228 Mr Ghree, On some experiments on [June 1, 



existent but more or less independent parts, a dark probably 

 convective discharge independent of chemical action, and a lu- 

 minous discharge. Lehmann's idea is something of the following 

 character. The luminous discharge is essentially disruptive and 

 intermittent whatever be the nature of the source of electricity. 

 Hittorf 1 and Home'n 2 it is true, employing for the source a large 

 number of cells, have imagined they got a steady luminous dis- 

 charge through the tube like the current in a metallic conductor. 

 But their reasons for this view such as the silence of a telephone 

 in the circuit are, according to Lehmann 3 , not conclusive, because 

 the steadiness of the current outside the tube does not necessarily 

 prevent its being intermittent inside. He regards the electrodes 

 as charging and discharging like condensers, requiring a certain 

 potential depending on the density, temperature, etc. of the gas 

 before the luminous discharge is possible. Simultaneously, how- 

 ever, the electrodes leak into the tube, the discharge being 

 carried off probably convectively without any necessary lumi- 

 nosity. An increased brilliancy in the tube only implies an 

 increased quantity of electricity passing at each individual lu- 

 minous discharge, and thus it accompanies whatever raises the 

 capacity of the electrodes or affords an obstacle to rapid dis- 

 charge. This explains the action of an air spark in the circuit 

 outside the tube. A diminished brilliancy may mean the passage 

 of a smaller current, or it may indicate an increased rapidity in 

 the succession of luminous discharges without any alteration in 

 the current outside the tube, or it may indicate the operation 

 of some agency facilitating the convective discharge. It is to 

 one or both of the latter causes that Lehmann would ascribe 

 the effects of heating a cathode or exposing it to ultra-violet 

 light. It is unquestionable that in many such cases the diminu- 

 tion in the luminosity of the tube is very striking. Lehmann 4 

 does not regard the luminous discharges at the anode and cathode 

 as having any necessary relation in the rapidity of their suc- 

 cession. He regards a red colour as merely indicating a strong, 

 a blue colour a weak discharge. Thus if, as usual, the positive 

 discharge is red and the negative blue, the difference is to be 

 accounted for either by the generally smaller cross-section of the 

 positive column, or by the interval between successive discharges 

 being longer at the anode than at the cathode. 



A good many writers while recognizing a convective discharge 

 ascribe it to the action of dust particles 5 . These may exist in 



1 Wied. Ann. 20, 1883, pp. 705—712, etc. 



2 Wied. Ann. 38, 1889, pp. 172 et seq. 



3 Molekularpliysik, Bd. n. pp. 234 — 7. 



4 Molekularpliysik, Bd. n. p. 257. 



5 See Lenard and Wolf, Wied. Ann, 37, 1889, pp. 443—456. 



