THE earliest industrial applications of the phenomena associated with strongly ionized gases, or ‘gaseous conductors’, appear to have been the introduction in England of the carbon arc for illumination in the early eighties, followed shortly by Moore's nitrogen and carbon dioxide discharge tubes in America. In the beginning of the twentieth century, electric discharges first began to be used on a considerable scale in chemical engineering, notably for the production of ozone in connexion with air and water purification, of nitric acid from the air, and of lubricants by the polymerization of light oils; about the same time discharges were also applied to the precipitation of dust and mist, and to the generation of high-frequency currents for radio communication. The phenomena associated with ‘free’ electrons in high vacua have been applied in the development of the present rich variety of X-ray tubes, wireless valves, and cathode ray tubes. The development of the transmission of electric energy has brought to light the importance of discharge phenomena in relation to the design of high-tension cables, insulators, condensers, and switchgear; and the physics and chemistry of the ionosphere are directly related to many problems of radio communication. Not the least important is the ignition system of all petrol-fed internal combustion engines, which depends on a gaseous electric discharge. Gaseous Electrical Conductors By Prof. E. L. E. Wheatcroft. (Oxford Engineering Science Series.) Pp. xi + 265 + 6 plates. (Oxford: Clarendon Press; London: Oxford University Press, 1938.) 21s. net.