X-Rays and the Atomic Weight of Nickel* 419 



disappear if we assumed for the atomic weight the value given 

 above. 



The results of experiments on arrays which are of interest 

 as bearing on the atomic weight of nickel are briefly stated 

 below : — 



(1) Curves connecting the atomic weight of an element 

 subject to a?-rays and the general penetrating power of the 

 secondary arrays emitted by it (various absorbing substances 

 being used) indicate for nickel an atomic weight of about 61'4. 



(2) Many experiments show that the secondary ^-radiation 

 from an element is specially penetrating to that element, and 

 to a less extent, to elements of neighbouring atomic weights — 

 the special power being a measure of the proximity. Experi- 

 ments show the proximity of nickel to copper and of cobalt 

 to iron. 



(3) Curves exhibiting the efficiency of different constituents 

 of a primary beam in producing secondary arrays, change in 

 character with the atomic weight of the radiator. The curve 

 for nickel appears to lie between those for cobalt and copper. 

 (This method is not at all sensitive, and the experiments are 

 not absolutely conclusive.) 



(4) Experiments by J. J. Thomson on the total ionization 

 produced by the easily absorbed corpuscular secondary rays 

 emitted by elements subject to arrays, give for nickel a result 

 which with the usually accepted atomic weight is the only 

 exception to the rule of increase of ionization with increase 

 in atomic weight of radiator. If we accept the atomic weight 

 given above it becomes perfectly regular. 



(5) The relation between the atomic weight of an element 

 and the absorption of a;- rays it produces is shown by a con- 

 tinuous curve exhibiting no clearly marked irregularities if 

 we except nickel. When a position between cobalt and 

 copper is assigned to nickel, it becomes quite normal in its 

 behaviour. 



In discussing these experiments we may dismiss at once 

 the possibility of the abnormal effects being due to impurities, 

 for in the first place the amount of impurity necessary to 

 produce such a change in the character of the secondary 

 radiation is absurdly large. Indeed, as the impurity could 

 not differ much in atomic weight from that of nickel itself — 

 a fact proved by the curves shown in fig. 5 — the quantity 

 would have to be enormous. 



Again, the various specimens, differing considerably in 

 purity, when subject to arrays emitted secondary rays which 

 were almost identical in character, within the limits of 

 experimental error. 



