Rays emitted by Substances exposed to 7 Rays. 625 



It will be seen that the curves representing the first and 

 second long periods are steepest in the middle ; they resemble 

 in this respect the curves obtained by Prof. J. J. Thomson * 

 with X rays. 



There is a comparatively great difference in the radiating 

 powers of carbon and boron, and of lead and bismuth, although 

 in both cases the atomic weights differ little from one another. 

 Thus the addition of a few electrons to a boron or lead atom 

 seems to produce a considerable change in the grouping of 

 the electrons, whereby either the absorption of the primary 

 7 rays, or the absorption of the secondary cathode rays, or 

 both, are affected to a considerable extent. It is interesting 

 that in the case of boron and carbon, the radiating power 

 decreases with increase of atomic weight, while it increases 

 with the atomic weight in the case of lead and bismuth. 



Prof. Thomson f found that with X rays the radiating 

 powers of bismuth and lead did not differ much from one 

 another, and the same was found to be the case with boron 

 and carbon. It ought to be mentioned that the radiation 

 that Prof. Thomson measured consisted of secondary X rays 

 and secondary cathode rays of small penetrating power. 



It will be observed that the radiating powers of silicon and 

 carbon are respectively smaller than those of aluminium and 

 boron. According to the periodic classification of the 

 elements, boron and carbon of the first short period corre- 

 spond to aluminium and silicon of the second short period, 

 boron and aluminium belonging to Group III. and silicon 

 and carbon to Group IV. Thus we might expect the 

 behaviour of aluminium and silicon to be similar to that of 

 boron and carbon. 



The curves show, as a whole, that the increase of secondary 

 radiation with atomic weight is, on the average, greater the 

 higher the atomic weight ; in other words, the average 

 steepness of the curves increases with the atomic weight. 



McClelland J has investigated the amount of secondary 

 radiation from various substances on which /3 and 7 rays of 

 radium were allowed to fall. The secondary radiation in his 

 experiments consisted principally of the secondary cathode 

 radiation due to the (3 rays, and a stream of reflected /3 rays 

 (these constituents amounting to about 80 per cent, of the 

 total radiation, according to the experiments of Eve) . The 

 remainder consisted of secondary 7 radiation, and secondary 

 cathode radiation due to the 7 rays. His results, therefore, 



* Proc. Canib. Phil. Soc. yoL xiv. pt. 1 (1906). 



t Loc. cit. 



% Trans. Roy. Soc. Dublin, vol. ix. p. 1 (1905), and p. 9 ^1906). 



