SALTS OF COBALT. 27 



COBALT BROMIDE IN METHYL ALCOHOL WITH WATER. (See Plate 16.) 



The concentration of cobalt bromide throughout was constant and 

 equal to 0.088. The percentages of water, beginning with the solution 

 whose spectrum is adjacent to the numbered scale, were 0, 0.5, 0.75, 1, 

 1.25, 1.5, 2, 2.5, 3, 3.5, 4, 5, 6, 8, 10, 12. 



The solution containing no water was slightly purplish in color, from 

 which the color changed rapidly with addition of water to a pink. After 



2 per cent of water had been added no appreciable change in color could 

 be noticed by the unaided eye. The depth of absorbing layer used through- 

 out was 2.0 cm., the exposures to the Nernst lamp and spark being 1 and 



3 minutes, respectively. 



The solution containing no water, and the one containing 0.5 per cent 

 of water, show considerable absorption from X 3900 towards the ultra- 

 violet, which at first sight does not agree very well with what is shown 

 by B, Plate 13. The discrepancy, however, disappears when it is noted 

 that when the series of exposures for the spectrogram now under discus- 

 sion was made, the workroom was at a temperature of about 30 C.; whereas 

 when the negatives for Plate 13 were made the temperature was about 

 20 C. Cobalt bromide dissolved in methyl or ethyl alcohol is very sensitive 

 to temperature change, a pink solution of proper concentration turning pur- 

 ple with a comparatively slight change of temperature. From the solution 

 containing 0.75 per cent of water to the one containing the largest amount 

 of water, the ultra-violet absorption recedes gradually from X 2850 to X 2580. 



The absorption band in the green is somewhat narrower than in the 

 solution of the chloride, but like that it narrows regularly with addition 

 of water. Measurements on the edges of the band in the strip pertaining 

 to the solution containing no water, give the wave-lengths X 5050 and X 5400, 

 locating the center of the band at X 5225. For the strip pertaining to the 

 solution containing 12 per cent of water the figures are X 5100 and X 5300, 

 locating the center at X 5200. The center, therefore, shifts slightly towards 

 the shorter wave-lengths with addition of water. 



The absorption in the red was not of sufficient intensity to be regis- 

 tered on the photographic plates, which, accordingly, show uniform and 

 complete transmission from X 5500 to X 7400. 



COBALT BROMIDE IN ETHYL ALCOHOL WITH WATER. (See Plate 17.) 



The concentration of cobalt bromide was constant throughout and 

 equal to 0.088. The percentages of water, beginning with the solution 

 whose spectrum is adjacent to the numbered scale, were 0, 1, 1.5, 2, 2.5, 

 3, 3.5, 4, 5, 6, 7, 7.5, 8, 9, 10, and 11. 



The solution containing no water was blue or purplish- blue; the one 

 containing 1 per cent of water was slightly purplish, from which the color 

 changed very rapidly to the regular pink of cobalt solutions. No color 

 change appreciable to the eye was noticed after the solution containing 

 3 per cent of water was reached. 



The cell was adjusted to a depth of 2.0 cm. and the exposures to the 

 Nernst lamp and spark were, respectively, of 1 and 3 minutes duration; 

 the slit was, as usual, 0.01 cm. wide. The solution containing no water 



