112 Dr. W. Makower and Dr. S. Russ on the 



air molecules before reaching the disk suspended above, then 

 it becomes a matter of chance whether such particles diffuse 

 on to the disk or back again on to the plate. In fact a small 

 current of air passing between the plate and disk might make 

 all the difference in deciding whether the disk received much 

 or little radium from the plate. The distance between the 

 disk and plate was therefore increased as far as was possible 

 without rendering the amount of radium C reaching the disk 

 immeasurably small. With the disk 2*5 centimetres from the 

 plate, and using the active deposit equivalent to *2 milligram 

 of radium bromide on the surface of the plate, it was found 

 just possible to make the required measurements. Even in 

 these circumstances the experiments w T ere difficult to perform 

 with any precision, for, as has been mentioned above, the 

 amount of radium expelled from a plate coated with a 

 definite quantity of radium B is a variable quantity, and it 

 was therefore necessary not only to test the quantity of active 

 deposit on the plate before starting an experiment but also to 

 determine its power of radiating radium C. Unfortunately, 

 the measurements were still further complicated by the 

 changes in the radiating power of the surface when left to 

 stand ; the surface had therefore to be tested after as well as 

 before an experiment, to make sure that it had not changed 

 enough to completely vitiate the results. Surfaces have 

 never yet been obtained which maintained their power of 

 radiating radium perfectly satisfactorily, but the changes 

 were erratic, being sometimes very large and sometimes 

 comparatively small. Only those experiments were made 

 use of in which the changes of surface were reasonably 

 small ; other experiments were rejected. 



The method of conducting an experiment consisted in 

 exposing a platinum plate, mounted on an iron disk to give it 

 rigidity, to some radium emanation for several hours. The 

 plate was then removed from the emanation and allowed to 

 .stand for three-quarters of an hour to allow radium A to decay 

 to an inappreciable amount, the emanation adhering to the 

 plate being driven off in the manner already described. The 

 activity of the plate having been measured by a y-ray 

 electroscope, it was mounted on the support A and the disk B 

 was suspended 2*5 cm. above it to receive the radium 

 expelled from the plate. The air in the vessel V was then 

 pumped out as quickly as possible by a Gaede pump, and after 

 20 minutes the disk B was removed and tested by a sensitive 

 a-ray electroscope. It was noticed that both sides of the disk 

 were radioactive when tested, though the activity of the front 

 side of the disk which had faced the radiating plate was always 

 oreater than that of the back. Now no active matter could 



