1 



ft 



Active Deposit of Radium. 399 



Further, the question of the charge of the radioactive 

 atoms expanding from the place has been investigated. The 

 amount of active matter deposited on the disk increased 

 many times when an electric field was established between 

 the plates A and B, but was found to be independent of the 

 direction of the field. On the other hand, a stream of air 

 maintained between A and B seemed to sweep away the 

 active matter in spite of a strong electric field between the 

 plates. In connexion with some experiments described in 

 another place it must be supposed that the expanding atoms 

 are uncharged, and are brought to the disk by the electric 

 wind*. 



A great number of experiments have been carried out with 

 the object of determining the relative quantity of active matter 

 expanding from the plate. Since the plates II during the first 

 experiments were usually slightly heated in a Bunsen flame or 

 washed in alcohol before being introduced into the apparatus, 

 it seemed necessary to find out to what extent the phenomenon 

 is affected by the process of heating or washing. It was 

 found that when the plate is introduced into the apparatus 

 without being heated or washed, the amount of active matter 

 expanding to the disk increases enormously, reaching in 

 some cases 20 times its normal value. It may be shown, 

 however, that this effect is not due to traces of emanation 

 which could adhere to the plate after its removal from the ex- 

 posure vessel and then diffuse towards the disk. Apart from 

 the fact that this supposition is not justified by the analysis of 

 the activity of the disk, the same effect can be observed when 

 the plate used is coated with RaB by recoil from RaA, and 

 has never been exposed to emanation. The experiments have 

 shown, however, that the activity given up by a plate once 

 slightly heated or washed decreases but slowly with further 

 heating or washing of the plate. 



Further experiments have been greatly complicated by the 

 lack of constancy in the relative quantity of active matter 

 given off by the plate R. When the experiments are carried 

 out under similar conditions, the ratio of activities on the 

 disk and the plate is sufficiently constant and independent of 

 the amount of emanation used ; but this ratio varies within 

 large limits with the time of exposure of the plate R to the 

 emanation, increasing considerably in case of small exposures. 

 This is more marked in the case of RaA, when the total 

 amount of active matter received by the disk is almost inde- 

 pendent of the time of exposure, so that a plate exposed to 

 emanation for a small fraction of a second gives up as much 



* Phil. Mag. xxxiv. November 1917. 



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