167 
of the yellow to the blue in the former, and the tint ought 
to pass from an emerald green to a bluish green. As this 
does not seem to be the case, it would follow that even if 
we mixed nickel and cobalt so as to obtain a perfect grey 
for a column of definite length, a column longer or shorter 
than this would still retain some colour. My experiments 
seemed to indicate a deficiency of blue in the mixture, and 
this I thought might be supplemented by another salt. So 
I tried the addition of sulphate of copper. After some trials 
I got a solution containing in 1,000 cub. c. 7*275 grms. 
N 1 SO 4 , 4,868 grms. C 0 SO 4 , and 11,468 CUSO 4 , the solutions 
also contained 30 cub. c. of strong sulphuric acid; this I 
added to guard against any possible formation of sub-salts 
on copious dilution. This solution seemed nearer to what 
I wanted than a solution of nickel and cobalt only ; it did 
not however appear wholly free from colour, and possibly 
a variation of the quantities might have given a better 
result; also the tint seemed to vary somewhat with the 
nature and intensity of the incident light. When in the 
failing light of approaching evening I held the containing 
bottle against the grey sky I thought that there remained 
a somewhat pinkish tint, whilst in the colorimeter when 
looking at an external white surface through a column 
sufficiently long to produce a perceptible absorption, I 
thought the solution had a bluish tint. When viewed 
against gas light it gave a greenish tint. Within the range 
of coloured fiuids in Chemistry there may be some which if 
combined would yield a mixture absorbing all colours in the 
same ratio, so as to be truly a soluble black. The pre- 
paration of such a fiuid would be an interesting problem in 
physics. It seems to me that we might also have such 
fluids which on spectral analysis would show not an 
absorption of all colours in the same ratio, but would be 
resolved into a violet and yellow, or an orange and blue, 
or red and green, or some other combination of colours of a 
complementary character. 
