Electroluminescence 295 



dark space " then a longer luminous region " the negative glow," 

 then the " Faraday dark space," and finally a long " positive 

 column " of luminosity with striations made up of dark and lumi- 

 nous regions, which extend to the anode. Occasionally another 

 dark space appears near the glowing anode. Details of the above 

 phenomena change with the length of tube, the current density, 

 and the pressure and kind of gas, and have been studied by many 

 workers. 



The experiments of Wm. Morgan (died 1833) , an insurance 

 actuary who experimented with electricity in 1785 are of consider- 

 able interest, since he stressed the different colors of the electric 

 light, depending on the gas pressure. He stated that a Mr. Walsh, 

 using a double barometer, had demonstrated " the impermeability 

 of the electric light through a vacuum." Morgan repeated the ex- 

 periment and showed that if small amounts of air are admitted the 

 color changes progressively from green to blue, to indigo, to violet, 

 to purple, " till the medium has at last become so dense as no longer 

 to be a conductor of electricity." He especially noticed that " the 

 degree of the air's rarifactions may be nearly determined by this 

 means," an observation which is still useful in judging the perfection 

 of a vacuum. 



During the early nineteenth century few observations on electro- 

 luminescence were made. With the discovery of the voltaic pile 

 (A. Volta, 1792) , attention turned to low voltage high current 

 electrical phenomena, the passage of electricity through solids and 

 liquids rather than gases. The trend was accelerated by Humphry 

 Davy's isolation of potassium and sodium metals in 1807. The 

 chemical experiments of Davy have eclipsed the fact that he was 

 particularly interested in the relation between electricity and matter 

 and endeavored to determine whether any electrical discharge could 

 occur in a " perfect vacuum." Morgan (1785) had previously held 

 that the electric light became less, the more perfect the vacuum, or 

 did not appear at all. In 1822 Davy prepared a short U-shaped tube 

 with a platinum wire fused in one end, filled with mercury (or tin, 

 which could be melted) and another platinum wire fused in the 

 glass near a stop-cock, so that the tube could be evacuated. When 

 the tube was pumped out, the mercury fell, leaving an evacuated 

 space and sealing the cock. Davy found that when the 



mercurial vacuum was perfect it was permeable to electricity, and was 

 rendered luminous by either the common spark, or the shock from a 

 Leyden jar, and the coated glass surrounding it became charged; but 

 the degree of intensity of these phenomena depended upon the tempera- 

 ture; when the tube was very hot, the electric light appeared in the tube 



