Absorption of X-B ays. All 



readjusted, and the position (d 2 ) of the screw is read again. 

 The change o£ the thickness of the water-sheet (d 2 —di) 

 depends evidently on the difference o£ absorption in the 

 solution and of a water-sheet o£ the same thickness (1 cm.). 

 If we suppose the intensity of the incident rays at the 

 absorption vessel to be I and at the comparator kl, and the 

 absorption coefficients of water and the solution to be /jl w and 

 fi s respectively, then it follows, after the first adjustment, 

 that 



and after the second adjustment, that 



whence it follows that 



fi s ^fi w = fi w (d 2 — d ± ) ; 



or, when the absorption coefficient of the dissolved substance 



— =d<> — d,» 



Now X. m. k. (p) is readily found if the number of molecules 

 in one litre is calculated. Since pure water contains 

 55'5 gram-molecules H 2 0, and the solution m gram-molecules 

 of the dissolved substance per litre, we obtain 



{d 2 —d 1 )55-5 .^ 



This formula presupposes that the water quantum of the 

 solution is the same as that of pure water, which case 

 only can be thought correct when working with diluted 

 solutions. When working with highly concentrated solu- 

 tions, a correction for the wanting water quantum must be 

 made. This correction is readily calculated when the specific 

 weight of the solution is known. The value of d 2 is aug- 

 mented by the thickness of the water sheet that should be 

 added in order to keep the water quantum of the absorption 

 vessel unaltered. Although, no doubt, part of the rays, 

 scattered from the absorption vessels, have penetrated into 

 the ionization-chambers, no special correction of the fault 

 originating from this source has been made. The intensity 

 of the scattered rays must needs be thought almost equal, 

 because the density of the solutions has very nearly been the 

 same as that of water and the mass-scattering coefficients of 

 light substances within' wide limits are independent of the 



