Mr Norman Campbell, The Radioactivity of Rubidium. 11 



The Radioactivity of Rubidium. By JSToRMAN Campbell, M.A., 

 Trinity College. 



[Bead 9 November 1908.] 



1. In the first paper by Mr Wood and myself* on the radio- 

 activity of the alkali elements, it was stated that rubidium as well 

 as potassium was found to be radioactive, and that the rays from 

 the former metal were less penetrating than those from the latter. 

 Owing to the difference in the penetrating powers of the rays, the 

 relative activity of the two elements can not be determined directly 

 by a comparison of the total ionisation caused by the rays from 

 a thick layer, but it can be determined by measurements of the 

 activity of layers of varying thickness. Such measurements on 

 potassium sulphate were described in a recent paperf : similar 

 measurements have now been made upon rubidium sulphate. 



2. The experiments were made in a manner exactly similar 

 to those on the potassium salt, and the same apparatus was used. 

 The sample of rubidium sulphate used was specially prepared by 

 Kahlbaum : its purity was tested by precipitation by barium 

 chloride : the weight of the sulphate precipitated agreed within 

 the limits of error (1 °U with that predicted from the accepted 

 atomic weight of Rb. Some difficulty was experienced in obtaining 

 layers of the requisite thinness and uniformity : the method 

 adopted ultimately consisted in scattering the finely powdered 

 salt over the surface out of a vessel of the nature of a pepper-pot. 



The following table gives the results obtained : 



These experimental points lie less closely on a theoretical ex- 

 ponential curve than the corresponding points for the potassium 

 salt : the rubidium rays are probably less homogeneous. 



Proc. Camb. Phil. Soc. xiv. 1, pp. 15 — 21. 



t Ibid. XIV. 6, pp. 557—567. 



