The chamber, 3, is filled with loose pieces of fire brick, which are soon heated white hot. Valves 9 and 10 are then closed and steam is taken from an upright boiler, 6, and carried by a small pipe, 7, to the incandescent mass in chamber 3, and passing down through it is superheated. This superheated steam passes from the bottom of chamber 3 to the bottom of chamber 1, and then up through the mass of red hot coal. The intensely hot steam is thus decomposed into hydrogen and oxygen, and the oxygen unites with the carbon of the coal to form carbonic oxide gas. As hydrogen and carbonic oxide burn with only a feeble blue flame, these gases are now made richer in light giving carbon at this point by the addition of oil contained in an elevated tank, 8. This, dripping on the incandescent coal in chamber 1, is volatilized, and at the same time enriches and combines with the hydrogen and carbonic oxide to form a permanent illuminating gas (water gas) that passes up pipe 5 and through the flues in boiler 6, to outlet 13, and thence on in the usual way to the condenser, scrubber and gas holder, which are not shown, and merely act to purify the gas. As the excessively hot water gas passes through the boiler flues it furnishes the necessary heat to generate the steam. The air used in the process is forced at 12 into a drum in the smokestack, 11, and is heated by the escaping products of combustion. In practical operation there are two (or more) of the steam superheating chambers 3, working alternately, and one of them is being heated up while the other is superheating the steam. Water gas has neither the illuminating nor the heating qualities of coal gas, and it is also much more poisonous. According to O. Wyss, one-tenth of 1 per cent. of uncarburetted water gas renders the air of a room injurious to health, and 1 per cent. is fatal to all warm-blooded animals. Notwithstanding these facts, however, its extreme cheapness and fairly satisfactory light have carried it into such general use that to-day it is said that two-thirds of all gas made in the United States is carburetted water gas. _Acetylene Gas_ is a combination of two parts carbon and two parts hydrogen. It was discovered in 1836 by Edmond Davy, who produced carburet of potassium, and evolved acetylene gas therefrom by decomposing it with water. It was long known as _klumene_, and when burned it produced an intense white light. For a long time it was only produced in a small way in the laboratory. It is now made commercially by the mutual decomposition of water and calcium carbide, the latter giving off, when brought in contact with the water, acetylene gas, which rises in bubbles. In the reaction the carbon of the carbide unites with a portion of the hydrogen of the water, producing acetylene gas (C₂H₂), while the calcium of the carbide unites with the oxygen of the water and the remaining portion of the hydrogen and forms calcium hydrate, or slaked lime, which precipitates as a slush.