In Maxwell's Electricity, published in 1873, there is a sentence in section 57, where the discharge of electricity through gases, particularly through rarefied gases, is discussed: "These and many other phenomena of electrical discharge are very important, and when they are better understood, they will likely shed much light on the nature of electricity as well as on the nature of gases and the medium that fills space." This prophecy has been abundantly fulfilled by the progress of science, and undoubtedly further advances in this direction are to be expected. First, the study of conduction through liquids and then the study of conduction through gases, in conjunction with the study of radiation phenomena, has significantly expanded our knowledge in recent years and opened a new chapter.