62 ABSORPTION SPECTRA OF SOLUTIONS. 



something like only 40 A.U. In the intermediate series the narrowing of 

 the band with dilution is still less, only about 25 or 30 A.U., and so may 

 very probably be due to a gradual shift in the position of the film during 

 the exposure. In the most dilute series no change in the width of the 

 region of absorption can be noted. The slight deviation observed in the 

 first series may perhaps be due to mutual influence of the absorbers, as 

 pointed out in the introductory chapter. 



Comparing these spectrograms with those of solutions in methyl alco- 

 hol, we find that the limit of transmission here is always a little nearer 

 the region of short wave-lengths, which is a little different from what we 

 have usually found. The rule has been that the absorbing power of any sub- 

 stance in ethyl alcohol is somewhat greater than in methyl alcohol, while 

 here the opposite is true. Ferric chloride is, however, a rather unstable sub- 

 stance in solution, and it is possible that the anomalies which we have noted 

 are due to some chemical change which has not been taken into account. 



FERRIC CHLORIDE IN ACETONE BEER'S LAW. (See Plate 55.) 



The concentrations of the solutions used in making the negative for A, 

 beginning with the one whose spectrum is adjacent to the numbered scale, 

 were 0.086, 0.064, 0.043, 0.029, 0.020, 0.014, and 0.011; the corresponding 

 depths of absorbing layer were 3, 4, 6, 9, 13, 18, and 24 mm. For B the 

 concentrations were 0.014, 0.010, 0.007, 0.0047, 0.0032, 0.0023, and 0.0017, 

 the depths of cell being the same as for A. 



The most concentrated solutions were red, from which on dilution the 

 color changed to yellow. The exposure, which was made only to the light 

 from the Nernst lamp, lasted 1 minute, the slit having the usual width 

 of 0.01 cm. 



It was observed that a solution of ferric chloride in acetone, on being 

 allowed to stand, changes color slowly with time, the color becoming 

 deeper. A solution which when freshly made up was yellow, was found 

 to be a clear orange after two days. In order to obtain the spectra photo- 

 graph before any appreciable change took place, it was necessary to make 

 the exposure just as soon as the solution was made up, and this was done, 

 the time elapsing between making up the series and completing the expos- 

 ures for the spectrogram being not more than 30 minutes. 



The negative for A shows a decrease of absorption with dilution, while 

 B shows no change in the width of the band. This is the same as what 

 we just found in the case of the solutions in ethyl alcohol, only the narrow- 

 ing of the band in the concentrated solutions is much greater with acetone. 



