Absorption of Rontgcns Rays by Solutions of Metallic Salts. 425 

 or, according to the above assumption, 



K = l-lQg(/W2 + ^) 



1 - log (AA + 



If K3K4, &c, are the values of K corresponding to the different 

 thicknesses £3, &c, of the solution, we have 



K 2 { 1 - log (A^ + ^)}= 1- log {X 2 U + m ) % 

 Kafl - log (X^ +/*!>} - l-log(V s + /zo). 



1 - log (Ai^i -f is a constant for any one plate = C (say) • hence, 

 subtracting, 



C(K 2 -K 3 ) = log(^±^. 



The value of /j. however (being that part of the absorption produced by 

 the walls of the cell and the air traversed) is known to be very small 

 compared with Xt, and may therefore in a first approximation be neg- 

 lected. The above equation then reduces to 



C (K 2 - K 3 ) = log 1 f = log t 3 - log U 



Below are given the results for the three plates used in the test 

 experiments. The relative intensities were obtained by means of an 

 arrangement similar to that described for the determination of the 

 relative intensities of the spots on the photographic plate in method I ; 

 but arrangements weie made to bring the illuminated images nearer to 

 each other to enable more accurate comparison to be made. 



The errors, when all precautions were taken, were not greater than 

 6 per cent, for the determination of the relative intensities, corre- 

 sponding to 3 per cent, on the determination of the distances of the 

 source of light necessary to produce equal illumination. 



In the following table — 



Column 1 gives the thickness of the solution traversed by the X rays 

 in millimetres. 



„ 2 gives the logarithms of these thicknesses, to base 10. 



,, 3 gives the differences of the logarithms. 



,, 4 gives the intensity of the image photographed through the 

 thickness of the solution given in column 1, in terms of 

 the intensity of the image photographed through 20 mm. 

 of water. 



„ 5 gives the differences of these intensities. 

 ,, 6 gives the ratio of the differences of the logarithms in 

 column 3 to the differences of the intensities in column 5. 



