320 Dr. T. Ewan on the Absorption- Spectra of 



and chromic acid have very nearly the same _ absorption- 

 spectrum, while that of potassium chromate is quite different. 



Erhardt * found that the violet solutions of potassium and 

 ammonium chrome-alums and of chromium sulphate have 

 identical absorption-spectra, while that of chromic chloride 

 is only very slightly different. The spectra of the other 

 salts examined were considerably different. 



As may be seen from the foregoing historical summary 

 the experimental material is hardly sufficient to decide 

 whether the absorptions of acid and base in aqueous solution 

 are really independent or not. In strong solutions, speaking 

 generally, they are not ; but on dilution changes, greater or 

 smaller, occur, and it is natural to inquire whether there is a 

 limiting dilution beyond which no further change in the 

 absorption occurs, and secondly, supposing that the limit 

 exists, whether the acid and base will exercise their absorptions 

 independently of each other in solutions more dilute than the 

 limit. 



To obtain answers to these questions, it was necessary to 

 make photometric measurements of the fraction of the! light 

 of each wave-length absorbed by a molecule of different salts 

 of the same metal in solutions of different concentrations. 

 The salts of copper were used, as they seemed to form a 

 suitable material for the measurements. 



A prehminary set of measurements was made by a method 

 not unlike that used by < Knoblauch t> but I believe slightly 

 more accurate, owing to the fact that it allows the spectra of 

 the two solutions which are being compared to be seen simul- 

 taneously, side by side, in the spectroscope. A short description 

 of it may therefore not be out of place. 



The spectroscope used was a one-prism graduated arc 

 instrument, by Hilger. The prism had a dispersion from 

 A to H of about 6° 20'. The solutions were contained in 

 troughs in such a way that a long layer of a dilute solution 

 could be compared with a short layer of a correspondingly 

 strong one. The lengths of the layers and the concentrations 

 of the solutions were always taken inversely proportional 

 to each other ; so that the light in both cases passed through 

 the same amount of copper salt. In order that it should also 

 pass through the same amount of water in each case, a long- 

 tube full of water was placed in front of the short tube con- 



* Erhardt, Inaugural Dissertation, Freiberg, 1875 ? 

 t These experiments were carried out in January and February 1891 ; 

 that is, several months before Knoblauch's paper was published. 



