposes, it is always enriched, that is, made richer in carbon. This is done by vaporizing a quantity of petroleum by heat, and injecting it into the hot gas before it leaves the generator, continuing the injection until the percentage of carbon in the gas is raised to the desired standard.

Pure water gas is very light, having a density of about .4 that of air. It has very little odor, and is, therefore, more dangerous than coal gas, because a considerable leak may exist without attracting much attention. In the process of manufacture, some of the impurities are allowed to remain, so as to give the gas an odor which is plainly perceptible.

201. Water gas is also made from crude petroleum by a continuous process. This is known as Archer gas, from the name of the inventor of the apparatus. The oil is pumped in a small stream into a red-hot retort, where it is quickly ieduced to vapor by the heat. The oil vapor is then mixed with a current of superheated steam, and the mixture is driven through a long coil of very hot pipe. The oxygen of the steam unites with the carbon of the oil, forming CO, and the hydrogen is set free. The resulting gas is permanent, and is of high value for heating purposes. It is produced at a pressure of 8 to 10 pounds per square inch.

202. Natural gas is obtained from holes or -wells which are drilled in the earth. It is found in large quantities in the vicinity of deposits of petroleum; and deposits of coal, both bituminous and anthracite, are always accompanied by greater or less quantities of gas of a very similar nature.