Electric Discharge in Gases. 95 



rays penetrate further into the tube, thus conditioning the 

 first phenomenon. 



The usual description of the phenomenon, according to which 

 the positive-light column is simply attracted to the magnet, 

 whilst new stratifications continually disengage themselves 

 from the positive electrode, whilst the first remains unchanged 

 and moves toward the kathode, therefore does not always apply. 



9. Behaviour of Poor Conductors as Kathodes. — A few ex- 

 periments relate to the behaviour of bad conductors as kathodes. 

 The questions to be decided were, first of all, whether such 

 poor conductors would be disintegrated in the same way as 

 good conductors, whether kathode-rays would appear, &c. 

 Paper and similar substances have been previously employed 

 as poor conductors, and the electrodes have been covered with 

 these substances. I have not employed these substances; for 

 so long as they are not perfectly dry, no vacuum at all perfect 

 can be obtained with them ; and if they are perfectly dry, 

 they are insulators which can only be broken through in dis- 

 ruptive discharges. 



I have therefore used lead chloride and lead iodide, which 

 at ordinary temperatures conduct very badly. In order to 

 cover the electrodes with these substances, the discharge-tube 

 was made of the form shown in fig. 13. An aluminium elec- 

 trode is melted into one end of the tube in the ordinary 

 manner, whilst at the other end at d a narrow tube was melted 

 on. In this a still smaller glass tube B fitted exactly. A 

 platinum wire was melted into one end a, into which a drop 

 of mercury (/3) was brought; and an aluminium wire (7) was 

 pushed into the tube and dipped in the mercury. In the 

 part e of $ not occupied by the aluminium wire, chloride or 

 iodide of lead was melted until the space e was completely 

 filled, when the tube S was pushed into the tube c and cemented 

 with sealing-wax. The electrode is completely covered by 

 glass and not by cement, so that it does not need further 

 melting in. This arrangement may be recommended, mutatis 

 mutandis, for the construction of other forms of discharge- 

 apparatus. 



With chloride or iodide of lead fine kathode-rays were ob- 

 tained accompanied by a very decided deposit upon the walls, 

 and especially upon the parts within the dark space round the 

 kathode, although the kathode-rays traversed the tube alto- 

 gether in the line of the axis. This deposit showed Newton's 

 rings plainly, and formed very rapidly. Whether its formation 

 is attended by electrolytic decomposition of the lead -salt 

 remains to be investigated. When iodide of lead is used, 

 peculiar spectral phenomena are observed, the description of 



