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CHAP. IV.

ON THE PHYSICAL CAUSES OF COLOURS, &c.

"The mind of Leonardo was, however, too active and capacious to be contented solely with the practical part of his art; nor could it submit to receive as principles, conclusions, though confirmed by experience, without first tracing them to their source, and investigating their causes, and the several circumstances on which they depended."

HAWKINS'S LIFE OF DA VINCI, p. 19.

IN treating of pigments, there is no speculation more natural than the inquiry, What are the physical causes of colour in general? We shall not therefore pass it over unnoticed, though it is a question of undoubted difficulty, which has never been satisfactorily answered; being perhaps too abstract and elementary to admit of sensible demonstration, and therefore incapable of popular solution. Hence the theory of colours has varied with the changes of philosophy, and the fashions of the mind.

According to common apprehension, colours are inherent or substantial qualities of bodies, or material things.* The more speculative have ascribed them to sense or vision only, regarding them as intellectual affections through the organs of sight; while a third class of inquirers has assigned them variously a medial station between vision and coloured objects; viz. in light.

Each of these doctrines has been sanctioned by great authorities, and they have given birth to the various hypotheses upon which the theories of colours have been founded and compounded; but, however various and ingenious these theories may have been, none has hitherto fulfilled the strict requisitions of reason or experiment.

It is probable, and perhaps demonstrable, that colours in the abstract belong neither exclusively to the object, the subject, nor the medium, but

* It may perhaps be ultimately found, in this as in other cases, that nature does not play the fool with our senses, but that the last accomplishments of science coincide with common apprehension.

refer in equal relation to them all; while they are capable of being regarded in either respect; physically regarded, they are material objects,metaphysically, they are intellectual impressions,—and, in an æsthetical sense, they are sensible representations of material objects to the mind.

Our present question is however purely physical, or that of the natural philosopher, in which view colours are either INHERENT, as belonging to their passive objects, as in pigments, &c. or TRANSIENT, as belonging to their concurrent medium, as in light, ocular spectra, &c. And in both they are here to be regarded as material and of the same nature; but the colours of light being the most pure, simple, and elementary, have afforded the most usual and elegible basis for this inquiry.

Yet sensible LIGHT is not a simple substance, but an effect of the concurrence of two elementary powers; one of which is the active principle of light; the other, passive or re-active, and to be regarded as the principle of shade, or darkness, the first, coincident if not identical with the oxygen of the chemist; the other, with hydrogen;* and, however exceptionable this may be to those who have been accustomed to regard darkness as a mere privation of light, yet as respects the artist, a principle of darkness, blackness, and shade, is as essential as is a principle of light.

Accordingly the sunbeam, as it arrives to us is a compound of these elements of light and shade, and it may be analyzed by refraction, and in other ways, into oxydizing or whitening rays, and hydrogenizing or blackening rays; and at the same time into others that are variously compounded of these, and variously coloured.

Light hence appears, as before remarked, to be in the sunbeam the effect of the concurrence or conjugation of two æthereal, electrical, or elementary substances or powers; the one, an agent, of which the sun appears to be the fountain or source; the other, a re-agent, existing in planetary or atmospheric space, analogous to shade ;-if so, the sun's light is a species of

* We adopt these terms for the two principles of light, not for their fitness, but because they have already been used in a similar elementary sense by the chemists;-otherwise either electrogen and thamogen, or phosphogen and sciogen, were more analogous terms; and it might be well that natural philosophers should agree upon general appellations for the two opposed or concurrent principles of light,—of electricity,—of galvanism,—of magnetism, and of every chemical or physical elementary science, since there can now be little doubt of their original identity; the experimental demonstration of which, and of our physical rationale of colours, is beside our present purpose, and pregnant with materials for volumes.

oxidation or combustion, a sort of flame attended by sensible or latent heat.

Light has nevertheless been considered, equally by the common and philosophic observers, as single or simple, and not as containing in itself any antagonist principles, but as having merely intension and remission.

Newton was the first who taught to regard the sunbeam as a compound of rays of various powers and colours, but still he regarded it singly, and its heat as accidental. Subsequent investigation has shown that his analysis was chemically defective ;—he erred also in regarding light as a compound of heterogeneous rays, of colours and powers essentially different; and in other respects, important to art on account of the great authority of his

name.

Scheele and others have demonstrated rays of an invisible kind, accompanying the colours of the sunbeam in the prismatic spectrum, which have been denominated variously deoxidizing, chemical, and phlogistic, or hydrogenizing rays,† and prove the existence of a tenebrous or dark principle, by which light is modified. Herschel has also investigated the calorific rays, or heat of the sunbeam; but these are to be regarded, with Newton, rather as accidental, or an effect, than as a constituent or principle of light; thus we have real and demonstrable principles in place of hypothesis, upon which to explain the various phenomena of light and colours.

We may therefore regard the transient colours of refracted light, and also light itself, as OXIDES OF HYDROGEN, produced by a species of combustion, attended by heat or caloric, as observed in the sunbeam and prismatic spectrum.

So also are the inherent colours of solids and liquids to be regarded upon the same analogy as oxides of hydrogen, or, what is the same, as of oxygen united with a phlogistic or inflammable principle. And thus the physical cause of all colours is to be explained upon the same elementary principle or reasoning. All substances too, whether solid, liquid, or elastic, are attractive or repulsive of oxygen and hydrogen,—of one or both, or they are neutral; and all substances are coloured. Hence the affinities of light determine it either to be wholly or partially reflected, transmitted, or refracted, or to enter into chemical combination with material substances.

See Exp. 1. 11. and particularly Exp. XIII. Chap. xxvI.

-

+ See Crell's "Journal," vol. III. p. 202. and Scheele's " Essays," p. 206.

From the foregoing constitution and properties of light, and a wide experimental induction, we infer the following propositions :

1. Neutral substances, or such as are in a state of indifference, neither attractive nor repulsive of the principles of light, are transmissive, or TRANSPARENT and ACHROMATIC, or colourless.

2. Substances entirely repulsive or reflective of the oxygenous and hydrogenous principles of light, are WHITE and OPAQUE.

3. Substances entirely attractive, or absorbent of, or having entire affinity for both principles of light, are BLACK and OPAQUE.

4. Substances having partial and equal affinities attractive or repulsive for both principles of light, according to the proportions in which they constitute light, are partially transparent or opaque; i. e. SEMI-PELLUCID and COLOURLESS, or grey.

5. If substances have unequal affinities for these oxygenous and hydrogenous principles, they are COLOURED and transparent, or opaque, according to the above conditions.

6. If, in consequence of this unequal affinity, a substance reflect, refract, or transmit light with one proportion of the hydrogenous principle in defect, it will be YELLOW-if with a second proportion, less deficient, it will be REDand if with a third proportion, but in excess, it will be BLUE; and of proportions intermediate, or compounded of these, will be constituted the secondary and intermediate colours, &c.

The relative proportions in which the primary colours combine in light, &c. in an achromatic or colourless state, as determined by the Metrochrome and denoted by the Scale of Equivalents, is approximately three of yellow, five of red, and eight of blue; † and since the two first belong to one extreme of the prismatic spectrum wherein the hydrogenous principle is in defect, and together amount to eight, and the last belonging to the other extreme wherein the hydrogenous principle is in excess, is also eight, it appears that the two principles of light are equal and complementary powers.

* Hitherto we have had no physical or rational explanation of transparency: every mechanical arrangement of parts is quite insufficient to account for this phenomenon. Transparency and opacity are entirely relative, there being no substance absolutely transparent or opaque. Glass and adamant reflect, and gold transmits, light and colours.

+ Exp. xxvII. Chap. XXVI.

Such in briefness we take to be the chemical constitution and physical causes of light and colours, upon which their chromatic relations and effects. depend; and our doctrine is illustrated and supported by many facts and experiments. Thus, in the oxygenation or oxidisement of metals, which have been not unaptly regarded as compounds of hydrogen, as well as in other inflammables also, the inferior degrees of oxidisement produce blacks, blues, greens, &c., but the higher degrees produce red, yellow, white, &c.; not uniformly indeed, but generally according to the unknown constitution of the bases of these inflammables themselves. So also in the colours of flame arising from hydrogen and other inflammable substances burning in air or oxygen, we observe at the base of the flame, in which the hydrogen abounds, colours tending to blue; and toward the apex of the flame, where it is more oxygenated, its colours tend to yellow; between which two colours lie tints abounding in red. Our principle of the evolution and absorption of the oxygenous element goes also to explain those changes of colour arising and disappearing, which take place by changes of temperature, or simply by heating and cooling, wetting or drying; changes which take place in many pigments, sympathetic inks, &c.

So again in the general and more permanent changes which pigments and colours undergo, oxygen bleaches or fades them, and hydrogen and inflammables deepen or darken them; while light and air, containing both principles, effect both these kinds of change variously, according to affinities already spoken of. The colours of all organic bodies, even to the plumage of birds and insects, depend upon the same principles, and we have found the same chemical effects uniformly in each of these subjects.†

Upon the same principles may be easily explained the production or evolution of the transient colours of refracted light, &c. Thus oxygen and hydrogen, having different affinities or activities in the luminous compound, are unequally affected or resisted in passing through transparent bodies, according to their various constitution; and consequently they are unequally refracted,—the oxygenous or more active principle being less so than the

* Davy's "Elements."

+ Indeed the first principles of light, in an extreme chemical view, seem to be the elements of all material things under different denominations, in remarkable accordance with the first work of creation, as recorded in Genesis c. I. vss. 2, 3, and 16, in which darkness and light were as principles before the sun was created.

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