32 ABSORPTION SPECTRA OF SOLUTIONS. 



sulphate solutions is greater. A comparison of Plate 19 B shows that the 

 absorption band in the green is of sensibly the same width, while the con- 

 centration of the nitrate solution pertaining to the seventh strip was about 

 40 per cent greater than that of the corresponding sulphate solution. As 

 the sulphate dissociates less strongly than the nitrate or chloride, it does not 

 seem altogether reasonable to ascribe the green band to the cobalt ions. 



Since Beer's law holds so accurately for the sulphate solutions, it was 

 evidently unnecessary to study sets of solutions of such concentrations 

 as to give a constant number of ions, or a constant number of molecules. 

 In the former case we should get a marked narrowing of the band and in 

 the latter case a widening. 



COBALT SULPHOCYANATE IN WATER BEER'S LAW. (See Plate 22.) 



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 2.17, 1.62, 1.08, 0.72, 0.50, 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 0.36, 0.27, 0.18, 0.12, 0.083, 0.060, and 0.045; the depths of absorb- 

 ing layer were the same as in set A. 



The color of solutions of the sulphocyanate is much deeper than of 

 solutions of other salts of cobalt used in the present work. The most con- 

 centrated solutions were very deep purple, from which, as concentration 

 decreased, the color changed to the usual pink. The exposures to the 

 Nernst lamp and spark were, respectively, 1J and 3 minutes, the slit being 

 adjusted to a width of 0.01 cm. 



The spectrogram shows a region of strong absorption in the ultra- 

 violet, which narrows rapidly w r ith dilution, the limit of transmission for 

 the most concentrated solution in A being X 3630, while for the most dilute 

 solution it is A 3370. The corresponding wave-lengths for B are A 3240 

 and X 3130. 



The absorption band in the green is very wide in A, extending in the 

 case of the most concentrated solution from A 4300 to A 6350, and in the 

 most dilute from A 4350 to A 5700. The violet edge moves towards the red 

 slowly but uniformly with dilution, while the red edge moves in the oppo- 

 site direction very rapidly from the first to the third solution, then more 

 slowly. There is apparently a band in the red not resolved from the green 

 band, which disappears very rapidly with dilution, and which causes the 

 apparent rapid narrowing of the green band observed in the spectrogram. 



In B the green band also narrows considerably, its limits in the first 

 solution being A 4900 and X 5430, and in the seventh solution X 5050 and 

 X 5320. The violet edge of the band is shaded considerably in the first 

 three solutions ; the transmission being incomplete as far as X 4500. In 

 the red this set of solutions does not show any absorption sufficiently 

 intense to be recorded on the photographic plate. 



A comparison of the fourth strip of B, Plate 22, with the fourth strip of B, 

 Plate 3, shows that the green band on the two has about the same intensity 

 and width. The depth of the two corresponding solutions was the same, 

 but the concentration of the chloride solution was 0.329, while that of the 



