X-Rays and the Atomic Weight of Nickel. 411 



tion of the radiation from nickel on the curve obtained from 

 observations on elements of neighbouring atomic weight, 

 values were found for the atomic weight which agreed 

 exceedingly closely for the various absorbers used — aluminium, 

 zinc, silver, tin, and platinum. 



Now as in general the percentage absorption of a beam of 

 Rontgen rays by a sheet of any substance is less after trans- 

 mission through any kind of matter than before, the absorption 

 coefficient calculated as for a homogeneous beam from the 

 relation I = I e -Aa; differs with the thickness of absorbing plate 

 usedj the thicker plate giving the smaller absorption co- 

 efficient X. Absorption coefficients determined in this manner 

 are therefore only rough measures of the absorbability of the 

 rays. The character of the secondary radiation from these 

 elements is, however, quite different, for experiments showed 

 it to be practically homogeneous, while similar experiments 

 exhibited most markedly the heterogeneity of the primary 

 beam. 



First dealing with the primary beam, the absorption by a 

 sheet of aluminium *01 centimetre in thickness amounted to 

 33*4 per cent. ; after transmission through one, two, three, 

 and four sheets of zinc, each "00131 cm. thick, this fell to 

 31-6, 29*8, 27*9, and 27-2 per cent, respectively. The absorp- 

 tions by a plate of zinc '00131 cm. thick corresponding 

 to the first four values were 59, 49*3, 41, 31 per cent, 

 respectively. 



Thus after transmission through successive layers of zinc the 

 rays were more and more penetrating to zinc, the last 12'6 

 per cent, being about twice as penetrating as the original 

 beam. The power of penetrating aluminium was, however, 

 not affected to nearly the same extent, the absorption 

 diminishing from 33*4 to 27*9 per cent. only. 



On the other hand after transmission through aluminium 

 plates the change in the penetrating power to aluminium was 

 much greater than that to zinc. Thus though what we may call 

 the general penetrating power is increased after transmission 

 through one of these substances, the power of penetrating- 

 further layers of the same substance is increased to a much 

 greater extent. 



Results similar to these were obtained by Walter for several 

 substances. 



If we investigate the secondary rays from copper in the 

 same way we find no such change, the whole radiation detected 

 by the electroscope at a distance of several centimetres from 

 the radiator being almost perfectly homogeneous. This is a 



