the Fluorescent Rant gen Radiations. 399 



The term fluorescent radiation seems, however, preferable to 

 either of the others. Radiation of this type may very readily 

 be observed from elements of higher atomic weight than 

 sulphur, when the observing electroscope is at a distance of 

 several centimetres from the substance exposed to the 

 primary rays. In most cases, unless special precautions are 

 taken, the ionizing effect of these radiations from any parti- 

 cular element is very much greater than that of the scattered 

 X-rays — indeed it completely swamps the effect of the latter. 

 But, as will be seen later, the scattered radiation m;iy be 

 isolated by using a primary beam which is incapable of 

 exciting this fluorescent X- radiation. 



In many cases the radiation which reaches an electroscope 

 situated several centimetres from the radiating element is an 

 almost perfectly homogeneous radiation. It is important to 

 point out that though very careful experiments have been 

 made to test this, both directly and indirectly, there is no 

 evidence of the slightest deviation from homogeneity of the 

 fluorescent radiations. There is not only no evidence of 

 lack of homogeneity, but striking evidence of homogeneity. 

 Perfect homogeneity is, of course, not to be expected in 

 these radiations any more than in light, for instance, but for 

 all practical purposes we may assume the radiations accurately 

 homogeneous. The reasons for regarding the radiations as 

 homogeneous will, however, be given. 



The direct experiment of successively placing thin sheets 

 of aluminium or other absorbing substance in the path of 

 the radiation and observing the absorptions produced, shows 

 that whereas the fractional diminution in the ionization pro- 

 duced by an ordinary primary beam becomes less and less as 

 plate after plate of absorbing substance is placed in its path 

 — owing to the more absorbable constituents being absorbed 

 by the earlier plates — the fractional diminution of the ioni- 

 zation produced by a secondary beam does not vary after 

 partial absorption. This was shown by Barkla and Sadler 

 (Phil. Mag. Oct. 1908) and need not be repeated here. The 

 very slight deviation from homogeneity which is observed 

 after a considerable portion of the radiation has been absorbed 

 is such as would be produced by the superposition of a small 

 amount of scattered radiation. This scattered radiation in 

 some cases produced no more than half of one per cent, of 

 the ionization produced by the homogeneous radiation. The 

 proportion of scattered to fluorescent radiation depends, 

 however, upon the particular primary beam used. The 

 radiations from elements of atomic weights in the neigh- 

 bourhood of iron, copper, zinc, &c, when subject to a radiation 



