26 ABSORPTION SPECTRA OF SOLUTIONS. 



A, respectively, and A 2920 and X 2820 for those in B. No trace of any 

 bands at X 3100 or A 3600, as found in the corresponding solutions of cobalt 

 chloride, can be seen. The green band may be seen in A, but unlike the 

 chloride solution there is a region of complete transparency on the red 

 side of it. In B the green band is so extremely weak that the slight shad- 

 ing noticeable near X 5250 may be due to the lack of sensibility of the 

 Seed film in this region. 



In A there is a wide region of complete absorption, which narrows 

 rapidly with dilution, breaking up into two absorption bands in B. The 

 limits of transmission for the most concentrated solutions of A are X 5750 

 and X 7200, while for the most dilute solutions the figures are X 6150 and 

 X 6900. It appears, therefore, that the red edge of the absorption band is 

 a little nearer the region of long wave-lengths than is the case for the cor- 

 responding solutions of cobalt chloride. 



B shows two absorption bands having their centers at X 6160 and X 6730, 

 respectively. Both narrow rapidly with dilution, disappearing practically 

 in the most dilute solution. No trace of the narrower bands seen in the 

 chloride solution is visible. 



COBALT BKOMIDE IN ACETONE BEER'S LAW. (See Plate 15.) 



The concentrations of the solutions used in making the negative for 

 A varied from 0.037 to 0.012; the depths of absorbing layer were 8, 9.5, 

 11.5, 14, 17, 20, and 24 mm. In B the concentrations were varied from 

 0.012 to 0.004, the depths of cell used being the same as in set A. The 

 strip corresponding to the most concentrated solution is in each case 

 adjacent to the numbered scale. 



The solutions were all blue, only the intensity of the color varying 

 with dilution. The exposures to the Nernst lamp and spark were, respec- 

 tively, 1J and 2J minutes. The absorption in the ultra-violet is of course 

 to be ascribed to the solvent, and has exactly the same limits as given 

 under Plate 6. 



In the red is a wide absorption band which in set A extends farther 

 into the red than the panchromatic plate is capable of recording. The 

 violet edge of the band lies at X 5750 and is fairly sharp. The red edge 

 was shown by visual observations to lie very close to X 7600. In B the 

 band has narrowed considerably, as some light is transmitted as far up in 

 the red as X 6200, the red edge being at approximately X 7050. The violet 

 edge of the band is very hazy, the transmission being greatly weakened 

 as far towards the violet as X 5850. The absorption shows no indication 

 of breaking up into smaller bands as did that of cobalt chloride in acetone. 

 Beer's law appears to hold very accurately, the absorption shown by all 

 the strips of one set being the same. 



In comparing this plate with Plate 6, it should be borne in mind that 

 the solutions used in making this plate had almost double the concen- 

 tration used in the other case. This accounts for the somewhat stronger 

 absorption shown by the bromide. The indications are, however, that the 

 absorbing power of the bromide in acetone is somewhat smaller than that of 

 the chloride, which agrees with what has been noticed in the other solvents. 



