and its Products of Transformation. 827 



activitj' of the two will obviously not decrease according to 

 an exponential law at first, since radium E x is changing to 

 radium E 2 . Finally, however, the decay will be exponential, 

 corresponding to the slower of the two periods. In the 

 experiments described later, the /3-ray product has been 

 separated from radium D, and has been found to decay 

 always according to an exponential law, with a period of 5 

 days, and only 5 days. At the same time the radium D, 

 inactive immediately after the separation, recovers its /3-ray 

 activity according to the same period ; consequently if 

 radium E x exist it must always have remained with radium D 

 in equilibrium amount, while radium E 2 must have been 

 completely separated. It is therefore impossible to disprove 

 the existence of radium Ej by such an experiment. The 

 question of its existence can, however, be definitely settled 

 by another method, depending on the observation of the rise 

 of the /3-ray activity in a preparation of initially pure 

 radium D. To this end the following experiment was made: — 

 A piece of platinum foil was exposed to the emanation cor- 

 responding to the equilibrium amount of 150 milligrams of 

 radium bromide. The quantity of emanation being very 

 large it was sufficient, in order to obtain a strong enough 

 preparation of radium D, to expose for 24 hours only. The 

 emanation occluded by the platinum was got rid of by dis- 

 solving the active deposit in acid and evaporating on a watch- 

 glass. The active deposit of short period decays in a few r 

 hours, and we have practically pure radium D left, with 

 only a very small /3-ray activity. The curve showing the 

 recovery of the /3-ray activity was very carefully determined 

 until a maximum value was reached. This curve was found 

 to be identical with the other recovery curves, being expo- 

 nential, and showing a period of 5 days. The results are 

 given in the following Table I., where the maximum activity 

 (that after 40 days) is taken as 100. If the rise of the 

 activity is due to the production of a siugle /3-ray product 

 the recovery curve of the activity is given by the equation 



where I is the maximum activity and I is the activity at 

 any time t. The theoretical numbers calculated for a. period 

 of 5 days are given in column B of Table I. They. show a 

 very good agreement with the experimental values given in 



3H2 



