418 Boltwood — Thorium Minerals and Salts. 



from the various solutions which had been obtained in the 

 above series of operations and were reworked for the separation 

 of any thorium which might have escaped the first treatment. 

 Only an insignificantly small amount of thorium was recovered 

 by this second treatment. The residues of sulphur from the 

 precipitates formed by the sodium thiosulphate were also ex- 

 amined for traces of thoria, but with negative results. Exactly 

 10 grams of monazite was taken for the analysis. 



The thorium oxide finally obtained in all cases was perfectly 

 white after intense ignition over the blast lamp. 



The percentage of uranium in the minerals was determined 

 in the case of thorianite by direct analysis, carried out by the 

 method which has been previously described.* In the case of 

 the other minerals, and also in the case of thorianite, the 

 uranium present was determined indirectly by comparisons of 

 the amounts of radium emanation produced in the minerals 

 with the amount of emanation produced in a standard, ana- 

 lyzed sample of uraninite.f With the thorianite the results 

 obtained by these two independent methods were in excellent 

 agreement. 



Radio-activity of the Minerals. 



The radio-activity of the different minerals was determined 

 by a method very similar to that which has already been 

 described by McCoy4 The minerals were ground to an 

 impalpable powder with redistilled chloroform in an agate 

 mortar. In the form of a thin paste with chloroform, the pow- 

 der was then painted with a camel's-hair brush on a thin plate 

 of aluminium. The sheets of aluminium were T - 5 cm wide, 

 9 cm in length and approximately 0"l mm in thickness, and 

 weighed about 2 grams each. After the chloroform had evap- 

 orated it was possible to determine the increase in weight due 

 to the film with considerable accuracy. In order to avoid the 

 necessity of making corrections for the absorption of the rays 

 by the material itself,§ the films were made very thin with a 

 weight of only about 5 milligrams of material on a surface of 

 60 sq. cm. The errors then due to the absorption of the rays 

 were very small and were within the limit of error of the other 

 measurements. It was found that the activities of films of 

 approximately equal weight prepared from the same material 

 by the above method were in good agreement, and that for 

 films weighing up to 10 milligrams the activities were quite 

 closely proportional to the weight of material taken. 



The ionization produced by the films was measured in two 

 different electroscopes, a smaller one having an ionization 



* Boltwood, Phil. Mag. (6), ix, 603, 1905. 



f Boltwood, loo. cit. 



t Jour. Am. Chem. Soc, xxvii, 391, 1905; Phil. Mag. (6), xi, 176, 1906. 



§ McCoy, loc. cit. 



