ELECTRIC DISCHARGE IN GASES 121 



South Kensington, and has extended over several years. The 

 most important facts were set forth in the Bakerian Lecture 

 given to the Royal Society in 1911, of which a condensed account 

 is given in the following paragraphs. 



ACTIVE NITROGEN 



It has been known for a long time that vacuum tubes fre- 

 quently show a luminosity of the contained gas after discharge 

 of electricity through the tube has taken place and has ceased. 

 In the case of air Strutt found that this effect is due to a phos- 

 phorescent combustion occurring between nitric oxide and 

 ozone, both formed during the discharge. He also found that 

 other phosphorescent combustions are observed in ozone, 

 notably of sulphur, sulphuretted hydrogen, acetylene, and 

 iodine. With a moderate discharge of electricity it was at first 

 supposed that pure nitrogen gave no afterglow, but when a jar 

 discharge was used with spark gap the glow was readily obtained. 



In order to examine the properties of the gas while showing 

 this phenomenon, the vacuum tube through which the discharge 

 passed was connected with an observing vessel, and a current of 

 gas was drawn into the latter by the operation of a powerful air 

 pump. One very remarkable effect on the appearance of the 

 glow is produced by change of temperature. If a long tube, 

 through which a stream of glowing nitrogen passes, is moderately 

 heated the glow is locally extinguished, but the luminosity is 

 recovered as the gas passes on into the cooler part of the tube. 

 If, on the other hand, the gas is led through a tube immersed in 

 liquid air it glows with increased brilliancy where it approaches 

 the liquid air, though the luminosity is finally extinguished 

 when it reaches the coldest part of the tube. It appears then 

 that the change, whatever it is which gives rise to the luminosity, 

 is promoted by cooling and retarded by heating. 



The glowing nitrogen has remarkable chemical properties. It 

 combines with common phosphorus at the same time producing 

 much red phosphorus. In this behaviour it resembles the 

 halogens, chlorine, bromine, and iodine. It also combines with 

 sodium, with mercury, and some other metals, in each case 

 developing the line spectrum of the metal concerned. 



It attacks nitric oxide, with formation, strangely enough, of 

 nitrogen peroxide, a more oxidised substance. This, hoAvever, 

 can be explained by supposing that the active ni'trogen prefers 



