10 Studies on Solution. 



where a is the absorption coefficient for the pure solvent, c is the concen- 

 tration in gram-molecules of salt per liter of solution, and A is called the 

 molecular absorption coefficient of the salt in the solution. From this 

 relation it follows that 



^ ^o /o\ 



A = (2; 



The present investigation has consisted of a systematic and thorough 

 study of the absorption coefficient a. This quantity has been measured 

 at intervals of 20/x/x to 40/x/z throughout the region of the spectrum from 

 GOOjuAi to l,300ju/>i for many solutions. The work has been restricted to 

 a study of inorganic salts in aqueous and alcoholic solution. All the 

 measurements have been carried out with solutions at room tempera- 

 ture. The values of a, when plotted as ordinates against the corre- 

 sponding wave-lengths as abscissas, form the absorption curve. For 

 each salt a series of solutions varying in concentration from satura- 

 tion to moderate dilution was prepared and the absorption curve has 

 been drawn for each solution. From the measured values of a and a 

 and from the known value of c, A has been calculated for each wave- 

 length by means of formula (2). The values of A for a given wave- 

 length have been plotted as ordinates against the corresponding values 

 of c as abscissas. The curves thus formed will be referred to as the 

 A-c curves. It was the purpose of the present investigation to deter- 

 mine the form of the A-c curves. 



HISTORICAL. 



The general problem of the absorption of radiation by solutions has 

 been the subject of many investigations. Only those papers are of 

 primary interest here which concern determinations of the numerical 

 values of the absorption coefficient as a function of the concentration. 



Beer 1 measured the absorption coefficient for red light of a number of 

 aqueous solutions of inorganic salts. The results of his experiments 

 showed that within the error of experiment A was a constant with 

 respect to c. The statement that "A is a constant" has been men- 

 tioned by subsequent workers in this field as "Beer's law." This 

 "law" has since been shown to be the exception rather than the rule, 

 and therefore in this paper but few references have been made to 

 "Beer's law." 



A paper by Rudorf 2 entitled " Lichtabsorption in Losungen vom 

 Standpunkt der Dissociationstheorie" reviews the literature up to the 

 year 1904 and gives a very good statement of the conclusions reached 

 at that time. Rudorf concluded the section of his paper concerning 

 Beer's law with the following observation: 



l Pogp. Ann., 86, 78 (1882). 



2 Sammlung Chemischer und Chemish-Tcchnischer Vortriige, 9, 1 (1904). 



