The discharge of gas from a coal seam or oil pool is at once the sign of a continuing change of composition in the fuel and an indication of the nature of the change. The process of change, which has been termed autometamorphism, is accelerated by warmth and pressure, and the chief by-products are carbon dioxide, methane, and water (Briggs, 1931). Other gases are found in fire-damp, but in proportions that are relatively so insignificant that, in the present connection, they may be altogether disregarded. The mixture known as “natural gas,” occurring in association with petroleum, usually contains other hydrocarbons besides methane; indeed, the “wet” gas set free from oil when the natural pressure is released may consist almost entirely of members of the paraffin series heavier than methane. Nevertheless the natural gas obtained from oilfields is chiefly CH4. The mean of 22 analyses of natural gas from oilfields in various parts of the world gives CH4, 86·3; CO2, 3·0; N2, 6·2 per cent. (Bacon and Hamor, 1916). In a generalised inquiry, then, it will be sufficiently accurate to reckon the whole of the inflammable ingredient of both natural gas and fire-damp as methane.