[ 857 ] 



XCIII. The Application of Anode Rays to the Investigation 

 of Isotopes. By G. P. Thomson, ALA., Fellow of Corpus 

 Christi College, Cambridge *. 



[Plate XXVI.] 



I HAVE shown in a previous paper f that anode rays are 

 susceptible to the ordinary method o£ positive ray 

 analysis, and that their impact affects a photographic plate. 

 Since the anode rays consist almost entirely of charged 

 metallic atoms, it follows that they can be used to investigate 

 the number and proportions of the isotopes of metals, pro- 

 vided that the resolving power of the analysing apparatus 

 used is sufficient. In the experiments to be described the 

 method of analysis was the ordinary parabolic one. The 

 limit of the resolving power of the common type of appa- 

 ratus is about 1 in 25, so that if an atom of atomic weight 

 greater than 25 had two isotopes differing by a single unit 

 in atomic weight, the parabolas corresponding to them would 

 not be distinct. Of the elements tested in these experiments, 

 Lithium and Beryllium ate well within this limit. Calcium 

 and Strontium are outside it, bat certain conclusions of 

 interest can be drawn from the results. Sodium could 

 probably have been dealt with, but as it has been shown 

 by Dr. Aston J to be single, no useful purpose would have 

 been served, and it was only used as a standard for the 

 measurement of other parabolas. 



In these experiments the anode rays were obtained by a 

 method due to Gehrcke and Reichenheim §. A mixture was 

 made of pondered graphite and some halogen salt of the 

 metal or metals to be tested. Some of the powder was 

 put into the end of a silica tube and well rammed down ; 



Fur. 1. 



Paste 



•65cm.-r5... 



-30 cm 



an aluminum wire was then pushed into it to serve as an 

 electrode (see fig. 1). When such an electrode is made the 

 anode in a discharge-tube with a pressure corresponding to 



* Communicated by Sir J. J. Thomson, O.M., F.R.S. 



t Proc. Camb. Phil. Soc. vol. xx. p. i>10. 



% 'Nature,' March 7th, 1921. 



§ Verh. D. Pkys. Gesell. vol. ix. p. 76. 



Phil. Mag. S. 6. Vol. 42. No. 251. Nov, 1921. 3 L 



