is not mixed with the water-gas until the latter has been purified; in others the petroleum is added directly to the coal. Anthracite coal, only, is used in the manufacture of water-gas, and great care is necessary to keep the temperature up to a white heat, since, if it falls too low, a large proportion of carbonic acid will be formed, and will injure the illuminating power of the gas unless it is removed by purification. Anthracite coal contains sulphur, and yields ammonia when distilled, so that purification is as necessary for water as for coal-gas, and therefore no saving is made in this respect. The real saving is in coal, since a large volume of steam can be decomposed by one ton.
It is necessary now to make a more particular inquiry into the nature of the different gases and compare them with each other. Coal-gas in its salable condition is composed about as follows:
|Olefiant and other hydrocarbons||7.27||4.75||4.91||3.80||13.00|
The hydrogen and carbonic oxide burn with a non-luminous flame, and marsh-gas burns with only a slightly-luminous one, the illuminating power coming almost entirely from the olefiant gas and other hydrocarbons. The oxygen, carbonic acid, and nitrogen, being incombustible, injure the illuminating power very greatly. The oxygen and nitrogen are admitted accidentally by the introduction of a little air in charging the retorts. Hence, other things being equal the illuminating power of the gas increases with the proportion of olefiant and other hydrocarbons, and these depend chiefly on the kind of coal used and the temperature at which it is carbonized. The gas-coals are the bituminous caking coals and cannel. The bituminous shales, like the boghead mineral of Scotland, and asphalt minerals, like the Albertite of Nova Scotia and the Grahamite of West Virginia, are used in small quantities for enriching. The yield of good gas-coal, like the Penn, is about 10,000 cubic feet of 15 to 16 candle-power gas to the ton (2,240 lbs.). The enriching coals yield a larger amount of richer gas, and the asphalt minerals from 13,000 to 15,000 cubic feet of 30 to 50 candle-power. The relative cost of enriching with these or with naphtha is a very important question. In a number of experiments made at the Boston Gaslight Works, for the purpose of testing the value of Albertite as an enricher, it was found that the