452 



Dr. C A. Sadler on Homogeneous 



gauge. A narrow pencil of the primary beam was passed 

 into an electroscope E^ 



The ratio of the readings on the electroscopes E 2 and E x 

 was taken at the ordinary pressure of the room, and again 

 when the pressure of the air in the cylinder had been reduced 

 to about 10 cm. of mercury, as indicated by the gauge. 



We may express the air in the cylinder in each case in 

 terms of a column of air of the same cross-section and 

 pressure of 76 cm. of mercury at 0° C. Let these columns 

 be of length 1^ and L 2 . The decrease in the ratio of the 

 readings on the electroscopes E 2 and E x which occurred 

 when the pressure was again made atmospheric, gave a 

 measure of the absorption of this particular secondary radia- 

 tion by a column of air of thickness L L — L 2 , and thus the 

 absorption coefficient in air at 76 cm. of mercury and U° C. 

 was determined. The values for copper and arsenic radiation 

 were respectively '01127 and '00536. 



It had been shown in a paper * on the absorption of 

 Rontgen radiation that the ratio of the absorption coefficients 

 of any two homogeneous beams is a constant for all absorbing 

 substances, in which no tertiary radiation of the homogeneous 

 type is excited by either of the beams under consideration. 

 It was to be expected, therefore, that the ratio of the absorp- 

 tion coefficients in aluminium and air would be constant for 

 beams from copper and arsenic. The ratio of the absorption 

 coefficient in aluminium for these radiations given in the 

 paper referred to is 2*12. The ratio of the coefficients in 

 air given above is 2*10. 



Taking advantage -of this fact the absorption coefficients 

 in air of the other secondary radiations used were deduced 

 from the absorption coefficients in aluminium. 



The values of R of equation (8) are tabulated below. 



Table I. — Values of R. 





Metals which serve as the source of homogeneous secondary 

 Rontgen Radiation. 



Tertiary 

 Eadiators. 



Fe. 



•007 



079 



Co. 



•009 

 •013 



•098 



Ni. 



•256 

 •119 



Cu. 



•262 

 ■018 

 •148 



Zn. 



As. 



Se. 



Sr. 



Mo. 



Rho. 



Ag. 



Sn. 



Aluminium... 



Iron 



Copper 



Silver 



•018 

 •267 

 •0594 



•178 



•036 

 •287 

 •386 

 •333 



•048 

 •305 

 •398 

 •410 



•115 

 •322 



•406 

 •672 



•264 



•380 

 •470 

 •972 



•400 

 •514 

 •563 

 108 



•500 

 •598 

 •645 

 1-15 



•680 

 •710 

 •746 

 2-19 





* Barkla and Sadler, Phil. Mag. May 1909. 



