10G Dr. W. Makower and Dr. S. Russ on the 



If no field existed between the plate and disk, this radium C 

 ■would diffuse to the disk and plate and to the walls o£ the 

 containing vessel, whereas in an electric field it might be 

 attracted to one or other electrode. Any such action would 

 have complicated the results, and the effect oH an electric 

 field of 340 volts per centimetre was therefore tried. It 

 will be seen, however, that ihe field had very little effect, 

 the time period of the activity received by the disk being 

 almost independent of the direction of the field, as may be 

 seen from curves II. and III. (fig. 3). 



The result just arrived at, that radium B and radium C 

 are mechanically projected from the plate as a secondary 

 effect due to the recoil of radium D from radium C, is not a 

 little surprising when we consider the possibility of the 

 formation of a double layer of atoms of active deposit. For 

 the area of the plate was about one square centimetre, and 

 if we take the diameter of a molecule to be 2xl0 -8 cm., 

 we see that it would require about 3 X 10 1:> active deposit 

 particles to completely cover the plate with a single layer of 

 atoms. But since the quantity of radium ( ) in equilibrium 

 with 1 gram of radium emits 3*4 x 10 10 a. particles per second *, 

 it follows that in this quantity of radium C there are 5*77 x 

 10 13 particles. Hence, since in the above experiments there 

 were about the same number of radium B and radium C 

 atoms on the plate, it would have required the amount of 

 these two products in equilibrium with 2G grams of radium 

 to form a single complete layer of atoms on the plate. This 

 is certainly 3000 times as much as was present on the plate 

 in the experiments, and to explain the observed effect it seems 

 necessary to suppose that the active deposit is not evenly 

 distributed over the plate but deposited in heaps at certain 

 places on the plate, the greater portion of which is entirely 

 free from deposit. Whether this localization of the activity 

 is due to the non -uniformity of the electric field during the 

 exposure of the plate to the emanation caused by the uneven- 

 ness of its surface, or to some other unknown cause, remains 

 a matter for speculation, but the existence of this aggregation 

 of the deposit is certainly striking. It may be mentioned 

 that an active glass surface behaved quite similarly to the 

 platinum surfaces generally used, so that the phenomena do 

 not appear to depend on the smoothness of the surface. 



To emphasize the extreme difficulty of getting absolutely 

 pure radium C by the recoil method, an experiment may be 

 cited in which a plate was exposed to a small quantity of 



* Rutherford Sc Geig-er, Proc. Roy. Soc. A. vol. lxxxi. p. 141. 



