Recoil of Radium C from Radium B. 



113 



have reached the back by direct radiation, and its activity was 

 at first attributed to traces of emanation left on the plate and 

 gradually escaping from it. But it was subsequently shown 

 that this was not the case ; for when the disk was exposed to 

 an active plate for a long time, so that all the radium B and 

 radium present had decayed, neither side or! the disk showed 

 the slightest activity. No doubt the real explanation o£ the 

 effect is that a portion of the radium G emitted by the plate 

 is stopped before reaching the disk by the residual air left 

 even at very low pressures and then diffuses round to the back 

 of the disk. In order to find the amount of radium C directly 

 shot on to the disk, the activity of the back of the disk was 

 subtracted from that on the front. 



After exposing a disk in this way another similar one was 

 then suspended above the plate, and the pressure of the air 

 in V adjusted to any desired value which was read on a 

 McLeod gauge. After 20 minutes this disk was removed and 

 tested, and a third one was inserted and exposed to the radiation 

 for 20 minutes in vacuo. Allowing for the decay of radium B 

 on the plate, it was easy to calculate the amount of radium 

 which should have reached this third disk in terms of the 

 quantity which reached the first disk exposed. If the quantity 

 of radium C found on the disk proved to be very different from 

 the calculated amount, the experiment was rejected ; in other 

 cases, the mean of the two readings obtained in vacuo was 

 taken. In this way it was possible to get some idea of the 



Table I. 



Pressure, 



in millimetres 



of Mercury. 



Activity 



in Arbitrary Units. 



Percentage of 

 the Mean of 



the two Vacuum 

 Readings. 



Front. 



Back. 



Front- 

 Back. 



•0114 



85 



36 



49 



88 



•010 



1040 



182 



858 



61 



•020 



252 



56 



190 



04 



•028 



220 



61 



105 



7-> 



•036 



347 



05 



282 



53 



•041 



186 



00 



120 



35 



040 



88 



44 



44 



40 



•045 



333 



145 



188 



49 



•066 



89 



31 



57 



24 



•073 



90 



38 



51 



14 



Phil Mar/. S. 6. Vol. 11). No. 109. Jan, 1910. 



