Light, according to the theory of Newton, is a substance consisting of very subtile particles, which are constantly emanating in straight lines from the sun, the fixed stars, and incandescent bodies. Some philosophers, however, following Hooke and Huygens, believe that light is merely the state of undulation of a highly rare, elastic medium, which, it is supposed, fills the whole of the universe. Both theories have their supporters; but the question may be regarded as being still undetermined. The size of the particles of light is too minute to be appreciated; but their velocity is estimated to be at the rate of 195,000 miles in a second. They appear to repel each other, like the particles of caloric.

Light, if not intercepted, is diffused in straight lines, in every direction, from every luminous point. The rays of light are reflected by aqueous vapour, clouds, liquids, and solid in various degrees: much depending on the condition of the surfaces of the reflecting bodies.

A ray of light falling obliquely upon a clean polished surface is reflected from it at an angle equal to the angle of its incidence. But when a ray of light falls at an angle of 35° 25' on a polished plate of glass; and, in being reflected from it, falls upon another plate of glass, so placed that its angle of incidence is also 35° 25', the second plate may be turned round its axis without varying the angle which it makes with the ray that falls upon it. If the two planes of reflection be parallel to each other, the ray of light is reflected in the same manner from both plates of glass; but if the second plate be turned round a quadrant of a circle, so as to make the plane of reflection perpendicular, the whole ray will pass through it, and none of it be reflected, yet, if this plate be turned round another quadrant of a circle, so as to make the reflecting planes again parallel, the ray will be now reflected by it as at first. The light can penetrate through the glass only when the reflecting planes are perpendicular, bat is reflected when they are parallel.

This property of light has been termed by Malus, by whom it was first discovered, its polarization.

Thomson's Chymistry, 4th edit. i. 546.

When the ray of light, moving in a straight line, passes within a certain distance of a body parallel to its direction, it bends towards the body, or is inflected; but when the body parallel to its course is at a greater distance, the ray is bent from it, or deflected. When it passes obliquely from one medium to another of a different density, it is bent a little from the line of its former direction, and assumes a new one, or is refracted. In passing into a denser medium it is refracted towards the perpendicular; but is refracted from the perpendicular when passing into a rarer medium. The refraction is proportional to the density of the medium; but, in that of a combustible, the refraction is greater than the ratio of its density; and when a liquid is converted into vapour, its refractive power diminishes at a greater ratio than its density diminishes.1

Every ray of light is resolvable into seven other distinct rays, each possessing a different degree of refrangibility; and consequently divisible from each other by the prism. The ultimate or component rays are distinguishable by the impression of colours they excite on the eye; and are arranged in the following order: red, orange, yellow, green, blue, indigo, violet. The red is the least refrangible, reflexible, and inflexible; the violet the most. The order in which the others are placed is that of their respective refrangibility The colour of bodies depends on their transmitting or reflecting those rays only which excite the impressions of their colour. The reflection of the whole prismatic rays constitutes "white; the absorption or suffocation of all, or the greater part, of these, occasions black, which is the total absence of light.

The illuminating power of the rays of light differs. Those towards the middle of the prismatic spectrum, as above arranged, possess the greatest illuminating power: the maximum lies in the yellow; this diminishes as the rays approach towards the extremities.

The calorific power of the rays of light differs. Of the visible rays, the red is the hottest: but the really hottest ray is beyond the point of the red ray of the prismatic spectrum. Such is the opinion of Herschel, Sir H. Davy, and some others; but Professor Leslie contended that the red ray is the hottest. It is a curious fact that the heating power of the prismatic rays varies according to the kind of prism used to separate them: but the result of the experiments made by all authorizes the conclusion that the hottest ray is the invisible one beyond the red, the next in degree the red, and the next the yellow.

1 See the experiments of Arago and Petit, in the Ann. de Chim. et Phys. torn. i. 1.

Light enters into combination with bodies; and, in some cases, is again extricated without any change being produced; as in pyrophori, or substances which absorb light, and emit it again when carried into a dark place. In some cases, however, the absorption of light by bodies occasions very sensible changes in them; the colour of plants, for example, their taste and odour, and the quantity of combustible matter they contain, depend on light; for a plant reared in the dark is nearly colourless, insipid, inodorous, and contains a very small proportion of combustible matter.

The natural sources of light are the sun and fixed stars; but it is also artificially produced by combustion, chymical combination, heat, and percussion. The sun's rays, the greatest source of light, have been found to be composed of three different species of rays: 1. rays which produce light and colour; 2. rays of mere heat; and, 3. rays which produce neither light nor colour, nor affect the thermometer, but which have the power of deoxidizing. Thus constituted, they produce very important chymical effects.

Light partially deoxidizes metallic oxides and salts. Thus it blackens chloride of silver; and as this takes place when the salt is placed beyond the violet ray, or out of the prismatic spectrum, the effect is apparently to be attributed to the action of a species of rays that excite neither heat nor light. It also reduces the nitro-muriatic solution of gold, when it is placed in contact with charcoal, or any other vegetable, or any animal matter; and the red oxides of mercury and of lead become much paler when exposed to the sun. The rays that produce these effects are the least refrangible. Dr. Wol-laston, however, has pointed out one exception to this effect of these rays, in guaiacum, which becomes green, or oxidized, in the least refrangible rays; and is again changed to yellow, or deoxidized, in the most refrangible.

Light has a powerful tendency to decompose nitric acid, which it renders red and fuming, even when it is contained in vessels accurately closed. Almost all the vegetable and animal colouring matters have their brilliancy and colour much impaired by long exposure to the sun's rays; and the colour and the properties of vegetable powders kept in clear glass bottles are also affected by them. Light even seems to have a strong influence on the process of crystallization; for, if light be only partially admitted to a crystallizing solution, the crystals will be larger and more numerous on the enlightened side; and often the whole mass will radiate towards this point. Chaptal1 found that by using a solution of a metallic salt, and shading the greater part of the vessel, capillary crystals shoot up the uncovered side, and the extent of the exposed part is distinctly marked by the limit of the crystallization.

Such are some of the properties of light, the chymical effects of the operation of which seem to be perfectly independent of its heating power; and there is even reason to believe, that the greatest chymical changes are produced by the invisible rays; for Ritter2 affirms that, by transmitting the coloured rays through different prisms, he has separated them from the invisible or chymical rays, and obtained a coloured spectrum devoid of any chymical power.