Bayley — On Colour Relations of Nickel and Cobalt. 131 



XXI. — On xhe Colotjk Eelations and Coloeimeteic Estimation of 

 Nickel and Cobaxt. By Thomas Baixet, Associate E. C. Sc. I. 



[Read, November 12, 1877.] 



The fact will have been observed by chemists that solutions of nickel 

 and cobalt salts are so far complementary in colour that, when they 

 are mixed together, the resulting liquid, if moderately dilute, is hardly 

 to be distinguished from pure water. I conceived this fact might be 

 made the basis of a method for estimating nickel and cobalt, and, 

 therefore, undertook the following experiments. 



A large hollow prism, filled with a moderately strong solution of a 

 nickel or cobalt salt, was placed immediately in front of the slit of the 

 spectroscope, and the thickness of the liquid traversed by the light 

 was regulated by moving the prism until the eye could most clearly 

 determine the dark absorption band caused by the metal in solution. 

 On referring to the accompanying diagram, which shows the absorp- 

 tion spectra of the two metals, it will be seen that cobalt and nickel 

 are almost exactly complementary in their relations to light. The 

 black band of cobalt is well defined at the edges, especially at the end 

 nearest to the red, while the absorption bands of nickel are not so 

 sharply defined, but fade away at each end. If the spectra were 

 exactly complementary, on superimposing the nickel spectrum upon 

 the cobalt spectrum, the dark part on the one would exactly cover 

 the light part on the other. This, however, though nearly the case, 

 is not exactly so, for the light band in the nickel spectrum overlaps 

 the dark cobalt band at the end nearest to the red, although with 

 diminished brilliancy. Consequently, when we employ a mixture of 

 nickel and cobalt salts in solution, we do not get a uniformly dark 



I ! { ' Sn ectrum ofliqh t 

 Ijl: passed thro: Ni: 



■ Sp-ecirum oflimt 

 passed thru: M: \Co', 



Snecirum ofliifhtnassed thro: Co: 



spectrum, but an excess of light coming through at the part where 

 the overlapping occurs, as seen in the diagram. This is why the so- 



