​NATURAL 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 olefin series are also present, especially in the gas of Baku. Varying amounts of carbon dioxide, sometimes as much as 10% 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.