The Absorption Coefficient of Solution for Monochromatic Radiation. 11 



"We have seen that in general Beer's law can be true only within certain 

 limits, though many observers believe that it holds accurately within wide 

 limits. The experimental data is in many cases unsatisfactory, and in still 

 others contradictory." 



A survey of the literature since 1904 bearing on the relation between 

 A and c yields few definite conclusions. The reason for the unsettled 

 state of the problem is not difficult to find. None of the researches 

 has been carried out with the necessary completeness. The investi- 

 gators have been content with a determination of the molecular absorp- 

 tion coefficient A for a few concentrations at a very limited number of 

 points of the spectrum. 



In 1906 M uller 1 measured A for three solutions of copper chloride in 

 water. The values of A were determined for 5 wave-lengths in that part 

 of the visible region of the spectrum where this solution was fairly trans- 

 parent. M tiller's results showed that A was not only variable with c, 

 but also that the rate of variation was different for each wave-length. 



Hantzsch 2 and his co-workers (the reference is to the final one of a 

 series of papers) have recorded the value of A for a number of solutions 

 of inorganic colored salts. A was measured for a single wave-length 

 for a few concentrations and was found in general to decrease with c in 

 the case of the monochromates, the ferrocyanides, and the permangan- 

 ates of the alkali metals, and to be fairly constant for dilute solutions 

 of certain organic colored salts. 



Sheppard, 3 in his researches, has included determinations of A for 

 alcoholic solutions of a few dyes. The values of A were constant 

 within the error of experiment, except for the most dilute solutions, 

 where they experienced a perceptible increase, which was ascribed to 

 chemical change taking place in the solution. 



Garrett 4 has recorded the values of A for aqueous solutions of a num- 

 ber of salts of copper. A was determined for 3 wave-lengths on the 

 violet side of the red absorption band for 3 concentrations and was 

 found in all cases to decrease with dilution. 



In the work thus far cited the values of A have been determined for 

 wave-lengths lying in the visible region of the spectrum by means of a 

 visual spectro-photometer. 



The photographic method of testing Beer's law, as used by previous 

 workers in this laboratory, 5 is applicable to both the ultra-violet and 

 visible regions of the spectrum. This method, however, yields informa- 

 tion concerning the variations of A with c only for those wave-lengths 

 on the edge of an absorption band. In studying a large number of 

 solutions in this way, many bands were found whose edges obeyed 

 Beer's law and many more whose edges did not. 



'Ann. d. Phys., 21, 515 (1906). 'Zeit. Elektrochem., 19, 1 (1913). 



*Zeit. phys. chem., 84, 321 (1913). 6 Carnegie Inst. Wash. Pub. No.s. 110, 130, 160, 190. 



Mourn. Chem. Soc., 95, 15 (1909); Proc. Roy. Soc., 82-A, 256 (1909). 



