Relation between Uranium and Radium. 211 



to be .80,000 years from the results with the third prepara- 

 tion, and 70,000 years from the results with the fourth. 

 Recalculating on the same data as are used later in this 

 paper, these periods must be increased by the factor 1*2, that 

 is to 96,000 and 84,000 years respectively. If part of the 

 growth of radium is derived from initial ionium the period 

 of ionium, naturally, is increased. Since it was thought 

 that even in these most highly purified preparations part of 

 the growth at least must be due to initial ionium, the period 

 of: the latter was estimated as probably at least 100,000 years 

 (now 120,000 years). In 1912, however, the direct experi- 

 ments only fixed a lower limit to the value of the period, 

 and gave no indication of the true period. 



Measurements on the rate of growth of radium in the 

 various preparations were continued till September 1914, in 

 Glasgow, under the same conditions and with the same 

 instrument as previously*, and indicated a clear increase of 

 the rate of growth in the fourth and largest uranium pre- 

 paration, containing 3 kilograms of uranium (element). 

 This increased rate has been confirmed by subsequent 

 measurements with a new instrument and under slightly 

 altered conditions of measurement in Aberdeen, whither the 

 preparations were all successfully transported. It is now 

 possible to say that a definite growth of ionium from uranium 

 has been experimentally observed, and to fix the true period 

 of ionium approximately. 



AfetJiod of Measurement. 



Until the summer of 1914, the method of measurement 

 was the same as previously adopted and described*. In 

 October 1914, the whole of the preparations were successfully 

 transported in their sealed flasks to Aberdeen, and a new 

 electroscope had to be set up for the measurements. Ad- 

 vantage was taken of this necessary break in the continuity 

 of the measurements to modify the method slightly to render 

 the observations less time-consuming. Hitherto, all the 

 measurements had been made with the leaf charged nega- 

 tively, maintaining the leaf charged as it leaked away during 

 the 3-hour interval between admission of the emanation and 

 measurement, to avoid errors through changes in the distri- 

 bution of the active deposit f. It is more convenient to 

 charge the leaf positively, though the sensitiveness of the 

 electroscope is thereby reduced, as then the instrument can 



* Phil. Map:. [G] xviii. p. 847 (1909). 

 t Ibid. p. 850. 



P2 



