Absorption Coefficients of X Rayt 



627 



stand out strongly from the background. The former is 

 troublesome, but is overcome as far as possible by multiplying 

 observations. It would be possible to remove this difficulty 

 altogether by using a balance method, and an instrument 

 has been constructed for the purpose. It is hoped that a 

 much more complete and accurate set of measurements will 

 be obtained by its use; the figures here given may be looked 

 on as preliminary. 



The results are given in the form of atomic absorption 

 coefficients. Each expresses the proportion of the energy of 

 an X-ray pencil which is absorbed on crossing a surface on 

 which lies one atom to every square centimetre. Expressed 

 in this way the coefficient should be more directly comparable 

 with other atomic characteristics. The ordinary mass ab- 

 sorption coefficient (X/p) can be obtained by dividing any 

 figure in the table by the assumed mass of: the corresponding 

 atom, which is given at the foot of the column to which 

 the figure belongs. 



It is interesting to observe that the mass absorption co- 

 efficients of the two wave-lengths emitted by Ag, IM, or I {li 

 lie one on each side of the one coefficient given by Barkla, or 

 to be deduced from his results. For example, it may be cal- 

 culated from the table, in the manner just explained, that 

 the mass absorption coefficients of the two Ag rays in zinc 

 are 21*8 and 30'3. Barkla gives 27*1 as the mass absorption 

 coefficient of silver rays in zinc. 



Table I. 

 Atomic absorption coefficients X 10 22 . 





Ray 



Wave-length. 



Absorber. 





Al. 



■813 

 l -on 

 L-22 

 1-20 

 1-41 

 1-57 



Fe. 



1.3-0 

 \Cv2 

 1 7 7 

 18-7 

 •214 

 238 



91*6 



Ni. 



Cu. 



Zn. 



Pel. 



Ag. 



Sn. 



Pt. 



Au. 



Silver 



harder 



softer 



0-491x10-8 

 0-508 „ 

 0-537 „ 

 0554 „ 

 0-576 „ 

 0-615 „ 



17 2 



20 8 

 22-6 



ill 

 27-0 

 30-9 



1)6-4 



21-8 



23 4 



•j;-! 

 27-8 

 32-2 

 36*1 



104-3 



23-4 

 268 

 30 1 

 32-4 

 36-4 

 41-6 



107-1 



101-5 

 200 

 22-4 

 23-8 

 28-3 

 31-5 



18-2 

 22-0 

 24-8 

 25-3 

 300 

 35-1 



23-8 



150 



155 

 188 

 187 

 213 



211 

 254 



Palladium ... 

 Rhodium ... 

 Silver 



29-7 J LC>7 

 32-1 ; 176 

 :;:>•-! 205 



Palladium ... 

 Rhodium ... 



390 



45-5 



195-2 



238 

 243 







Mass of 



atomXlO-- 4 



4,5 



175-0 



177 



320-3 



323-5 



It has been pointed out by Barkla that the ratio of the 

 absorption coefficients of any two absorbers for the same wave- 

 length is independent of the wave-length over considerable 

 ranges. This is well shown by the results given here ; and 



