1891.] liquid electrodes in vacuum tubes. 237 



Electrodes Hg. and Al. 



The aluminium electrode was a flat circular plate of about 

 two-thirds the internal diameter of the tube. The following ob- 

 servations were taken on several occasions: 



The distances are in millimetres, and were all measured from 

 the centre of the cathode surface. At the higher pressures the 

 upper limit of the negative glow was pretty distinct and so has 

 been recorded above. Observations in the same horizontal line 

 were taken in rapid succession as in the case of Table in. The 

 ratio of the mean distance from the cathode to the end of the 

 positive column when the cathode was mercury to the corre- 

 sponding mean distance when the cathode was aluminium is as 

 1"37 : 1, and so is very much less than the ratio found when the 

 solid electrode was platinum. Since no such striking difference 

 between the metals platinum and aluminium as electrodes seems 

 to have been noticed by previous observers, the inference would 

 seem to be that we are here concerned with the size and shape 

 rather than with the material of the cathode. 



Owing to the appearance of a slight crack in the tube with 

 the Al. electrode its base had to be immersed in mercury con- 

 tained in a paraffin cup. Thus the length of the Crookes' space 

 when short could not be accurately observed without lowering 

 the cup. This was not done in the cases recorded in the table, 

 to avoid the risk of leakage, variation of current, etc., but on 



