The gases are then passed on through numerous other devices to remove remaining traces of impurities, and are finally collected in a circular chamber known as the gas-holder, from which they are distributed to the consumer. Fig. 23.

If the purification is not perfect, the coal gas will contain sulphur compounds, and these on burning produce oxide of sulphur, which is further changed by moisture and the air into sulphuric acid. The quantity produced may be very minute and yet in time may be sufficient to damage books and fabrics.

The materials which collect in the hydraulic main and the condensers contain many useful substances, one of the most valuable being ammonia. Among the most interesting substances obtained from coal tar is aniline from which beautiful dyes are made. Aniline itself is a colorless liquid, but in combination with other chemical substances it yields a wide range of beautiful colors now used in dyeing. Other useful substances obtained from the distillation of coal tar are carbolic acid, a disinfectant, and naphthalene which is sold in the form of moth balls.

In some cities what is known as water gas forms the basis of the illuminating gas. This is made by passing very hot steam over red hot anthracite coal or coke. The oxygen of the water unites with the carbon of the coal, forming carbon monoxide - a compound of one part oxygen and one part carbon - and the hydrogen of the water is set free. Both the gases thus formed Will burn, but in burning they produce a colorless flame. It is therefore necessary to mix with them some gases containing much more carbon which will give light when burning. The mixture is stored and distributed like coal gas.

This gas is cheaper to manufacture in most localities, but it contains much more carbon monoxide which is a very poisonous gas. Much discussion has arisen as to the safety of using water gas and in some places its manufacture is forbidden by law.

The destructive distillation of vegetable and animal life in the depths of the earth, caused by the great heat within the earth, has in some places given rise to petroleum and natural gas. The gas gave a cheap and convenient fuel, but unfortunately the supply is becoming rapidly exhausted.

An illuminating gas of growing importance today is acetylene. This is a compound of carbon and hydrogen and is prepared by the action of water upon calcium carbide, which is a compound of carbon and the element calcium. Calcium carbide is manufactured in large quantities at Niagara Falls where pure lime mixed with powdered charcoal is fused at an intense heat. A dark gray crystalline solid results which, when mixed with water, produces acetylene gas and slaked lime.

Acetylene is a colorless gas of characteristic odor, soluble in water, and explosive if mixed with air. With an ordinary burner it makes a yellowish smoky flame, but with a properly constructed burner, it gives a brilliantly white light, very like sunlight. Colors appear at their true values seen in this light. The flame is an intensely hot one. In acetylene burners the gas escapes through two very minute holes directed obliquely towards each other, as shown in Fig. 24.

Fig. 22a. The Effect of Wire Gauze on a Gas Flame

Fig. 22 Aniline Water Gas

Fig. 23. Manufacturing Of Coal Gas

The gas has been somewhat in disrepute because of lack of a suitable arrangement for making and storing it. Many generators are upon the market, it is true, but very few of these are really safe. As soon as a reliable one is obtainable, the gas will be widely used for lighting. It may also be used for cooking, but at present is rather expensive. One form of generator is illustrated in Fig. 25. The calcium carbide in lumps is fed automatically into water as long as the gas is used. When the storage tank is nearly full the supply of carbide is automatically shut off. In another style, which is also automatic, water is fed on to the lumps of carbide. Both styles have their advocates, but the lump feed generator is most generally recommended. The apparatus costs from about $65.00 for a 10 light plant to $300.00 for a 100 light plant.

Acetylene Generators

A cheaper gas than acetylene is gasoline gas, sometimes called carburetted air gas because it is common air impregnated with the vapors of gasoline. It burns with a rich, bright flame similar to coal gas and is conducted through pipes and fixtures in the same manner. It may be used in an ordinary gas stove.

The gas machine consists of a generator containing evaporating pans, an automatic air pump operated by a heavy weight or by a water motor, together with a regulator or mixer. The general arrangement is shown in Fig. 26, the generator being entirely outside the building in which the gas is used. All such machines require intelligent care, for several disastrous explosions have taken place when such care has not been given to the apparatus.