﻿78 Dr. H. L. Bronson on the Periods of 



variation in x and ?/, and the. nature of the experiment made 

 it very difficult to obtain great precision in these curves. In 

 order to get a check on these results, the ratio of the /3-ray 

 to the a-ray activity was measured after the greater part of 

 radium B had been removed by volatilization. In this case 

 the a- and /3-ray curves were approximately parallel, which 

 means that x is nearly zero and that y is nearly equal to r. 

 The values obtained for r in the two experiments tried were 

 0*80 and 1*00, a very satisfactory agreement with the other 

 results. As has been mentioned above, the results are not 

 accurate ; and furthermore there would be no particular 

 advantage in having them so, for they would be quite dif- 

 ferent under different experimental conditions. However, 

 under ordinary experimental conditions, when there is no 

 covering over the active wire, it would seem safe to say that 

 the /3 rays from radium C produce nearly 1 per cent, as much 

 ionization as the a rays, and that the /3 rays from radium B 

 produce somewhat more than this, when the two products are 

 in equilibrium. 



Measurement and Calculation of Decay Curves. 



In obtaining the experimental decay curves of the active 

 deposit, the most satisfactory results were obtained by using 

 a cylindrical testing-vessel with a central electrode, and by 

 measuring the ionization current with an electrometer, using 

 the constant-deflexion method. A saturation-potential of 

 200 volts was in general used on the testing-vessel. This was 

 found to produce practical saturation in all cases. In fact, 

 almost no difference in the curves could be detected when 

 600 volts were used instead of 200. In the case of the /3-ray 

 curves, the active wire was carefully covered with lead or 

 aluminium-foil and used as the central electrode. In the case 

 of the a-ray curves the best results' were obtained by putting 

 the emanation itself into the testing-vessel and then blowing- 

 it out suddenly, after a long or short exposure, depending on 

 the experiment. The short exposure was never over 30 

 seconds. In this paper the time is always reckoned from the 

 removal of the emanation. 



The calculations were all made on the assumption that the 

 three products radium A, B, and C are successive. The 

 theory of three successive changes for both long and short 

 exposure has been given by Rutherford (Phil. Trans. 1904, 

 p. 181, and Radioactivity, 2nd ed. p. 331). The periods of 

 the three products were taken as 3, 26, and 19 minutes 

 respectively. These values for the periods had been pre- 

 viously obtained by direct experiment. The period of radium 



