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cylinder covered externally with black cloth except a cir- 
cular aperture of 8 mm. diameter at the bottom. When I 
looked through the column of fluid at a white surface, the 
colour was decided, resembling somewhat the pigment known 
as yellow ochre. Also with a less proportion of cobalt to 
nickel, namely 20 cub. c. of cobalt solution to 50 cub. c. of 
nickel solution, I still obtained a tint in which yellow seemed 
to predominate. Had I employed solutions so dilute that 
no colour was perceptible in the mixture, this would not 
strictly imply that the colours were complementary, but 
that the resulting tint was too feeble to produce the 
impression of colour, and if we filled a long tube with such 
a dilute solution the colour might again become manifest. 
Moreover, my aim was not to mix two coloured solutions so 
as to obtain a fluid which exercised no perceptible absorption 
of light, but to obtain a fluid which would exercise a 
considerable absorption subject to a certain condition. The 
following consideration seemed to me to render it hopeless 
to obtain a soluble black by nickel and cobalt only. A 
solution of cobalt when dilute is pink, but if we look through 
a considerable thickness or through a concentrated solution, 
the pink shows a tendency to pass into a scarlet. This 
shows that as the quantity of the salt increases, the ratio 
of the yellow to the red increases. The colour of the 
undissolved salt is brownish red, and the colour of the 
solution seems to approximate towards this as the concen- 
tration increases. Hence the colour of a solution of cobalt 
alters not only in intensity but also in kind as the amount 
of the salt is increased. On the other hand, the green of a 
solution of nickel varies in intensity, but does not seem to 
vary in character, at least in any marked manner, as the 
quantity of the salt increases. In order that it should be 
generally complementary in character to cobalt, any 
inconstancy in the ratio of the red to the yellow of the 
latter would require a corresponding variation in the ratio 
