SALTS OF COBALT. 33 



sulphocyanate was only 0.12. This would indicate for the sulphocyanate 

 solution an absorbing power about 2.5 times as great as for the chloride 

 solution, considering only the green band. It is also worth noticing that 

 whereas the green band in the chloride solutions remains practically con- 

 stant for concentrations below 0.4 normal, when the conditions for Beer's 

 law are fulfilled, the same band in the sulphocyanate solutions continues 

 to narrow rapidly, even where the concentration is as small as 0.05 nor- 

 mal. The percentage dissociation of solutions of the two salts having 

 equal concentrations is not very different in amount. 



COBALT SULPHOCYANATE IN WATER MOLECULES CONSTANT. (See Plate 23.) 



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, 1.78, 1.34, 1.01, 0.78, 0.62, and 0.51; the corresponding 

 depths of absorbing layer were 3, 4, 6, 9, 15, 18, and 24 mm. For B the 

 concentrations were 0.51, 0.40, 0.30, 0.215, 0.165, and 0.10; the depths 

 of cell were the same as for A. The exposures to the Nernst lamp and the 

 spark were, respectively, 1^ and 3 minutes, the width of slit being 0.01 cm. 



The ultra-violet absorption still narrows quite markedly with dilution, 

 the limits of transmission for the most concentrated and most dilute solu- 

 tions of A being ^ 3630 and A 3430; the corresponding limits for B were 

 A 3290 and A 3220. There is, then, a tendency for this band to remain con- 

 stant in width as the dilution is increased, since the narrowing is very 

 much less in B than in A. 



The green band widens somewhat, though less than in the case of some 

 of the other cobalt solutions studied. In A the widening of this band 

 towards the violet and the narrowing of the ultra-violet absorption pro- 

 duce the curious result that a rather narrow region of transmission moves 

 continuously towards the ultra-violet with dilution, its width remaining 

 sensibly constant. By varying the depth of layer the transmission may 

 be made to have almost any width desired. 



The limit of the violet edge of the band in A changes from ^ 4350, in 

 the most concentrated solution, to A 4220 in the most dilute. The red 

 edge is at A 6400 in the first solution. It moves rapidly towards the shorter 

 wave-lengths at first, reaching A 5900 in the fifth solution. From this 

 point it moves gradually towards the red with further dilution. The appar- 

 ent narrowing of this band at first is undoubtedly due to the band located 

 in the red, which disappears rapidly with dilution. If this band were 

 absent we should find that the green band would widen continuously, 

 though slowly, with decrease in concentration when molecules are kept 

 constant. 



In B the limits of the green band in the most concentrated solution 

 are ^ 4630 and ^ 5500, while in the most dilute solution they are A 4580 

 and ^ 5520, thus showing a widening of about 70 Angstrom units. The 

 widening is somewhat unsymmetrical, which is to be explained, however, 

 by the lack of uniformity in the sensibility of the Seed plate in the region 

 occupied by the red edge of the band. 

 3 



