Total sterility is rarely achieved by ultra-violet radiation, the method being usually aimed at the reduction of bacterial population in air, water and on surfaces-particularly in sterile rooms and inoculation cabinets. The most effective wavelengths are 240-280 nm; the rays, emitted by mercury vapour lamps, possess little penetrating power but are readily transmissible in air and water. Their use is limited by the danger of exposure of skin or eye to the radiation. The life of the lamps must be monitored, as visible light will give an appearance of efficiency long after decline of the ultra-violet emission. Water is treated by passage through an annular tube surrounding a mercury arc quartz tube in a quartz cylinder.