•±16 Bolt /rood — Thorium Minerals and Salts. 



a proportionately larger amount of the characteristic thorium 

 emanation, appeared at first to add a further complication to 

 the question as to the radio-activity of ordinary thorium salts. 

 Halm has suggested,* however, that this novel radio-active 

 body, which has been named " radio-thorium," is itself a dis- 

 integration product of ordinary thorium. If this hypothesis is 

 correct, it is to be expected that, in the natural minerals con- 

 taining thorium, the parent substance and its product will have 

 reached a state of radio-active equilibrium and the total activ- 

 ity of the thorium and its products will be proportional to the 

 absolute amount of the element thorium which is present. 



The experiments which are described in the present paper 

 were undertaken with the object of determining what propor- 

 tion of the total a-ray activity of radio-active minerals was 

 produced by the thorium present and whether the activity due 

 to thorium was in all cases proportional to the actual amount 

 of this element contained in the mineral. 



Composition of the Minerals. 



The minerals used in these experiments were thorianite, 

 thorite, orangite and monazite. The determination of the per- 

 centage of thorium oxide in each mineral was carried out in 

 the following manner. The thorianite was dissolved in con- 

 centrated nitric acid and the thorite and orangite in dilute 

 hydrochloric acid. The solutions were evaporated to dryness 

 and the residues heated to render the silica insoluble. The 

 residues were then moistened with concentrated hydrochloric 

 acid, boiling water was added and after a short period of diges- 

 tion the insoluble portions were removed- from the solutions by 

 filtration. The filtrates were again evaporated to dryness, the 

 residues treated with a few drops of hydrochloric acid followed 

 by a small quantity of water and the solutions thus obtained 

 were filtered to remove traces of separated silica. The solu- 

 tions were cooled and diluted, and then treated with an excess 

 of hydrogen sulphide. The precipitated sulphides were filtered 

 off, the filtrates were boiled to remove the excess of hydrogen 

 sulphide, and were poured into boiling solutions containing a 

 considerable excess of oxalic acid. The mixed solutions were 

 then allowed to stand at ordinary temperature and at the end 

 of 24 hours the precipitates of oxalates of the rare earths were 

 collected on filters. The oxalates were converted into nitrates 

 and the excess of nitric acid in the solutions was removed by 

 evaporation. A solution of ammonium oxalate saturated in 

 the cold was prepared and portions of this containing a con- 

 siderable excess of the salt over the amount necessary to pre- 

 * Jahrb. d. Badioaktivitat, ii, 425, 1905. 



