X.-liays and the Atomic Weight of Nickel. 409 



was approximately a linear one, and that a small variation 

 in the atomic weight of the radiator was accompanied by a 

 considerable variation in the character of the radiation 

 emitted. 



As more recent work showed that within these limits very 

 consistent results were obtained from repeated observations, 

 the method seemed exceedingly sensitive. 



When the elements were examined successively under 

 similar conditions it was found that nickel behaved irregularly, 

 only falling into line when an atomic weight considerably 

 greater than that of cobalt was assigned to it. 



This exception to what otherwise seemed a general decline 

 of absorbability with increase in atomic weight between the 

 limits named was so striking that we considered it called for 

 more detailed investigation. 



To determine if this apparent anomaly was due to any special 

 relationship between the radiating and the absorbing sub- 

 stances, similar experiments were made with the same radiators 

 and a number of different absorbers and curves were drawn — 

 one for each absorbing substance experimented upon. It was 

 found that platinum, tin, silver, and zinc absorption curves 

 were very similar to that obtained for aluminium, showing 

 that the relation between atomic weight of the radiating 

 substance and the absorption by aluminium previously found, 

 was a relation between the nature of the radiator and general 

 absorbability of the rays it emits, and not a relation depending 

 on the nature of the absorbing substance. In each of these 

 cases it was evident from the curves that the behaviour of 

 nickel would be perfectly regular if an atomic weight 

 approximately mid-way between those of cobalt and copper 

 were assigned to it. 



It had previously been shown that the absorbability of 

 secondary #-rays is an atomic property simply, that is it 

 depends on the nature of the atoms from which the rays 

 proceed, and a mixture or even combination with other 

 elements does not affect the nature of the radiation as it pro- 

 ceeds from the radiating atom itself. From this we should 

 conclude that small or even moderate quantities of impurity 

 would not materially affect the results. After preliminary 

 experiments on good commercial specimens, however, samples 

 of the elements of the highest order of purity were obtained 

 and all the experiments were repeated. The results were 

 almost exactly the same as before, indicating that the 

 irregularity was not in any way due to the presence of foreign 

 substances. 



