Recoil of Radium C from Radium B. 103 



leaving the plate by recoil from radium A might be com- 

 parable with the number of radium G particles. It will be 

 seen later that at least one-tenth of the radium B particles 

 which recoil from radium A actually leave the plate. 



The second explanation of the phenomenon seems probable 

 if there is deposited on the plate sufficient active deposit to 

 form a double layer of atoms, in which case a radium B 

 particle, if deposited on the top of a radium (J particle, 

 might easily be mechanically carried off the plate during 

 the formation of radium D. It will be seen later that each 

 of the two causes suggested is in all probability partly 

 responsible for the ejection of radium B from the active 

 plate. 



On account of these considerations it was thought that in 

 order to obtain pure radium C on a disk exposed to an active 

 plate, as long a time as possible should be allowed after 

 removing the plate from the emanation to allow the radium A 

 on it to decay to an inappreciable quantity. Moreover, it 

 seemed desirable to use the smallest quantities of emanation 

 that would permit of the subsequent measurements being- 

 made with sufficient accuracy. The following series of 

 experiments shows how a coudition can thus be reached in 

 which the active plate radiates practically pure radium C to 

 a disk exposed to it in a high vacuum. The plate to be 

 rendered active was exposed for some hours to a considerable 

 quantity of emanation, after which it was withdrawn and 

 heated in a vacuum for 27 minutes to remove any adhering 

 emanation. The quantity of radium C on the plate was then 

 measured on a 7 ray electroscope, and found to be equal to 

 the amount in equilibrium with 8*05 mgms. of radium 

 bromide. Six minutes later it was mounted on the support 

 A (fig. 1), and a disk suspended a few millimetres above it 

 to receive the radiant active matter emitted by it. After an 

 exposure of 20 minutes the disk was removed, and the rate 

 of decay of the activity collected on it measured by a 

 quadrant electrometer. The results obtained are shown in 

 fig. 2, curve I. (p. 104) . It will be seen that the activity of the 

 disk fell to half value in about 33 minutes, indicating that there 

 were both radium B and radium C on it. After 80 minutes 

 from the time at which the active plate was removed from 

 the emanation, a second disk was exposed to the radiating 

 plate, and a similar series of observations made with it after 

 an exposure of 20 minutes. The results are given in curve II. 

 and show that even after this time the disk had received 

 some radium B as well as radium ( \ for the activity took 

 24J minutes to fall to half its initial value, although the 



