EMISSION AND TRANSMISSION OF BONTGEN EAYS. 



141 



primary rays, and on the evidence offered by Messrs. BARKLA and SADLER themselves 

 (p. 412) as to the homogeneity of their secondary radiations, that such homogeneity 

 is only rigidly true when screens and radiator have the same atomic weight or ones 

 closely adjoining. If this be the case, the relative absorptions produced by any one 

 thickness of a metal screen cannot safely be regarded as characteristic of the radiators ; 

 a screen of very different thickness would in general furnish another and a different 

 set of relative absorptions. 



Prof. J. J. THOMSON'S* anomalously high result for nickel in his work on Secondary 

 Rontgen Radiation can, as he lias pointed out, be equally well explained by 

 supposing the radiation value for cobalt to be too low. This would be so if the cobalt 

 (which was in the form of fine powder) were partly oxidised ; recently, evidence was 

 forthcoming that this was the case. 



The results bearing on the point, obtained in the present work, are suggestive. 

 From figs. 7, 8, and 9 (Al, Cu, and Pt screens respectively) mean absorption 

 coefficients were obtained from the earliest portions of the curves (i.e., the thinnest 

 screens) for iron, nickel, and copper radiations. These are tabulated below : 



For each screen the mean X for nickel radiation is nearer the X. for copper than that 

 for iron radiation, and each set of numbers plotted against atomic weight gives an 

 atomic weight of almost exactly 61 '4 for nickel. 



But if we come to the last portion of the curves and work with thicker screens, and 

 in a region which has been shown to be much freer from anomaly than is the case 

 with thin screens, then we get different results. With both aluminium and platinum 

 screens the absorption coefficients for iron, nickel, and copper radiators are almost 

 identical (X about 40 for Al screens, and 3500 for Pt screens). Thus the absorption 

 coefficients vary from those tabulated above for thin screens to a practical equality 

 for thick screens. The method is therefore useless, at any rate in the case of primary 

 rays, to evaluate atomic weights. With copper screens (fig. 8) the final portions of 

 the curves are not yet out of the region of selective transmission, but it may be noted 

 that the mean absorption coefficients for the thickest screens employed are X = 580 

 for iron radiation, 540 for nickel radiation, and 470 for copper radiation a set of 

 values which makes nickel perfectly normal. Thus for thick screens, nickel offers no 



* J. J. THOMSON, 'Proc. Camb, Phil. Soc.,' XIV., 1, p. 109, Nov., 1906. 



