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XLIX. The rMass Spectra of the Alkali Metals. By F. 

 W. Aston, D.Sc, F.R.S.. Fellow of Trinity College, 

 Cambridge* . 



[Plate XV. j 



POSITIVE rays of the metallic elements cannot be 

 obtained by the ordinary discharge-tube method, since 

 in general they have extremely low vaponr-pressures and 

 are incapable of forming stable volatile compounds. Mer- 

 cury is a notable exception to this rule, and its rays are 

 exceedingly easy to produce. 



Positively charged rays which appeared to be atoms of the 

 alkali metals were first observed by Gehrcke and Reichen- 

 heim (Ver. D. Fhys. Gesell. viii. p. 559 (1906): ix. pp. 76, 

 200, 376 (1907); x. p. 217 (1908)). They obtained them 

 by two distinct methods : the first, which may be con- 

 veniently called the " Hot Anode " method, consisted in 

 using as anode of the discharge-tube a platinum strip 

 coated with a salt of the metal and electrically heated by an 

 external battery. The second device, with which they per- 

 formed most of their pioneer work on Anode Rays, was to 

 use a composite anode of special construction which worked 

 without the need of external heating. 



A modified form of the first has been subsequently used 

 by Dempster (Phys. Rev. xi. no. 4, April 1918). The "Com- 

 posite Anode" method is now being employed by Gr. P. 

 Thomson in the Cavendish Laboratory in investigations on 

 anode rays. 



Preliminary experiments were" undertaken by the writer 

 and showed that the " Hot Anode " method was capable of 

 yielding the required results with the alkali metais when 

 difficulties of obtaining the necessary conditions had been 

 overcome. The present paper is an account of results- 

 obtained by the analysis of the metals of the lithium group. 



Apparatus and Method. 



The apparatus for producing the rays was very simple, 

 and will be readily understood from the figure. The hot 

 anode A is a strip of platinum foil *03 mm. thick, about 

 2 mm. wide by 7 mm. long, welded to the two stout platinum 

 leads which are fused through the glass at C. It was raised 

 to the required temperature by current from one large 

 storage-cell connected through a rheostat as shown. As the- 



* Communicated bv the Author. 



