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

This beautiful blue is to be arrested if the steel is suddenly taken out of the heat and buried in ashes. The blue steel works are produced in this way. If, again, the steel is held longer over the fire, it soon becomes a light blue, and so it remains.

475.

These colours pass like a breath over the plate of steel; each seems to fly before the other, but, in reality, each successive hue is constantly developed from the preceding one.

476.

If we hold a penknife in the flame of a light, a coloured stripe will appear across the blade. The portion of the stripe which was nearest to the flame is light blue; this melts into blue-red; the red is in the centre; then follow yellow-red and yellow.

477.

This phenomenon is deducible from the preceding ones; for the portion of the blade next the handle is less heated than the end which is in the flame, and thus all the colours which in other cases exhibited themselves in succession, must here appear at once, and may thus be permanently preserved.

478.

Robert Boyle gives this succession of colours as follows:--"A florido flavo ad flavum saturum et rubescentem (quem artifices sanguineum vocant) inde ad languidum, postea ad saturiorem cyaneum." This would be quite correct if the words "languidus" and "saturior" were to change places. How far the observation is correct, that the different colours have a relation to the degree of temper which the metal afterwards acquires, we leave to others to decide. The colours are here only indications of the different degrees of heat.--Note R.

479.

When lead is calcined, the surface is first greyish. This greyish powder, with greater heat, becomes yellow, and then orange. Silver, too, exhibits colours when heated; the fracture of silver in the process of refining belongs to the same class of examples. When metallic glasses melt, colours in like manner appear on the surface.

480.

Seventh condition.--When the surface of glass becomes decomposed. The accidental opacity (blindwerden) of glass has been already noticed: the term (blindwerden) is employed to denote that the surface of the glass is so affected as to appear dim to us.

481.

White glass becomes "blind" soonest; cast, and afterwards polished glass is also liable to be so affected; the bluish less, the green least.

482.

Of the two sides of a plate of glass one is called the mirror side; it is that which in the oven lies uppermost, on which one may observe roundish elevations: it is smoother than the other, which is undermost in the oven, and on which scratches may be sometimes observed. On this account the mirror side is placed facing the interior of rooms, because it is less affected by the moisture adhering to it from within, than the other would be, and the glass is thus less liable to become "blind."

483.

This half-opacity or dimness of the glass assumes by degrees an appearance of colour which may become very vivid, and in which perhaps a certain succession, or otherwise regular order, might be discovered.

484.

Having thus traced the physical colours from their simplest effects to the present instances, where these fleeting appearances are found to be fixed in bodies, we are, in fact, arrived at the point where the chemical colours begin; nay, we have in some sort already passed those limits; a circumstance which may excite a favourable prejudice for the consistency of our statement. By way of conclusion to this part of our inquiry, we subjoin a general observation, which may not be without its bearing on the common connecting principle of the phenomena that have been adduced.

485.

The colouring of steel and the appearances analogous to it, might perhaps be easily deduced from the doctrine of the semi-opaque mediums. Polished steel reflects light powerfully: we may consider the colour produced by the heat as a slight degree of dimness: hence a bright yellow must immediately appear; this, as the dimness increases, must still appear deeper, more condensed, and redder, and at last pure and ruby-red. The colour has now reached the extreme point of depth, and if we suppose the same degree of semi-opacity still to continue, the dimness would now spread itself over a dark ground, first producing a violet, then a dark-blue, and at last a light-blue, and thus complete the series of the appearances.

We will not assert that this mode of explanation will suffice in all cases; our object is rather to point out the road by which the all-comprehensive formula, the very key of the enigma, may be at last discovered.--Note S.

PART III.

CHEMICAL COLOURS.

486.