652 Miss Hath Pirret and Mr. F. Soddy on the 



Evidence is also given of a limit to the penetrating power 

 of Rontgen radiation. Unless the laws of scattering some- 

 where break down, the lowest possible value for — is about 



•2. This has been approached in the case of absorption by 

 carbon. 



LXXVII. The Ratio between Uranium and Radium in 

 Minerals. II. By Ruth Pirret, B.Sc, and Frederick 

 Soddy, M.A., F.R.S.* 



IN a previous paper on this subject (Phil. Mag. 1910 [6] 

 xx. p. 345), a short account was given of the determi- 

 nation of the ratio of radium to uranium in Ceylon thorianite 

 and a specimen of Portuguese autunite. The preliminary 

 results went to confirm those of Mile. Gleditsch (Compt. 

 Rend. 1909, cxlviii. p. 1451 ; cxlix. p. 267) in that the ratio 

 in autunite was found to be considerably lower than in 

 pitchblende ; but the results with thorianite w T ere not equally 

 conclusive. Only one specimen of thorianite was compared 

 with the old pitchblende standards prepared some years ago. 

 Further investigations with several different specimens of 

 thorianite, pitchblende, and autunite were therefore carried 

 out on the same lines. 



Uranium Analysis. — The methods of estimating the uranium 

 in thorianite and autunite have already been described. In 

 the case of the pitchblendes the mineral was first dissolved 

 in nitric acid, the solution diluted, filtered, evaporated to 

 dryness, the residue dissolved in hydrochloric acid, treated 

 with sulphuretted hydrogen and filtered. The filtrate, after 

 heating and oxidizing, was poured into a mixture of ammonium 

 hydrate, sulphide, and carbonate, corked, and left over night. 

 The filtrate from this precipitate was heated, acidified by 

 nitric acid, and the uranium precipitated by microcosmic salt 

 and sodium thiosulphate in presence of acetic acid. The 

 precipitate was ignited in a porcelain crucible and weighed 

 in the form of a green compound of constant composition. It 

 was then converted, by means of a few drops of strong nitric 

 acid, into uranium pyrophosphate and weighed again after 

 ignition at a dull red heat (Brearley's 'Analytical Chemistry 

 of Uranium/ p. 7). In some cases Patera's Method (Fre- 

 senius, ' Quantitative Analysis,' vol. ii. p. 310) was employed, 

 or a modification of it in which, instead of the uranium being- 

 weighed as sodium uranate, it was, after separation by 



* Communicated by the Authors. 



