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Before we consider the general nature of Gas-Light, it will be necessary to give a short sketch of the theory and action of the instruments of illumination employed for supplying light, together with some other facts connected with the artificial production and distribution of light; such a proceeding will enable us to understand the general nature of the new system of illumination which it is the object of this Essay to explain.

To procure light for the ordinary purposes of life, we are acquainted with no other ready means than the process of combustion.

The rude method of illumination consists, as is sufficiently known, in successively burning certain masses of fuel in the solid state: common fires answer this purpose in the apartments of houses, and in some light-houses. Small fires of resinous wood, and the bituminous fossil, called canel-coal, are in some countries applied to the same end, but the most general and useful contrivance is that in which fat, or oil, of an animal or vegetable kind is burned by means of a wick, and these contrivances comprehend candles and lamps.

In the lamp the combustible substance must be one of those which retain their fluidity at the ordinary temperature of the atmosphere. The candle is formed of a material which is not fusible but at a temperature considerably elevated.

All these substances must be rendered volatile before they can produce a flame, but for this purpose it is sufficient to volatilize a small quantity of any of them, successively; for this small quantity will suffice to give a useful light, and hence we must admire the simple, yet wonderful contrivance of a common candle or lamp. These bodies contain a considerable quantity of the combustible substance, sufficient to last several hours; they have likewise, in a particular place, a slender piece of spongy vegetable substance, called the wick, which in fact is the fire-place, or laboratory where the whole operation is conducted.

There are three articles which demand our attention in the lamp—the oil, the wick, and the supply of air. It is required that the oil should be readily inflammable; the office of the wick appears to be chiefly, if not solely, to convey the oil by capillary attraction to the place of combustion; as the oil is decomposed into carburetted hydrogen gas and other products, other oil succeeds, and in this way a continual current and maintenance of flame is effected.

When a candle is for the first time lighted, a degree of heat is given to the wick, sufficient first to melt, and next to decompose the tallow surrounding its lower surface; and just in this part the newly generated gas and vapour is, by admixture with the air, converted into a blue flame; which, almost instantaneously encompassing the whole body of the vapour, communicates so much heat to it, as to make it emit a yellowish white light. The tallow now liquefied, as fast as it boils away at the top of the wick, is, by the capillary attraction of the same wick, drawn up to supply the place of what is consumed by the cotton. The congeries of capillary tubes, which form the wick, is black, because it is converted into coal; a circumstance common to it with all other vegetable and animal substances, when part of the carbon and hydrogen which enter into their composition having been acted on by combustion, the remainder and other fixed parts are by any means whatever covered and defended from the action of the air. In this case, the burning substance owes its protection to the surrounding flame. For when the wick, by the continual wasting of the tallow, becomes too long to support itself in a perpendicular situation, the top of it projects out of the cone formed by the flame, and thus being exposed to the action of the air, is ignited, loses its blackness, and is converted into ashes; but that part of the combustible which is successively rendered volatile by the heat of the flame is not all burnt, but part of it escapes in the form of smoke through the middle of the flame, because that part cannot come in contact with the oxygen of the surrounding atmosphere; hence it follows, that with a large wick and a large flame, this waste of combustible matter is proportionately much greater than with a small wick and a small flame. In fact, when the wick is not greater than a single thread of cotton, the flame, though very small, is, however, peculiarly bright, and free from smoke; whereas in lamps, with very large wicks, such as are often suspended before butchers’ shops, or with those of the lamp-lighters, the smoke is very offensive, and in great measure eclipses the light of the flame.

A candle differs from a lamp in one very essential circumstance; viz. that the oil or tallow is liquefied, only as it comes into the vicinity of the combustion; and this fluid is retained in the hollow of the part, which is still concrete, and forms a kind of cup. The wick, therefore, should not, on this account, be too thin, because if this were the case, it would not carry off the material as fast as it becomes fused; and the consequence would be, that it would gutter or run down the sides of the candle: and as this inconvenience arises from the fusibility of the tallow it is plain that a more fusible candle will require a larger wick; or that the wick of a wax candle may be made thinner than that of one of tallow. The flame of a tallow candle will of course be yellow, smoky, and obscure, except for a short time after snuffing. When a candle with a thick wick is first lighted, and the wick snuffed short, the flame is perfect and luminous, unless its diameter be very great; in which last case, there is an opake part in the middle, where the combustion is impeded for want of air. As the wick becomes longer, the interval between its upper extremity and the apex of the flame is diminished; and consequently the tallow which issues from that extremity, having a less space of ignition to pass through, is less completely burned, and passes off partly in smoke. This evil increases, until at length the upper extremity of the wick projects beyond the flame and forms a support for an accumulation of soot which is afforded by the imperfect combustion, and which retains its figure, until, by the descent of the flame, the external air can have access to the upper extremity; but in this case, the requisite combustion which might snuff it, is not effected; for the portion of tallow emitted by the long wick is not only too large to be perfectly burned, but also carries off much of the heat of the flame, while it assumes the elastic state. By this diminished combustion, and increased afflux of half decomposed oil, a portion of coal or soot is deposited on the upper part of the wick, which gradually accumulates, and at length assumes the appearance of a fungus. The candle then does not give more than one-tenth of the light which the due combustion of its materials would produce; and, on this account, tallow candles require continual snuffing. But if we direct our attention to a wax candle, we find that as its wick lengthens, the light indeed becomes less. The wick, however, being thin and flexible, does not long occupy its place in the centre of the flame; neither does it, even in that situation, enlarge