I9II-] BOLTWOOD— RADIOACTIVITY. 343 



radium. ^Measurements of the rate of production of helium by a 

 radium salt have been carried out by Sir James Dewar and have 

 given results somewhat in excess of this, namely 182 and 169 cubic 

 millimeters. As a confirmation of the accuracy of Rutherford and 

 Geiger's values, however, it may be stated that an investigation of 

 the production of helium by radium made last year by Professor 

 Rutherford and myself gave results in excellent agreement with the 

 calculated value. An account of these experiments will be published 

 shortly. 



In connection with these results there is, however, one rather 

 important point which should be mentioned. This is the fact that 

 tne rate of production of helium and the half value period of radium 

 as calculated by Rutherford and Geiger from the results of their 

 counting experiments, are directly dependent of the purity of the 

 salt of radium used as a standard in their measurements. They 

 assumed that the material of their radium standard was pure anhy- 

 drous radium bromide containing 58.5 per cent, of radium. If this 

 was not the case and the material used as their standard contains 

 less than the theoretical amount of radium, their calculation of the 

 number of alpha particles emitted per gram of radium and the rate 

 of production of helium is too low, and their estimate of the half 

 value period of radium is too high. If, on the other hand, the mate- 

 rial of their standard consists in part of some other compound of 

 radium containing a higher proportion of this element than is con- 

 tained in the bromide, their value for the number of alpha particles 

 emitted and the rate of production of helium is too high and their 

 calculated rate of disintegration of radium is too low. 



There are certain reasons which lead me to believe that the 

 radium standard used by Rutherford and Geiger actually contains a 

 higher proportion of the element radium than they have assumed in 

 their calculation, and that the true half value period of radium is 

 greater than 1,760 years as they have deduced it. In 1908 I pub- 

 lished an account of some experiments on the growth of radium in 

 ionium preparations, which pointed to two thousand years as the 

 half-value period of the former. This estimate was quite indepen- 

 dent of any radium standard and I am of the opinion that it is nearer 

 the true value than is the estimate made by Rutherford and Geiger. 



