Transformation Products of Radium. 291 



At the time of publication a sufficient interval had not 

 elapsed to experimentally determine the rate of decay of the 

 activity of these substances, but it was calculated from purely 

 radioactive data that radium D should be half transformed in 

 about 40 years, and radium E in about one year. 



The present paper contains an account of further experi- 

 ments upon the variation of activity of the different products 

 with time, and of the isolation of a new product which was 

 previously overlooked. 



If a body is left for some time in the presence of the radium 

 emanation, the activity after removal, measured by the a, ft or 

 7 rays, rapidly decays with the time. At the end of 24 hours, 

 the products radium A, B, and C, deposited on the plate, have 

 been almost completely transformed. There then remains a 

 small residual activity, which comprises both a, and ft rays. 

 The magnitude of this residual activity depends upon the 

 quantity of the emanation, and upon the time of exposure, 

 but is, in general, of the order of one millionth of the activity 

 observed immediately after removal. 



It was shown that the a ray activity of the body was small 

 at first, but increased nearly proportionately with the time 

 over a period of two months. In another set of experiments, 

 it was shown that the a ray activity was still increasing after 

 standing for nine months. 



The ft ray activity was examined about a month after 

 removal, and was then found to remain sensibly constant for a 

 further period of nine months. It was considered advisable to 

 see whether the ft ray activity remained constant during the 

 first few weeks after removal. For this purpose, the active 

 deposit was obtained on a platinum plate exposed to a large 

 quantity of radium emanation for 3*8 days. Observations 

 were begun on the ft ray activity 24 hours after removal. The 

 activity was measured by placing the plate under an electro- 

 scope, the base of which was covered with a sheet of aluminium 

 of sufficient thickness to absorb all the a. rays. 



It was found that the ft ray activity was small at first, but 

 increased with the time, reaching a practical maximum about 

 40 days later. The results are shown graphically in fig, 1, 

 where the ordinates represent the activity, and the abscissse 

 time in days. The time was reckoned from the middle of 

 the time of exposure to the emanation. The first observation 

 was, in consequence, made after an interval of 2'9 days. If 

 the curve is produced backwards to the origin, its shape is 

 very similar to the recovery curves of UrX and of other 

 radioactive products, and the activity L at any time t can be 



