acetylene, the simplest and best-known member of the hydrocarbon series containing one or more pairs of carbon atoms linked by triple bonds, called the acetylenic series, or alkynes. It is a colourless flammable gas widely used as a fuel in oxyacetylene welding and the cutting of metals and as raw material in the synthesis of many organic chemicals and plastics; its chemical formula is C2H2. Pure acetylene is a colourless gas with a pleasant odour; as prepared from calcium carbide, it usually contains traces of phosphine that cause an unpleasant garliclike odour. Acetylene can be decomposed to its elements with the liberation of heat. The decomposition may or may not give rise to explosion, depending on conditions. Pure acetylene under pressure in excess of about 1.05 kilograms per square centimetre (15 pounds per square inch) or in liquid or solid form explodes with extreme violence. Mixtures of air and acetylene are explosive over a wide range, from about 2.5 percent air in acetylene to about 12.5 percent acetylene in air. When burned with the correct amount of air, acetylene gives a pure white light, and for this reason it was at one time used for illumination in locations where electric power was not available—e.g., buoys, miners’ lamps, and road signals. The combustion of acetylene produces a large amount of heat, and, in a properly designed torch, the oxyacetylene flame attains the highest flame temperature (about 3, 300 °C, or 6, 000 °F) of any known mixture of combustible gases.