to settle behind the miners, no sustaining pillars of coal being left. This, when well planned, is the safer, both as regards facility of ventilation and less liability to accidents from falls. At a Durham colliery, working the Harvey seam, 3½ feet in thickness, 5,185 tons of coal were obtained when working by the long-wall system and 5,052 tons when working by the post-and-stall system. In thick and highly inclined beds it is usual to remove the coal by horizontal slices and to fill the excavation with waste material. In some instances blast furnace slag is used for the purpose.
The great depth and size of modern collieries necessitate the raising of vast quantities of coal through a single shaft and the winding engines of modern erection are of extraordinary power.
Production.—The total coal production of the United States in 1919 was 544,263,000 short tons. Of this 458,063,000 tons were bituminous and 86,200,000 tons were anthracite. This production was a decrease of 133,949,000 tons over that of 1918. Pennsylvania produced the largest amount of coal for 1919, 145,300,000 tons of bituminous coal and 86,200,000 of anthracite coal. West Virginia was second with 75,500,000 tons; Illinois third with 64,600,000 tons; and Ohio fourth with 35,050,000 tons. Other States producing over 10,000,000 tons were Alabama, Indiana, and Colorado. The number of employees in the coal mines in the country in 1918 was 762,426. Of these 147,121 were employed in the anthracite mines and 615,305 were employed in the bituminous mines. The total value of the coal produced in 1918 was $1,828,290,287.
During the participation of the United States in the World War, the production of coal in sufficient quantity became an important problem. In 1918-1919 strikes in various fields produced a shortage of coal, and only by the most rigorous methods of distribution was it possible to obtain sufficient quantities to keep industrial plants running and to supply domestic demands. During 1918 industrial plants were shut down for certain periods owing to a shortage of coal. In 1920 conditions had greatly improved, and there was no alarming shortage of coal during that year. On Aug. 30, 1920, President Wilson approved a report of an anthracite wage commission which awarded from 17 to 20 per cent. increase over their previous pay to men employed in the anthracite coal mines. The men refused to accept the provisions of the commission and undertook a strike in September. The President refused to reopen the question of the wage award, and the strike subsided. The coal output for the first 8 months of 1920 indicated an increase in production over 1919. For 205 working days the production of bituminous coal was 347,406,000 tons, and nearly 30,000,000 tons more than were produced in the same period of 1919.
History.—The use of coal does not seem to have been known to the ancients, nor is it known at what time it began to be used for fuel. Some say that it was used by the ancient Britons, and at all events it was to some extent an article of household consumption during the Anglo-Saxon period as early as A. D. 852. There is reason for thinking that England was the first European country in which coal was used to a considerable extent. About the end of the 13th century it began to be used in London, but at first only in the arts and manufactures, and the innovation was complained of as injurious to health. In 1316 Parliament petitioned the king, Edward II., to prohibit the use of coal, and a proclamation was accordingly issued against it; but owing to a high price of wood its use soon became general in London. It was for a long time known there as Sea-Coal, because imported by sea.
COALDALE, a borough of Pennsylvania, in Schuylkill co. It is on the Lehigh and New England, and the Central of New Jersey railroads, and is the center of an important coal mining region. Pop. (1910) 5,154; (1920) 6,336.
COAL GAS, a mixture of gases produced by the destructive distillation of coal at regulated temperatures. It is used in lighting streets, houses, etc., and for cooking and heating purposes. Coal gas is colorless and has a disagreeable smell. It is purified from H2S by ferric hydrate, which is moistened with FeSO4 and H2SO4 to remove ammonia. The carbon disulphide can be removed by passing it through an iron tube filled with iron turnings and heated to redness. Coal gas consists of a mixture of hydrogen, 40 to 50 per cent., carbon-monoxide about 5 per cent., marsh gas (CH4) about 40 per cent., which contribute nothing to the illuminating power of the gas; it depends upon the presence of heavy hydro-carbons, principally C2H4 ethene, ordinary gas containing about 4 per cent., and cannel gas about 8 per cent. Coal gas also contains small quantities of acetylene, butylene, etc., and aromatic hydro-carbons, as benzine, etc. The percentage of nitrogen is very variable. When gas is burned a large quantity of water is formed, hence, if a gas stove is used to dry a room there must be sufficient ventilation to carry off the
