234 



Mr Chree, On some experiments on 



[June 1, 



The distances are in mm., and are all measured from the 

 upper extremity of the cathode. The observations recorded in 

 the same horizontal line were taken in immediate succession, 

 the make and break regulator of the induction coil remaining 

 untouched. The uniformity of the pressure and of the make 

 and break was tested by reversing the current twice, which 

 showed whether the appearances at the surface which was first 

 cathode had altered durirjg the observations. The experiments 

 extended over a considerable interval and the tubes were cleaned 

 out and refilled more than once, thus the difference between 

 the cathodes shown by the table, which was confirmed by nu- 

 merous observations in which accurate measurements were not 

 taken may, I think, be fully relied on. The table shows that 

 the distance above the cathode of the head of the positive 

 column was on an average about thrice as much when the 

 cathode was mercury as when it was platinum. The difference 

 was greater the lower the pressure. At the highest pressures 

 when the Faraday space first became distinct no exact measure- 

 ments were recorded, but the difference though then not so 

 striking as in the table was still conspicuous. As the exhaustion 

 proceeded very slowly and both tubes were at intervals heated 

 up by a burner, the difference can hardly be attributed to any 

 great extent to a difference in the gaseous contents of the two 

 vertical tubes. 



The difference may be due to the difference in the material 

 of the electrodes or to the dissimilarity of their sizes and shapes. 

 According to Goldstein the effect of the difference in size of the 

 cathodes should have tended in the opposite direction. For 

 reasons explained in treating of the next case, I am inclined to 

 attribute the difference in considerable measure to the difference 

 in shape. 



