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. i. ii. and particularly Exp. xiii. Chap xxvi. 4to.ed of this work.

† 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 homogeneous 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.

* Hitherto we have had neither physical nor 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; for glass and adamant reflect, and gold transmits, light and colour: and, as effects of vision, we have no doubt of their being chemico-mechanieal qualities; transparency being indicative of perfect chemical with mechanical union, which are disturbed and wanting in all cases of opacity.

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 red - and 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, etc.

The relative proportions in which the primary colours combine in light, etc. in an achromatic or colourless state, as determined by the Metro-chrome, and denoted by the Scale of Chromatic Equivalents given in the preceding plate, 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. 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 the innumerable dark bands or lines mixed with the light of the solar spectrum, and abounding most at its cold extreme, discovered by Fraun-hof'er, indicate the two prime elements of light,* 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, etc, but the higher degrees produce red, yellow, white, etc.; 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, etc.

* See our Exp. xxvii. Chap. xxvi. 4to. edit.

* Exley's "Optics," p. 31. † Davy's "Elements."

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, etc. 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 hydrogenous, the refraction of hydrogen being about six and a half times more than that of oxygen when they are of equal densities;* and being thus variously dispersed and compounded, they produce colours, as we see, in passing through prisms and lenses,† etc.

* Indeed the first principles of light, under different denominations, in an extreme chemical view, seem to be the elements of all material tilings, in remarkable accordance with the first work of creation, as recorded in Genesis, i. 2, 3, and 16, in which darkness and light were as principles before the sun was created; and with Ecclesiastic us, xvi. 16, " He separated his light from the darkness with an adamant;" and, Psalm ixviii. 34, "God's strength is in the skies [Shahakim, or alters in conflict]": and, with the antient doctrine of Zareta the Chaldean, who held that Light and Darkness were the father and mother who engendered all things. - D'Argens, vol. i, p. 195. This doctrine has also been extensively illustrated according to the latest facts and principles of modern chemistry, in Kyan'a "Elements of Light, and their identity with those of matter radiant and fixed."