Me Coy and Ashman — Urano-TJranic Oxide. 529 



It is well known that the potential gradient required to pro- 

 duce a saturation current increases with increasing activity, 

 the recombination of the ions being greater for a given poten- 

 tial gradient the more intense the ionization. It was, there- 

 fore, possible that the observed current for a standard film was 

 below the maximum on this account. We made the following 

 experiment to throw light on this point. Films of TJ 3 8 were 

 made in the usual way on a pair of semi-circular plates, made 

 by cutting an ordinary T cm plate into halves. The activity of 

 each half-film was measured separately and compared with 

 that observed when the two were placed side by side to make 

 a circular film. The sum of the separate activities was 0*33 per 

 cent greater than that of the two together. The experimental 

 error of the activity measurements did not exceed 0"05 per cent. 

 The experiment shows that appreciably greater recombination 

 of the ions takes place when the two plates act simultaneously, 

 due to the more intense ionization. It follows from this, that 

 the ionization current calculated above for one sq. cm. of U 3 8 

 is somewhat smaller than that which actually would be observed 

 for a film of unit area. However, the error which thus arises 

 is eliminated in the calculation of the ionization current of unit 

 mass of uranium or thorium ; since in such- a case the specific 

 activity is calculated from the value based on the activity of an 

 infinitely thin film* which would produce a vanishingly small 

 ionic concentration. Consequently the ions would suffer no 

 recombination and therefore the calculated ionization current 

 is that which would be produced by all of the ions formed. 



Boltwoodf has determined the relative activity of radium 

 and uranium by direct comparison of the activity of a minute 

 known quantity of radium with the activity of very thin films 

 of known weight of U 3 8 . It was found that radium (free 

 from its products) is 1*30 X 10 6 times as active as an equal 

 weight of uranium. Rutherford % found that the ionization 

 current of a thin film of 0-184: mg. of pure EaBr 2 , free from 

 its active products, was 8*4 X 10" 8 amp. as measured by a sen- 

 sitive galvanometer. Considering half of the a-rays to have 

 been absorbed by the plate carrying the film, this is equivalent 

 to a current of 5*94 X 10~ 4 amp. for 1 g. of pure radium. We 

 have found the total ionization current of 1 g. of uranium to 

 be 4*61 X 10" 10 amp. Therefore the a-ray activity of radium 

 (free from its products) is 1*29 X 10 6 times that of an equal 

 weight of uranium, a result which is in good agreement with 

 that found by Boltwood. 



* McCoy, Jour. Amer. Chem. Soc, xxvii, 402, 1905. 

 t This Journal, xxv, p. 296, 1908. 

 JPhil. Mag., x, p. 207, 1905. 



