54 Messrs. J. H. L. Johnstone and B. B. Boltwood on th 



le 



of the thorium products present. A specimen of uraninite 

 from Spruce Pine in the possession of the authors was con- 

 sidered to fulfil all the necessary requirements. It consisted 

 of essentially unaltered material selected with much care 

 from a considerably larger quantity. It contained less than 

 0*2 per cent of silica and residue insoluble in dilute nitric 

 acid. A determination of the uranium content was made by 

 one of the authors and by Ledoux & Co., of New York City*. 

 The mineral contained 1*9 per cent, of thorium oxide. 



In determining the activity of uranium a very pure speci- 

 men of uranoso-uranic oxide was used. This had been pre- 

 pared from a specimen of especially pure uranium nitrate 



Fig-. I. 



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obtained by fractional recrystallization of a much larger 

 quantity (see p. 52). The oxide was made from the nitrate 

 with all the precautions which have been mentioned in an 

 earlier papert. It is very important to note that this 

 oxide was used as a standard in the analytical determina- 

 tion of uranium (both volumetric and gravimetric) in the 



* The authors wish to express here their obligation to Ledoux & Co. 

 for this favour and to state their appreciation of the value of this carefully 

 conducted analysis. 



t Boltwood, Am. Journ. Sci. xxv. p. 278 (1908). 



