276 Prof. Barkla and Miss White on tie 



As a consequence when the wave-length is small, scattering 

 is the predominant cause of loss of energy except in the 

 heaviest elements. In general, the lower the atomic weight 

 the greater is the proportion of loss by scattering to the total 

 loss of energy. 



Scattering and Absorption. 



In paraffin-wax, water, and paper scattering accounts for 

 almost the whole " absorption " within the range of the 

 shortest wave-lengths here dealt with. This is shown by 

 the following facts : — 



(1) The total absorption coefficient in each substance 

 varies little with the wave-length of the radiation absorbed ; 

 (2) the total absorption coefficient is almost independent of 

 the nature of the light-absorbing substance ; (3) there is 

 close agreement between the magnitude of the coefficient 

 and that directly calculated from measurements of the 

 energy of radiation scattered. 



The small variation of absorption in light elements with a 

 change in wave-length is illustrated by a comparison of the 

 absorptions in paraffin-wax and in copper — the former in- 

 creasing only by about 20 per cent., while the absorption 

 in copper increases by 400 per cent. 



The absorbability in light substances as paraffin-wax, 

 paper, water, or even aluminium is thus very unsuitable as 

 a measure of the character (wave-length) of such high- 

 frequency radiations even though the effect of scattering is 

 not lost sight of, as it unfortunately has been both in the 

 derivation and in the application of formulae connecting 

 wave-lengths and absorptions. 



We see, too, that the absorption coefficients (/z/p) in paraffin- 

 wax, paper, and water all become only slightly less than *2 

 for X-radiation of very short wave-length. This agrees 

 exceedingly well with the value *2 obtained many years ago** 

 from measurement of the energy of the X-radiation of longer 

 wave-length scattered in light substances (with the exception 

 of hydrogen). Owing to the constituent hydrogen the 

 scattering coefficients for these three substances as then 

 determined for somewhat longer waves are slightly greater 

 than *2. 



Further, the value '2 agrees almost perfectly with that 

 obtained by calculation, on the assumption that the number 

 of electrons per atom is equal to the atomic number. Or, 

 putting it another way, the value *2 when applied to 

 Sir J. J. Thomson's calculated expression for the energy 



* Barkla, Phil. Mag. May 1904. 



