1026 



fociissed on the kathode by means of a quartz lens in order to 

 prevent retardation in the discharge. ') 



/r^ 



Fig. i. 



In the case of pure argon we used a discharge tube of 7 cms 

 diameter with electrodes of 13 mms at a distance of 6,3J mm. 

 The radius of curvatnie of the opposed surfaces was 7 cms; the 

 centre of curvalure coincided with the sealing places of the supply- 

 wires, so that changes in the position of tiie electrodes had no 

 intluence on the distance. 



The discharge tube was connected to a Langmuir condensation 

 pump and to the apparatus in which the gases were kept and 

 purified. The argon-nitrogen mixtures were prepared from fractionated 

 argon and nitrogen*): the composition was determined by means 

 of a baroscope. The last remaining traces of oxygen were burnt 

 out by means of a glowing tungsten wire. The gases were freed 

 from water-vapour, mercury-vapour and carbon dioxide by means 

 of cooling tubes in liquid oxygen. Pure argon was prepared in a 

 potassium-cell according to Gehlhoff'j; the smaller discharge-tube 

 was used in this case in order not to use too much gas. The pressure 

 in the discharge-space was measured with a mercury gauge; every 

 now and then we used a Mac Leod gauge. The potential was 



M E. Warburg, Ann. d. Phys. (62), 385, 1897. When an arc between iron 

 electrodes was used, irregular results were obtained, probably in consequence of 

 an effect of the heat. 



-) Our thanks are due to Mr. H. Filippo Jzn. for the preparation of the mixtures. 



3) We made use of the same large potassium-cell as Avas used by Dr. L. Hamburger 

 (Diss. Delft 1917). 



