Passage of Electricity through Gases. 1G5 



air at a pressure of from 0*5 to 1 millim. ; and it was furnished 

 at both ends with platinum electrodes having their axes per- 

 pendicular to that of the tube. When the tube was insulated, 

 and both terminals directly connected with the conductors of 

 a Holtz machine, both electrodes were covered with a bright 

 halo. From the positive there proceeded a somewhat con- 

 tracting discharge, A, which at some distance from the electrode 

 showed a dark spot, B, and then slightly decreasing (C) through 

 the entire tube, passed to the neighbourhood of the negative 

 electrode, from which it was separated by the dark space D. 



On putting the positive electrode to earth, the dark spot B 

 becomes smaller, while the discharge A spreads somewhat 

 further. On putting the negative to earth the dark space D 

 becomes larger, and the part C of the discharge contracts in 

 a spindle-shape towards it. 



A small air-spark about 1 millim. in length was then in- 

 troduced in the conducting wire of one of the electrodes. If 

 the spark was at the negative electrode, the dark space D 

 appeared much more sharply defined and more broadly 

 developed, while from the small dark spot B close to the 

 positive electrode the red luminous discharge C, at first some- 

 what diminishing and then again increasing, extended through 

 the tube to D. 



If the positive electrode was put to earth, the dark spot B 

 disappeared almost entirely ; otherwise the appearance re- 

 mained nearly unchanged. If the spark was increased, the 

 dark space D became larger ; and when it was considerably 

 increased, a bright streak extended from the lowest point of 

 the electrode to the other luminous discharge. If the current 

 flowing to the negative electrode was retarded by a column of 

 water, the extent of the dark space D diminished more and 

 more. 



If the negative electricity of the machine was put to earth 

 before the air-spark, the dark spot B at the positive electrode 

 became smaller, the succeeding part of the discharge C 

 sharpened to a point towards the centre of the tube where the 

 contraction had taken place shortly before, and at the same 

 time it increased in brightness. But before coming to a point 

 it passed into a feebler column of light, which filled the tube, 

 and was separated from the negative electrode by the very 

 sharply and broadly marked space D. 



When the spark was introduced at the positive electrode, 

 that electrode appeared covered with a brighter halo than the 

 negative. From the former there proceeded a column of grey 

 light, which filled the whole tube ; this slightly contracted 

 conically towards the negative electrode, and became brighter 



