GAS, the name given to the inflammable gas occurring in petroliferous formations. It consists mainly of hydrocarbons of the paraffin series, principally marsh gas, which constitutes from 50 to 90% of the Pennsylvanian gas. Members of the olefine series are also present, especially in the gas of Baku. Varying amounts of carbon dioxide, sometimes as much as io% or more, and small quantities of carbon monoxide, nitrogen, hydrogen and oxygen are also found. For particulars of the geological occurrence, and the collection and distribution, of natural gas, see Petroleum.