364 



Mr. D. C. H. Florance on 



The experimental arrangement was similar to that shown 

 in fig. 2 (Phil. Mag. Feb. 1914), except that the electroscope 

 was kept fixed and either the radiator or the emanation 

 tube, which acted as a source of 7 rays, was altered in 

 position. The emanation tube was surrounded by lead 

 1 cm. thick to absorb completely the 7 rays from radium B. 

 The thickness of the lead radiator was 0*05 cm., and a corre- 

 sponding thickness was taken for the carbon, aluminium, 

 zinc, copper, and tin radiators, so that the mass per unit area 

 of each radiator was the same. 



In the following table are included the results of a set of 

 observations in which the electroscope and emanation tube 

 were kept in a fixed position, and the position of the radiator 

 was changed so that the " emergent " or " returned " radiation 

 could be examined at an angle of about 90°. 



Badiator. 

 Carbon 



Side of electroscope 

 0"202 cm. aluminium. 



Side of electroscope 

 0084 cm. lead. 



Emergent. 

 90 

 94 

 90 

 95 

 98 

 126 



Beturned. 

 68 

 69 

 70 

 76 

 76 

 105 



Emergent. 

 80 

 80 

 80 

 80 

 77 

 76 



Beturned. j 

 59 

 59 

 59 

 59 

 55 

 55 



Aluminium 



Copper 



Zinc 



Tin 



Lead 





The apparent amount of secondary radiation emitted by 

 the radiators depended to a great extent on the thickness 

 of the side of the electroscope through which the radiation 

 was transmitted. When this side was thick enough to absorb 

 the soft radiation emitted by the lead radiator, the' amount of 

 scattered radiation from all radiators was the same. When 

 aluminium 0'202 cm. thick was used as the side of the 

 electroscope, the lead radiator was found to produce more 

 secondary radiation than did the elements of low atomic 

 weight. 



From the results of a large number of experiments similar 

 to the one recorded, it is evident that equal masses of dif- 

 ferent elements scatter the 7 rays from radium to the same 

 extent, or, in other words, the scattering per atom is propor- 

 tional to the atomic weight of the different elements. It is 

 also to be noticed that when the radiator subtends the same 

 solid angle at the source of the primary 7 rays, whether the 

 returned or the emergent radiation is being examined, 



