22 ABSORPTION SPECTRA OF SOLUTIONS. 



COBALT BROMIDE IN WATER BEER'S LAW. (See Plate 10.) 



The concentrations of the solutions used in making the negative for 

 A were 2.14, 1.60, 1.07, 0.71, 0.49, 0.36, and 0.27, the corresponding depths 

 of cell being 3, 4, 6, 9, 13, 18, and 24 mm. For B the concentrations were 

 0.71, 0.53, 0.35, 0.24, 0.164, 0.12, and 0.09; the depths of cell were the 

 same as in A. The strip corresponding to the most concentrated solu- 

 tion is in each case adjacent to the numbered scale. 



The exposures to the Nernst lamp lasted H minutes, while those to 

 the spark lasted 3 minutes, the slit having a width of 0.01 cm. 



The most concentrated solution had a deep reddish-brown color. With 

 dilution the color gradually changed to the usual pink. No exposures 

 were made to the red end of the spectrum, since even the most concen- 

 trated solution in layers of 2 cm. or more failed to show any absorption 

 in this region. 



The spectrogram shows two regions of absorption, one in the extreme 

 ultra-violet and the usual one in the green. 



The most concentrated solution of set A transmits the ultra-violet 

 light of the spark as far as ^ 2720, while the most dilute solution trans- 

 mits as far as A 2570. The absorption decreases with dilution much more 

 rapidly at first, as is shown by the rounded appearance of the edge. The 

 most concentrated solution of B transmits ^ 2470, while the most dilute 

 one lets through some light of wave-length ^ 2350, being, therefore, almost 

 perfectly transparent to all wave-lengths emitted by the spark used. No 

 trace of an absorption band near ^ 3300, such as was observed in aqueous 

 solutions of cobalt chloride, could be noticed in the aqueous solutions of 

 the bromide. In fact, the bromide is much more generally transparent to 

 light of all wave-lengths than the chloride, and especially so for the ultra- 

 violet region of the spectrum. The band in the green behaves very much 

 like that due to cobalt chloride. It narrows rapidly at first with dilution, 

 then more slowly, and finally remains of practically constant width in the 

 four most dilute solutions of set B. The middle of the band in B is very 

 near ^ 5200, which is the same as in the case of cobalt chloride. The band 

 is, however, considerably narrower in the bromide than in the chloride solu- 

 tions, due in part to the slightly smaller concentration; but as this is hardly 

 sufficient to account for the whole effect it indicates an intrinsic difference 

 in the behavior of the two salts. 



COBALT BROMIDE IN WATER MOLECULES CONSTANT. (See Plate 11.) 



No data giving the dissociation of cobalt bromide were at hand, and, 

 accordingly, since it is a general rule that the chlorides and bromides do 

 not differ greatly in this respect, the dissociation of the bromide was 

 assumed to be the same as for cobalt chloride. Now it is possible that the 

 actual value of the dissociation for any given concentration of the bro- 

 mide might differ somewhat from the corresponding value for the chloride, 

 and still the rate of change of dissociation with concentration might be 

 sensibly the same for the two salts. And, evidently, in the calculation of 

 the series of concentrations required in order to make the number of mole- 

 cules in the path of the beam of light constant, for an arbitrary set of 



