750 Mr. & Mrs. Soddy and Mr. A. S. Russell on the 



Fig. 15. 



7-rays of radium are practically exponentially absorbed 

 between a range of 2 2 to 12'0 cm. of lead (X=0*51), but 

 that from 12*0 to 18*0 the value of X decreases continuously. 

 The work described above suggests that the cause of this 

 decreasing value of X is not due to the heterogeneous 

 character of the 7-rays, as has been supposed, but that the 

 peculiar secondary radiation is giving an increasing effect 

 relative to that due to the primary 7-rays as the thickness of 

 the absorbing material is increased. Tuomikoski worked 

 with an electroscope of aluminium, which behaves towards 

 this radiation very like brass. In the light of these results, 

 experiments on the absorption of the 7-rays, from 2 ±0 22 cm. 

 in total thickness of lead, were undertaken. In order to 

 prevent any secondary radiation from the stand, bench, or 

 wails of the room from entering the one vital spot in the lead 

 electroscope already described, namely the windows, the 

 following experiments were conducted in the middle of the 

 laboratory. Fig. 15 shows the disposition employed. An 

 iron tripod of height 27 cm., its 

 top being a narrow circular ring, 

 18' 3 cm. diameter, was placed on 

 the work-table and all except the 

 lower part of its legs covered with 

 sheet lead (0*14 cm. thick). On 

 the top of the tripod was placed a 

 flat slab of lead 18 cm. square and 

 1*56 cm. thick. On the slab was 

 placed the lead electroscope sur- 

 rounded by the two circular screens 

 of lead. The slab would absorb 

 any secondary radiation due to the 

 radium coming from the table, the 

 reading microscope, or the iron 

 stand, though such, if any, has been 

 shown to be very small, while the 

 circular screens protect the sides. 

 31 mg. of radium bromide were 

 mounted in a lead disk and placed 

 in a cylindrical stand of lead 12 - 5cm. 

 in diameter and 4*1 cm. high. Over the radium were 

 placed the lead absorbing screens, each of which was about 

 1*25 cm. thick and 12'5 cm. in diameter. The range 

 of thickness of lead kid on between 10 and 20 cm. was 

 investigated. The absorption was at first exponential 

 (\ = 0*50), but it departed latterly just as it had done in 

 fig. 12. It was found that if the natural leak were increased 

 by a constant amount equal to half itself and the sum 



rffc 



/ 



