400 



Mr. S. Ratner on some Properties of ' tlie 



RaA as a plate exposed for several minutes. This effect is 

 always observed whether the plate is heated or not. As 

 stated above, however, the quantity of RaA received by the 

 collecting disk is proportional to the amount of emanation 

 used. 



In the case of very short exposures the relative quantity of 

 RaA expanding from the plate is so large that it becomes 

 comparable with the amount of RaB projected from the 

 plate by recoil. This is clearly shown in curve I (fig. 2), 



Fig. 2. 



10 20 



Time in minutes 



which is a curve of decay of the active matter collected by 

 the disk, in the case when the plate R is exposed to emanation 

 for a small fraction of a second *, and the direction of the 

 electric field in the apparatus is such as to enable the recoil 

 atoms of RaB to reach the disk. If under the same conditions 

 the field in the apparatus be reversed, the curve III is 

 obtained for the activity of the disk. Curve I is evidently 

 the sum of the two curves, II and III, corresponding to RaB 

 and RaA, and it may be easily seen from the curves that the 

 amounts of RaB and RaA deposited on the disk are as 

 10 to 1 respectively. Putting 0'6 for the efficiency of 



* This may be easily realized when Wertenstein's exposure vessel is 

 used. Wertenstein, These, Paris 1912. 



