170 Dr. Tycho E:son Auren on 



If ^/jj q designates the molecular absorption coefficient 

 of a certain substance (A) in relation to the molecular 

 coefficient of water, then when the said substance is 

 supposed to contain m and water 55*5 mols per litre, 

 we obtain the following formula for the calculation of this 

 coefficient : 



55*5<r? ..,, 



^A/F.O- - ^ W 



As shown by researches by Winaver and Sachs*, Glockerf, 

 Barkla and White t, etc., the absorption coefficient of water 

 increases with increasing wave-length much more slowly 

 than is the case with absorption coefficients of substances 

 containing elements of higher atomic weight. In section IV. 

 I shall give a detailed account of the matter and its cause. For 

 that reason it is less suitable, in values obtained when the 

 composition of radiation has been varied, to use the molecular 

 absorption coefficient of water as a unit. The coefficients 

 calculated after formula (1) have therefore been recalculated 

 so as to give values corresponding to the atomic absorption 

 coefficient of copper. If ^ Cu / H designates the atomic 

 absorption coefficient of copper in relation to the molecular 

 absorption coefficient of: water, we thus obtain 



K = ■ ...... (2) 



K CuJR 2 



Benoist was the first to advance the hypothesis that in 

 chemical compounds, as well as what has been ascertained 

 in regard to mixtures, the total absorption can be calculated 

 from the absorption of the respective components by simple 

 addition. This assumption has later on been regarded as 

 satisfactory, a fact which is also in accordance with my 

 former experiments. If in a molecule of a certain substance 

 (A) there are n x atoms of the element a 1? n 2 atoms of the 

 element a 2 , &c, we consequently suppose that 



& a in —n-i/c in . -\-n»/c Ir . -\-n-iK irt +.... . . (3) 



A/Cu l aJCvx l aJCn ' 6 aJCu ^ ' 



When chemical compounds are used it is evidently 

 most profitable for attaining a greater accuracy to choose 



* Winaver and Sachs, Phys. Z. xvi. p. 258 (1915). 



t Glocker, Phys. Z. xviii. p. 332 (1917). 



% Barkla andWhite, Phil. Mag. xxxiv. p. 270 (1917). 



