of Electricity through Hot Gases. 361 



of communicating a charge of electricity to a gas in a normal 

 state is afforded by Blake's discovery (Wied. Ann. xix. p. 51 8), 

 confirmed by Sohncke (Wied. Ann. xxxiv. p. 925), that the 

 vapour arising from an electrified liquid is not electrified. 



The effect on the electric discharge of the disintegration of 

 the electrodes seems to have been too much neglected. Me- 

 tallic electrodes give off particles of the metal freely under 

 many circumstances. Thus, for example, it has been known 

 for a long time that they do so when they glow ; and the 

 recent experiments of Lenard and Wolff (Wied. Ann. xxxvii. 

 p. 443) prove that they do so when exposed to ultra-violet 

 light. Now a gas when exposed to ultra-violet light allows 

 electricity to pass through it. This might either be due to 

 the decomposition of the gas by the ultra-violet light (of which, 

 however, there does not seem any direct evidence), or the 

 discharge may be carried by the particles which the electrodes 

 give off under the influence of ultra-violet light. The dis- 

 covery by Wiedemann and Ebert, that electricity will not 

 pass through the gas unless the cathode is illuminated, how- 

 ever brightly the rest of the field may be illuminated, points 

 to this explanation ; as Lenard and Wolff found that a posi- 

 tively charged plate does not give off particles of metal when 

 illuminated, though a negatively electrified plate does so freely, 

 and an uncharged one also, though not to so great an extent as 

 a negatively electrified one. 



Dr. Schuster has lately found (Proc. Roy. Soc. xlii. p. 371) 

 that a gas through which an electric discharge is passing is 

 unable to insulate the smallest electromotive intensity due to 

 a source independent of that producing the discharge. This 

 is easily explained on the preceding view ; for the discharge 

 splits up the molecules, provides a supply of atoms, and thus 

 turns the gas into a conductor. In this case the atoms are 

 supplied by the splitting up of the molecules by the electric 

 field. There are, however, other ways of splitting up the 

 molecules, of which raising the temperature is one ; it is 

 therefore interesting to study the effect of heat on the elec- 

 trical conductivity of gases. The following experiments were 

 made for that purpose. 



Results of previous Experiments. 



Previous experiments on the electrical conductivity of hot 

 gases have given apparently conflicting results. Thus 

 Becquerel, whose experiments have been confirmed by Blond- 

 lot, found that hot air conducted ; while Grove could not 

 detect any conduction between white-hot platinum wires sur- 



