572 Prof. J. J. Thomson on 



fields, we get rays for which e/m = 5 x 10 3 ; but this is not a 

 peculiarity of helium : all the gases which I have tried show 

 exactly the same effect. 



Argon. 



When the discharge passed through argon the effects 

 observed were very similar to those occurring in air. The 

 sides were perhaps a little more curved, and there was a 

 tendency for bright spots to develop. The measurements of 

 the electric and magnetic deflexion of these spots gave 

 e/m = 10 4: , the value obtained for other cases. There was no 

 appreciable increase of luminosity in the positions corre- 

 sponding to e/m—lOy^O, as there would have been if an 

 appreciable number of the carriers had been argon atoms. 



Positive Hays in Gases at very low pressures. 



As the pressure of the gas in the discharge-tube is gradually 

 reduced, the appearance of the deflected phosphorescence 

 changes : instead of forming a continuous band, the phos- 

 phorescence breaks up into two isolated patches ; that part 

 of the phosphorescence in which the deflexion was very 

 small disappears, as also does the phosphorescence produced 

 by the negatively electrified portion of the rays. 



In the earlier experiments considerable difficulty was 

 experienced in working at these very low pressures ; for 

 when the pressure was reduced sufficiently to get the effects 

 just described, the discharge passed through the tube with 

 such difficulty, that in a very few seconds after this stage was 

 reached sparks passed from the inside to the outside of the 

 tube, perforating the glass and destroying the vacuum. In 

 spite of all precautions, such as earthing the cathode and all 

 conductors in its neighbourhood, perforation took place too 

 quickly to permit measurements of the deflexion of the 

 phosphorescence. 



This difficulty was overcome by taking advantage of the 

 fact that, when the cathode is made of a very electropositive 

 metal, the discharge passes with much greater ease than when 

 the cathode is made of aluminium or platinum. The electro- 

 positive metals used for the cathode were (1) the liquid alloy 

 of sodium and potassium which was smeared over the cathode, 

 and (2) calcium, a thin plate of which was affixed to the front 

 of the cathode. With these cathodes the pressure in the tube 

 could l)e reduced to very low values without making the dis- 

 charge so difficult as to lead to perforation of the . tube by 

 sparking, and accurate measurements of the position of the 

 patches of phosphorescence could be obtained at leisure. 



The results obtained at these low pressures are very in- 

 teresting. Whatever kind of gas may be used to fill the 



