180 DR. JAMES BOTTOMLEY ON COLORIMETRY. 



we mixed nickel and cobalt so as to obtain a perfect grey 

 for a column of a 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 looo cub. c. 7*275 grms. 

 NiSO^, 4'868 grms C0SO4, and 1 1 -468 CuSO^ : the solutions 

 also contained 30 cub. c. of strong sulphuric acid ; this I 

 added to guard against any possible formation of subsalts 

 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 gaslight, it gave a greenish tint. Within the range 

 of coloured fluids 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 fluid 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. 



