Manchester Memoirs, Vol. Iv. (1910), No. %. 3 



calculated theoretically. It will be seen that curve B, 

 which represents the results of the experiment just 

 described, hardly rises at all, remaining nearly constant at 

 first, and beginning to decay after about 20 minutes. 

 The curve indicates that more than half of the active 

 matter reaching S was radium C, and not radium B. This 

 result can be explained if, when the radium B impinges 

 on the reflector, a small portion of it is scattered on to S, 

 but the greater part remains on the reflector, and subse- 

 quently gives rise to radium C, a small fraction of which 

 is then directly projected on to the plate S. That the 

 admixture of radium B with radium C on S is to be 

 attributed to reflection is probable, since the matter reach- 

 ing a surface by direct radiation from a wire coated with 

 radium A consists only of radium B. An experiment was 

 performed under these conditions, and the decay curve of 

 the activity collected on a surface after an exposure of ten 

 minutes to the radium A was obtained. The points lying on 

 the curve A {Figure 2) were determined in this way, and 

 we have seen that the curve itself was obtained by calcu- 

 lation for these experimental conditions. In an experi- 

 ment in which a plate was situated so as to receive 

 radium B from a source of radium A, only after a number 

 of reflections, the proportion of radium C reaching the 

 plate was even greater than in the case already cited. 

 The fact that in the case of a single reflection considered 

 above, radium B and radium C reached the plate S in 

 almost equal proportions was a little surprising ; for it 

 has been shown that when a surface is coated with radium 

 B, under normal conditions the number of atoms of radium 

 C which succeed in escaping from the surface by recoil is 

 only of the order of one thousandth of the total number 

 formed.* The composition of the activity on the plate S 



* Makower & Russ, r/iiL Mag., Jan., J910. 



