tlie Fluorescent Rontgen Radiations. 405 



— about J of the energy of the primary beam absorbed, and 

 quite 100 times the energy of the scattered radiation. 



The isolation of any particular fluorescent radiation may 

 usually be effected very simply. If it is a fairly penetrating- 

 radiation such as that from tin which it is wished to study, 

 then a primary beam should be used with a fair proportion 

 of constituents of more penetrating type. This, as shown, 

 excites the desired radiation and ail softer radiations charac- 

 teristic of the particular substance. But as all the characteristic 

 radiations of a particular element appear to be widely different 

 in penetrating power, the softer constituents may be absorbed 

 by a sheet of aluminium without diminishing to any con- 

 siderable extent the intensity of the particular radiation 

 studied. Thus, as shown by the absorption curve fig. 1, after 

 the first absorbing plate the remaining radiation was homo- 

 geneous and of absorbability given by — = # 8. The preli- 

 minary filtering out of the easily absorbed constituents is 

 frequently effected by the surface layers of the radiator 

 itself, and by the air separating it from the detecting- 

 electroscope. 



In order to obtain a very soft fluorescent radiation, such 

 as that from tin, antimony, &c, a very soft X-ray tube should 

 be used to give the primary radiation. This is then not 

 sufficiently penetrating to excite the more penetrating fluo- 

 rescent, radiation, and provided the testing electroscope is 

 placed near to the radiator the more absorbable fluorescent 

 radiation may be readily studied. This radiation is, however, 

 usually mixed with a large quantity of more penetrating 

 scattered radiation. The quantity of this may be estimated 

 by finding how much is transmitted through sheets of alumi- 

 nium of sufficient thickness to completely absorb the soft 

 fluorescent radiation. By comparison of this with the intensity 

 of radiation transmitted through similar plates of aluminium 

 from a substance such as carbon giving only scattered radia- 

 tion, the intensity of scattered radiation superposed on the 

 soft fluorescent radiation, and passing through any number 

 of absorbing plates, may be got by proportion from the 

 corresponding intensity from carbon. 



After subtraction of the ionization due to the scattered 

 radiation, it is found that the successive sheets by absorption 

 diminish the effect of the radiation transmitted by the same 

 fraction, showing the homogeneity of the soft characteristic 

 radiation (Barkla and Nicol, 'Nature,' Aug. 4, 1910). 



Phil. Mag. S. 6. Vol. 22. No. 129. Sept.'ldll. 2 E 



