Methane, known also by the names light carburetted kydroyen and marsh gas, is the first member of the group of hydrocarbons known as the paraffins. It is a colourless, odourless gas, slightly soluble in water. If subjected to cold and pressure it may be liquefied. Its chemical composition is represented by the formula CH4, and its density is hence 8; or, if referred to air, -56. It burns with a yellowish flame, with the production of carbonic acid and water. In the neighbourhood of the petroleum wells of America and the Caucasus the gas escapes from the earth, and in some districts (Baku) has been kept burning for years as saoed fires. It is evolved as a product of the decomposition of vegetation, and is thus found in swampy and boggy regions. [Marsh Gas.] It is frequently found in coal-mines and known to the miners as fire-damp. As it forms an explosive mixture with air, its presence is a source of considerable danger in mines, and has been the cause of many fatal explosions. It is also of interest from the fact that the study of this and some allied hydrocarbons first led Dalton to the formulation of the atomic theory.