Rutherford and Boltwood — Radium and Uranium. 55 



Art. VII. — The Relative Proportion of Radium and 

 Uranium in Radio-active Minerals / by E. Rutherford 

 and B. B. Boltwood. 



The experiments made by one of us* have shown that 

 within the limit of experimental error the amount of radium 

 present in radio-active minerals is proportional to the content 

 of uranium. The amount of radium corresponding to each 

 gram of uranium in a mineral is thus a definite constant which 

 is of considerable practical as well as theoretical importance. 



The proportionality between the content of uranium and 

 radium in radio-active minerals strongly supports the view that 

 radium is a decomposition product of uranium. According to 

 the disintegration theory, the amount of radium per gram of 

 uranium present in a mineral should be a constant whose value 

 can be approximately deduced if the relative activity of pure 

 radium and pure uranium is known. 



In order to determine the amount of radium associated with 

 one gram of uranium it is only necessary to compare the 

 activity of the emanation produced by a standard quantity of 

 pure radium bromide with that produced by a quantity of 

 mineral containing a known weight of uranium. 



In the experiments which are to be described, a standard 

 solution of radium bromide was prepared from a specimen of 

 radium bromide, which had been found experimentally by 

 Rutherford and Barnes to give out heat at a slightly greater 

 rate than 100 gram-calories per hour. The radium bromide 

 was, therefore, probably pure. About one milligram of the 

 salt was taken and weighed as accurately as possible on a bal- 

 ance. The weighing was confirmed by comparing the relative 

 gamma-ray effect produced on an electroscope by the sample 

 in question and the effect produced by a quantity of radium 

 bromide weighing 23*7 milligrams. The determinates by the 

 two methods were found in good agreement. 



The known weight of radium bromide was dissolved in water 

 and solutions were successively made up which contained 10" 2 

 and 10" 4 milligram of radium bromide per cubic centimeter. 

 Of the more dilute solution a quantity equivalent to l*581 cc 

 was carefully weighed out, transferred to a glass bulb having 

 a capacity of about 100 cc and diluted to a volume of about 50 cc 

 with pure, distilled water. The bulb was sealed and allowed 

 to stand for about 60 days in order that the maximum quan- 

 tity of emanation might accumulate. At the end of this 

 period the emanation was completely removed by boiling the 

 * Boltwood, Phil. Mag. (6), ix, 599, 1905. 



