226 Prof. Barkla and Miss J. Gr. Dunlop on the 



coefficient, were obtained from the various scattering sub- 

 stances. Owing to the different qualities of the fluorescent 

 (characteristic) X-radiations from these substances, the details 

 of the methods adopted depended upon the particular 

 substance. 



Copper. — The characteristic radiation (series K) has a 

 wave-length about 1*5 x 10 ~~ 8 cm. This was excited by all the 

 primary radiations used, these being of shorter wave-length, 

 consequently the complex secondary radiation was passed 

 through a sufficient thickness of aluminium to absorb all the 

 characteristic radiation and leave only the scattered radiation 

 to traverse the measuring electroscope. The intensity of 

 this, as shown by the ionization produced in the electroscope, 

 was compared with that from aluminium when this also had 

 been transmitted through an equal thickness of aluminium. 

 It was seen that the two radiations measured — one from 

 copper and the other from aluminium — after transmission 

 through the particular thickness of aluminium were of the 

 same penetrating power. When the radiation was soft, 

 however, perfect similarity of the two secondary radiations 

 was never obtained, though it was known that all the " K " 

 characteristic radiation from copper was cut off. The 

 difference in penetrating powers was, however, very small 

 and was accounted for, as seen later, by the fact that the 

 primary beam was not quite homogeneous, and the softer 

 constituents were scattered from copper in somewhat greater 

 intensity than the harder. When the primary radiation was 

 of a penetrating type, the two secondary radiations were as 

 nearly as measurable identical. This indicated that they 

 were not characteristic radiations but purely scattered 

 radiations. The thickness of aluminium used varied from 

 about *08 cm. to 2'2 cm. according to the penetrating power 

 of the X-radiation experimented upon. 



A series of comparisons was made between the intensities 

 of scattered radiations, through a long series of penetrating 

 powers of the primary radiation, by both methods mentioned 

 above. 



Silver. — The radiation of series K characteristic of silver 

 has a wave-length about *56 x 10~ 8 cm. When a very soft 

 primary radiation was used — one not containing an appreci- 

 able constituent of shorter wave-length — this characteristic 

 radiation was not excited, consequently the secondary 

 radiation from silver, after transmission through a very thin 

 absorbing sheet in order to absorb the " L " radiation, was 

 entirely scattered radiation. When the primary radiation 

 became a little harder (with constituents of shorter wave- 



