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meets with a medium of different density, and most copiously when this difference is greatest. As the thin plate of air between the convex and plane glasses, mentioned before, transmitted or reflected the rays of different colours, according to its different thicknesses: and as the concentric rings of colours appearing in the bubble of water, continually varied with the changing thickness of the shell of water: this shows that the particular colour of any ring depends upon the particular thickness of the plate of air or shell of water, where the incident light is reflected to the eye. In the same manner, the transparent parts of bodies, according to their several sizes, reflect rays of one colour and transmit those of another; on the same grounds as the thin plates of air or water transmit or reflect them: and this seems to be the foundation of their different colours. For, if a transparent substance, of the thickness proper to produce any colour, should be cut into slender threads, or broken into fragments, we can see no reason why every fragment should not keep its own particular colour, and a heap of such fragments constitute a body of that particular colour. So that the different sizes of the particles, of which bodies are composed, must be the reason of their appearing to be of different colours. This is farther confirmed by the analogy between the colours of natural bodies and the colours of the rings exhibited in the bubbles of water and the glasses above mentioned. These plates of air or water do not appear of the same colour at the same place, when viewed obliquely, as when seen directly; for they will be observed to expand and grow larger with the increasing obliquity, with which they are viewed. This shows, that the transparent substance between the glasses does not exhibit the same colour, at the same thickness, in all situations of the

eye: just as we observe that the colours of the same part of a peacock's tail vary with the position in which it is seen. The colours of variable silks, and many other substances, change in like manner with the position of the eye. Besides, we find the colours of many substances growing faint and diluted, by being wetted with water or oil, which can intimately penetrate their pores, and recovering their former lustre upon being dried; as we find the colours reflected from thin pieces of Muscovy glass grow faint, by wetting the glass with water, and grow vivid again, when dry.

To all this it may be added, that many bodies undergo a change of colour from trituration and a comminution of their parts. Thus the colours, which painters use, will be a little changed by being ground elaborately, which is doubtless owing to their having their par. ticles broken and lessened. Mercury, by being variously tortured in the fire by the chemists, may have the size of its particles so changed as to appear sometimes red, or yellow, or white; and lead, by a similar process, sometimes appears red and sometimes white.

Copper, in the mass, appears red; but when dissolved by a strong acid, appears blue. Mineral waters are tried by the change of colour they effect in the syrup of violets, which is naturally blue: if an acid salt prevail in the water, it will change its colour into red, by intimately incorporating with the mineral, and coalescing into larger globules, which always reflect the red rays of the sun more copiously, than particles of a lesser size: but if an alkali prevail in the water, it will not so closely cohere to the particles of the syrup, and the particles of the compound will be of a lesser size, such as are adapted to the reflexion of a green colour; and therefore the mixture will appear green. If the fumes

of a strong acid, which are its most subtile and energetic particles, reach a green cloth, they will break and lessen the particles in its surface to such a degree, as to make it appear blue; whereas, if it reach the cloth in substance it will operate less violently upon it, and change it into a yellow colour.

Upon the same principles, we may account for that surprising change of colour produced by the mixture of various fluids, where the particles of the one, by their action upon the other, in different degrees, either make them greater or less, so as to dispose them to reflect the rays of this or that particular colour more copiously than the rest, and thereby cause the body to appear of a particular colour. Hence it is, that for the production of black, the particles must be smaller than for the exhibition of any colour, viz. of a size, answering to the thickness of the plate of air, where, by reflecting little or no light, it became dark and opake. Yet the particles must not be too small, for that will make them transparent through a deficiency of reflexion in the internal parts. They must be of a size bordering on that which is disposed to reflect a blue, as we see that all blacks partake a little of that colour. Hence, we see the reason why bodies dissolved by fire, that most subtile dissolvent, generally become black; why the same follows upon putrefaction; why, in setting a razor upon a hone, or in grinding glasses upon plates of copper, the particles of the metal, glass, or sand, usually become very black; and also, why black substances so easily communicate their hue to others: as the particles of such bodies, from their extreme subtileness and minuteness, readily overspread any surface and fill up the inequalities arising from their roughness.

On the same principle, we account for the different

tints of the clouds, which they receive from the rising or setting sun, together with many other phenomena observable in the atmosphere; such as that vapours, when first raised, do not destroy the transparency of the air, being divided into particles too small to cause any reflection from their superficies. But when, in order to compose drops of rain, they begin to coalesce into larger globules, of size sufficient to reflect some rays, and to transmit others, they may form clouds of differ. ent colours, according to their sizes.

Hence, we see the reason, why two transparent and colourless fluids may, upon being mixed together, become opake; as the mixture of the spirits of wine and the spirits of hartshorn produces an opake coagulated substance, called the offa alba: as the mixture is not so intimate, as to leave the pores small enough for the transmission of light. Two coloured fluids also, one red, and the other blue, the first transmitting the red rays of light only, while the other transmits the blue, will, when mixed together, become opake; or when one is held behind the other, no object can be seen through both: because the rays which pass through the one are stopped by the other. Thus, also, leaf-gold appears yellow by reflected light, but of a bluish tint by transmitted light. There are also some sorts of liquors, which reflect one sort of rays, while they transmit others, as a tincture of lignum nephriticum, and thereby will appear of different colours: while other fluids appear of the same colour, both by transmitted and reflected light; as the same rays, which pass through the fluid, are reflected again from the farther surface.

THE RAINBOW.

WE have now paved the way for the explication of that coloured arch which we often see in the cloud, denominated the rainbow; when the sun, on the one hand, shines bright on the drops of descending rain, on the other..

This phenomenon, though observed in the heavens ever since the deluge, remained inexplicable, until the different refrangibility of the rays of light was discovered by Sir Isaac Newton. Antonio de Dominis, archbishop of Spalato, and after him Descartes, expressly showed that the rainbow was formed by the reflexion of the sunbeams from the drops of falling rain, but neither of them could account for the diversity of its colours. They could indeed easily confirm their opinion, that the rainbow was formed by the reflexion of the sun's rays from the drops of rain, by a well known experiment, of hanging up a glass globe filled with water opposite to the rays of the sun; for by varying the position of the eye, or by elevating or depressing the globe, the various colours of the rainbow would successively appear in the globe; but they had no idea of these colours being an inherent property of the rays of light, when separated by refraction. So that the cause of the different colours remained an inexplicable secret, until Sir Isaac Newton unravelled the web of light, and accounted for every circumstance.

There are two rainbows generally appearing at the same time, both of which depend upon the same cause, but they are formed in a different manner.

The interior or primary bow is formed by two refractions and one intermediate reflexion of the rays of light in the drops of falling rain: but the exterior bow

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