438 Dr. F. W. Aston on the 



The mechanism of the discharge is extremely obscure, for the 

 current intensity is, of course, enormously in excess of that 

 to be expected from the ordinary thermionic release of 

 positive ions from the hot anode. There was very little 

 visible glow in the tube, the X-radiation was small and, 

 although a faint cloud of sodium light nearly always appeared 

 in front of the red-hot anode, the pressure was too low for 

 the anode rays to be visible ; their point of impact with the 

 cathode could, however, be inferred from the scintillations on 

 its surface. 



Observations of this effect lead to the conjecture that the 

 bulk of the rays originate not from the surface of the salt 

 itself but from that of the heated platinum, and also that some 

 points on this are much more active than others, giving rise 

 to jets of rays. The direction of these jets seemed to depend 

 on the local configuration of the strip and was beyond prac- 

 tical control. The obvious device of moving the anode about 

 by means of the ground joint to get a radiant point in the 

 required place could not be applied, for the parabolas were 

 never bright enough to be visible on the willemite screen. 

 To add to these difficulties the salt disappeared very rapidly, 

 in some cases in a few minutes. Consequently exposures 

 were very limited in duration, and even in the most favourable 

 cases the results rarely had a satisfactory intensity. 



The preliminary experiments were done with sodium 

 phosphate, and before long encouraging results were obtained. 

 In all the successful exposures only a single parabola 

 appeared, and this showed that — although the method on 

 account of the number of inevitable failures is an exas- 

 perating one to use — as a means of identifying isotopes it 

 has the great merit of producing the positive rays of the 

 metals and no others. This characteristic seems to be due 

 to the very low pressure employed and also possibly to the 

 position of the anode itself, which prevents any positive rays 

 generated in more distant parts of the tube from ever 

 reaching the perforation in the cathode in the necessary 

 axial direction. 



Such a selective action has two very important results. 

 In the first place, it eliminates the many ambiguities of the 

 ordinary mass spectrum due to multiply charged rays, or to 

 hydrogen and other addition products ; but, in the second, it 

 prevents the use of the oxygen line as a comparison standard. 

 As soon as it was demonstrated beyond any reasonable doubt 

 that sodium was a simple element (and its chemical atomic 

 weight is so exactly integral on the oxygen scale as to be 

 conclusive corroboration) it was taken as standard at 23. 



