MARSH GAS (methane), the first and simplest member of the paraffin hydrocarbons, owes its name to the fact that it is a constituent of the gas which arises in marshy districts from the decomposition of vegetable matter under the surface of water.
It occurs also as a component of "fire damp" of coal mines. It is found in volcanic gases, in petroleum and in human intestinal gases, and is a product of the destructive distillation of complex organic matters (coal, lignite, bituminous shale, peat, wood, etc.) so that it is present to the extent of 3o to 4o% in coal gas.
Methane, is a colourless, inodorous gas of specific gravity 0.559 (air= 1). At --160° C it has been condensed to a colour less liquid, specific gravity 0•415 (water= I). It boils at C/736 mm. and freezes at —186° C to white needles. One volume of water at t° C absorbs 0.05449-0.001807t volumes of methane, and it is nearly ten times as soluble in alcohol.
The preparation of industrial methane is effected by passing a mixture of hydrogen with carbon dioxide or, preferably, carbon monoxide over a heated nickel catalyst. With the dioxide the reaction requires temperatures between 30o° and 400° C, but with the monoxide the change proceeds to completion at 230 250° C. As the other product is steam, which can be condensed, the methane can be obtained almost pure.
Other methods of preparation are of scientific interest. Pure
methane is obtained on decomposing aluminium or beryllium carbide with water (see CARBIDES) and also by the action of water on zinc dimethyl (E. Frankland) or on magnesium methyl iodide. (See GRIGNARD REAGENTS.) The removal of sulphur by red-hot copper from a mixture of sulphuretted hydrogen and carbon disulphide vapour leads to the formation of methane and is a classical instance of an early organic synthesis (M. Berthelot, 1856). In the laboratory, methane is often prepared by heating dry sodium acetate with soda-lime but the product is contaminated with hydrogen and ethylene. Methane has been obtained by direct combination of carbon and hydrogen at 1,200° C (Bone, Jerdan, and Coward, Methane forms with air an explosive mixture of gas, the "fire damp" of the coal mines and a frequent cause of colliery ex plosions. (See Publications of the Safety in Mines Research Board, vol. ii., 1926, H.M. Stationery Office, London.) The oxidation of methane may be caused to occur in stages, and methyl alcohol and formaldehyde have been identified as inter mediate products of its combustion. Successive chlorination of methane leads to methyl chloride, methylene chloride, chloro form, and finally carbon tetrachloride.