402 Prof. C. G. Barkla on the Spectra of 



experiments on absorption by elements Fe, Ou, &c, which do 

 emit homogeneous fluorescent radiations. (See fig. 2.) 



One of the most interesting facts concerning each of these 

 homogeneous fluorescent X- radiations is that it is not excited 

 by radiation of lower penetrating power. [The penetrating- 

 power must be measured in some substance whose charac- 

 teristic radiations are not within the range of penetrating- 

 power experimented upon, and not near to this range on 

 its more absorbable side.] This was first shown by Barkla 

 and Sadler in the case of the secondary radiation from silver. 

 When a heterogeneous primary beam was used it was fre- 

 quently found that the homogeneous fluorescent radiation 

 from silver was more penetrating than the primary as a 

 whole ; but however absorbable the primary beam was, a 

 plate of aluminium placed in that primary beam diminished 

 the intensity of fluorescent radiation less than the same plate 

 placed so as to intercept the secondary radiation itself, 

 showing that the secondary beam was more absorbable than 

 the portion of the primary beam producing it. Again, in 

 transmitting the homogeneous radiation from copper through 

 iron, it was found that a certain transformation had taken 

 place in the transmitted radiation, due to the excitation in 

 iron of its slightly softer characteristic radiation and the 

 superposition of this on the truly transmitted radiation ; 

 whereas when iron radiation was transmitted through copper, 

 that is a very soft radiation through a substance whose cha- 

 racteristic fluorescent X-radiation is of slightly more pene- 

 trating type, the radiation emerging was purely iron radiation 

 without the admixture- of radiation characteristic of copper. 

 Similar experiments were performed with copper and zinc. 

 The radiation from zinc being slighly more penetrating than 

 that from copper excited in copper its fluorescent radiation, 

 but the process could not be reversed. Dr. Sadler later 

 verified the law with considerable accuracy for a number of 

 radiators, by using homogeneous radiations to excite the 

 fluorescent radiation. All experiments have only further 

 shown the generality and the accuracy of this law, which is 

 analogous to Stokes's law of light fluorescence. 



Not only is there this direct evidence, but a considerable 

 amount of indirect evidence obtained from observation of the 

 relation between the absorption in a given substance of 

 X-rays, and their absorption in aluminium, say ; also from 

 the relation between ionization produced in different gases 

 by rays of varying penetrating power. 



To take the phenomenon of absorption first, it has been 

 shown by the writer and Dr. Sadler that the general variation 



