the gas, whose combustion constitutes the flame, and the flame will be too small.

The access of air is of the first importance in every process of combustion. When a lamp is fitted up with a very slender wick, the flame is small, and of a brilliant white color: if the wick be larger, the combustion is less perfect, and the flame is brown: a still larger wick not only exhibits a brown flanie, but the lower internal part appears dark, and is occupied by a portion of volatilised matter, which does not become ignited until it has ascended towards the point. When the wick is either very large or very long, part of this matter escapes combustion, and shows itself in the form of coal or smoke. The different intensity of the ignition of flame, according to the greater or less supply of air, is remarkably seen by placing a lamp with a small wick beneath a shade of glass not perfectly closed below, and more or less covered above. While the current of air through the glass shade is perfectly free, the flame is white; but, in proportion as the aperture above is diminished, the flame becomes brown, long, wavering, and smoky; it instantly recovers its original whiteness, when the opening is again enlarged. The inconvenience of a thick wick has been long since observed, and attempts made to remove it; in some instances by substituting a number of small wicks instead of a larger; and in others by making the wick flat, instead of cylindrical. The most scientific improvement of this kind, though perhaps less simple than the ordinary purposes of life demand, is the well-known lamp of Argand. In this the wick forms a hollow cylinder or tube, which slides over another tube of metal, so as to afford an adjustment with regard to its length. When this wick is lighted, the flame itself has the figure of a thin tube, to the inner as well as the outer surface of which the air has access from below. And a cylindrical shade of glass serves to keep the flame steady, and in a certain degree to accelerate the current of air. In this very ingenious apparatus many experiments may be made with the greatest facility. The inconvenience of a long wick, which supplies more oil than the volume of flame is capable of burning, and which consequently emits smoke, is seen at once by raising the wick; and, on the other hand, the effect of a short wick, which affords a diminutive flame merely for want of a sufficient supply of combustible matter, is observable by the contrary process.

The best form of the Argand lamp is that contrived by Mr. Quarrill, under the name of the sinumbra, or shadowless lamp. It is represented at fig. 1, plate LAMPS, from which it will be apparent that the oil reservoir is so shaped as to conform with the direction of right lines issuing from the brightest part of the flame, a portion of the light of which is thrown down by a small reflector upon the circular plate of ground glass which fills the lower part of the lamp, and which is surrounded by the oil vessel. The chimney of the lamp is constructed as usual, and the whole is surmounted by a ground-glass lightdistributor, so formed as to do away all shadow from any portion of the lamp, and, at the same

time, not to offend the eye by any want of elegance in shape or dimension."

Light-houses are now generally lighted with Argand lamps; which have hollow cylindrical wicks placed before reflecting mirrors. Several of these lamps are fixed on a frame, and protected from the weather by glass windows. The lamps of light-houses are fed with oil, and in some places with pit-coal gas, as in a lighthouse near Trieste.

In many of the light houses on the British coast the frame on which the lamps are fixed is made to revolve by means of clock-work, so that to a spectator, situated in the circle of which the light-house is the centre, the light appears at its brightest at the end of a stated period of time, which is generally one or two minutes. The revolving of the light enables seamen to distinguish the light-house from the light of lime-kilns or other fires upon the coast. This dictinction is of great importance, for shipwrecks have happened in consequence of mistaking the light of lime-kilns for the light of a light-house. The light is, in some light-houses, made of a red tinge, to distinguish it from some other lighthouse not far distant. The red color of the light is produced by placing windows of red glass before the lamps. Red is the only color that can be given to the light in this way. When stained glass of other colors is placed before the lamp, it is not found to produce a change in the color of light seen at a distance; the blue or green color of the glass becomes insensible when seen through a great body of air, which has itself a blue color.

The flame in a lamp never consumes the wick, till the wick is exposed to the air by the flame's falling downward; and from thence it may be inferred, that a way found out to keep the fuel, and consequently the flame, at the same height upon the wick would make it last a long time. Many ways have been devised to arrive at this important desideratum, but it seems only possible to be done, in any degree of perfection, by reference to hydrostatic pressure. Thus, let a lamp be made two or three inches deep, with a pipe coming from the bottom almost as high as the top of the vessel; let it be filled so high with water, as to cover the whole of the pipe at the bottom, to the end that the oil may not get in at the pipe, and so be lost. Then let the oil be poured in, so as to fill the vessel almost to the surface, which must have a cover, pierced with as many holes as there are wicks designed. When the vessel is thus filled, and the wicks are lighted, if water falls in by drops at the pipes, it will always keep the oil at the same height, or very near; the weight of the water being to that of the oil as 20 to 19, which in two or three inches makes no great difference. If the water runs faster than the oil wastes, it will only run over at the top of the pipe, and what does not run over will come under the oil, and keep it at the same height.

In the lamp contrived by Mr. Kier, of Kentish Town, the oil is raised to the wick, and sustained by a column of a solution of salt in water. This liquid, being as we have already stated, of

a greater specific gravity, a column of it counterbalances a taller column of oil. The solution of salt is made of such a specific gravity, that it will support a column of oil four-thirds of its own height. This is nearly the specific gravity of the heaviest saline solution that is known to exist in any great body of natural water, namely, in the Dead Sea; the weight of the waters of this sea, of distilled water, and of oil, being in the relative proportions of 120, 100, 92. To convey an idea of this lamp, we may suppose a syphon with two upright branches, and the junction of the branches at the bottom. The shortest branch has a bulb at top. The longest branch has a bulb near its lower extremity. The shortest branch is filled with a solution of salt, whose upper surface is in the superior bulbs. The longer branch contains the oil, and in its upper extremity the wick is placed. In the lower bulb the surface of the oil rests upon the surface of the solution of salt.

The bulbs serve as reservoirs, prolonging the action of the machine; by means of the bulbs, and the greater specific gravity of the solution, it is effected, that the abstraction of a considerable quantity of oil by the combustion in the wick occasions but a small depression in the upper surface of the solution; the height of the sustaining column of solution will become shorter in proportion as the column of oil which it counterbalances is consumed; but this diminution of the height of the column of oil will be slow, and therefore the column of oil will for a considerable time be of sufficient length to reach the wick. Suppose an inverted syphon, of equal diameter throughout, the shorter leg of which contains a column of solution of salt, whose height is seventy-five, and this counterbalances a column of oil whose height is 100, in the longer leg; if now the column of oil in the longer leg be diminished in height by ten, the counterbalancing column of solution will diminish to 67.5, being 7.5 shorter than at first. But if the syphon, instead of being of equal diameter, has two dilatations or reservoirs, whose horizontal section is ten times the area of the tube of the syphon, one of the reservoirs being placed at the top of the short branch, so as to contain the upper surface of the solution of salt, and the other at the bottom of the long branch, so as to contain the surface where the oil rests upon the solution, then, if the same quantity of oil, as in the former example, is taken from the top of the longer leg of the syphon, the column of oil will only fall one-tenth of what it did in the undilated syphon of equal diameter, and the solution of salt will diminish one-tenth of what it did in the syphon of equal diameter.

The oil reservoir and the wick remain stationary, and do not descend as the oil is consumed. This descent takes place in two lamps now to be mentioned, because in these two lamps the oil reservoir swims in liquid that acts as a counterpoise.

In the lamp contrived by the Chevalier Edelcrantz, of Stockholm, the oil reservoir floats in mercury, and the column of oil is maintained at the requisite height by the counterpoise of a column of mercury; in proportion as the oil is

consumed, the oil reservoir and the wick which is connected with it sink.

The general structure of this lamp may be understood by conceiving a flask, with a long narrow neck, and enlarged at the under part. The flask is heavy enough to swim, when it is placed in mercury, with part of its under part immersed. The bottom of the flask is open. The flask, being placed in mercury, is made to float with its neck perpendicular. Oil is poured in at the neck till the flask is full. Then the surface of the mercury at the bottom of the flask and within the flask will be depressed by the weight of the column of oil that rests upon it and the surface of the mercury on the outside of the bulb, or lower part of the flask, will stand higher than the surface of the mercury within the flask. The height which measures the difference of level of the two surfaces of mercury will be the height of a column of mercury o equal weight with the column of oil that is in the flask; and, as mercury is about 14 times the weight of oil, the difference of level of the two surfaces of mercury will be of the height of the oil in the flask. In proportion as oil is abstracted from the upper end of the tube, by the combustion in the wick, the height of the column of oil is thereby diminished, and the two surfaces of mercury will come nearer to each other, the flask sinking a little in the mercury. As the area of the horizontal section of the lower part of the flask is much greater than the area of the section of the neck, and as the specific gravities of mercury and oil are very different, it follows, that, to restore the equilibrium after the abstraction of a column of oil from the neck, the surface of the mercury within the lower part of the flask will rise by a much shorter column.

In the lamp invented by Mr. Barton, comptroller of his majesty's mint, a solution of salt in water is used as a counterpoise to the oil. The combination consists of a light flask, open at the bottom, floating in a solution of salt, so that, when oil is poured into the flask, the surface of the oil in the neck of the flask stands at a higher level than the surface of the saline solution in which the flask swims. The wick is at the upper end of the neck of the flask, and as the area of the horizontal section of the bulb or lower part of the flask is much greater, suppose twenty times greater, than the area of the section of the neck of the flask, it will happen, that, when a column of oil an inch high is abstracted from the neck of the flask, the height of the rise of the surface of the solution in the bulb or lower part of the flask will be only of an inch

At Paris, oil of rape-seed, and oil of poppyseed, are clarified for the lamp by filtering through cotton, wool, and other processes. In the south of France and Italy the inferior kind of olive orl is used in lamps, and sometimes the oil of the plant called arachis hypogana, or earth-nut. In Italy, lamp-oil has been pressed from the stones of the grape. In Piedmont, walnut-oil is used for lamps. On the eastern and southern coasts of the Mediterranean, and in China, the inferior kind of oil of sesamum. In tropical countries, cocoa-nut oil, which in the temperature of heat of Britain, is solid and white like

tallow. It is burnt in lamps made of the shell of the cocoa-nut, and of bamboo. Much of the oil used in China is obtained by expression from the seeds of the tree called by botanists camellia oleifera, which is extensively cultivated for that purpose, as is the shrub called croton sebiferum, for the solid oil or tallow that the Chinese press from its fruit. Essential oils, extracted from plants by distillation, are too volatile, and, in consequence of their volatility, are too easily inflamed.

The lamp commonly used in rooms at Florence consists of a round reservoir, with four beaks projecting from as many opposite points of its circumference; through the middle of the reservoir a vertical stalk passes, and on this stalk the lamp may be raised or slid down. The stalk is fixed in a foot that rests on the table. The whole is made of brass.

A lamp which affords a faint light is made of a waxed wick, an inch long, passed through the centre of a thin round piece of cork, and of a piece of card placed above the cork. Some oil is placed on the surface of water in a glass tumbler, and the cork, with its wick, is laid upon the surface of the oil. This lamp, called a veilleuse, is commonly used in Paris for burning in bedrooms during the night, as rush-lights are in London.

There is a very useful little lamp which is now much employed cailed the self-generating gas lamp. In this simple apparatus, a small cup is furnished with a tube capable of supplying as much oil as is required for combustion, and the capillary attraction, aided by a small addition of hydrostatic pressure, serves to raise the oil to the surface of the aperture. The best mode of lighting this lamp is to take a piece of small twine, and, having immersed one extremity in the oil, apply it when inflamed to the top of the tube. The moment that this is heated, sufficiently to decompose the oil, flame results. A section of the best form of this lamp is given at fig. 2.

The method of measuring the comparative intensities of light is one of the first requisites in an enquiry concerning the art of illumination. Two methods of considerable accuracy are described in the Traité d'Optique of Bouguer, of which an abridged account is given by Dr. Priestley in his Optics. The first of these two methods has been used by others since that time, and probably before, from its very obvious nature, but particularly count Rumford, who has given a description and drawings of an instrument called the photometer, in the Philosophical Transactions for 1794. The principle it is grounded upon is, that if two lights shine upon the same surface at equal obliquities, and an opaque body be interposed, the two shadows it will produce must differ in blackness or in intensity in the same degree. For the shadow formed by intercepting the greater light will be illuminated by the smaller light only, and reversely the other shadow will be illuminated by the greater light. That is to say, in short, the stronger light will be attended with the deeper shadow. But it is easy, by removing the greater light to a greater distance, to render the illumination it produces at the common surface,

equal to that afforded by the less. Experiments of this kind may be conveniently made by fastening a sheet of white paper against the wall of a room. The two lights or candles, intended to be compared, must then be placed so that the ray of light from each shall fall with nearly the same angle of incidence upon the middle of the paper. In this situation, if a book or other object be held to intercept part of the light which would have fallen on the paper, the two shadows may be made to appear as in fig. 3, where A represents the surface illuminated by one of the lights only; B, the surface illuminated by the other light; C the perfect shadow from which both lights are excluded. It will easily be understood that the lights about D and E, near the angle F, will fall with equal incidences when the double shadow is made to occupy the middle of the paper: and, consequently, if one or both of the lights be removed directly towards or from the paper, as the appearances may require, until the two shadows at E and D have the same intensity, the quantities of light emitted by each will be as the squares of the distances from the paper.

By experiments of this kind many useful particulars may be shown. Thus, for example, the light of a candle, which is so exceedingly brilliant when first snuffed, is very speedily diminished to one-half, and is usually not more than one-fifth, or one-sixth before the uneasiness of the eye induces us to snuff it. Whence it follows, that, if candles could be made so as not to require snuffing, the average quantity of light afforded by the same quantity of combustible matter would be more than doubled. In the same way, likewise, since the cost and duration of candles, and the consumption of oil in lamps, are easily ascertainable, it may be shown, whether more or less of light is obtained at the same expense during a given time, by burning a number of small candles instead of one of greater thickness.

It is almost unnecessary to describe a thing so universally known as a candle. This article is formed of a consistent oil, which envelops a porous wick of fibrous vegetable matter. The cylindrical form and dimensions of the oil are given either by casting it in a mould, or by repeatedly dipping the wick into the fused ingredient. Upon comparing a candle with a lamp. two very remarkable particulars are immediately seen. In the first place, the tallow itself, which remains in the unfused state, affords a cup or cavity to hold that portion of melted tallow which is ready to flow into the lighted part of the wick. In the second place, the combustion, instead of being confined, as in the lamp, to a certain determinate portion of the fibrous matter, is carried, by a slow succession, through the whole length. Hence arises the greater necessity for frequent snuffing the candle; and hence also the station of the freezing point of the fat oil becomes of great consequence. For it has been shown that the brilliancy of the flame depends very much on the diameter of the wick being as small as possible; and this requisite will be most attainable in candles formed of a material that requires a higher degree of heat to fuse it. The wick of a tallow candle must be

made thicker in proportion to the greater fusibility of the material, which would otherwise melt the sides of the cup, and run over in streams. The flame will therefore be yellow, smoky, and obscure, except for a short time immediately after snuffing. Tallow melts at 92° of Fahrenheit's thermometer; spermaceti at 133°; the fatty matter formed of flesh after long immersion in water melts at 127°; the pela of the Chinese at 145°; bees' wax at 142°; and bleached wax at 155°. Two of these materials are well known in the fabrication of candles. Wax in particular does not afford so brilliant a flame as tallow; but, on account of its less fusibility, the wick can be made smaller; which not only affords the advantage of a clear perfect flame, but from its flexibility it is disposed to turn on one side, and come in contact with the external air, which completely burns the extremity of the wick to white ashes, and thus performs the office of snuffing. We see, therefore, that the important object to society of rendering tallow candles equal to those of wax, does not at all depend upon 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, 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, lastly, the tallow itself must be rendered less fusible by some chemical process.

A lamp for burning tallow has been contrived by Mr. Close. Its construction will be best understood by reference to fig. 4. A represents a cup made in the form of a cone; it contains the tallow, and is supported, with the point downwards, by the thumb-screw i, upon the piece of iron D, which is firmly fixed into the circular wood bottom E. The widest diameter of this cup is about two inches and a half, and the diameter of a small aperture in the point is rather less than one-eighth part of an inch. This cup must be made of iron, brass, or copper, and the joint on the side closed with hard solder. A circular plate of iron is made to fit the widest diameter, and firmly fixed therein by the sides of the cup being turned over it a little near one side of this circular plate a circular hole less than one inch in diameter is made, and into it is fixed the ring d, which forms the mouth of the cup, and may be closed with a cork, &c.

ab represents a piece of wire which passes through a hole made in the circular piece, and through the aperture in the point of the cup. This wire is rather more than one-eighth part of an inch in diameter; it converges near the point, and exactly closes the same aperture when thrust down: it serves to regulate the descent of tallow into the cup B, according to the quantity consumed by the flame: and therefore, when it is required not quite to close the aperture, it is drawn up a little, and a small spring of brass, in the inside of the cup at C, presses against it, and holds it in the place.

The spring passes through the circular plate, and is fixed on the outside by a small screw.

B represents a small cup, in which the tallow is burned: it is about one inch in diameter, and about half an inch in depth. Into the bottom of this cup is soldered the tube f, which is about two inches and a half long, and slides into the tube g, which is soldered into the bottom of the cup c.

e represents a piece of bent wire, which supports the wick of the lamp. The ends of this piece of wire are thrust into a piece of soft wood, fitted into the tube f. Another tube, represented by h, is soldered to one side of the cup B, above the brim: the use of this tube is to contain a quantity of clean wick, and to serve for a handle to lift the tube f out of the socket g when the lamp is to be lighted.

C represents a cup to receive any tallow that may chance to run over the sides of the cup B. It is rather more than one inch deep, and two inches in diameter.

Lastly, by the help of the thumb screws i, i, the height of the cups B, C, and A, and the distance between B and A, may be regulated at pleasure.

A wick of cotton being put into the tube h, and brought through the ring e; a quantity of tallow put into the conical cup; and the small cup filled with melted tallow; the lamp may be lighted: if the point of the cup A be raised two or three inches above the brim of the cup B, and the air in the room at rest, the tallow in the cup A will be fused in a few minutes, and if the wire a b be properly adjusted, a constant supply of tallow will drop to the flame.

Small particles of dust and other impurities in the tallow sometimes impede the drops, but the light continues undiminished, until all the tallow in the little cup is consumed, and the cup is then easily filled again by holding up the wire, after which the drops must be regulated again. To lessen this inconvenience as much as possible, nothing but clean tallow, or hog's lard, must be burned. Tallow and hog's lard will burn with a very clear bright flame, of the same intensity, for a long time.

The small cup, when detached from the rest of the apparatus, will supply the different uses of a candle: it may be carried about by the tube ǹ, or occasionally placed with the tube ƒ in a small wooden stand. The tallow is not very liable to be spilled.

Every time before the lamp is lighted, a new portion of wick must be drawn through the ring. This is not easily effected when the tallow is cold; therefore to avoid this trouble, it is the best method always to draw up the wick immediately before the flame is extinguished; for which purpose, a small forceps, made of one piece of bended iron, will be most convenient.

Lastly, it may be necessary to remark, that after the lamp is lighted, when the tallow in both cups is cold, the tallow in the small cup must be broken and stirred up, that a sufficient quantity may be fused immediately to supply the flame; and, the sooner to fuse the tallow in the conical cup, the flame should be raised near to its point,