On Secondary Homogeneous X Radiation. 807 



one value of n is 



1 

 = 4 n ex 



* {*m)}> 



71 4- 1 



and the corresponding value of /3 is —= — . 



The least value that this maximum pressure can have is 

 when n is about 1*8 ; the value of this particular maximum 

 pressure is about 15*1 ; and the corresponding critical volume 

 is about *777. The values of 7 corresponding to a = are 

 respectively zero and that given by y*=4(w+l). It will be 

 seen that increasing n lowers both the upper and low r er parts 

 of the curve obtained by plotting the reduced temperature 

 against the reduced pressure as in fig. 3 ; these portions are 

 remarkably straight except near the turning-point and for 

 small values of a on the lower portion. 



Conclusion. 

 The foregoing results show unmistakably that Dieterici's 

 equation gives a suitable first approximation to the positions 

 of the inversion-points of many gases. In a further part we 

 shall discuss a second approximation which at the same time 

 suggests a law for those gases for which Dieterici's equation 

 is not satisfactory. 



XCIII. On Secondary Homogeneous X Radiation, By J. 

 C. Chapman, B.Sc, and IS. H. Piper, King's College, 

 London *. 



IT is well known that all bodies while exposed to Rontgen 

 radiation emit secondary X rays. Barkla f has shown 

 that these secondary rays are of two types — a scattered 

 radiation having the same penetrating power as the primary 

 beam and resembling it in that it is heterogeneous, and an 

 X radiation characteristic only of the element emitting it 

 and independent of the penetrating power of the primary 

 beam. Barkla and Sadler J have shown that this radiation 

 is homogeneous and is only excited by a primary beam more 

 penetrating than itself. The former type of radiation has 

 been found to be such as would be emitted by electrons 

 when accelerated simply by the electric forces in the primary 

 pulses §. Though much is known of the homogeneous 



* Communicated bv Prof. C. G. Barkla. 



t Phil. Mag'. June/1906. 



J Phil. Mag. Oct. 1008. 



§ 'Conduction pi Electricity through Gases 1 (Second Edition), 



