carried by the a and ft Rays of Radium. 203 



In order to eliminate as far as possible the error due to 

 absorption of the /3 rays in the radium itself and the envelope 

 containing it, I employed a different method. Instead of 

 using the radium compound itself, a body made active by 

 exposure in the presence of the radium emanation was used 

 as a source of ft rays. A lead cylinder 1 cms. long and 4 mms. 

 in diameter was made the negative electrode in a vessel con- 

 taining a large quantity of radium emanation. After about 

 three hours exposure, tiie excited activity reaches a maximum. 

 The lead rod was then removed, and the intensity of the 7 

 rays from it was compared directly by means of an electro- 

 scope with that due to a known weight of pure radium bromide 

 in radioactive equilibrium. The 7 rays, rather than the 

 ft rays, were chosen as a means of comparison, as the ab- 

 sorption of the 7 rays in the lead rod or tlie radium envelope 

 is very small. Suppose, for example, that the 7 ray effect 

 from the active deposit in the lead rod was equivalent to 

 m milligrams of radium bromide. Now the ft and 7 rays from 

 the radium or the active deposit on the rod arise only from 

 the one product radium C. Since the ft and 7 rays always 

 occur together and in the same proportion, the total number 

 of ft particles emitted by the lead rod is equivalent to the 

 number emitted by m milligrams of radium bromide. Since 

 the active deposit on the rod is extremely thin, half of the 

 ft rays projected from it escape without absorption. The 

 stoppage of the ft particles by the active matter itself, such 

 as would occur if radium bromide were directly used, is thus 

 avoided. 



Immediately after testing, the lead cylinder was wrapped 

 with a thickness of aluminium-foil just sufficient to com- 

 pletely absorb the a. rays. It was found experimentally that 

 the u rays were completely stopped by 13 layers of foil, each 

 of thickness "00031 cms. 17 layers of foil, that is a thick- 

 ness of aluminium of "0053 cm. were added in all. This 

 ensured that the absorption of the ft particles in the aluminium 

 screen was a minimum, consistent with complete absorption 

 of the a rays. The lead rod was made the central electrode A 

 in the apparatus of fig. 2. The outer cylinder B was con- 

 nected with one pole of a battery, the other pole of which 

 was earthed, and the central electrode with a Dolezalek elec- 

 trometer, using a suitable capacity in parallel. The air was 

 exhausted as rapidly as possible, and measurements were 

 begun usually about 20 minutes after the removal of the 

 active rod from the emanation. 



The potential of the external cylinder was alternately re- 

 versed, and the currents measured. As we have seen in the 



