Dictionarium polygraphicum. Colours of natural bodies.

Dictionarium Polygraphicum:
Or, The Whole Body of Arts Regularly Digested.
Vol I.
London: Printed for C. Hitch and C. Davis in Pater-noster Row, and S. Austen in St. Paul's Church Yard. MDCCXXXV.
1735
Bodies only appear of different Colours, as their surfaces are dispos'd to reflect rays of this or that Colour alone, or of this or that Colour more abundantly than any other; hence bodies appear of that Colour which arises from the mixture of the reflected rays.

All natural bodies confist of very thin, transparent lamellæ which if they be so dispos'd in regard to each other, as that there happen no reflections or refractions in their interstices, those bodies become pellucid or transparent, but if their intervals be so large, and those are fill'd with such matter; or so empty (in respect to the density of the parts themselves) as that there happen a number of reflections and refractions within the body, the body in that case becomes opake.

The rays which are not reflected from an opake body penetrate into it, and there suffering innumerable reflections and re fractions at length unite themselves, to the particles of the body itself.

Hence an opake body grows hot the sooner, as it reflects light less copiously; whence we understand the reason why a white body, which reflects almost all the rays that strike upon it, heats much more slowly than a black one, which reflects scarce any.

In order to determine that constitution of the surface of bodies, wherein their Colour depends; it must be observ'd, that the smallest corpuscles or first particles of which surfaces are made up, are most thin and transparent, and separated by a medium of a different density from the particles themselves.

So that in the surface of every colour’d body are innumerable fmaller, thin plates, corresponding to those of bubbles; wherefore, what has been observ'd of those may be understood of these.

Hence it is gathered, that the Colour of a body depends upon the density and thickness of the parts of the body, between the pores of the surface, that the colour is more vivid and homogeneous, as the parts are thinner; that cæteris paribus the said parts are the thickest when the body is red, and the thinnest when violet; that the parts of bodies are usually much denser, than the medium contain’d in their interstices; but that in the tails of peacocks, some silks, and generally, in all bodies whose Colour varies, according to the situation of the eye, it is less; and that the Colour of a body is the less vivid to the eye, as it has a denser medium within its pores.

Now of the several opake bodies, those consisting of the thinnest lamellæ are black, those consisting either of the thickest lamellæ, or of lamellæ very different from each other in thickness, and on that account fitted to reflect all Colours, as the froth of water, &c. are white.

Those again consisting of lamellæ, most of which are of some intermediate thickness, are blue, green, yellow or red, inasmuch as they reflect the rays of that particular Colour, much more copiously than that of any other Colour; most of which last they either absorb or extinguish, by intercepting them, or else they transmit light.

Hence it is, that some liquors, v. g. an infusion of lignum nephriticum appear red or yellow, if view’d by reflected light, and blue by transmitted light; and leaves of gold yellow if view’d by reflected light, but green or blue in the latter.

To this we may add, that some of those powders, us’d by painters, have their Colour chang'd by being very finely grounds; which must be caus’d by the commination, or breaking of their small parts into others still smaller, just as a lamella has its Colour altered by altering its thickness.

In short, those odd phænomena arising from the mixture of liquors of different Colours, can no way be better accounted for, than from the various actions of the saline, &c. corpuscles of one liquor with the coloured corpuscles of another; if they unite, the mass will either swell or shrink, and thereby its density will be altered; if they ferment, the size of the particles may be dimi nished, and thereby the coloured liquor may become transparent; if they coagulate, an opake liquor may be produc’d of two transparent ones.

Hence it is easy to conceive, why a colour'd liquor in a glass of a conical figure, plac'd between the eye and the light, appears of a different Colour in different parts of the vessel; there being more and more rays intercepted, as they pass through a longer or shorter section of the vessel, till at the base they are all intercepted, and none seen but those that are reflected.

From the various Colours of natural bodies, sir Isaac Newton observes, the bigness of their component parts may be estimated; for that the parts of bodies do properly exhibit the same Colour with a lamella of equal thickness provided the density in both be the same.