404 Properties of A ctive Deposit of Radium. 



surface of the plate, so that small air-disturbances could 

 easily set them free. This is apparently supported by the 

 fact that the washing or heating of the plate considerably 

 reduces the effect. It must be remembered, however, that 

 the phenomenon has been observed by Russ and Makower 

 (loc. cit.) at high vacua where no air-disturbances could 

 arise. Furthermore, the plate after being repeatedly heated 

 and washed several times still continues to give off the active 

 matter. In some experiments the active surface of the plate 

 (not heated or washed) was exposed to a violent stream of gas 

 coming from a high-pressure bottle at 80 atmospheres, and 

 this did not reduce appreciably the amount of expanding 

 active matter, although the stream of gas was certainly 

 strong enough to remove the slight^ attached particles from 

 the active surface. 



A number of experiments have been made in order to 

 ascertain whether the phenomenon is affected by physical 

 or chemical conditions on the surface of the plate. The 

 results were entirely in the negative. A clean and well- 

 polished platinum surface was found to give up the same 

 amount of active matter, and with the same period T as a 

 rough surface of brass oxidized in air or covered with 

 grease. 



5„ General Conclusions. — If the experiments carried out in 

 this work have failed to disclose the nature of the phenomenon, 

 they give nevertheless a detailed description of some occur- 

 ences taking place in the active deposit of radium, which for 

 some time now have served as a grave source of error in 

 many investigations in radioactivity. In the work of Fajans 

 this source of error could be overcome owing to the fact that 

 the amount of the branch product given up by RaC is not 

 too small compared with the quantity of RaC expanding 

 to the collecting disk. In other cases, however, this source 

 of error renders the investigation impossible. That, for 

 instance, is the case in some work carried out with the 

 object of investigating the recoil phenomena due to /3-rays. 

 Various experiments described in this paper show clearly 

 that, if the recoil of RaC from RaB does exist in reality, the 

 amount of RaC given up by this process must be vanishingly 

 small, compared with the activity expanding from the surface 

 coated with the active deposit. A survey of the work 

 dealing with the questions of recoil of RaC from RaB 

 leads to the conclusion that this phenomenon has hardly 

 ever been observed. Unless the source of error referred 

 to above is completely eliminated, all attempts to detect 



