6 Johnstone and Bolhvood — Relative Activity 



mentioned in an earlier paper. 13 It is very important 

 to note that this oxide was used as a standard in the 

 analytical determination of uranium (both volumetric 

 and gravimetric) in the mineral, so that the analytical 

 determinations and radioactive measurements are in 

 direct accord with one another, although it was assumed 

 for purposes of calculation that the uranoso-uranic 

 oxide contained 84-8% of uranium. 

 It has been found by McCoy 14 that considerable ab- 



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sorption of the radiation takes place in the film itself 

 when its thickness is appreciable. In earlier work by 

 one of us 15 it was shown that by the use of thin films 

 any necessity for an absorption correction could be 

 avoided. To further demonstrate this fact a series of 

 films of uranoso-uranic oxide weighing from 0-0019 g. 

 to 0-04 g. were prepared and their activities were 

 measured. The results are given in table I and are 

 shown graphically in Fig. 1. In films weighing not more 



13 Boltwood, this Journal, 25, 278, 1908. 



14 Jour. Am. Chem. Soc, 27, 391, 1905; Phys. Rev., 1, 393, 1913. 



15 Boltwood, this Journal, 25, 176, 1908. 



