4. Red, purple, and blue flowers, were also digested in spirit of wine, all of which yielded their colouring matter to the spirit, and became white by being deprived of it. From most of these flowers, however, the spirit acquired either no tinge at all, or only a very faint one; but when acidulated, it became red, and by the addition of an alkali appeared blue, purple, or green, according to the quantity of alkali, and the nature of the infusion. In these states, all of them, when viewed by transmitted light, or poured upon a white ground, showed their colours, but universally appeared black by reflection.

5. Red, purple, and blue flowers, were digested in water slightly acidulated with nitrous acid. Thus, red infusions were obtained, which, by saturation with sea-salt, might be preserved for many years.

6. The same liquors were changed green, blue, or purple, by the addition of an alkali; but here the case was the same as before; all of them yielding vivid colours by transmission, but none by reflection. In making this experiment, care must be taken to add the alkali very gradually; for if too much is put in at once to the red liquor, the immediate colours between the red and the green will be wanting. To half an ounce of the red infusion it is proper to add, at once, only the smallest quantity that can be taken upon the point of a pen; repeating this addition slowly, until each of the colours be produced.

7. The flowers, after having been repeatedly macerated in acidulated water, lost their colouring matter, and became white.

8. Yellow flowers also communicated their colours to water and to spirit of wine. The infusions and tinctures of these flowers were subjected to the same experiments as had been employed in the examination of the liquors already mentioned; and appeared yellow by transmitted light, but did not reflect any colour.

9. White paper, linen, &c. may be tinged of any of these colours, by dipping them in the infusions; and the consideration of the manner in which the colours are imparted to the linen, affords much insight into the manner in which natural colours are produced. It has already been observed, that, when the colouring matter of plants is extracted from them, the solid fibrous parts, thus divested of their covering, display their natural whiteness. White linen, paper, &c. are formed of such fibrous vegetable matter; which is bleached by dissolving and detaching the heterogeneous colouring particles. When these are dyed or painted with vegetable colours, it is evident that they do not differ in their manner of acting on the rays of light from natural vegetable bodies; both yielding their colours by transmitting, through the transparent coloured matter, the light which is reflected from the white ground. This white matter frequently exists, without any considerable mixture, in plants, while they are in a state of vegetation; as cotton, white flowers, the pith, wood, seeds, roots, and other parts of several kinds of vegetables. When decayed trees, &c. have been long exposed to the atmosphere, their coloured juices are sometimes so perfectly extracted, that the fibres appear white. This white matter is not distinct from the vegetable earth to which plants are reduced by burning. Mr Delaval has rendered ashes intensely white, by carefully calcining them, and afterwards grinding with a small proportion of nitre, and exposing them to such a degree of white heat as would cause the nitre delagrate with the remaining quantity of phlogiston. Lastly, the ashes were digested with muriatic acid, in order to dissolve the ferruginous matter diffused through them, and repeatedly washing the remainder in water. Mixing ashes thus purified with borax, and applying a vitrifying heat, an opaque enamel is obtained, remarkable for its whiteness.

Hence it appears, that the earth which forms the substance of plants is white, and separable from that earth of substance which gives to earth its peculiar colour; that plants, whenever it is pure and unmixed, or diffused through pure, in colourless media, it shows its native whiteness; and is then that the only vegetable matter endowed with a reflective reflects the power. It may be discovered, however, by other light means than that of burning: thus, roles may be whitened by exposing them to the vapour of burning sulphur; an effect which cannot be attributed to the sulphuric acid, but to the phlogiston contained in that vapour. This was proved to be the case, by exposing several kinds of red, and purple flowers to the phlogistic vapour issuing from hepar sulphuris; and by this every one of them was whitened; their colour being afterwards restored by the addition of an acid either mineral or vegetable.

Thus (says Mr Delaval) it appears, that the colouring matter of the flowers is not discharged or removed, but only dissolved by carbonic acid; and carbonic thereby divided into particles too minute to exhibit any colour. In this state, together with the vegetable juice in which they are diffused, they form a colourless transparent covering, through which the white matter of the flowers is seen untinged. The colouring particles of plants consist principally of inflammable matter, and their solubility in carbonic acid, and union with it, are analogous to the action of other inflammable bodies upon each other. Thus, rather dissolves all essential and expressed oils, animal empyreumatic oils, and resins. Sulphur, camphor, and almost all substances abounding in phlogiston, are soluble in oils, ardent spirits, or other inflammable menstrua. The manner in which the red colour of vegetable flowers is restored, appears to be explicable from known chemical laws. When acids are applied to the whitened flowers, they unite with the phlogiston which the sulphur had communicated, and disengage it from the colouring particles; which, being thus extricated, resume their original magnitude and hue. A change of the same kind is also produced by fixed alkali, which, like the acids, has a strong attraction for phlogiston, always changes the whitened flowers to a blue, purple, or green colour.

In like manner, the action of the rays of light operates upon coloured bodies. Thus, dyed silk, or other substances of that kind, when exposed to the light of the sun's fun's sun's light, are deprived of their colour in every part on which the rays are allowed to act; whilst those preserve their colour which are defended from the light by the folds of the cloth, or intervention of any opaque body. The colours thus impaired, may be restored, if acids are applied while the injury is recent; but they are afterwards apt to fly off, on account of that volatility which is constantly imparted by inflammable matter to any other with which it is united."

Our author now proceeds, at considerable length, to prove the identity of the solar light and carbonic acid; but as recent experiments have shown that these two are essentially distinct, we omit his argumentation upon this head. The error of his theory in this respect, however, does not in the least affect the doctrine concerning colours above laid down; on the contrary, the latest experiments have determined, that carbonic acid in its gaseous form, viz. that of common charcoal, manifests a surprising power of whitening various substances; which, according to Mr Delaval's theory, proceeds from the power it has of dissolving the colouring matter with which they are impregnated. This solvent power, according to our author, is manifest in many other instances besides those already mentioned. Silk is whitened by the carbonated vapours of sulphur; and this operation does not appear to differ from the change effected on flowers by the same vapour. The light of the sun is found to be a necessary and essential agent in bleaching linen, wax, and various other substances; some part of the colouring matter which impairs the whiteness of these bodies not yielding to any other solvent. Red flowers are whitened by the electric spark, of whose inflammable nature we cannot entertain the least doubt; for the spark itself is a bright flame, and yields the same smell which all other carbonated matters impart. The electric spark, in like manner, changes the blue infusion of turpentine to red (v). The effects which it produces on the turpentine, and on red flowers, do not differ from each other, except in degree only. For when vegetable matter is dissolved, it is changed from blue to red; and, when farther dissolved, it is divided into particles too minute to exhibit any colour.

Solutions effected by means of phlogiston frequently are wrongly attributed to the operation of supposed acid menstrua, as several kinds of substances are capable of being dissolved indiscriminately both by acids and phlogiston. For the purpose of distinguishing, therefore, in any case between the action of the acid solvents and that of the inflammable menstrua, it is proper to examine the nature of the matter by which either of these principles are furnished. It appears from various chemical processes, that alkalies are rendered mild, and capable of crystallization, in proportion as they are united to carbure. The carbonated alkaline lixivium, when saturated, is perfectly mild; and by a flight evaporation is reduced to a concrete crystalline mass, which does not deliquesce or imbibe the least moisture from the air, and no longer retains any alkaline property. M. Beaume, by an elegant and ingenious experiment, has proved the presence of carbure in mild alkalies, and has shown that their power of crystallizing depends on their union with that principle. He heated in a silver vessel a lixivium of mild alkali, which imparted to the silver a covering or coating of inflammable matter, by which its surface was tarnished and became black. The lixivium was several times poured out of the silver vessel, and after the surface of the metal had been freed from the tarnish, the lixivium was replaced in it, and again heated, by which the tarnish was renewed; and this was repeated till the lixivium no longer communicated any stain to the silver. The causticity of the lixivium was increased in proportion as it imparted its carbure to the silver; and at the end of the process the alkali became perfectly caustic and incapable of crystallizing.

From the preceding experiments (says he) it appears, that the colouring particles of flowers and leaves are soluble in acid, alkaline, and carbonated menstrua. The other parts of vegetables consist of materials similar to those which are contained in their flowers and leaves, and undergo the same changes from the same causes. Having extracted from logwood its colouring particles by repeatedly boiling it in water, the wood was thus deprived of its yellow colour, and assumed a brown hue similar to that of oak-wood. Some pieces of it thus deprived of its colour were then macerated in nitric acid; and after they had undergone the action of that acid, they were washed in a sufficient quantity of water. The wood was thus reduced to whiteness.

Here our author observes, that though most authors who treat of colouring substances describe logwood as affording only a red colour, he was never able to procure any other colour from it than yellow. It imparts yellow and orange colours to distilled water. Other waters extract a red tinge from it by means of the alkali which they contain. These observations are also applicable to the other dyeing woods, kermes, and various other articles of the materia medica. By a similar treatment, fustic wood also lost its colouring matter, and became white.

The results of all the experiments above related are, that the colouring matter of plants does not exhibit any colour by reflection, but by transmission only; that their solid earthy substance is a white matter; and that it is the only part of vegetables which is endowed with a reflective power; that the colours of vegetables are produced by the light reflected from this white matter, and transmitted from thence through the coloured coat or covering which is formed on its surface by the colouring particles; that whenever the colouring matter is either discharged or divided by solution into particles too minute to exhibit any colour, the solid earthy substance is exposed to view, and displays that whiteness which is its distinguishing characteristic.

Mr Delaval next proceeds to examine the coloured parts of animal substances, and finds them exactly similar, with regard to the manner in which the colour animal substances.

(b) This effect of the electric spark is now known to be produced, not by its carbonated nature, but by the generation of an acid. is produced, to the vegetable bodies already treated of. The tinctures and infusions of cochineal and of kermes yield their colours when light is transmitted through them, but show none by reflection. On diluting fresh ox-gall with water, and examining it in the phials already mentioned, that part of it which was in the neck of the phial, and viewed by transmitted light, was yellow; but the anterior surface was black and reflected no colour. Flesh derives its colour entirely from the blood, and when deprived of it, the fibres and vessels are perfectly white; as are likewise the membranes, finesse, and bones, when freed from their aqueous and volatile parts; in which case they are a mere earth, unalterable by fire, and capable of imparting an opaque whiteness to glass.

On examining blood diluted with water in one of the phials formerly described, it transmitted a red colour, and the anterior surface was almost, but not entirely, black; for it received a slight hue of brown from some coagulated particles that were suspended in the liquor. In order to procure blood sufficiently diluted, and at the same time equably and perfectly dissolved, he mixed as much sugar with spirit of salt ammonia as imparted a bright colour to it. The liquor being then viewed in the phial, that part which was contained in the neck, and transmitted the light, appeared of a fine red; but the anterior part reflecting no light, was intensely black. Hence it appears, that the florid red colour of the flesh arises from the light which is reflected from the white fibrous substance, and transmitted back through the red transparent covering which the blood forms on every part of it.

Blood, when recently drawn, does not assume the appearance common to transparent coloured liquors; for these, when too mazy to transmit light from their farther surfaces, always appear black; but blood, when recently drawn, always shows a fine red colour, in whatever way it be viewed. This is occasioned by a white matter diffused through the blood; and which is easily separated from the coagulum, by dividing it after coagulation into a number of thin pieces, and washing in a sufficient quantity of pure water. Thus the water acquires a red colour, and ought to be changed daily. In a few days it will acquire no more tinge; and the remaining masses of the coagulum are no longer red, but white.

In like manner, the red colour of the shells of lobsters, after boiling, is no more than a mere superficial covering spread over the white calcareous earth of which the shells are composed, and may be easily removed from the surface by scraping or filing. Before the application of heat, this superficial covering is much denser, inasmuch that, in some parts of the shell, it appears quite black, being too thick to admit the passage of the light to the shell and back again; but where this transparent blue colour of the unboiled lobster is thinner, it constantly appears like a blue film. In like manner, the colours of the eggs of certain birds are entirely superficial, and may be scraped off, leaving the white calcareous earth exposed to view.

The case is the same with feathers, which owe their colours entirely to a very thin layer of some transparent matter upon a white ground. Our author obtained this by scraping off the superficial colours from certain feathers which were strong enough to bear the operation; and thus separated the coloured layers from the white ground on which they had been naturally spread. The lateral fibres of the feathers cannot indeed have their surfaces separated in this manner; but their texture, when viewed by a microscope, seems to indicate, that the colours are produced upon them by no other means than those already related. In the examination of some animal subjects, where the colouring matter could not be separated by chemical means, our author had recourse to mechanical division; but this can only be employed when the principal part of the white substance is unmixed with the coloured coat or covering which is spread upon its surface. All of them, however, by whatever means their colours could be separated, showed that they were produced in the same manner, namely, by the transmission of light from a white ground through a transparent coloured medium.

The coloured substances of the mineral kingdom are of the very numerous, and belong principally to two classes, viz. earths and metals. The former, when pure, are nearly all perfectly white, and their colours arise from carbuncles or metallic mixtures. Calcareous earths, when indurated, constitute marble, and may be tinged with various colours by means of metallic solutions; all which are similar in their nature to the dyes put upon silk, cotton, or linen, and invariably proceed from the same cause, viz. the transmission of light through a very thin and transparent coloured medium. Flints are formed from siliceous earths, and owe their colour to carbon. When sufficiently heated, they are rendered white by the loss of the inflammable matter which produced their colour. When impregnated with metals, they form agates, cornelian, jasper, and coloured crystals. The coloured gems also receive their different hues from metals; and all of them may be imitated by glasses tinged with such carbuncle or metallic matters as enter into the composition of the original substances.

Thus our author concludes, that the coloured earths, gems, &c. exhibit their various tints in the same manner with other substances; viz. by the transmission of light reflected from a white ground. Our author, however, proceeds farther; and affirms, that even the colours of metals themselves are produced in the same manner.

"Gold (says he) exhibits a white light, which is tinged with yellow. I have used this expression, because it appears from experiment that gold reflects a white light, and that its yellow colour is a tinge superadded to its whiteness. The experiment is thus set forth by Sir Isaac Newton. Gold in this light (that is, a beam of white light) appears of the same yellow colour as in day light, but by intercepting at the lens a due quantity of the yellow-making rays, it will appear white like silver, as I have tried; which shows, that its yellowness arises from the excess of the intercepted rays, tinged that whiteness with their colour when they are let pass.

"I have already shown, by numerous experiments, in what manner coloured tinges are produced; and it uniformly appears, from all these experiments, that colours do not arise from reflection, but from transmission only. A solution of silver is pellucid and colourless. A solution of gold transmits yellow, but reflects This metal also, when united with glass, yields no colour by reflection, but by transmission only. All these circumstances seem to indicate, that the yellow colour of gold arises from a yellow transparent matter, which is a constituent part of that metal; that it is equally mixed with the white particles of the gold, and transmits the light which is reflected by them, in like manner as when silver is gilt, or foils are made by covering white metals with transparent colours. But these fictitious coverings are only superficial; whereas the yellow matter of gold is diffused throughout the whole substance of the metal, and appears to envelope and cover each of the white particles. In whatsoever manner the yellow matter of gold is united to its white substance, it exists in a rare state; for it bears only the same proportion to the white particles of the gold as that of the yellow-making rays which were intercepted bear to all the other rays comprised in the white light of the sun.

"Sir Isaac Newton has shown, that when spaces or interstices of bodies are replenished with media of different densities, the bodies are opaque; that those superficies of transparent bodies reflect the greatest quantity of light which intercede media that differ most in their refractive densities; and that the reflections of very thin transparent substances are considerably stronger than those made by the same substances of a greater thickness. Hence the minute portions of air, or of the rarer medium which occupies spaces void of other matter, reflect a vivid white light whenever their surfaces are contiguous to media whose densities differ considerably from their own; so that every small mass of air, or of the rarer medium, which fills the pores or interstices of dense bodies, is a minute white substance. This is manifest in the whiteness of froth, and of all pellucid colourless bodies; such as glass, crystal, or salts, reduced to powder, or otherwise flayed: for in all these substances a white light is reflected from the air or rarer medium which intercede the particles of the denser substances whose interstices they occupy."

From these principles our author takes occasion to explain the reason why the particles of metals, which yield no colour by incident light when suspended in their solvents, are disposed to exhibit colours when separated from them. Hence also we see why opaque white substances are rendered pellucid by being reduced to uniform masses, whose component parts are everywhere nearly of the same density; for as all pellucid substances are rendered opaque and white by the admixture of pellucid colourless media of considerably different densities, they are again deprived of their opacity by extricating these media which kept their particles at a distance from each other: thus froth or snow, when resolved into water, lose their whiteness, and assume their former pellucid appearance. In like manner, by proper fluxes, the opaque white earths are reduced to pellucid colourless glazes; because all reflections are made at the surfaces of bodies differing in density from the ambient medium, and in the confines of equally dense media there is no reflection.

As the oxides of metals are enabled to reflect their colours by the intervention of the particles of air; so, when mixed with oil in the making of paints, they always assume a darker colour, because the excess of the density of oil over that of air forms a sensible difference when comparatively considered with respect to the specific gravity of the rarer metals. From this cause perceptibly less light is reflected from the molecule of oil than from those of air, and consequently the mass appears darker. The case, however, is different with such paints as are formed of the denser metals; as vermillion, minium, &c., for though oil differs very considerably from air in its specific density, yet it also differs very much in this respect from the denser metallic powders; and the molecules of oil which divide their particles act upon the light so strongly, that the reflection occasioned by them cannot be distinguished from those which are caused by rarer media. Hence though we mix vermillion or minium with oil, the colour is not sensibly altered.

This part of our author's theory, however, seems liable to objection: for though it be true, that the oxides of some metals are denser than others, yet that is not the case with all; nor is even the difference of density between oil and the oxides of the heavier metals at all comparable to that between the density of air and oil. Thus, though the oxide of iron may be 10 or 11 times more dense than oil, yet, as the latter is between 500 or 600 times denser than air, the small difference between the oil and metallic oxide ought to be imperceptible. In this respect, indeed, there are considerable differences with regard to the oils employed, which cannot be supposed to arise from the mere circumstance of density. Thus the colour of vermillion, when mixed with turpentine varnish, is much brighter than with linseed-oil; and yet the difference between the densities of linseed-oil and turpentine-varnish is very trifling. The mere action of heat likewise has a surprising effect in this case. Thus the red oxide of iron, called scarlet ochre, by being only heated a certain degree, appears of a very dark purple, refusing its red colour when cold; and this variation may be induced as often as we please by only heating it over the fire in a trowel. In like manner, by gradually heating red-lead, it may be made to assume a most beautiful crimson colour; which growing gradually darker, becomes at last almost quite black. On cooling, if the heat has not been raised too high, it gradually returns through the same shades of colour, until at last it fixes in its original hue. These immense differences in colour cannot by any means be attributed either to the expulsion of air, or to an alteration in density. The fire indeed does certainly expand these oxides as well as other bodies; but as the medium interposed between their particles is thus also expanded, the colour ought at least to remain the same, if not to become lighter, on account of the superior expansion of air to that of metal by the same degree of heat. It would seem, therefore, that the action of the element of fire itself has a considerable share in the production of colours; and indeed its share in the operations of nature is so great, that we might well think it strange if it should be entirely excluded from this.

With regard to semipellucid substances, which appear of one colour by incident, and another by transmitted light, our author likewise endeavours to show that no reflection is made by the coloured matter, but only only by the white or colourless particles. They consist of pellucid media, throughout which white or colourless opaque particles are dispersed. The latter are disposed at such distances from each other, that some of the incident rays of light are capable of passing through the intervals which intercede them, and thus are transmitted through the semipellucid mass. Some sorts of rays penetrate through such masses, while others which differ from them in their refrangibility are reflected by the white or colourless particles; and from thence are transmitted through the pellucid part of the medium which intervenes between the reflecting particles and the anterior surface of the mass. On the same principle our author explains the blue colour of the sky, the green colour of the sea, and other natural phenomena; and from his numerous experiments on this subject at last concludes, "that the power by which the several rays of light are transmitted through different media is inherent in the particles themselves, and therefore is not confined to the surfaces of such media.

For if the transmissive force was exerted at the surface only, the thinner plates of coloured substances would act upon the rays as powerfully as thicker masses. But it appears from experiment, that in proportion as the rays pass through different thicknesses of coloured media, they exhibit colours differing not only in degree, but frequently in species also.

"The sun's light, by which bodies are illuminated, consists of all the rays of which a white light is compounded. These rays, in their entire and undivided state, are incident upon the opaque particles of semipellucid substances, and upon the colouring particles of transparent-coloured substances, whenever these media are exposed to the light. When the rays accede to the opaque particles of semipellucid substances, some sorts of them are reflected back from the anterior surface of those particles; the other sorts of rays, which are not reflected back, are diverted from the direction which is opposite to the anterior surface of the opaque particles, and passing through the intervals between the particles, are transmitted through the mass.

"When the rays are incident upon the particles of transparent coloured bodies, none of them are reflected back; because the colouring particles are not endowed with any reflective power; but some of the rays are either stopped at the anterior surface of the particles, or are diverted into such directions as render them incapable of passing towards the further side of the mass; and consequently such rays cannot be transmitted. The rays which are not thus intercepted or dispersed, are transmitted in the same manner as those which pass through semipellucid media. Thus it is evident, that the coloured rays which are transmitted through semipellucid substances are reflected by the opaque particles; and those which are transmitted through transparent-coloured substances are reflected by the colouring particles. From the preceding observations likewise it appears, that the particles of coloured media reflect the several sorts of rays according to the several sizes and densities of the particles; also in proportion to the inflammability of the media which owe their colour to them; and it is manifest that the transmission of coloured rays depends upon their reflection. All these observations are conformable to Sir Isaac Newton's doctrine, that the rays of light are reflected, refracted, and inflected, by one and the same principle acting variously in various circumstances."

The most remarkable part of Mr Delaval's doctrine is that concerning the metals; for the better understanding of which we shall premise a short abstract of his general doctrine concerning white bodies, and the manner in which light is reflected by them. "All the earths (he observes), which in their natural state are of manner in a pure white, constitute transparent colourless media which light when vitrified with proper fluxes, or when dissolved from white substances menstrua; and the saline masses obtainable from their solutions are transparent and colourless, while they retain the water which is essential to their crystallization, and are not flawed or reduced to powder; but after their pores and interfaces are opened in such a manner as to admit the air, they become then white and opaque by the entrance of that rare medium. The earthly particles which form the solid parts of bodies generally exceed the other in density; consequently these particles, when contiguous to the rare media already mentioned, must reflect the rays of light with a force proportionate to their density. The reflective power of bodies does not depend merely upon their excess of density, but upon their difference of density with respect to the surrounding media. Transparent colourless particles, whose density is greatly inferior to that of the media they come between, also powerfully reflect all sorts of rays, and thereby become white. Of this kind are the air or other rare fluids which occupy the interfaces of liquids; and in general of all denser media into whose interfaces such rare particles are admitted.

"Hence we may conclude, that white opaque bodies are constituted by the union or contiguity of two or more transparent colourless media differing considerably from each other in their reflective powers. Of these substances we have examples in froth, emulsions, or other imperfect combinations of pellucid liquors, milk, snow, calcined or pulverized fats, glaas or crystal reduced to powder, white earths, paper, linen, and even those metals which are called white by mineralogists and chemists: for the metals just mentioned do not appear white unless their surfaces be rough; as in that case only there are interfaces on their surfaces sufficient to admit the air, and thus make a reflection of a white and vivid light.

"But the polished surfaces of metallic mirrors reflect the incident rays equably and regularly, according to their several angles of incidence; so that the reflected rays do not interfere with each other, but remain separate and unmixed, and therefore distinctly exhibit their several colours. Hence it is evident, that white surfaces cannot act upon the light as mirrors; because all the rays which are reflected from them are blended in a promiscuous and disorderly manner.

"The above-mentioned phenomena give much insight into the nature and cause of opacity: as they cause clearly how, that even the rarest transparent colourless substances, when their surfaces are adjacent to media differing greatly from them in refractive power, may thereby acquire a perfect opacity, and may assume a resplendency and hue so similar to that of white metals, that the rarer pellucid substances cannot by the sight..."

Light be distinguished from the dense opaque metals. And this similarity to the surfaces of metals occurs in the rare pellucid substances, not only when, from the roughness of their surfaces, they resemble unpolished metals in whiteness, but also when, from their smoothness, they resemble the polished surfaces of metals.

"Metals seem to consist entirely of transparent matter, and to derive their apparent opacity and lustre solely from the copious reflection of light from their surfaces. The analogy between the metals and transparent media, as far as respects their optical properties, will appear from the following considerations."

1. All metals dissolved in their proper menstrua are transparent. 2. By the union of two or more transparent media, substances are constituted which are similar to metals in their opacity and lustre, as plumbago and marcasites. 3. The transparent substances of metals, as well as those of minerals, by their union with carbons, acquire their strong reflective powers from which their lustre and opacity arise. 4. The surfaces of pellucid media, such as glass or water, assume a metallic appearance, when by their smoothness, difference of density with respect to the contiguous media, or any other cause, they are disposed copiously to reflect the light.

From all these considerations it is evident, that opaque substances are constituted by the union or contiguity of transparent colourless media, differing from one another in their reflective powers; and that, when the common surface, which comes between such media, is plane, equal, and smooth, it reflects the incident rays equally and regularly as a mirror; but when the surface is rough and unequal, or divided into minute particles, it reflects the incident rays irregularly and promiscuously in different directions, and consequently appears white."

From all these experiments we can only conclude, that the theory of colours seems not yet to be determined with certainty; and very formidable, perhaps unanswerable, objections might be brought against every hypothesis on this subject that hath been invented. The discoveries of Sir Isaac Newton, however, are sufficient to justify the following

APHORISMS.

1. All the colours in nature proceed from the rays of light. 2. There are seven primary colours; which are red, orange, yellow, green, blue, indigo, and violet. 3. Every ray of light may be separated into the seven primary colours. 4. The rays of light in passing through the same medium have different degrees of refrangibility. 5. The difference in the colours of light arises from its different refrangibility: that which is the least refrangible producing red; and that which is the most refrangible, violet. 6. By compounding any two of the primary colours, as red and yellow, or yellow and blue, the intermediate colour, as orange or green, may be produced. 7. The colours of bodies arise from their dispositions to reflect one sort of rays, and to absorb the other; those that reflect the least refrangible rays appearing red; and those that reflect the most refrangible, violet. 8. Such bodies as reflect two or more sorts of rays appear of various colours. 9. The whiteness of bodies arises from their disposition to reflect all the rays of light promiscuously. 10. The blackness of bodies proceeds from their incapacity to reflect any of the rays of light (c).

Entertaining Experiments, founded on the preceding Principles.

I. Out of a single colourless ray of light to produce seven other rays, which shall paint, on a white body, the seven primary colours of nature.

Procure from an optician a large glass prism DEF, well polished, two of whose sides must contain an angle of about sixty-four degrees. Make a room quite dark, and in the window shutter AB, cut a round hole, about one-third of an inch in diameter, at C, through which a ray of light LI passing, falls on the prism DEF; by that it is refracted out of the direction IT, in which it would have proceeded into another GH; and, falling on the paper MNSX, will there form an oblong spectrum PQ, whose ends will be semicircular, and its sides straight; and if the distance of the prism from the paper be about eighteen feet, it will be ten inches long, and two inches wide. This spectrum will exhibit all the primary colours; the rays between P and V, which are the most refracted, will paint a deep violet; those between V and I, indigo; those between I and B, blue; those between B and G, green; those between G and Y, yellow; those between Y and O, orange; and those between O and R, being the least refracted, an intense red. The colours between these spaces will not be everywhere equally intense, but will incline to the neighbouring colour; thus the part of the orange next to R will incline to a red, that next to Y to a yellow; and so of the rest.

II. From two or more of the primary colours, to compose others that shall, in appearance, resemble those of the former.

By mixing the two homogeneous colours red and yellow, an orange will be produced, similar in appearance to that in the series of primary colours; but the light of the one being homogeneous, and that of the other heterogeneous, if the former be viewed through a prism it will remain unaltered, but the other will be resolved into its component colours, red and yellow. In like manner, other contiguous homogeneous colours may compound new colours; as by mixing yellow and green, a colour between them is formed; and if blue be added, there will appear a green, that is the middle colour.

(c) From hence it arises, that black bodies, when exposed to the sun, become sooner heated than all others. colour of those three. For the yellow and blue, if they are equal in quantity, will draw the intermediate green equally toward them, and keep it, as it were, in equilibrium, that it verge not more to the one than to the other. To this compound green there may be added some red and violet; and yet the green will not immediately cease, but grow less vivid; till by adding more red and violet it will become more diluted; and at last, by the prevalence of the added colours, it will be overcome, and turned into some anomalous colour.

If the sun's white, composed of all kinds of rays, be added to any homogeneous colour, that colour will not vanish, nor change its species, but be diluted; and by adding more white, it will become continually more diluted. Lastly, if red and violet be mixed, there will be generated, according to their various proportions, various purples, such as are not like, in appearance, to the colour of any homogeneous light; and of these purples, mixed with blue and yellow, other new colours may be composed.

III. Out of three of the primary colours, red, yellow, and blue, to produce all the other prismatic colours, and all that are intermediate to them.

Provide three panes of glass of about five inches square; and divide each of them, by parallel lines, into five equal parts. Take three sheets of very thin paper; which you must paint, lightly, one blue, another yellow, and the third red (n). Then paste on one of the glasses five pieces of the red paper, one of which must cover the whole glass, the second only the four lower divisions, the third the three lower, the fourth the two lowest, and the fifth the last division only. On the other glasses five pieces of the blue and yellow papers must be pasted in like manner. You must also have a box of about six inches long, and the same depth and width as the glasses; it must be black on the inside; let one end be quite open, and in the opposite end there must be a hole large enough to see the glasses completely. It must also open at the top, that the glasses may be placed in it conveniently.

When you have put any one of these glasses in the box, and the open end is turned toward the sun, you will see five distinct shades of the colour it contains. If you place the blue and yellow glasses together, in a similar direction, you will see five shades of green distinctly formed. When the blue and red glasses are placed, a bright violet will be produced; and by the red and yellow, the several shades of orange.

If, instead of placing these glasses in a similar position, you place the side AB of the yellow glass against the side BD of the blue, you will see all the various greens that are produced by nature (e); if the blue and red glasses be placed in that manner, you will have all the possible varieties of purples, violets, &c.; and, lastly, if the red and orange glasses be so placed, there will be all the intermediate colours, as the marigold, aurora, &c.

IV. By means of the three primary colours, red, yellow, and blue, together with light and shade, to produce all the gradations of the prismatic colours.

On seven square panes of glass, paste papers that are painted with the seven prismatic colours, in the same manner as in the last experiment. The colours for the orange, green, indigo, and violet, may be made by mixing the other three. Then with bistre (f), well diluted, shade a sheet of very thin paper, by laying it light on both its sides. With pieces of this paper cover four-fifths of a glass, of the same size with the others, by laying one piece on the four lowest divisions, another on the three lowest, a third on the two lowest, and the fourth on the lowest division only, and leaving the top division quite uncovered. When one of the coloured glasses is placed in the box, together with the glasses of shades, so that the side AB of the one be applied to the side BC of the other, as in fig. 3, the several gradations of colours will appear shaded in the same manner as a drapery judiciously painted with that colour.

It is on this principle that certain French artists have proceeded in their endeavours to imitate, by designs printed in colours, paintings in oil: which they do by four plates of the same size, on each of which is engraved the same design. One of these contains all the shades that are to be represented, and which are painted either black or with a dark gray. One of the three other plates is coloured with blue, another with red, and the third with yellow; each of them being engraved in those parts only which are to represent that colour (g); and the engraving is either stronger or weaker, in proportion to the tone of colour that is to be represented (h).

These four plates are then passed alternately under the

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(n) Water-colours must be used for this purpose: the blue may be that of Prussia, and very bright; the red, carmine; and the yellow, gamboge, mixed with a little saffron. These colours must be laid very light and even, on both sides of the paper.

(e) In the first position of the glasses, the quantity of blue and yellow being equal, the same sort of green was constantly visible; but by thus inverting the glasses, the quantity of the colours being constantly unequal, a very pleasing variety of tints is produced.

(f) The bistre here used must be made of root, not that in stone.

(g) When a red drapery is required, it is engraved on the plate assigned to that colour; and so of yellow and blue: but if one of the other colours be wanting, suppose violet, it must be engraved on those that print the red blue; and so of the rest. The plates of this kind have been hitherto engraved in the manner of mezzotinto; but these, unless they are skilfully managed, are soon effaced. Engravings in the manner of the crayon will perhaps answer better.

(h) The principal difficulty in this sort of engraving arises from a want of a skilful management, in giving each plate that precise degree of engraving which will produce the tone of colour required. If a bright green is the press, and the mixture of their colours produces a print that bears no small resemblance to a painting. It must be confessed, however, that what has been hitherto done of this kind falls far short of that degree of perfection of which this art appears susceptible. If they who engrave the best in the manner of the crayon, were to apply themselves to this art, there is reason to expect they would produce far more finished pieces than we have hitherto seen.

V. To make figures appear of different colours successively.

Make a hole in the window-shutter of a dark room, through which a broad beam of light may pass, that is to be refracted by the large glass prism ABC, which may be made of pieces of mirrors cemented together, and filled with water. Provide another prism DEF, made of three pieces of wood, through the middle of this there must pass an axis on which it is to revolve. This prism must be covered with white paper; and each of its sides cut through in several places, so as to represent different figures; and those of each side should likewise be different. The inside of this prism is to be hollow, and made quite black, that it may not reflect any of the light that passes through the sides into it. When this prism is placed near to that of glass, as in the figure, with one of its sides EF perpendicular to the ray of light, the figures on that side will appear perfectly white; but when it comes into the position g b, the figures will appear yellow and red; and when it is in the position k l, they will appear blue and violet. As the prism is turned round its axis, the other sides will have a similar appearance. If, instead of a prism, a four or five sided figure be here used, the appearances will be still further diversified.

This phenomenon arises from the different refrangibility of the rays of light. For when the side EF is in the position g b, it is more strongly illuminated by the least refrangible rays; and wherever they are predominant, the object will appear red or yellow. But when it is on the position k l, the more refrangible rays being then predominant, it will appear tinged with blue and violet.

VI. The solar magic lantern.

Procure a box, of about a foot high, and eighteen inches wide, or such other similar dimensions as you shall think fit, and about three inches deep. Two of the opposite sides of this box must be quite open; and in each of the other sides let there be a groove, wide enough to pass a stiff paper or pasteboard. This box must be fastened against a window on which the sun's rays fall direct. The rest of the window should be closed up, that no light may enter. Provide several sheets of stiff paper, which must be blacked on one side. On these papers cut out such figures as you shall think proper; and placing them alternately in the grooves of the box, with their blacked sides towards you, look at them through a large and clear glass prism; and if the light be strong, they will appear to be painted with the most lively colours in nature. If you cut on one of these papers the form of the rainbow, about three quarters of an inch wide, you will have a lively representation of that in the atmosphere.

This experiment may be farther diversified, by pasting very thin papers, lightly painted with different colours, over some of the parts that are cut out: which will appear to change their colours when viewed through the prism, and to stand out from the paper, at different distances, according to the different degrees of refrangibility of the colours with which they are painted. For greater convenience, the prism may be placed in a stand on a table, at the height of your eye, and made to turn round on an axis, that when you have got an agreeable prospect, you may fix it in that position.

VII. The prismatic camera obscura.

Make two holes F, f, in the shutter of a dark Fig. 5 chamber, near to each other; and against each hole place a prism ABC, and a b c, in a perpendicular direction, that their spectrums NM may be cast on the paper in a horizontal line, and coincide with each other; the red and violet of the one being in the same part with those of the other. The paper should be placed at such a distance from the prisms that the spectrum may be sufficiently dilated. Provide several papers nearly of the same dimensions with the spectrum; cross these papers, and draw lines parallel to the divisions of the colours. In these divisions cut out such figures as you shall find will have an agreeable effect, as flowers, trees, animals, &c. When you have placed one of these papers in its proper position, hang a black cloth or paper behind it, that none of the rays that pass through may be reflected and confuse the phenomena. The figures cut on the paper will then appear strongly illuminated with all the original colours of nature. If, while one of the prisms remains at rest, the other be revolved on its axis, the continual alteration of the colours will afford a pleasing variety; which may be further increased by turning the prism round in different directions.

When the prisms are so placed that the two spectrums become coincident in an inverted order of their colours, the red end of one falling on the violet end of the other; if they be then viewed through a third prism DH, held parallel to their length, they will no longer appear coincident, but in the form of two distinct spectrums, p t and n m (fig. 6.), crossing one another in the middle, like the letter X: the red of one spectrum and the violet of the other, which were coincident at NM, being parted from each other by a greater refraction of the violet to p and m, than that of the red to n and t.

This experiment may be further diversified by adding two other prisms, that shall form a spectrum in the same line, and contiguous to the other; by which not only the variety of figures, but the vicissitude of colours, will be considerably augmented.

VIII.

is to be represented, there should be an equal quantity of engraving on the red and yellow plates: but if an olive green, the yellow plate should be engraved much deeper than the red. VIII. The diatonic scale of colours.

The illustrious Newton, in the course of his investigations of the properties of light, discovered that the length of the spaces which the seven primary colours possess in the spectrum, exactly corresponds to those of chords that sound the seven notes in the diatonic scale of music: As is evident by the following experiment.

On a paper in a dark chamber, let a ray of light be largely refracted into the spectrum AFTMGF, and mark the precise boundaries of the several colours, as \(a, b, c, \&c\). Draw lines from those points perpendicular to the opposite side, and you will find that the spaces \(M r F F\), by which the red is bounded; \(r g e f\), by which the orange is bounded; \(g p e d\), by which the yellow is bounded, &c., will be in exact proportion to the divisions of a musical chord for the notes of an octave; that is, as the intervals of these numbers \(1, \frac{3}{4}, \frac{2}{3}, \frac{3}{5}, \frac{4}{5}, \frac{5}{6}, \frac{6}{7}, \frac{7}{8}\).

IX. Colorific music.

Father Caffel, a Frenchman, in a curious book he has published on chromatics, supposes the note \(ut\) to answer to blue in the prismatic colours; the note \(re\) to yellow, and \(mi\) to red. The other tones he refers to the intermediate colours; from whence he constructs the following gamut of colorific music:

| Ut | Blue | |--------|------------| | Ut sharp | Sea green | | Re | Bright green | | Re sharp | Olive green | | Mi | Yellow | | Fa | Aurora | | Fa sharp | Orange | | Sol | Red | | Sol sharp | Crimson | | La | Violet | | La sharp | Blue violet | | Si | Sky blue | | Ut | Blue |

This gamut, according to this plan, is to be continued in the same manner for the following octave; except that the colours are to be more vivid.

He supposes that these colours, by striking the eye in the same succession as the sounds (to which he makes them analogous) do the ear, and in the same order of time, they will produce correspondent sensations of pleasure in the mind. It is on these general principles, which F. Caffel has elucidated in his treatise, that he has endeavoured, though with little success, to establish his ocular harpsichord.

The construction of this instrument, as here explained, will show that the effects produced by colours by no means answer those of sounds, and that the principal relation there is between them consists in the duration of the time that they respectively affect the senses.

Between two circles of pasteboard, of ten inches diameter, AB and CD, inclose a hollow pasteboard cylinder E, 18 inches long. Divide this cylinder into spaces half an inch wide, by a spiral line that runs round it from the top to the bottom, and divide its surface into six equal parts by parallel lines drawn between its two extremities: as is expressed in the figure.

Let the circle AB, at top, be open, and let that at bottom, CD, be closed, and supported by an axis or screw, of half an inch diameter, which must turn freely in a nut placed at the bottom of a box we shall presently describe. To the axis just mentioned adjust a wooden wheel G, of two inches and a half in diameter, and that has 12 or 15 teeth, which take the endless screw H. Let this cylinder be inclosed in a box LMN (fig. 9.) whose base is square, and at whose bottom there is a nut, in which the axis F turns. Observe that the endless screw H should come out of the box, that it may receive the handle O, by which the cylinder is to be turned.

This box being closed all round, place over it a tin covering A, which will be perforated in different parts; so placed that they may strongly illumine the inside of the cylinder. In one side of this box (which should be covered with pasteboard) cut eight apertures \(a, b, c, d, e, f, g, h\), of half an inch wide, and fig. 9. \(f\) of an inch high; they must be directly over each other, and the distance between them must be exactly two inches. It is by these openings, which here correspond to the musical notes, that the various colours analogous to them are to appear; and which being placed on the pasteboard cylinder, as we have shown, are reflected by means of the lights placed within it.

It is easy to conceive, that when the handle O is turned, the cylinder in consequence rising half an inch, if it be turned five times round, it will successively show, at the openings made in the side of the box, all those that are in the cylinder itself, and which are ranged according to the direction of the inclined lines drawn on it. It is therefore according to the duration of the notes which are to be expressed, that the apertures on the cylinder are to be cut. Observe, that the space between two of the parallel lines drawn vertically on the cylinder, is equal to one measure of time; therefore, for every turn of the cylinder, there are six measures, and thirty measures for the air that is to be played by this instrument.

The several apertures being made in the side of the cylinder, in conformity to the notes of the tune that is to be expressed, they are to be covered with double pieces of very thin paper, painted on both sides with the colours that are to represent the musical notes.

This experiment might be executed in a different manner, and with much greater extent; but as the entertainment would not equal the trouble and expense, we have thought it sufficient to give the above piece, by which the reader will be enabled to judge how far the analogy supposed by F. Caffel really exists.