ART. 12 CAENOTITE FEOM UTAH — -HESS AND FOSHAG 5 



tion of the uranium to form lead in chemically measurable quantity 

 unless much more carnotite than was available could be used. How- 

 ever, that some lead is present was shown spectroscopically by E. G. 

 Zies, who was kind enough to test the mineral for us. He found also 

 copper and tin, together with gold and silver, the amount of the last 

 two being very small. 



The problem of the age of the mineral had therefore to be at- 

 tacked through the proportional equilibrium of the radium present. 

 Eutherford- estimated that equilibrium of radioactivity — that is, 

 the maximum possible quantity of radium present with its ancestral 

 uranium — was reached after a period of 10,000,000 years. Mr. C. W. 

 Davis, of the Reno station of the Bureau of Mines, was therefore 

 asked to make a determination of the proportional radioactivity, 

 which he kindly did, using a part of the analyzed material. Con- 

 cerning the determination Mr. Davis wrote: 



The carnotite from Foshag's analysis contains 65.6 per cent UO3, which, 

 using the atomic weight of uranium as given in the international table of 

 atomic weights for 1925,^ gives the uranium content of 54.60 per cent. 



Standard pitchblende, part of the sample used by Lind and Roberts in their 

 determination of the absolute value of the radium : uranium ratio,* containing 

 66.12 per cent U, was used to standardize the instruments and the Ka/U ratio 

 found by them (3.40 by 10—') was used in my calculations. 



Four samples of the carnotite of about 60 milligrams each and four samples 

 of the standard pitchblende of about 50 milligrams each were accurately 

 weighed into thin glass bulbs on an assay balance, and the bulbs, which were 

 provided with a neck, were sealed off by drawing out the neck. After from 

 35 to 38 days (it took 4 days to make the final determinations) these samples 

 were analyzed for radium. 



The method described by Lind and Whittemore ^ as the " solution emanation 

 method in a single operation " was chosen as the most satisfactory for the 

 purpose. This prevents any loss of radium that might occur froni the long 

 standing of solutions and eliminates errors that might occur in determining 

 the " emanating power " of the minerals used. It also makes the use of equi- 

 librium tables unnecessary. The details of boiling off, collection, and measur- 

 ing of the radon are given by Lind.* 



The electroscope chambers were treated so that the natural leak was satis- 

 factory (0.033 divisions per second), and a blank test on the reagents and 

 apparatus gave precisely the same rate of leaf fall. This natural leak remained 

 constant during the four days on which the tests were made. 



Fifteen electroscopic readings were made for each determination, the pres- 

 sure in the chambers being kept at less than atmospheric until about 30 minutes 

 before readings. 



At the times during which readings were taken the barometric pressure was 

 within the limits 644 mm. and 642.9 mm., and the temperature was within 

 the limits 20° C. and 18.5° C, so that no correction is necessary for these 

 factors. 



* Rutherford, E. Radioactive snbstai.ces and then- radiations. Cambridge, p. 431, 1913. 

 3 Journ. Amer. Ch«EQ. Soc, vol. 47, p. 600, 1925. 



*Idpm, vol. 42, p. 1170, 1920. 



* Bureau of Mines Technical Paper 88, pp. 12-13, 1915. 



« Jonrn. Industrial & Engineering Chemistry, vol. 7, p. 1024, 1915. 



