of Gamma Rays from Radium B and Radium C. 729 



absorbing material equivalent in amount to about 14 cm. of 

 air. Any very soft type of y radiation which might be 

 present would be largely absorbed in traversing this material. 

 It is essential, however, in the experiments to use sufficient 

 absorbing material to stop completely the a rays from 

 radium 0, which have a range of 7 cm. in air. Since it is 

 well known that, for equal masses, X rays pass with much 

 less absorption through elements of small atomic weight, 

 arrangements were made to absorb the a rays mainly by 

 hvdrogen and carbon. The source of radiation was the 

 active deposit of radium deposited from the emanation on 

 both sides of a very thin mica plate. This was placed in 

 a brass vessel which was closed at one end by a thin 

 mica plate equal in stopping power to 1*5 cm. of air. This 

 mica plate also formed one side of the ionization chamber, 

 shown iii fig. 1. The active matter was deposited on mica 

 to avoid the excitation of detectable characteristic X rays. 

 For a similar reason, the inside of the brass vessel was lined 

 entirely with thick cardboard. A continuous current of 

 hydrogen was sent through the brass> vessel. Sheets of 

 india-paper were interposed in the path of the rays of just 

 sufficient thickness to stop entirely the a. rays. The ioniza- 

 tion in the detecting vessel filled with methyl iodide was 

 then carefully examined when thin aluminium screens were 

 introduced. It was found that the ionization at first de- 

 creased more rapidly than corresponded to an exponential 

 law of absorption of the radiation for which ;>t = 40 in 

 aluminium. This initial drop could be accounted for by 

 assuming the presence of a very soft y radiation for which 

 yu, = 230 about in aluminium. Since the ionization in methyl 

 iodide due to this radiation corresponded to only 10 per 

 cent, of the total effect, the initial slope of the curve could 

 not be determined with much certainty. It is difficult to 

 decide whether this soft radiation has an independent 

 existence, or whether it is due to an initial drop in the 

 absorption curve of the radiation corresponding to /z = 40. 

 It will be seen later that a rapid initial drop of the absorption 

 curve is always observed when lead is used as an absorbing 

 material. It is possible that aluminium may show a similar 

 effect for a comparatively soft radiation. 



From the rate of decay of this very soft radiation it was 

 clear that it arose from radium B. It was always propor- 

 tional in amount to the radiation /z, = 40. It should be 

 pointed out that decay of the active deposit measured under 

 these experimental conditions is initially far more rapid than 

 that calculated by Moseley and Makower (loc. cit.). This is 



