the Absorption of X-Rays. 169 



sometimes happened to undergo small changes, which have 

 had a disturbing effect on the measurements. These 

 changes seem likely to depend on the position of the focus 

 beiDg changed. To be independent, in the highest degree 

 possible, of this source of errors I have used two cuvettes 

 beside each other (E, and E 2 ), which have been so arranged 

 as to be pushed alternately into the path of the rays. 

 One of the cuvettes has always been filled with water, while 

 the other has been filled with the liquid whose absorption 

 was to be determined. When solid elements were examined 

 the latter cuvette was emptied and in front of it a plate of 

 the element in question was placed. 



The procedure of measuring has been as follows : — When 

 the cuvette filled with water had at first been placed in the 

 path of the rays and the thickness of the water layer in 

 comparator (cuvette F, fig 1) had been adjusted so that the 

 electrometer gave no indication, the other cuvette, con- 

 taining the liquid whose absorption was to be determined, 

 was immediately pushed into the same position ; whereupon 

 a new adjustment was effected. The difference between the 

 two adjustments then indicates the difference of absorption 

 in the two cuvettes. As a rule several such adjustments 

 have been made, which, however, have seldom differed by 

 more than in maximo 5 per cent. The absorption coefficients 

 given in the following pages are calculated from the mean 

 values of the experimental observations. The thickness of 

 the liquid layers contained in both the cuvettes was about 

 1 cm. By special observations the thicknesses of the layers 

 have been accurately determined. From these determina- 

 tions I could measure the thickness of a water layer that 

 absorbed just as much as a layer of the thickness of 1 cm. 

 of a liquid or a layer of known thickness of the solid 

 substance in question.. 



If the thickness of the water layer whose absorption is 

 equal to a layer of the thickness of 1 cm. of a certain sub- 

 stance be supposed to be d cm., and the respective absorption 

 coefficients of the water and of the liquid in question to be 

 fi w and fi s , then by means of the absorption law I = I e-/^, 

 where I stands for the intensity of transmitted radiation and 

 I for the intensit}' of incident radiation, we obtain the 

 equation : — 



I e-^ d = Joe'' 1 ', 

 from which follows that 



