Theory of Colours — trans. Charles Lock Eastlake (1840)

The exhibitions on the _minus_ side are less frequent. Copper slightly oxydized appears blue. In the preparation of Prussian-blue, alkalis are employed.

516.

Generally, however, these appearances of colour are of so mutable a nature that chemists look upon them as deceptive tests, at least in the nicer gradations. For ourselves, as we can only treat of these matters in a general way, we merely observe that the appearances of colour in metals may be classed according to their origin, manifold appearance, and cessation, as various results of oxydation, hyper-oxydation, ab-oxydation, and de-oxydation.[2]

[1] See par. 478.

[2] As these terms are afterwards referred to (par. 525), it was necessary to preserve them.

XXXVIII.

AUGMENTATION OF COLOUR.[1]

517.

The augmentation of colour exhibits itself as a condensation, a fulness, a darkening of the hue. We have before seen, in treating of colourless mediums, that by increasing the degree of opacity in the medium, we can deepen a bright object from the lightest yellow to the intensest ruby-red. Blue, on the other hand, increases to the most beautiful violet, if we rarefy and diminish a semi-opaque medium, itself lighted, but through which we see darkness (150, 151).

518.

If the colour is positive, a similar colour appears in the intenser state. Thus if we fill a white porcelain cup with a pure yellow liquor, the fluid will appear to become gradually redder towards the bottom, and at last appears orange. If we pour a pure blue solution into another cup, the upper portion will exhibit a sky-blue, that towards the bottom, a beautiful violet. If the cup is placed in the sun, the shadowed side, even of the upper portion, is already violet. If we throw a shadow with the hand, or any other substance, over the illumined portion, the shadow in like manner appears reddish.

519.

This is one of the most important appearances connected with the doctrine of colours, for we here manifestly find that a difference of quantity produces a corresponding qualified impression on our senses. In speaking of the last class of epoptical colours (452, 485), we stated our conjecture that the colouring of steel might perhaps be traced to the doctrine of the semi-transparent mediums, and we would here again recall this to the reader's recollection.

520.

All chemical augmentation of colour, again, is the immediate consequence of continued excitation. The augmentation advances constantly and unremittingly, and it is to be observed that the increase of intenseness is most common on the _plus_ side. Yellow iron ochre increases, as well by fire as by other operations, to a very strong red: massicot is increased to red lead, turbith to vermilion, which last attains a very high degree of the yellow-red. An intimate saturation of the metal by the acid, and its separation to infinity, take place together with the above effects.

521.

The augmentation on the _minus_ side is less frequent; but we observe that the more pure and condensed the Prussian-blue or cobalt glass is prepared, the more readily it assumes a reddish hue and inclines to the violet.

522.

The French have a happy expression for the less perceptible tendency of yellow and blue towards red: they say the colour has "un œil de rouge," which we might perhaps express by a reddish glance (einen röthlichen blick).

[1] Steigerung, literally _gradual ascent_. See the note to par. 523.

XXXIX.

CULMINATION[1]

523.

This is the consequence of still progressing augmentation. Red, in which neither yellow nor blue is to be detected, here constitutes the acme.

524.

If we wish to select a striking example of a culmination on the _plus_ side, we again find it in the coloured steel, which attains the bright red acme, and can be arrested at this point.

525.

Were we here to employ the terminology before proposed, we should say that the first oxydation produces yellow, the hyper-oxydation yellow-red; that here a kind of maximum exists, and that then an ab-oxydation, and lastly a de-oxydation takes place.

526.

High degrees of oxydation produce a bright red. Gold in solution, precipitated by a solution of tin, appears bright red: oxyde of arsenic, in combination, with sulphur, produces a ruby colour.

527.

How far, however, a kind of sub-oxydation may co-operate in some culminations, is matter for inquiry; for an influence of alkalis on yellow-red also appears to produce the culmination; the colour reaching the acme by being forced towards the _minus_ side.

528.