278 



Prof. Barkla and Miss White on the 



other three substances through their constituent hydrogen 

 contain slightly more electrons per unit mass, and so scatter 

 to a greater extent mass for mass. Such a lower value appa- 

 rently persists throughout the results of all experiments on 

 the absorption of 7 rays (see Table II.). 



Table II. 

 Coefficients of Absorption of 7 Bays. 



(Soddy & Russell.) 



Absorbing Sheet. 



Mercury 



Lead 



Tin 



Zinc 



Copper 



Brass 



Iron 



Sulphur 



Aluminium ... 



Slate 



Glass 



Magnesia Brick 

 Paraffin-Wax 



Thorium D, 



Radiurn 0. 



III p. 



nip. 





•0472 



•0405 



•0438 



0326 



•0388 



•033 



•0393 



•0334 



•0398 



•0325 



•0S89 



•0328 



•0399 



0369 



•0438 



•0324 



•0406 



•0337 



•0414 



•0352 



•0416 



•0323 





•0361 



•0464 



Mesothorium 2. 



Uranium X. 



p/p. 



Pip. 





•0612 



•0544 



•0636 



•0421 



•0470 



•0424 



•0465 



•0423 



•0472 



•0425 



•0470 



•0415 



•0472 



•0465 



•0516 



•0421 



•0469 





•0469 



•0448 



•0484 



.0469 



•0478 



•0580 



•0502 



Absorption and Scattering of 7 Rays. 



The explanation of the approximate equality of the mass 

 coefficients of absorption in different substances of X-rays of 

 short wave-length thus leads to the explanation also of the 

 approximate equality in various substances of the mass 

 coefficient of absorption of 7 rays. The results with these 

 short waves bridge the gap between the very different 

 absorptions in various substances of X-rays of longer wave- 

 length and the approximately equal absorptions of 7 rays in 

 these substances. Evidently the absorption of 7 rays in 

 substances of low atomic weight is almost entirely absorp- 

 tion by scattering, which is proportional to the number of 

 electrons in the substance traversed *. Thus, as we see 

 from Table II., jx/p varies little with the atomic weight of the 

 absorbing substance except in the case of the heaviest 

 elements^ in which the value may be as much as 40 per cent, 

 greater. 



It is probable, however, that the " fluorescence absorption " 



* Mr. Ishino thinks there is a large true absorption. This is probably 

 true of mercury, the substance upon which he made further experiments. 

 With regard to the lighter elements, all evidence appears to point the 

 other way. 



