746 Prof. C. G. Barkla and Mr. G. Shearer on the 



follows from Whiddington's Law that the path varies as the 

 fourth power of the velocity.) As the corpuscular radiation 

 from substances which from their behaviour under exposure 

 to ultra-violet light might be expected to give well marked 

 variations had not previously been examined, the first ex- 

 periments were made to test for such a variation. 



Cooksey & Innes *, experimenting with ordinary hetero- 

 geneous beams of X-rays, had, by the magnetic deflexion 

 method, found small differences between the velocities of 

 emission from the various substances Zn, Ag, Pt, Au, Pb. 

 The results, however, showed primarily the constancy of the 

 value of the velocity for a given radiation ; the variations 

 observed were slight and appear to have been due to the 

 experimental inaccuracies. 



Sadler |, investigating the emission under homogeneous 

 X-rays, had found the absorbabilities in atmospheric air of 

 the electronic radiations from Al, Fe, Cu, Ag, when these 

 were exposed to a definite homogeneous X-radiation, to be 

 practically the same. The conclusion was that the velocities 

 of emission were the same. 



Whiddington % had later calculated the velocity of emis- 

 sion by applying his Fourth Power Law to estimates of the 

 maximum path of the electrons obtained from Beatty's § 

 experimental results. In these experiments the ionization in 

 a narrow chamber due to the corpuscular radiation from the 

 ends was found for a series of pressures of the contained air. 



In the experiments undertaken by the writers, the last 

 method was adopted, pressure-ionization curves being 

 obtained for air ionized by a homogeneous X-radiation in a 

 short ionization-chamber backed in one case by calcium and 

 in the other by zinc. The ionization-chamber was in the 

 form of a shallow square box, 10 cm. edge and 3'0 cm. deep 

 (fig. 1). An insulated electrode in the form of a square 

 frame of aluminium crossed by very fine aluminium threads 

 was situated about midway between the two faces of the 

 ionization-chamber. This was connected by insulated leads 

 through ebonite plugs to an electroscope. Inlet and outlet 

 tubes were connected so that the chamber could be exhausted 

 or filled with any gas. The front window, which was 

 supported inside by a strong perforated brass plate, was of 

 thin aluminium lined inside with filter-paper. At the back 

 of the chamber was placed a screen of any desired material 

 with its face in a definite plane. 



The secondary X-radiation characteristic of a given metal 



* Roy. Soc. Proc. Aug. 1907. t Phil. Mag. March 1910. 



t Roy. Soc. Proc. Jan. 1912 § Phil. Mag. Aug. 1910. 



