Absorption of Hard X-Rays in the Lightest Elements. 735 



coefficient can be calculated, i£ the corresponding coefficient 

 of oxygen relative to water is known. As for the validity of 

 the law of additivity, I have not, in a considerable number 

 of my experiments in which widely ditferent substances have 

 been investigated, found anything in any case indicating that 

 the law should not be strictly in force. On the other hand, it 

 must be said that the determination of /eo/H o is liable to an 

 error of great importance in the calculation of a:h/h 2 o« This 

 error is owing to the fact that this coefficient could only have 

 been calculated from the difference of the molecular absorp- 

 tion coefficients of substances, which, in regard to their 

 chemical composition, differ from each other by one atom of 

 oxygen, because the experimental errors of both coefficients 

 appear in this difference. Since the value of /ch/h 2 o is 

 calculated by the subtraction of the coefficient of ato/h o from 

 tfH 2 o/H 2 o or 1, its value will be liable to error, because of the 

 inaccuracy of aco/HoO? an d also because /^h/h 2 o is in itself a 

 small quantity compared to ko/h^O' However, I have pre- 

 viously shown that absorption can be determined with great 

 accuracy in organic compounds, and it may, therefore, be 

 of more advantage to calculate the absorption of hydrogen 

 directly from these determinations. So I have made a new 

 series of absorption determinations in various organic sub- 

 stances at the effective wave-lengths recorded in Table I. In 

 these researches, only pure preparations free from water have 

 been used. In Table II. are recorded absorption coefficients 

 (/e A /H" 2 o), calculated by the formula * 



55'5d 



tfA/H 2 0=^-; (1) 



where d stands for thickness in cm. of a water layer of an 

 absorption power equal to a layer 1 cm. thick of the substance 

 to be examined, and m means the number of gram-molecules 

 per litre of the same substance. It appears from Table II. 

 that a variation of wave-length has very little influence on the 

 determination of a: a/H20 . The circumstance, therefore, that 

 radhition has not been quite homogeneous cannot have been 

 of any essential significance in these measurements. Cer- 

 tainly the fact that the compositions of both ray pencils after 

 passing through the liquid layers have somewhat differed, 

 cannot have exercised any noteworthy influence on the 

 results. Many experiments have shown the determination of 

 «a/u 2 o for a certain metal to be independent of the composition 

 of the solution, and a very close agreement has been obtained 



* Loo. cit. p. 170. 

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