XLVI.

COMMUNICATION, ACTUAL.

572.

Having now provided the colouring materials, as before shown, a further question arises how to communicate these to colourless substances: the answer is of the greatest importance from the connexion of the object with the ordinary wants of men, with useful purposes, and with commercial and technical interests.

573.

Here, again, the dark quality of every colour again comes into the account. From a yellow, that is very near to white, through orange, and the hue of minium to pure red and carmine, through all gradations of violet to the deepest blue which is almost identified with black, colour still increases in darkness. Blue once defined, admits of being diluted, made light, united with yellow, and then, as green, it approaches the light side of the scale: but this is by no means according to its own nature.

574.

In the physiological colours we have already seen that they are less than the light, inasmuch as they are a repetition of an impression of light, nay, at last they leave this impression quite as a dark. In physical experiments the employment of semi-transparent mediums, the effect of semi-transparent accessory images, taught us that in such cases we have to do with a subdued light, with a transition to darkness.

575.

In treating of the chemical origin of pigments we found that the same effect was produced on the very first excitement. The yellow tinge which mantles over the steel, already darkens the shining surface. In changing white lead to massicot it is evident that the yellow is darker than white.

576.

This process is in the highest degree delicate; the growing intenseness, as it still increases, tinges the substance more and more intimately and powerfully, and thus indicates the extreme fineness, and the infinite divisibility of the coloured atoms.

577.

The colours which approach the dark side, and consequently, blue in

## particular, can be made to approximate to black; in fact, a very

perfect Prussian blue, or an indigo acted on by vitriolic acid appears almost as a black.

578.

A remarkable appearance may be here adverted to; pigments, in their deepest and most condensed state, especially those produced from the vegetable kingdom, such as the indigo just mentioned, or madder carried to its intensest hue, no longer show their own colour; on the contrary, a decided metallic shine is seen on their surface, in which the physiological compensatory colour appears.

579.

All good indigo exhibits a copper-colour in its fracture, a circumstance attended to, as a known characteristic, in trade. Again, the indigo which has been acted on by sulphuric acid, if thickly laid on, or suffered to dry so that neither white paper nor the porcelain can appear through, exhibits a colour approaching to orange.

580.

The bright red Spanish rouge, probably prepared from madder, exhibits on its surface a perfectly green, metallic shine. If this colour, or the blue before mentioned, is washed with a pencil on porcelain or paper, it is seen in its real state owing to the bright ground shining through.

581.

Coloured liquids appear black when no light is transmitted through them, as we may easily see in cubic tin vessels with glass bottoms. In these every transparent-coloured infusion will appear black and colourless if we place a black surface under them.

582.

If we contrive that the image of a flame be reflected from the bottom, the image will appear coloured. If we lift up the vessel and suffer the transmitted light to fall on white paper under it, the colour of the liquid appears on the paper. Every light ground seen through such a coloured medium exhibits the colour of the medium.

583.

Thus every colour, in order to be seen, must have a light within or behind it. Hence the lighter and brighter the grounds are, the more brilliant the colours appear. If we pass lac-varnish over a shining white metal surface, as the so-called foils are prepared, the splendour of the colour is displayed by this internally reflected light as powerfully as in any prismatic experiment; nay, the force of the physical colours is owing principally to the circumstance that light is always acting with and behind them.

584.

Lichtenberg, who of necessity followed the received theory, owing to the time and circumstances in which he lived, was yet too good an observer, and too acute not to explain and classify, after his fashion, what was evident to his senses. He says, in the preface to Delaval, "It appears to me also, on other grounds, probable, that our organ, in order to be impressed by a colour, must at the same time be impressed by all light (white)."

585.

To procure white as a ground is the chief business of the dyer. Every colour may be easily communicated to colourless earths, especially to alum: but the dyer has especially to do with animal and vegetable products as the ground of his operations.

586.

Everything living tends to colour--to local, specific colour, to effect, to opacity--pervading the minutest atoms. Everything in which life is extinct approximates to white (494), to the abstract, the general state, to clearness[1], to transparence.

587.

How this is put in practice in technical operations remains to be adverted to in the chapter on the privation of colour. With regard to the communication of colour, we have especially to bear in mind that animals and vegetables, in a living state, produce colours, and hence their substances, if deprived of colours, can the more readily re-assume them.

[1] Verklärung, literally _clarification_.