A form of gas called natural gas is obtained from the earth by drilling a deep well. Such gas is formed as the result of decomposition of organic matter under pressure and heat. It comes to the surface often under great pressure and requires but little preparation for use. In different districts natural gas is of different composition, but its principal constituent is always "marsh gas," a compound of carbon and hydrogen that has a very low lighting but a very high heating value. It is used for both heating and lighting.