165 
various shades, verging towards blackness as the length of 
the column increased. Such a fluid we might call a soluble 
black. I am not aware of any single fluid that fulfils the 
above conditions. It might be said, why not use ink ? but 
such specimens of ink as I have examined are bluish or 
violet on copious dilution. Moreover the colour alters with 
the degree of oxidation; also it seems to be colouring 
matter in suspension rather than in solution. I had some 
hopes of succeeding by mixing solutions of nickel and 
cobalt salts. On reference to the Phil. Magazine, vol. YI., 
page 15 , I find that the colour relations of nickel and cobalt 
had been studied by Mr. Thomas Bayley with a view to the 
quantitative determination of these metals founded upon 
the complementary character of their colours. He states, 
“ 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.” After considering the nature of the absorption 
spectra of nickel and cobalt salts, he states, “ If the spectra 
were exactly complementary, on superimposing the nickel 
spectrum upon the cobalt spectrum the dark part on the 
one would cover exactly the light part on the other. This, 
however, though nearly the case, is not exactly so this 
is why the solution obtained by mixing strong solutions of 
nickel and cobalt is not grey, but reddish brown in colour.” 
Some experiments which I made seemed to confirm the 
opinion of Mr. Bayley, the nickel solutions contained O'Oo 
grms. of NiS04 per cub. c. and the cobalt solution contained 
0*05 grms. C0SO4 per cub. c. A mixture consisting of 50 
cub. c. of cobalt solution with 100 cub. c. of nickel solu- 
tion contained in a white porcelain basin, seemed to 
be a grey tinted with pink in the shallower parts, and 
having a tendency to pass into a yellowish tint as the 
depth increased. I now poured the fluid into a tall glass 
