Flame.--Artificial light is obtained, for common purposes, by the combustion of substances, which afford a permanent and luminous flame. All flames ate not equally luminous. Those substances which, during combustion, produce chiefly gaseous, or volatile matter, emit from their flame a very feeble light, as is seen in burning hydrogen, or sulphur. Those, on the other hand, which produce particles of solid matter during their combustion, yield a whiter flame, and a greater illumination. Sir Humphrey Davy is of opinion, that the brilliancy of the flames, used for illumination, is owing to the decomposition of the gaseous matter, towards the interior of the flame, by which solid charcoal is produced, and strongly ignited, before it is burnt. In a conical flame, like that of a candle, the combustion takes place most rapidly towards the surface, where the inflammable gas mixes with the atmospheric air. At the centre of the base, there is a darker portion, which consists of the matter, which is volatilized, but not yet fully on fire. In the interior, or most luminous part, the solid particles are brought to a white heat, just before they are burnt. The degree of their ignition is very powerful, since it is found, that the flame of a common candle is hot enough to melt a small filament of platinum.

Support of Flame.--That a flame may burn steadily, and produce a uniform light, it is necessary, that the supply of combustible matter should be constant and uniform. For this purpose, the combustible must be in a liquid, or gaseous state, when it approaches the flame, so that it may flow in an uninterrupted current. This current is commonly sustained, either by capillary attraction, or by mechanical pressure, operating on the reservoir which contains the combustible.

Torches and Candles.--The rudest material used for affording light, is the torch, composed of the resinous part of wood of the pine, or fir. In such torches, the turpentine, or melted resin, oozes out through the pores of the wood, and is gradually burnt, the wood interposing a vehicle, which regulates the supply, and prevents it from being consumed at once; thus sustaining a dull and irregular light, with much smoke, for some time. A common candle is an improvement upon this natural mechanism. It consists, as is well known, of a fusible solid, as tallow, wax, or spermaceti, formed into a cylinder, having a wick of cotton, or some other porous substance, for its axis. As the tallow melts by the radiated heat of the flame, it is carried upward by the capillary attraction of the wick, and is converted into vapor, as fast as it reaches the surface. The end of the wick, although it is blackened by the heat, is prevented from consuming, merely because it is surrounded by inflammable vapor, so that the oxygen of the atmosphere has no access to it. If the wick be turned to one side, so as to project from the blaze into the atmospheric air, it is immediately burnt off. Tallow, being more fusible than wax, requires to be burnt with a larger wick. The reason why this wick requires continual snuffing is, that, if it is suffered to become long, it divides the blaze, and intercepts a part of the light; it also cools the flame, by its radiation, obstructs the combustion, and thus causes the escape of smoke, and the deposition of charcoal. Wax and spermaceti, being less fusible, may be burnt with a smaller wick, which, if made sufficiently slender, bends out of the flame, and burns off, so as not to require snuffing.

Lamps.--When the combustible used is fluid, at common temperatures, a vessel is necessary to contain this fluid, and supply it to the flame. In this country, and in England, whale oil is the principal fluid which is burnt in lamps.[A] In France, and the south of Europe, the oil of poppies, of nuts, rape seed, and the inferior kinds of olive oil, are used for this purpose. The volatile oils are but seldom burnt, since they exhale a strong odor, and throw off soot, during their combustion. They are also liable to take fire, over their whole surface, unless guarded with great care. Naphtha, however, as it is found native, or as it is distilled from pitcoal, is used for supplying street lamps, in some of the cities of Europe.

Reservoirs.--As the flame of a lamp is intended to consume no more oil, than is attracted upward by the capillary action of the wick, it is necessary that a sufficient body of oil should be so placed, as to keep its surface, permanently, at a small distance below the level of the flame. The Greeks and Romans employed lamps of various forms, having the wick projecting from a sort of beak, at the side, nearly on a level with the surface of the oil. A similar plan is now practised in our street lamps. At the present day, portable lamps, of small size, are made with a central wick, having the reservoir of oil immediately below the flame. These reservoirs, if small, require frequent filling, and if large, cast an inconvenient shadow. All closed lamps require a minute hole, for the admission of air; otherwise, the pressure of the atmosphere will prevent the oil from ascending the wick. If this hole be obstructed, the oil will also sometimes overflow, from the expansion of the confined air, when heated.

[A] The oil which is extracted in cold weather, and called Winter-strained oil, remains fluid at low temperatures. The Summer-strained oil is liable to congeal in Winter. To obviate this inconvenience, lamps have been contrived for melting the Summer oil by the heat of the blaze. This is done, either by placing the reservoir of oil immediately over the blaze, or by conducting the heat by a metallic bar, which extends from the flame into the reservoir.

Astral Lamp.--With a view to get rid of the effect of shadow, various contrivances have been introduced, in which the reservoir is placed at a distance from the flame. In the astral and sinumbral lamps, the principle of which was invented by Count Rumford, the oil is contained in a large horizontal ring, having a burner at the centre, communicating with the ring by two or more tubes, placed like rays. The ring is placed a little below the level of the flame, and, from its large surface, affords a supply of oil for many hours. A small aperture is left, for the admission or escape of air, in the upper part of the ring. When these lamps overflow, it is usually because the ring is not kept perfectly horizontal, or else because the air hole is obstructed, a circumstance which may even happen from filling the lamp too high with oil.

Hydrostatic Lamps.--In several cases, the laws of hydrostatics have been applied to raise oil to the flame, from a reservoir, placed so far below the wick, as to be out of the reach of its effective capillary attraction. One of these hydrostatic lamps is constructed on the principle of Hero's fountain. It is composed of three vessels, or cavities, occupying different heights, and communicating by tubes, or syphons. One portion of oil, by descending gradually from the middle vessel, A, to the lower vessel, B, causes another portion of oil to ascend from the upper vessel, C, to the flame, at D, the hydrostatic equilibrium being kept up by the intervention of the column of air, B C.

The lamps of Girard de Marselle, and of King, are on this principle, though the form of their apparatus is that of a cylinder, with internal tubes, opening into different cavities.

Other hydrostatic lamps are constructed, so as to contain, in one part, a column of some fluid, the specific gravity of which is considerably greater than that of oil; such, for example, as water saturated with salt. This fluid acts in such a manner as to raise the oil, by its greater weight. Thus, if an inverted syphon contain oil in one part, and salt water in another, the surfaces of the two fluids will stand at different heights, inversely proportionate to their specific gravities. In the diagram, A, represents the surface of the heavier fluid, and B, that of the oil. The bulbs serve as reservoirs, to prolong the action. Mr. Kiers' lamp is constructed on this principle. Those of Barton and Edelkrantz depend on the same principle; but, in their construction, an open tube of oil is made to float in an upright vessel, containing a heavier fluid, which, in some cases, is salt water, in others, mercury. As the oil consumes, the tube, with the wick and light, descend in the supporting fluid, and follow the surface of the oil, as it lowers.

Automaton Lamp.--The automaton lamp of Porter, is a simple and effectual contrivance for keeping the surface of the oil near the level of the blaze. It consists of an oblong tin box, having the wick tubes at one end, this end being thus rendered heavier than the other. The box is suspended on pivots, placed a little out of the centre, and toward the tubes, so that, when the lamp is full of oil, the box will hang level. As the oil burns out, however, the end containing the tubes will preponderate, so as to keep the flame always near the surface of the oil. The annexed figures show the position of the lamp when full, and when half exhausted. This lamp is of cheap construction, and is said to be extensively used in cotton mills, and other manufactories, in the north of England.

Mechanical Lamps.--Some lamps are manufactured in France, in which the oil is raised from a large reservoir below, to a small one near the flame, by means of a pump. This, in some instances, is worked by hand, and in others is carried by clock-work, the motion being deprived from a spring, which is wound up as often as necessary.

Pressure Lamp.--A lamp has been lately introduced from France, which represents a column, having the reservoir of oil in the pedestal. A piston, which is wound up at proper intervals, descends by a spring upon the surface of the oil, and forces out a continual stream, which ascends in a side passage to the level of the flame, where it escapes slowly, through a minute aperture. A part of it feeds the combustion, while the remainder overflows, and sinks back to the bottom of the lamp. These lamps are usually provided with long glasses. They consume much oil, and give a brilliant light. They are objectionable, on account of the frequent winding up, which is necessary, and on account of the liability of the small aperture, through which the oil is delivered, to become clogged with impurities, so as to cut off the supply, and cause the extinction of the lamp.

Fountain Lamp.--The most common mode of disposing of the oil, in large lamps, is to place the reservoir above the level of the flame, so that the burner, or part containing the wick, may be supplied in small quantities, as fast as its oil is consumed. These reservoirs are constructed on the principle of the bird fountain. They are open-at bottom; but the oil is kept from running out at once, by the pressure of the atmosphere. The reservoir commonly terminates in a neck at bottom, with an opening on one side. This neck is immersed beyond the opening, in a small cavity, which contains oil nearly on a level with the burners, and communicates with them by tubes. So long as the whole of the opening is immersed, no oil can descend from the reservoir, because no air can enter, to take its place. But, whenever the oil in the lower cavity is consumed, so far as to sink below the upper edge of the opening, a bubble of air will enter the neck, and ascend into the reservoir; at the same time displacing an equal bulk of oil, which descends to feed the lamp. For convenience, the opening is commanded by a sliding valve; and, when the reservoir is to be filled, it is unscrewed from the lamp, inverted, and the oil poured in at the neck. When these lamps overflow, it is commonly owing to an increase in the heat of the room, which causes the air in the upper part of the reservoir to expand, and drive out a portion of oil. As it is not easy to prevent this occurrence, lamps are usually provided with receptacles at bottom, to receive the waste oil which runs over at the wick.

Argand Lamp.--This name is applied, after one of the inventors, to all lamps with hollow or circular wicks; and, of course, most of the lamps already described, may be also Argand lamps, if furnished with a circular burner. The intention of the Argand burner is, to furnish a more rapid supply of air to the flame, and to afford this air to the centre, as well as the outside, of the flame. It is constructed by forming a hollow cylindrical cavity, which receives oil from the main body of the lamp, and, at the same time, transmits air through its axis, or central hollow. In this cavity is placed a circular wick, attached at bottom to a movable ring. This ring is capable of being elevated, or depressed, by means of a rack and pinion, or, more commonly, by a screw; so that the height of the wick may be varied, to regulate the size of the flame. On the outside is placed a glass chimney, which is capable of transmitting a current of air, on the same principles as a common smoke-flue. When this lamp is lighted, the combustion is vivid, and the light intense, owing to the free and rapid supply of air. The flame does not waver, and the smoke is wholly consumed. The brilliancy of the light is still further increased, if the air be made to impinge laterally against the flame. This is done, either by contracting the glass chimney, near the blaze, so as to direct the air inwards, or by placing a metallic button over the blaze, so as to spread the internal current outward.

Submarine Lamp.--A lamp, ingeniously contrived to burn under water, has been connected with a diving apparatus, for examining the Thames tunnel. A box, containing the lamp, is made air-tight, with a glass in front, and a reflector behind. A quantity of alkali is placed in the box, and a reservoir of condensed oxygen is attached. The oxygen is admitted in a small stream, to support the flame. The products of the combustion are water, and carbonic acid. The water is condensed, and the carbonic acid combines with the alkali, so that room is continually made for the fresh supply of oxygen.

A common lamp may be made to burn under water, by enclosing it in a lantern, through which a current of air is continually forced, by means of a pump, and an elastic pipe.

Hydro-oxygen Light.--If a stream of oxygen and hydrogen be directed, while in combustion, upon a mass of quicklime, the result is an intense degree of ignition, attended with a most brilliant light, which is said to be visible at a greater distance than any other artificial light. It has been modified, in various ways, by conducting a stream of oxygen through alcohol, or oil of turpentine, and by using it in the combustion of common oil.

Spirit Lamp.--It has been found, that certain volatile and inflammable liquids are capable of burning with a bright light, but are objectionable, on account of the generation of smoke and lampblack, during their combustion. This defect has been obviated, by burning them in combination with other fluids, and in a lamp of particular construction. Oil of turpentine, when mixed with a certain proportion of alcohol, and, perhaps, with other fluids, burns with a clear, white light, in a lamp properly constructed. As these liquids are cheaper than common sperm oil, lamps for burning them were atone time extensively introduced. But they were found objectionable, on account of the volatility and extreme inflammability of the liquid, by which serious accidents occurred, when it was spilt upon the dress, and took fire. A mixture, which appears to resemble the foregoing, in some respects, has been introduced under the name of "chemical oil," for the use of light houses. It gives an intense and brilliant light, much exceeding that of common oil.

Reflectors.--For obvious reasons, a lamp yields most available light, when it is placed in the centre of a room, or space, to be illuminated. In this situation, if a reflecting surface be brought near to it, this surface, by its reflection, will increase the amount of light in one direction, at the expense of intercepting it in another, so that the total advantage is not increased by the reflector. But, when a lamp is placed near a wall, so that a part of its rays are wasted, by falling immediately upon the wall,--in this case, if a polished surface be placed behind the flame, it reflects back most of the rays, which would otherwise be lost upon the non-reflecting wall; and thus it increases the effect of the light. The familiar fact, that rooms, with light-colored walls, are most easily lighted, is owing to the greater reflective power which such walls possess, when compared with darker surfaces.

Hanging of Pictures.--As the surface of varnished paintings has a considerable reflecting power, it happens, that when the spectator stands in the way of the reflected light, his eye is dazzled, and rendered incapable of distinctly perceiving the picture. Paintings, therefore, should not be hung opposite to lights, nor in any situation in which a line, drawn from the place intended for spectators, will make the same angle with the surface of the picture, as a line drawn from a window, or other illuminating point; the angle of reflection being always equal to the angle of incidence. As a general rule, a picture will be in a bad light, with regard to a spectator, whenever the image of a window could be seen by him in a looking-glass, occupying the same place as the picture.

Transparency of Flame.--If two lamps be placed by the side of each other, the flame of the one, when clear of smoke, does not intercept the light of the other, and casts little or no shadow. Count Rumford found, that the brilliancy of flame is, in some high ratio, proportionate to its elevation of temperature. If several concentric circular wicks, or several parallel flat wicks, be burnt near together, they produce more light, in consequence of the accumulation of heat, than they would do, if burnt separately.

Glass Shades.--To relieve the eye from the glare of light, produced by bright lamps, shades of roughened glass are frequently used. A rough surface upon glass maybe produced by grinding it with sand or emery, by corroding it with fluoric acid, or by covering it with powdered glass, and exposing it to heat, till the particles adhere. Glass shades have the effect to disperse the rays of light, by the numerous reflections and refractions which they occasion; till, at length, the light issues from all parts of their surface, and it appears as if the glass itself were the luminous body.

Sinumbral Lamp.--The reservoir of the sinumbral lamp is constructed on the same general principles with that of the astral. The ring, however, which holds the oil, is so formed, as to oppose the smallest diameter of its section to the rays of light. A large shade of ground glass is used, which nearly encloses the light, and, by the different refractions and reflections given to the rays, by the ground glass, they escape in all directions, so that there is no perceptible shadow, at a small distance from the ring. Reflectors are sometimes added, when it is desired to throw the principal mass of light in one direction.

Measurement of Light.--The following method of measuring the comparative illuminating power of different lights is founded on the law, that the amount of rays, thrown on a given surface, is inversely as the square of the distance of the illuminating body. Place two lights, which are to be compared with each other, at the distance of a few feet, or yards, from a screen of white paper, or a white wall. On holding a small card near the wall, two shadows will be projected on it, the darker one, by the interception of the brighter light, and the fainter shadow, by the interception of the duller light. Bring the fainter light nearer to the card, or remove the brighter light farther from it, till both shadows acquire the same intensity, which the eye can judge of with great precision, particularly from the conterminous shadows at the angles. Measure now the distances of the two lights from the wall or screen, and the squares of these distances will give the ratio of illumination. Thus, if an Argand flame and a candle stand at the distances of ten feet, and four feet, respectively, when their shadows are equally deep, we have the square of ten, and the square of four, or one hundred, and sixteen, as their relative quantities of light. In this experiment, the spectator should be equidistant from each shadow.

Light Houses.--Light houses are permanent beacons, erected along the seacoast, for the guidance of mariners in the night. Their general form is that of a small tower, with a lantern at top. Some of these have been erected, with great difficulty, on sunken reefs, or small rocks, exposed to the violence of the sea. Such is the case with the Eddystone, and Bell-rock light houses, on the coast of Great Britain, in the construction of which, much expense and great architectural skill were necessary, to insure their stability. Floating lights are sometimes used, to give notice of shoals. They consist of vessels, which are moored in the requisite situations, having lanterns fixed at their mast head.

Light houses were used by the ancients. The celebrated Pharos, of Alexandria, accounted one of the wonders of the world, appears to have existed as early as three hundred years before Christ. In England, they were in use, in the reign of Henry VIII., and in Scotland, in the time of James VI.

The lanterns, which form the top of light houses, are usually of an octagonal form, with windows for sides. The best are made of plate glass, with sashes of iron, or bronze. Within these are placed lamps, with Argand burners, the number and character of which are varied, so as to distinguish one light house from another.

The lights are either white or red, stationary or revolving, intermittent, flashing, double, or leading. Their intensity is increased by reflectors, placed behind them, which are generally of a parabolic form, made of copper, plated with silver, and highly polished; or, in cheaper constructions, made of tinned iron.

The revolving light is produced by the motion of a frame, carried by clock-work, having several faces, furnished with reflectors behind. As the frame revolves, the lights are observed gradually to increase, till they arrive at their greatest intensity, after which they gradually decline and disappear; and are succeeded by others in the same manner. The flashing light is constructed on the revolving principle, but the revolutions being more rapid, and the light, in some cases, thrown through tubes, it is characterized by a succession of bright, transient flashes. The intermittent light is distinguished by its bursting suddenly on the view, and continuing steady for a short time, after which it is eclipsed for, perhaps, half a minute. This effect is produced by the periodical interposition of shades, by which the light is alternately hid and displayed. The double light consists of two lights, one above the other, displayed from the same tower. The leading lights are exhibited from two towers, one higher than the other, and, when seen in one line, they form a direction for the course of the shipping.

Gas Lights.--In the flame of a common lamp, or candle, the combustible matter is not burnt, until it has first been converted into vapor, or inflammable gas. This gaseous matter is burnt, as fast as it is generated, in consequence of being brought immediately in contact with the atmospheric air, and set on fire by the same heat which produces it. It is found, however, if certain combustibles be exposed to heat, and if the inflammable gas, which they yield, be kept separate from the atmospheric air, that this gas may be conveyed in pipes to any distance, and burnt for light in any place, where a stream of it is discharged into the atmosphere. In this way, various combustibles may be used, which are not capable of being burnt in lamps, and a brilliant and economical light obtained from them. The materials chiefly employed for this purpose, are pitcoal> and animal oil. Various other substances are capable of supporting gas lights, such as bitumen, resin, oleaginous vegetable seeds, other oily or resinous bodies, and even wood, and turf. The inflammable gas, which is procured from all these substances, is chiefly carburetted hydrogen. Of this, two kinds are known, the first, sometimes called defiant gas, and the other, subcarburetted hydrogen. Mr. Brande, however, considers the last species as merely a mixture of the first with hydrogen. The fitness of a mixed gas, for purposes of illumination, is dependent on the quantity of carburetted hydrogen which it contains, other things being equal.

Coal Gas.--The use of coal gas, for purposes of illumination, appears to have been first introduced by Mr. Murdoch, in 1792, although its power of affording a luminous flame was known much earlier. It is found that the bituminous coals, and particularly cannel coal, afford the most, and the best, illuminating gas. Some of the anthracites, according to Professor Silliman, afford as much gas as Liverpool coal; but it burns with a feeble flame, and is unfit for the purposes of illumination.

In the manufacture of coal gas, the coal is placed in iron retorts, which are subjected to a strong heat in a furnace. The gas is thus driven off, mixed with the vapor of tar, oil, and ammoniacal water, and in this state is conducted by pipes, first into a horizontal trunk of cast-iron, called the hydraulic main, and from thence into a condensing apparatus, surrounded with cold water, where the vapors of the tar, oil, and water, are condensed, and fall down, while the gaseous product is conveyed along, containing several impure gases, such as sulphuretted hydrogen, and carbonic acid.

In order to separate the carburetted hydrogen from these impurities, various contrivances have been adopted. The usual method of purifying coal gas, is to make it pass through a mixture of lime.and water, called cream of lime, which absorbs, or combines with, the contaminating gases. For this purpose, a considerable number of purifiers are erected, and the lime and water are kept in a state of constant agitation, either by a steam-engine, or by one or two men, till the gas is rendered sufficiently pure. Sometimes, the purification is effected, by causing the gas to pass in contact with solid lime, newly slaked; and, sometimes, by passing it through retorts, containing clippings of iron, made red hot. When thus purified, the gas is conveyed, by a pipe, to the gasometer.

The Gasometer, is a large inverted vessel, made of malleable iron, or copper, either of a cylindrical or rectangular form, and suspended over a reservoir of water, of a little larger size, by means of counter-weights. The gas is introduced by pipes, ascending from the bottom of the reservoir, and rising a little above the surface of the water. While the gasometer is filling with gas, it gradually rises out of the water, until it is filled, after which no more gas is admitted, and its contents are ready to be distributed through the pipes, by which it is to be conveyed to the place intended to be illumined by burning it. As the gas is forced out by the weight of the gasometer, and is burned, the gasometer descends, gradually, in the water, till the whole of its contents are expelled, when it is again filled, by the same process as before.

The gas being thus ready for use, must be carried off by pipes, the diameter of which is proportional to the degree of light required. It has been found that a pipe one inch in diameter, will, under a pressure of a column of water from five eighths to three fourths of an inch, supply gas equal to one hundred candles; and if there was no friction, or mechanical impediment, the number of candles would be found, for other diameters of pipe, by multiplying the square of the diameter of the pipe, in inches, by one hundred. The friction, however, or obstruction, diminishes so rapidly with the diameter of the pipe, that the number of candles is always greater than this rule gives. Thus, a pipe three inches in diameter will supply light equal to one thousand candles; a pipe four inches, two thousand; a pipe six inches, five thousand; and a pipe ten inches, about fourteen thousand.

When the gas is to be burned in rooms, shops, or streets, it is allowed to escape through small circular apertures, of from one fortieth to one sixtieth of an inch in diameter, which may be arranged in various ornamental ways, or disposed in a circle, like an Argand burner, with a current of air running between them. The lights thus produced are equal, steady, and of the most brilliant kind. When the supply of gas is cut off, they are instantly extinguished. When it is restored, the invisible current flows out, and may be instantly lighted again by the contact of flame.

Oil Gas.--It has been long known to chemists, that wax, oil, tallow, &c, when passed through ignited tubes, are resolved into combustible gaseous matter, which burns with a bright light. Of late years, this gas has been used for purposes of illumination. Oil gas is considered, in many respects, superior to coal gas, and free from its inconveniences. The material, from which it is produced, containing no sulphur, or other matter, by which coal gas is contaminated, it never produces a suffocating smell in rooms; so that the costly operation of purifying the gas, by lime, and other means, is avoided. Nothing is contained in oil gas, which can injure the metal, of which the conveyance pipes are made.

The oil gas has a further advantage over coal gas, in containing a greater proportion of carburetted hydrogen, so that one cubic foot of oil gas is said to go as far as two or three of coal gas. This circumstance is of importance, as it reduces, in the same proportion, the size of the gasometers, which are necessary to contain it. Oil gas contains about seventy-five per cent, of carburetted hydrogen, while purified coal gas but seldom contains more than forty per cent.

In procuring this gas, a quantity of oil is placed in an air-tight vessel, in such a manner, that it may pass slowly into retorts, or iron tubes, which are kept at a moderate red heat. Fragments of coke, or brick, are usually enclosed in the tubes. The oil, in its passage through the retorts, is principally decomposed, and converted into gas, proper for illumination, carrying with it, however, some oil, in the state of vapor. To purify the gas from this oil, which is suspended in it, and which occasions an empyreumatic smell, it is conveyed into wash vessels, where, by bubbling through water, or through fresh oil, it is cooled, and rendered fit for use. It then passes, by a proper pipe, into a gasometer, from which it is suffered to pass off in pipes, in the usual manner, to its places of destination.

The poorest kinds of oil, which are unfit for burning in lamps, produce excellent gas. This is, indeed, the chief source of economy in the process, which, otherwise, is too expensive.

According to Mr. Brande, a light, equal to ten wax candles, for one hour, requires for its production, two thousand and six hundred cubic inches of pure carburetted hydrogen, or defiant gas, four thousand eight hundred and seventy-five cubic inches of oil gas, or thirteen thousand one hundred and twenty cubic inches of coal gas.

Gasmeter.--In dispensing gas, for the illumination of particular rooms, it was found necessary to possess some method of measuring the quantity expended in each place. An ingenious instrument, called the gasmeter, has been introduced for this purpose. It consists of a horizontal cylinder, partly filled with water, within which, another cylinder revolves, on an axis, having its interior surface divided into several compartments. These compartments, being successively filled with the gas, as it passes through, rise out of water, like inverted buckets of an overshot wheel, and cause the inner cylinder to revolve. The number of revolutions is registered by machinery; and thus the quantity of gas which escapes, in a given time, is estimated.

Portable Gas Lights.--The magnitude and expense of gas works prevents the use of them, except in cases where a large number of lights are wanted, within a convenient distance from the gasometer. The gas, however, may be conveyed to any distance, by condensing it in strong vessels of iron or copper, made of a small or portable size. The gas is forced into these vessels by a condensing pump, and, when afterwards suffered to escape, through a small orifice, is capable of supporting a flame for many hours. The economy, however, of this process has, with reason, been doubted.

Safety Lamp.--In coal mines, an inflammable gas is generated, called fire damp by the miners, and composed chiefly of carburetted hydrogen. This gas, when mixed with atmospheric air, is liable to take fire from the flame of a lamp, or candle, and to explode with great violence. Terrible accidents have happened, and many lives have been destroyed, from these explosions. To prevent such accidents, several troublesome and circuitous modes of obtaining light were resorted to by the miners; such as striking sparks from a wheel, and enclosing a lamp within a tight lantern, which was supplied with air from a bellows. All these are now superseded by the safety lamp of Sir Humphrey Davy. This important invention consists simply of a lamp, the flame of which is wholly enclosed in a cylinder of fine wire gauze. Its operation depends on the principle discovered by Sir H. Davy, that explosive mixtures cannot be inflamed through minute apertures, in metallic surfaces, or tissues. The.wire gauze, being a powerful conductor and radiator of heat, cools a flame which is in contact with it, so as to deprive it of the power of producing an explosion on the other side. If this lamp be immersed in an explosive mixture, the gas will be inflamed, and burn on the inside of the gauze cylinder, but not on the outside. In these cases, the flame of the lamp first enlarges, and is then extinguished, the whole of the cage being filled with a lambent blue light. If the supply of gas be withdrawn, this appearance gradually ceases, and the wick becomes rekindled.

Recently, it has been found, that the Davy lamp is not a protection against explosion, when exposed to much motion, or to a current of air. It has, therefore, been improved by Messrs. Upton and Roberts, so as to keep, between the flame and the external air, a layer of carbonic acid, a cylinder of wire gauze, and a cylinder of glass.

Lamp without Flame.--This curious instrument may be made, by winding upon the wick of a lamp, containing alcohol, a fine wire of platinum, not more than a hundredth part of an inch in thickness. There should be about sixteen spiral turns, one half of which should surround the wick, and the other half rise above it. Having lighted the lamp for an instant, on blowing it out, the wire will become brightly ignited, and will continue to glow, as long as any alcohol remains, without the blaze being any more renewed. The principle depends upon the slow combustion which is found to take place, in inflammable or explosive mixtures, at a lower temperature than is necessary to produce inflammation. This combustion is not visible; but the heat is, nevertheless, sufficient to ignite minute solids, exposed to its influence. In the lamp, which has been described, the explosive mixture is the vapor of alcohol and atmospheric air. But the experiment may be varied, by using ether, camphor, &c, and by substituting platinum leaf, for wire.

Modes of procuring Light.--To obtain light and fire, when wanted, in an expeditious manner, various instruments have been introduced, constructed on optical, mechanical, and chemical, principles. The methods, by which they operate, are, chiefly, the following. 1. By concentration of the solar rays, as in the focus of a common lens, or burning glass. 2. By friction. Dry wood takes fire, if rubbed violently, in the manner practised by savages, or if it be held against the surface of a wheel which revolves rapidly. Phosphorus takes fire by very slight friction, and, on this account, is used in the phosphoric fire bottles, the matches of which, after being charged with a minute quantity of phosphorus, take fire by rubbing them on the cork. Other matches, now very common, have their ends coated with a mixture of phosphorus and sulphur. 3. By percussion. When hard bodies, such as flint and steel, are brought into collision, small particles of ignited matter are struck off, in the form of sparks, which are sufficiently hot to set fire to tinder, gunpowder, &c. Common firelocks, tinder-boxes, &c, operate on this principle. 4. By compression. If a piece of tinder is confined in a small cavity, at the end of a condensing syringe, it will take fire, if the piston of the syringe be driven down with a stroke, so as suddenly to condense the air. The tinder, commonly used for this purpose, is what is called German tinder, made of a fungus that grows on trees, (Boletus igniarius,) boiled in a solution of nitre, and dried. 5. By chemical action. In the oxymuriatic fire boxes, the matches are charged with chlorate of potash, mixed with sulphur, or some other combustible. When these are brought into contact with sulphuric acid, a violent chemical action takes place, and the match takes fire. Homberg's pyrophorus takes fire, on exposure to the air. It may be made by calcining alum with less than an equal quantity of flour, or sugar, until the smoke and flame disappear. It is then kept in close-stopped bottles; and, if a little of it be shaken out, upon any light combustible, as cotton or tow, it causes it to inflame. The platinum lights depend on a remarkable property, discovered by Dobereiner, in platinum, by which a sponge, made of that metal, becomes ignited, when exposed to a stream of hydrogen gas.

WORKS OF REFERENCE.--ACCUM, on Gas Light, 8vo. 1816;--PECKSTON, on Gas Lighting;--RUMFORD'S Works;--NICHOLSON'S Philosophical Journal, vol. i. 4to. vol. xiv. 8vo., &c--REES' Cyclopedia;--URE'S Chemical Dictionary;--Ditto, Dictionary of Arts and Manufactures, 8vo. 1839.