588 Professor J. J. Thomson [March 22, 



10^. It is true that, at very low pressures and with strong electric 

 fields, we get rays for which — = 5 x 10^ ; but this is not a pecu- 

 liarity 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 deflection 



of these spots 2"ave — = 10"^, the value obtained for other cases. 

 ^ m 



There was no appreciable increase of luminosity in the positions 

 correspondino- to — = — , as there would have been if an appreci- 

 able number of the carriers had been argon atoms. 



Positive Rays in Gases at very loiv pressures. 



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

 duced, the appearance of the deflected phosphorescence changes : 

 instead of forming a continuous band, the phosphorescence breaks up 

 into two isolated patches ; that part of the phosphorescence in which 

 the deflection was very small disappears, as also does the phosphores- 

 cence 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 out- 

 side of the tube, perforating the glass and destroying the vacuum. 

 In spite of all precautions, such as earthing the cathode and all con- 

 ductors in its neighbourhood, perforation took place too quickly to 

 permit measurements of the deflection 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 dis- 

 charge passes with much greater ease than when the cathode is made 

 of aluminium or platinum. The electropositive 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 be reduced to very low values without making the 

 discharge so difficult as to lead to perforation of the tube by sparking, 

 and accurate measurements of the position of the patches of phosphor- 

 escence could be obtained at leisure. 



