200 JONES-STRONG— THE ABSORPTION SPECTRA [April 24, 



chromatic light. For ordinary white light one would have to 

 integrate this equation over the range of wave-lengths used. The 

 equation would then have the form 



/=/o r 



The quantity ^ is called the index of absorption and A the molecular 

 extinction coefficient. If the absorption is proportionately greater 

 in the more concentrated solutions, then Beer's law fails and A 

 decreases inversely as the concentration. 



From photometric measurements Settegast* and Sabatier* con- 

 clude that the absorption spectrum of potassium dichromate is the 

 same as that of chromic acid, and that the absorption spectrum of 

 potassium chromate is entirely different. This is corroborated by 

 the present work. Settegast finds that Beer's law does not hold 

 for potassium chromate and potassium dichromate, the coefficient A 

 decreasing with increasing concentration. Griinbaum^ finds the 

 following values of A and e where e^c/A. 



Potassium Dichromate. 

 Value of ^. Value of ^. 



A c = .034 c = .0034 



509 62.4 58.0 



521 28.7 26.2 



538 7.24 6.2 



It will be seen that the deviation here from Beer's law is in the 

 opposite direction from that of Settegast. Grunbaum finds that c 

 and therefore A depends on the depth of layer. 



An example will be given where the same concentration was used 

 and different depths of the solution. 



A Values of e for c = .0034 



25 cm. layer. 12 cm. layer. 5 cm. layer. 



521 .0758 .0818 .0884 



521 .0761 .0830 .0897 



Our work indicates that Beer's law holds for all small concentra- 

 tions and usually the deviations for concentrated solutions is very 



^Wied. Ann., 7, pp. 242-271, 1879. 



* C. R., 103, pp. 49-52, 1886. 



^ Ann. d. Phys., 12, pp. 1004, loii, 1903. 



