containing Copper, Zinc, and Nickel. 



15 



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 solu- 

 tion 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 accom- 

 panying diagrams, which show the absorption-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 



Spectrum of light passed through Co. 



Spectrum of light passed through Ni and Co. 



The black parts here represent the bright parts of the spectrum, and vice versa. 



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 

 bright 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 

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

 where the overlapping occurs, as seen in the diagram. This 

 is why the solution obtained by mixing strong solutions of 

 nickel and cobalt is not grey, but reddish brown in colour. 



Having so far demonstrated the complementary character of 

 the two metals, I next endeavoured to find in what proportions 

 they must be mixed in order to neutralize each other. For 

 this purpose a tall glass cylinder (150 cubic centims. capacity), 



