472 Dr. Tycho E:son Auren on the 



just mentioned, made use of the atomic absorption coeffi- 

 cient that 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. This kind 

 of expression will in many cases be preferable to the mass- 

 absorption coefficient, because the atomic absorption coeffi- 

 cient can be more easily than that coefficient compared with 

 other characteristic constants of the atom. As the mass of 



an atom can be easily calculated — is obtained by dividing 



the atomic absorption coefficient by the mass of the corre- 

 sponding atoms. 



The relation of the absorption coefficient and the wave- 

 length (\) can be expressed approximately by the formula 



£=A\*, (2) 



where A and x are constants, x seems to have the same 

 value (about 3) in all materials. 



The relation of the atomic absorption coefficient (a) and 

 the atomic number is given by Bragg and Pierce in the 

 formula 



« = CN 4 A>, (3) 



where C is a constant and N the atomic number. 



From the formula (2), it follows that the ratio of the 

 absorption coefficients of two materials must be constant, 

 i. e. independent of the wave-length, and according to the 

 formula (3), this ratio might be expressed by the ratio of 

 the fourth powers of the atomic numbers of the respective 

 elements. The fact that the ratio in question is really constant 

 in the case of rays of widely different wave-lengths was first 

 pointed out by Barkla and Sadler * and, later on, it has been 

 confirmed by Bragg and Pierce |. 



In the following table, the values found by the last-named 

 scientists for the ratio of the atomic absorption coefficients of 

 a few elements are compared partly with the corresponding 

 values which I have calculated from the values found by 

 Barkla and Sadler of the mass-absorption coefficients, partly 

 with the values I have found by means of the method 

 mentioned below. In column 1 are noted the average 

 values found by Bragg and Pierce by means of the charac- 

 teristic rays from Ag, Rh, Pd ; in column 2, those found by 

 Barkla and Sadler by similar radiation from Ag; and in 



* Barkla and Sadler, Phil. Mag. xvii. p. 739 (1909). 

 t Bragg and Pierce, loc. cit. 



