296 History of Luminescence 



of a bright green colour, and of great density; as the temperature di- 

 minished, it lost its vividness; and when it was artificially cooled to 

 20° below zero of Fahrenheit, it was so faint as to require considerable 

 darkness to be perceptible. 



When air was let in the color of the light changed. Davy also 

 obtained a light above tin, coming to the conclusion that the heat 

 of the discharge volatilized the metals and that some vapor must be 

 present. There appeared to be conduction of current in the best 

 vacua obtainable at the time. 



Ever since Bose (1743) distinguished the spark, the brush (a 

 multitude of minute sparks) and the glow discharge, the relation 

 of the three forms had been a subject for speculation. A great deal 

 of attention was paid to the type of discharge by Michael Faraday 

 (1791-1867) , successor to Davy at the Royal Institution, in 1838 and 

 1839. After pointing out that: " Rarefactions of the air wonderfully 

 favors the glow discharge " and that " To obtain a negative glow in 

 air at common pressures is difficult " but " In rarified air the nega- 

 tive gloiu is easily obtained," Faraday showed that the spark passes 

 to the brush sooner when the surface of the electrode is negative, 

 but a positive brush passes to a glow sooner than a negative brush. 

 The glow, which could never be analyzed as minute sparks like the 

 brush, occurred in all gases tried (air, No, Oo, Ha, coal gas, COo, 

 HCl, SOo, and NH3) but differences in color were to be observed. 

 Faraday was of the opinion that in the highest vacua obtainable, 

 the surface of the glass might conduct the cuiTcnt. 



An important observation was the " dark discharge," Avhich re- 

 ferred to a dark space in rarified gas between the positive and nega- 

 tive glow, which was later to bear his name, the Faraday dark space. 

 The electric light thus becomes a complicated phenomenon, depen- 

 dent on many factors— different conditions of temperature and pres- 

 sure and different materials. New tools were obviously necessary 

 for new advances. These turned out to be the perfection of glass 

 blowing techniques and the rise of spectroscopy. 



JULIUS PLUCKER AND LATER RESEARCH 



With increasing interest in spectra of all kinds, together with the 

 perfection of the spectroscope, it was natural to continue the study 

 which Priestley had hoped to undertake years ago, the spectrum of 

 electroluminescent glow.'^^ However, the varying colors which ap- 

 pear during the discharge of electricity through vacuum tubes were 

 not adequately analysed for spectral composition until the late fifties. 



*^ See Chapter VI for the history of spectral investigation. 



