Читать книгу A Practical Treatise on Gas-light - Friedrich Christian Accum - Страница 9

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The flame of burning bodies consists of such inflammable matter in the act of combustion as is capable of existing in a gazeous state. When all circumstances are favorable to the complete combustion of the products, the flame is perfect; if this is not the case, part of the combustible body, capable of being converted into the gazeous state, passes through the luminous flame unburnt, and exhibits the appearance of smoke. Soot therefore always indicates an imperfect combustion. Hence flame is produced from those inflammable substances only, which are either totally volatile when heat is applied to them, so as not to alter their chemical habitudes—or which contain a quantity of combustible matter that is readily volatilized into vapour by heat, or the elements necessary for producing such vapour or gazeous products, when the chemical constitution of the body is altered by an increase of temperature. And hence the flame of bodies is nothing else than the inflammable product, either in a vaporous or in a permanently elastic gazeous state. Thus originates the flame of wood and coal, when they are burned in their crude state. They contain the elements of a quantity of inflammable matter, which is capable of assuming the gazeous state by the application of heat, and subsequent new chemical arrangements of their constituent parts.

As the artificial light of lamps and candles is afforded by the flame they exhibit, it seems a matter of considerable importance to society, to ascertain how the most luminous flame may be produced with the least consumption of combustible matter. There does not appear to be any danger of error in concluding, that the light emitted will be greatest when the matter is completely consumed in the shortest time. It is therefore necessary, that the stream of volatilized combustible gazeous matter should pass into the atmosphere with a certain determinate velocity. If the quantity of this stream should not be duly proportioned; that is to say, if it be too large, its internal parts will not be completely burned for want of contact with the air. If its temperature be below that of ignition, it will not, in many cases, burn when it comes into the open air. And there is a certain velocity at which the quantity of atmospherical air which comes in contact with the vapour will be neither too great nor too small; for too much air will diminish the temperature of the stream of combustible matter so much as very considerably to impede the desired effect, and too little will render the combustion languid.

We have an example of a flame too large in the mouths of the chimneys of furnaces, where the luminous part is merely superficial, or of the thickness of about an inch or two, according to circumstances, and the internal part, though hot, will not set fire to paper passed into it through an iron tube; the same defect of air preventing the combustion of the paper, as prevented the interior fluid itself from burning. And in the lamp of Argand we see the advantage of an internal current of air, which renders the combustion perfect by the application of air on both sides of a thin flame. So likewise a small flame is always whiter and more luminous than a larger; and a short snuff of a candle giving out less combustible matter in proportion to the circumambient air; the quantity of light becomes increased to eight or ten times what a long snuff would have afforded.

The light of bodies burning with flame, exists previously either combined with the combustible body, or with the substance which supports the combustion. We know that light exists in some bodies as a constituent part, since it is disengaged from them when they enter into new combinations, but we are unable to obtain in a separate state the basis with which it was combined.

That in many cases the light evolved by artificial means is derived from the combustible body, is obvious, if we recollect that the colour of the light emitted during the process of combustion varies, and that this variation usually depends not upon the medium which supports the process of combustion, but upon the combustible body itself. Hence the colour of the flame of certain combustibles, even of the purest kind may be tinged by the admixture of various substances.

The flame of a common candle is far from being of an uniform colour. The lowest part is always blue; and when the flame is sufficiently elongated, so as to be just ready to smoke, the tip is red or brown.

As for the colours of flames that arise from coals, wood, and other usual combustibles, their variety, which hardly amounts to a few shades of red or purple, intermixed with the bright yellow light, seems principally to arise from the greater or less admixture of aqueous vapour, dense smoke, or, in short, of other incombustible products which pass through the luminous flame unburnt.

Spirit of wine burns with a blueish flame. The flame of sulphur has nearly the same tinge. The flame of zinc is of a bright greenish white. The flame of most of the preparations of copper, or of the substances with which they are mixed, is vivid green. Spirit of wine, mixed with common salt, when set on fire, burns with a very unpleasant effect, as may be experienced by looking at the spectators who are illuminated by such light. If a spoonful of spirit of wine and a little boracic acid, or nitrate of copper be stirred together in a cup, and then be set on fire, the flame will be beautifully green. If spirit of wine be mixed with nitrate of strontia, it will, afterwards, on being inflamed, burn with a carmine red colour. Muriate of lime tinges the flame of burning spirit of wine of an orange colour.[2]

[2] See Chemical Amusement, comprising minute instructions for performing a series of striking and interesting chemical experiments, p. 8, &c.

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 the diameter of the flame, so as to prevent the access of air to its internal part. When its length is too great for the vertical position, it bends on one side; and its extremity, coming in contact with air, is burned to ashes; excepting such a portion as is defended by the continual afflux of melted wax, which is volatilized, and completely burned, by the surrounding flame. Hence it appears, that the difficult fusibility of wax renders it practicable to burn a large quantity of fluid by means of a small wick, and that this small wick, by turning on one side in consequence of its flexibility, performs the operation of snuffing itself, in a much more accurate manner than can ever be performed mechanically. From the above statement it appears, that the important object to society of rendering tallow candles equal to those of wax, does not at all depend on the combustibility of the respective materials, but upon a mechanical advantage in the cup, which is afforded by the inferior degree of fusibility in the wax: and that, in order to obtain this valuable object, one of the following effects must be produced: either the tallow must be burned in a lamp, to avoid the gradual progression of the flame along the wick; or some means must be devised to enable the candle to snuff itself, as the wax-candle does; or the tallow itself must be rendered less fusible by some chemical process. The object is, in a commercial point of view, entitled to assiduous and extensive investigation. Chemists in general suppose the hardness or less fusibility of wax to arise from oxygen. Mr. Nicholson[3] is led by various considerations to imagine, that the spontaneous snuffing of candles made of tallow or other fusible materials, will scarcely be effected but by the discovery of some material for the wick, which shall be voluminous enough to absorb the tallow, and at the same time sufficiently flexible to bend on one side.