52 Messrs. J. H. L. Johnstone and B. B. Boltwood on the 



disintegrates to form radium, followed successively by radium 

 emanation, radium A, radium 0, and polonium. When these 

 are all present in equilibrium proportions, as is the case in a 

 non-emanating, old, radioactive mineral, then certain com- 

 paratively simple relations will exist between the a-ray 

 activities of the different constituents. It has been shown 

 by Geiger* that the ionizing power of an a-particle is propor- 

 tional to the two-thirds power of its range. The ionization 

 produced by equal numbers of a-particles emitted by two 

 different radio-elements will therefore be proportional to the 

 two-thirds power of the ranges of these particles. In a series 

 of successive products in equilibrium, each product emits 

 the same number of a-particles in unit time. The relative 

 ionization (and therefore the relative activities) due to each 

 of these products should therefore be proportional to the two- 

 thirds power of the range of the>espective a-particles. 



This relation has been shown f to hold quite closely in the 

 case of radio-thorium and its a-ray products, and also in the 

 actinium and the radium series of products. The chief object 

 of the work described in the present paper was to apply the 

 same methods to the case of the uranium-radium series with 

 the expectation that the results would throw some light on 

 the obscure relations of the earlier members of the series. 



The Radioactive Measurements. 



The determinations of the radioactivity of the different 

 solids examined were carried out in an electroscope which 

 has already been described J. In the present experiments a 

 telemicroscope was used for observing the position of the 

 gold-leaf. The natural leak of the instrument was small 

 and over a period of about six months varied from 0'4 to 0'7 

 scale division per minute. Before and after each series of 

 measurements the sensibility of the electroscope was deter- 

 mined by measurements of the activity of a standard refer- 

 ence film of pure uranoso-uranic oxide. This film was 

 carefully preserved throughout the entire period of the 

 measurements here recorded, and all the results given in this 

 paper are given in terms of this film as the standard. 



The method of preparing the radioactive materials for 

 measurement was essentially the same, with certain modifi- 

 cations, as that described by Boltwood. The material to be 



* Proc. Eoy. Soc. A Ixxxiii. p. 505 (1910). 



t McCov and Viol, Phil. Mag. xxv. p. 383 (1913); McCoy and Leman, 

 Phys. Rev.' iv. p. 409 (1914) ; ibid. vi. p. 185 (1915). 

 X Boltwood, Am. Jonrn. Sci. xxv. p. 272 (1908). 



