[Du.] Matter is capable of existing in the three forms known as solid, liquid, and gaseous. The gaseous condition of matter is defined as that which is capable of unlimited expansion -that is to say, that a very small quantity of any gas, if introduced into a large empty space, will always expand so as to fill the entire space. A gas may therefore be defined as matter in the gaseous state. An important property of gases is that they may be liquefied, and all gases, even the volatile hydrogen, have been reduced to the liquid state, and many of them to the solid. Illuminating gas is an inflammable gas produced by distillation from coal, petroleum, or other carbonaceous material. It is very largely used in cities for lighting and heating purposes, being conveyed in pipes from a central generating station, carried into houses, and burned at a small opening in a gas burner. Natural gas arises from wells in the earth in petroleum districts, and is similarly burned for house lighting, heating, and manufacturing purposes. Gas was first used for lighting in England about 1800, in the United States, at Boston, in 1822 ; New York in 1827, and Philadelphia in 1835.