X-Rays and the Atomic Weight of Nickel. 417 



By replacing the primary beam as used by Benoist, or the 

 secondary beam from a substance of low atomic weight like 

 carbon, by one of these secondary beams from metals, a curve 

 showing similar characteristics was obtained (see fig. 5), but 

 a strongly marked deviation occurred in the neighbourhood 

 of the atomic weight of the radiator, exhibiting the special 

 power of the rays emitted by each substance of penetrating 

 further layers of that substance. 



[The ordinates for the primary curve have been reduced to 

 one-third their true value to make this curve comparable with 

 the true secondary curves.] 



Treating the radiation from cobalt in this way it will be 

 seen that the curve obtained approximated much nearer to 

 that obtained for the radiation from iron than that from 

 copper, and exhibited the special power of the cobalt rays of 

 penetrating iron, denoting closer proximity in atomic weight 

 to that of iron than of copper. On the other hand the curve 

 obtained by experiments on the absorption of the rays from 

 nickel was very similar to that obtained from copper, and 

 exhibited the special power possessed by these rays of pene- 

 trating copper and zinc, but not iron. This then indicated 

 the proximity in atomic weight of nickel to that of copper. 

 The two series of experiments on cobalt and nickel when 

 taken together appear to furnish very strong evidence that 

 the atomic weight of nickel is considerably nearer that of 

 copper than is that of cobalt, that is, that the atomic weight 

 of nickel is considerably higher than that of cobalt. 



Not only do the relative positions of the atomic weights 

 agree with those indicated in the previous series of experiments, 

 but though this method dees not furnish accurate numerical 

 results, it will be seen that the value previously given would 

 bring it again into harmony with all the experiments on this 

 special property. 



In investigating the efficiency of different portions of a 

 heterogeneous primary beam in producing secondary rays 

 from metals, experiments were made to determine the extent 

 to which the secondary radiation was diminished by absorbing 

 portions of .the primary beam before it was incident on the 

 radiating substance. 



Thus when an aluminium plate was placed in the path of 

 the primary beam before it fell on the secondary radiator, 

 the ionization produced by the primary in a given space was 

 diminished by x per cent, and the ionization produced by the 

 secondary diminished by y per cent. Thus by plotting per- 

 centage reduction of the ionizing power of the primary as 

 abscissae, and consequent reduction of the ionizing power of 



