1 



Active Deposit of Radium. 401 



i 



? 



I 



recoil of' RaB from the surface of the plate, it appears that 

 thrf relative quantity of RaA expanding from the plate under 

 the described conditions is about 3 per cent. In the case 

 when the plate is exposed to emanation for several minutes, 

 the same quantity was found by direct measurements to be 

 of the order of one ten-thousandth. To obtain distinctly the 

 first part of curve I the disk must be exposed in the apparatus 

 for a short time not exceeding one minute. 



The order of magnitude of the amount of expanding active 

 matter in the case of Ra(B + C) is well illustrated by the 

 f ol lo wing experiments. If the plate R coated with Ra (B -f C) 

 be thoroughly washed in water and alcohol and then strongly 

 heated for a considerable time in a Bunsen flame in order to 

 reduce as much as possible the amount of expanding matter, 

 the activity of the collecting disk, when measured by /3-rays, 

 shows a well-marked decrease during the first 3 or 4 minutes. 

 This is undoubtedly due to the presence of RaC 2 on the disk, 

 since the effect is observed only when the disk is negatively 

 charged. When the plate is but slightly heated this fall 

 in the activity of the disk cannot be detected. It appears 

 that the amount of active matter expanding from the plate is 

 usually large compared with the amount of RaC 2 given up 

 by recoil from the active deposit, and that only under special 

 conditions does it diminish to the same order of magnitude. 

 Direct measurements show that the relative quantity of 

 Ra(B + C) expanding from the plate varies from 3 

 to — I . 



25.000 



Some experiments were also made with aplaie coated with 

 a strong layer of polonium (in equilibrium with RaD). No 

 traces of activity could be detected on the collecting disk 

 after an exposure of more than two weeks. 



For a more complete study of the phenomenon, it appeared 

 necessary to investigate the rate of change with time of the 

 amount of active matter expanding from the plate; and for 

 this purpose experiments have been undertaken in order to 

 determine the period T (see Sec. 2) . Most surprising results 

 were obtained in case of RaA, when this period appeared to 

 be of striking regularity and constancy under different expe- 

 rimental conditions. The same period T, viz. 14 min., for 

 RaA was found in the preliminary, as well as in the final, 

 experiments, although they were carried out in two different 

 laboratories and after an interval of more than three years. 

 In Table I. the results are shown of one series of these expe- 

 riments. Four disks, I, II, III, and IV, were exposed in the 

 apparatus to a plate coated with RaA for 1 min. each and 



