SALTS OF COBALT. 25 



aluminium chloride was used as a dehydrating agent, thus indicating 

 that the difference in the spectra which we have just considered is to 

 be referred to the colored salt and not to the dehydrating agent. 



COBALT BROMIDE IN METHYL ALCOHOL BEER'S LAW. (See Plate 13.) 



The concentrations of the solutions used in making the negative for 

 B were 0.228, 0.191, 0.158, 0.130, 0.108, 0.091, and 0.076; the corre- 

 sponding depths of absorbing layer were 8, 9.5, 11.5, 14, 17, 20, and 24 

 mm. For set A the concentrations were 0.114, 0.096, 0.079, 0.065, 0.054, 

 0.045, and 0.038. The depths of absorbing layer were the same as for set 

 B. The concentrations were accordingly about 9 per cent smaller for B 

 than was the case in the similar set for cobalt chloride, and about 18 per 

 cent smaller in case of A. 



The most concentrated solution was slightly purplish in color, from 

 which the color changed to the usual pink of cobalt solutions. The con- 

 centrated solutions had very little general absorption, differing in this 

 respect markedly from those of cobalt chloride in the same solvent. The 

 exposures to the Nernst lamp and spark were, respectively, 1J and 3 

 minutes, the slit being, as usual, 0.01 cm. in width. 



The edge of the ultra-violet band is perfectly straight in both sets, 

 showing that Beer's law holds rigidly. The limit of transmission in B 

 is X 2850, while for A it is /I 2760, thus indicating much stronger absorp- 

 tion in this region for the bromide than for the chloride. The chloride, 

 although transmitting some light of shorter wave-length, does, however, 

 exert a greater amount of absorbing action in the region below X 3700, 

 since it has two bands, one at X 3100 and one at X 3600, both of which are 

 entirely absent in the bromide solution. The band in the green is not as 

 intense as in the chloride solution. Its center is at X 5250, and it narrows 

 slightly with dilution in both B and A. 



The slight absorption in the red, which was undoubtedly present in 

 the most concentrated solution, was not sufficient to be registered on the 

 photographic plate. This, accordingly, indicates complete transparency 

 from the green band to beyond X 7400. 



COBALT BROMIDE IN ETHTL ALCOHOL BEER'S LAW. (See Plate 14.) 



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

 were 0.146, 0.122, 0.102, 0.084, 0.069, 0.058, and 0.044, the depths of cell 

 being 8, 9.5, 11.5, 14, 17, 20, and 24 mm. The concentrations for B were 

 0.058, 0.049, 0.040, 0.033, 0.026, 0.023, and 0.019; the depths of cell were 

 the same as for A. The concentrations were accordingly almost the same 

 throughout as those used in making the negative for Plate 5, so that the 

 two are directly comparable. 



All the solutions were blue, the color becoming very faint, however, 

 in the most dilute solutions of set B. Exposures and slit-width were the 

 same as for Plate 13. 



There is strong absorption in the extreme ultra-violet which, how- 

 ever, narrows slightly with dilution; the limits of transmission being 

 X 3060 and X 3000 for the most concentrated and most dilute solutions of 



