Pittsburg is decreasing its use of natural gas for manufacturing, as the diminishing supply and greater distance it has to be brought are making it too costly for the iron and glass works, which are returning again to coal and coke, but the city is still said to use forty-five thousand millions of cubic feet in a year, mostly for domestic purposes. Pittsburg stands in a great but partly exhausted natural-gas district. The gas is stored under pressure beneath strata of rock, being set free when these are pierced. This is a gaseous member of the paraffin series, of which petroleum is a liquid member, and is mainly marsh-gas, the "fire-damp" of the miner. It originates in the decomposition of animal and vegetable life, and usually has but little odor, whilst its illuminating power is low, but in fuel value eight cubic feet equal one pound of coal. It was first used at Fredonia, New York, in 1821, for lighting purposes, being procured from a well. The natural-gas region is the part of Pennsylvania west of the Alleghenies, extending into New York, Ohio and West Virginia; and gas is also found in Indiana, Illinois, Kentucky and Kansas. It is held under enormous pressure within the pockets beneath the rocks, and when first reached in drilling, the tension has been known to equal a thousand pounds per square inch. It is not uncommon, when a well is drilled, to have all the tools and casing-pipe blown out, while an enormous thickness of masonry has to be constructed to hold down the cap that covers the well. Its use began in Pittsburg in 1886, the chief field of supply then being Murrysville, about twenty miles east of the city, while there are also other fields southwest and east of Pittsburg. The pipes underlie all the streets, and a main route of supply is along the bed of the Allegheny River. There are said to be about sixteen hundred miles of pipes laid down to lead the gas to Pittsburg from the different fields. PETROLEUM.