746 Mr. & Mrs. Soddy and Mr. A. S. Eussell on the 



range but finally became convex as before. The value of the 

 leak at this point of departure from the straight line 

 (T = 5*5 cm.) is about 10 divisions per minute. The 

 departure in this case is not due to the same small effect as 

 had caused the departure in the first experiments (fig. 12). 

 Some measurements to illustrate the effect of the circular 

 screens on the radiation are given in Table III. 





Table III. 





Thickness of Lead. 



Intensity, 



Intensity, 



(cm.) 



no lead screens. 



with lead screens 



7-415 



23-21 



16*24 



8-591 



17-38 



9-99 



9-856 



14-03 



6-22 



11-086 



11-65 



4-27 



Thus at the end of the range the radiation entering the 

 brass electroscope is reduced to 0*37 of its value by merely 

 placing lead screens round the side of the electroscope. 

 Similar screens of brass of equivalent thicknesses were sub- 

 stituted for the lead but they had practically no effect on the 

 leak in the electroscope. It may be noticed that the 

 difference in the values in columns 2 and 3 of Table J. 

 is practically constant (7*4 divisions per minute), as though 

 caused by a constant reflected or secondary radiation entering 

 the instrument, which is cut down by lead but which is 

 capable of penetrating brass. Such a type of radiation has 

 already been indicated by Kleeman (Phil. Mag. 1907, xiv. 

 p. 643) and Eve (Phil. Mag. Aug. 1909, p. 283). 



The origin of this effect was sought next. With disposition 

 otherwise identical, the radium was now mounted not in a 

 block of wood but was placed in a shallow groove in a lead 

 plate of the same diameter as the absorbing plates and with 

 3'5 cm. thickness of similar plates below. When the 

 absorbing plates are now put on the radium it is surrounded 

 in every direction by at least 1 cm. of lead. The absorp- 

 tion curve now obtained was exponential over the range 1 to 

 10*5 cm. (\ = 0'50). The screens of lead round the brass 

 electroscope have now practically no effect, a very small 

 difference between the leaks with or without lead round the 

 electroscope being due to secondary 7-radiation reflected 

 from the wood of the stand, the existence of which has been 

 established by Kleeman. This latter secondary 7-radiation 

 is quite distinct from the peculiar secondary radiation under 

 discussion . 



