K X-rays of Silver in Gases and Gaseous Mixtures. 385 



The absorption chamber in this experiment was evacuated 

 with an oil-pump to < 1 mm. Hg ; the stopcocks were 

 lubricated with a brominated grease from paraffin wax and 

 vaseline, to avoid leaks from action of the iodide vapour. 



II. Results. 



(1) Absorption Coefficients. 



If I x is the measure of the intensity of the rays entering 

 the ionization vessel (as recorded by the observed potential 

 on the electroscope) for a pressure of gas p± mm. of mercury 

 in the absorption chamber, I 2 is the corresponding measure 

 for p 2 mm. pressure, i* is the linear absorption coefficient of 

 the gas at atmospheric pressure P, d the length in cm. of 

 the absorbing path, 



then I 2 ==Ii.* V ' 



i. e. 



^ 2-3x76 >< ]og 10 I 1 /I 2 

 d (P2-P1) ' 



The quantity d is the measure of the average length 

 traversed in the chamber — in this experiment the difference 

 between this and the axial length (50*8 cm.) was negligible. 

 Inserting the value for d, we have 



3 ^oli-logiol 2 . 



(P2-.Pl) 



This coefficient represents the total loss suffered in the- 

 particular apparatus. Some of the radiation scattered in a 

 forward direction is collected by the ionization chamber, and 

 thus reduces the absorption as measured. In the lack of 

 definite data for scattering by various elements at different 

 wave-lengths, a rough estimate of the amount so collected 

 was made, assuming the mass scattering coefficient to be *2, 

 using Oowther's * distribution figure and taking a mean 

 value for the solid angle of rays collected. Adding this 

 correction, a value for the total absorption is obtained. 



A further correction is necessary for the lack of homo- 

 geneity of the rays used. As will be seen from figure 1, 

 over the range of absorption used, the rays appear homo- 

 geneous but with a lower absorption coefficient in aluminium 



* J. A. Crowther, Proc. Eoy. Soc. lxxxvi. A, 1912. 

 Phil. Mag. S. 6. Vol. 43. No. 254. Feb. 1922. 2 C 



