424 Lord Blytliswood and Dr. E. W. Marchant. 



Thickness of Solution necessary to prodnce the same Absorption as 

 20 mm. of Water. 



Strength of 

 solution. 



Potassium 

 bromide. 



Lead 

 acetate. 



Iodide in potas- 

 sium iodide. 



Nickel 

 chloride. 



N 



3-6 



2 





8-2 



N/2 



4*4 



3-4 



2-4 



11 



* /6 



7-4 



5-4 



5-0 



14 



N/8 





7-4 



6-9 



16 



N/10 



11-0 



9-3 



8-3 





N 



2xN 



5xN ...... 



Sodium 

 thiosulphate. 



10 -6 



8-5 

 7'5 





Effect of Thickness of Solution traversed. 



Various experiments have been made in order to verify the state- 

 ment made by Messrs. Gladstone and Hibbert and others* that the 

 amount of absorption varies logarithmically with the thickness traversed 

 by the ray — 



r = log(^ + /x), 



where r = percentage of the rays absorbed by the solution under con- 

 sideration. The results obtained appear to confirm their conclusion. 

 As a preliminary experiment, the intensities at different distances 

 along the strips on three of the plates above considered were carefully 

 compared with the intensity of the strip photographed through 20 mm. 

 of water. Now proceeding to consider these results — 



Let A be the original intensity of the X rays 



Ai the amount of the rays absorbed by the water. 

 A 2 the amount of the rays absorbed by a given thickness of the 

 solution. 



Ai be the coefficient of absorption for water, 



A, 2 be the coefficient of absorption for the solution. 



Then the intensity of the strip obtained by passing the rays through 

 20 mm. of water is proportional to A - Ai, and the intensity of the 

 photographic image at the distance along the strip corresponding to 

 the given thickness of the solution to A-A 2 , and for the relative 

 intensities of the photographic images (K 2 ), we have 



* A. Buguet, 'Comptes Kendus,' vol. 125, pp. 398—400; F. Ee, loc, cit. 



