METHODOFASCERTAINING THE ILLUMINATING POWEROFCANDLES, LAMPS, GAS-LIGHTS,ANDOTHER LUMINOUS BODIES.

[3]Philosophical Journal, 4to Series, Vol. I. p. 70.

Though the eye is not fitted to judge of the proportional force of different lights, it can distinguish, in many cases with great precision, when two similar surfaces, presented together, are equally illuminated. But as the lucid particles are darted in right lines, they must spread uniformly, and hence their density will diminish in the duplicate ratio of their distance. From the respective situations, therefore, of the centres of divergency, when the contrasted surfaces become equally bright, we may easily compute their relative degrees of intensity.

For this purpose it is assumed as a principle, that the same quantity of light, diverging inall directions from a luminous body, remains undiminished in all distances from the centre of divergency. Thus we must suppose, that the quantity of light falling on every body, is the same as would have fallen on the places occupied by the shadow; and if there were any doubt of the truth of the supposition, it might be confirmed by some simple experiment. Therefore, it follows, that, since the shadow of a square inch of any surface occupies at twice the distance of the surface from the luminous point the space of four square inches, the intensity of the light diminishes as the square of the distance increases. If, consequently, we remove two sources of light to such distances from an object that they may illuminate it in equal degrees, we may conclude that their original intensities are inversely as the squares of the distances.

Hence, if two lights of unequal illuminating powers shine upon the same surface at equal obliquities, and an opake body be interposed between them and the illuminated surface, the two shadows produced, must differ in blackness or intensity in the same degree. For the shadow formed by intercepting thegreater 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, the stronger light will be attended with the deeper shadow. Now it is easy, by removing the stronger light to a greater distance, to render the shadow which 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, of whatever nature they are, 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 this figure;

Overlapping shadows

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 some experiments made in this way, the degree of illumination of different lights may readily be ascertained to the tenth part of the whole. And, by experiments of this kind, many useful particulars may be shewn. For, since the cost and duration of candles, and the consumption of oil in lamps, are easily ascertainable, it may be shewn whether more or less light is obtained at the same expence during a given time, by burning a number of small candles instead of one or more of greater thickness. It will thereforebe easy to compare the power of different kinds of lamps or candles, or gas lights, so as to determine the relative cost of each particular kind of the combustible substance employed for furnishing light:—for example, if a candle and a gas-burner supplying coal-gas, adjusted by a stop-cock, produce the same darkness of shadow, at the same distance from the wall, the strength or intensity of light is the same. An uniform degree of intensity of the gas-light may readily be produced, by opening or shutting the stop-cock, if more or less be required, and the candle is carefully snuffed to produce the most regular and greatest quantity of light. The size of the flame in experiments of this kind of course becomes unnecessary, and will vary very much with the quality of the coal gas. The bulk of the gas consumed, and the quantity of tallow used, by weighing the candle before and after the experiment, furnish the data for ascertaining the relative costs of tallow and gas-light, when compared with each other.

From experiments made by CountRumford, concerning the quantity of materials requisite for producing a light of a certain intensity fora given time: it was found that we must burn of wax 100, of tallow 101, of oil, in an Argand’s lamp, 129, of an ill-snuffed tallow candle 229 parts, by weight. And with regard to the quantity of carburetted hydrogen, or coal-gas, I have found that from 18 to 20 cubic feet (according to the purity of the gas) are required to give a light equal in duration and in illuminating powers to 1lb. of tallow candles, six to the pound, provided they were set up and burnt out one after another.[4]

[4]112lbs. of Newcastle coal, called Tanfield Moor, produce, upon an average, from 250 to 300 cubic feet of gas, fit for illumination.

It is sufficiently known that the light of a candle, which is so exceedingly brilliant when first snuffed, is very speedily diminished toone-half and is usually not more than one-fifth or one-sixth before the uneasiness of the eye induces us to snuff it.[5]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.

[5]Ezekiel Walker.—Nicholson’s Journal, Vol. IV. 8vo. Series.

When a lighted candle is so placed as neither to require snuffing or produce smoke, it is reasonable to conclude that the whole of the combustible matter which is consumed is converted to the purpose of generating light; and that the intensities of light afforded in a given time, by candles of different dimensions, are in proportion to the quantity of matter consumed. That is to say; when candles are made of the same materials, if one candle produce twice as much light as another, the former will in the same time lose twice as much weight as the latter.

To prove the truth of this position, Mr. Walker made the experiments contained in the following

TABLE.

These experiments, Mr. Walker informs us, were made in the following manner:—

Three candles, the dimensions of which are given in the table, against 1, 3, and mould. These were first weighed, and then lighted at the same instant. At the end of the time inserted in the third column of the above table, they were extinguished and weighed again,and the loss of weight of each candle is contained in the fourth column.

The three first experiments were made under such favourable circumstance, that there was little doubt of their results being more accurate than what practical utility requires, but the fourth experiment cannot be depended on so much, in consequence of the variable light of No. 5. This candle was moved so often to keep the two shadows equal, that it was found necessary to set down its mean distance from the wall by estimation; but as this was done before the candles were weighed, the experimenter’s mind could not be under the influence of partiality for a system.

The method which Mr. Walker employed in comparing one light with another in each experiment, was that which has been describedpage 24.

1. The experiments were made at different times, and the light of the mould candle was made the standard, with which the lights of the others were compared; but it must not be understood, that this candle gave the same strength of light in every experiment.

2. The sign + in the 5th column, signifiesthat the candle against which it is placed, gave a stronger light than the others.

From the experiments contained in the table, it appears to be an established law, where combustion is complete, that the quantities of light produced by tallow candles, are in the complicate ratio of their times of burning and weights of matter consumed.

For if their quantities of matter be equal, and times of burning the same, they will give equal quantities of light,by the experiments.

And if the times of burning be equal, the quantities of light will be directly as their weights of matter expended.

Therefore the light is universally in the compound ratio of the time of burning and weight of matter consumed.

If the law which Mr. Walker has endeavoured to prove, both by reason and experiment, be admitted, we have a standard with which we may compare the strength of any other light.

Let a small mould candle, when lighted, be so placed as neither to produce smoke nor require snuffing, and it will lose an ounce of its weight in three hours. Let this quantityof light produced under these circumstances, be represented by 1.00.

Then should this candle at any other time, lose more or less of its weight in three hours than an ounce, the quantity of light will be still known, because the quantity of light in a given time is directly as the weight of the candle consumed.[6]

[6]To investigate rules for this purpose, 1. Let M represent the mould candle,aits distance from the wall, on which the shadows were compared,xits quantity of matter consumed in a given time, (t) and Q the quantity of light emitted by M in the same time: 2. Letmrepresent any other candle,bits distance from the same wall, andyits quantity of matter consumed, in the timet.Then as the intensities of light are directly as the squares of the distances of the two candles from the wall, we have asa2: Q∷::b2:b2+ Qa2= the quantity of light, emitted bymin the time.Then let us suppose that the quantities of light are directly as the quantities of matter consumed in the timet, and we have, Asx: Q∷::y:y+ Qx= the quantity of light emitted bymin that time, by hypothesis.Now, whenb2+ Qa2(Theo. 1.) is =Y + QX(Theo. 2.) the quantities of light of M andmare directly as their quantities of matter consumed in any given time.

Then as the intensities of light are directly as the squares of the distances of the two candles from the wall, we have asa2: Q∷::b2:b2+ Qa2= the quantity of light, emitted bymin the time.

Then let us suppose that the quantities of light are directly as the quantities of matter consumed in the timet, and we have, Asx: Q∷::y:y+ Qx= the quantity of light emitted bymin that time, by hypothesis.

Now, whenb2+ Qa2(Theo. 1.) is =Y + QX(Theo. 2.) the quantities of light of M andmare directly as their quantities of matter consumed in any given time.

Mr.Ezekiel Walkerhas shewn that, if a trifling alteration be made in the method of using common tallow candles, they will become excellent substitutes for those of wax.

A common candle, weighing one-tenth of a pound, containing fourteen single threads of fine cotton, placed so as to form an angle of 30 degrees[7]with the perpendicular, and lighted, requires no snuffing; and what is much more valuable for some purposes, it gives a light that is nearly uniform in strength without the least smoke. These effects are thus produced:

[7]Candlesticks may be made to hold the candle at this angle, or they may be so contrived as to hold the candle at any angle at pleasure.

When a candle burns in an inclined position, most part of the flame rises perpendicularly from the upper side of the wick, and when viewed in a certain direction, it appears in the form of an obtuse angled triangle. And as the end of the wick projects beyond the flame at the obtuse angle, it meets with the air, and is completely burnt to ashes: hence it is rendered incapable of acting as a conductor to carry off part of the combustible matter in the form of smoke. By this spontaneous mode of snuffing, that part of the wick which is acted upon by the flame continues of the same length, and the flame itself very nearly of the same strength and magnitude[8].

[8]The wick’s not being uniformly twisted throughout, may occasion a little variation in the dimensions of the flame.

The advantages which may be derived from candles that require no snuffing and afford no smoke, may be readily understood; but these candles have another property which ought not to be passed over in silence. A candle snuffed by an instrument gives a very fluctuating light, which, in viewing near objects is highly injurious to the eye; and this is aninconvenience which no shade can remove. But when a candle is snuffed spontaneously, it gives a light so perfectly steady and so uniformly bright, that the adjustments of the eye remain at rest, and distinct vision is performed without pain, and without uneasiness.

Candles, on which Mr.Walkerhas made experiments, are described in the following

TABLE.

Number 1, 2, and 3. These candles, when lighted and placed to form an angle of 30° with the perpendicular, require no snuffing: they give lights which are nearly equal, and combustion proceeds so regularly, that no part of the melted tallow escapes unconsumed, except from accidental causes.

No. 4, placed at the angle mentioned above, and lighted, requires no snuffing: it gives alight very little stronger than No. 1, but its colour is not quite so white, nor its flame so steady.

No. 5. This candle, placed at an angle of 30°, and lighted, requires no snuffing; its flame is rather fluctuating, and not so white as No. 4, nor is its strength of light much greater than No. 1. The melted tallow sometimes overflows when the air in the room is put in motion; yet the light of this candle is much improved by being placed in an inclined position.

The mould candle, treated in the same manner, affords a very pure steady flame, without smoke and without snuffing, and its strength of light is about equal to that of No. 1.

The experiments have not been sufficiently numerous to determine with precision which of these candles affords the most light at a given expence, but the few experiments which have been made seem to indicate, that the quantity of light is nearly as the quantity of combustible matter consumed, and thus a candle which is used in the manner pointed out gives more light than a candle of the same dimension set perpendicularly and snuffed,because one part of a candle that is snuffed, is thrown away, and another part flies off in the form of smoke. And this is not the only inconvenience that attends the using candles in this manner, and which the other method is free from, for the light which it gives is of a bad quality, on account of its being variable and undulating.

From the time that a candle is snuffed till it wants snuffing again, its strength of light scarcely continues the same for a single minute. And that variation which frequently takes place in the height of the flame, is a matter of still more serious consequence.

The flame of a long candle placed vertically when it is snuffed burns steadily, is about two inches high, but it very frequently rises to the height of four inches or upwards; drops down again in a moment, till it is less than three inches, and then rises again. In this manner the flame continues in motion for some time before it returns to its original dimensions. But it does not continue long in a quiescent state before it begins a new series of undulations. In this manner the candle burns till the top of the wick is seen near the apex ofthe flame, carrying off clouds of smoke. In this state of things the eye becomes uneasy for want of light, and the snuffers are applied to remove the inconvenience.

Mr.Walkerfurther observes, that it is these sudden changes, and not the nature of candle-light itself, that do so much injury to the eye of the student and artist; and that that injury may be easily prevented, by laying aside the snuffers, and in the place of one large candle, let two small ones be used in the manner stated.

The following observations on this subject are copied from the Monthly Magazine, 1805, p. 206.

“It is scarcely necessary to observe, that the combustion of candles proceeds the quicker in proportion as the inclination is greater. From the experiments which I have made, I should consider an angle of forty degrees with the perpendicular as the maximum of inclination, beyond which several considerable inconveniencies would occur; and I should take 25 degrees as the minimum of inclination, less than which does not sufficiently expose the point of the wick to the action of the air.

“By those who are much in the habit of reading or writing by candle-light, it will also be esteemed no inconsiderable addition to the advantages already mentioned, that the trouble of seeking and applying the snuffers is superseded. A candle of common size in a vertical position, requires the application of the snuffers forty-five times during its complete consumption.

“But I found an obstacle to the adoption of Mr.Walker’s plan, which, from the inclined position of the candle, it did not immediately occur to me by what means to counteract. Any agitation of the air of the room, occasioned either by the opening or shutting of a door, or by the quick passage of a person near the candle, caused the melted tallow to run over, or, in more familiar language, caused the candle to gutter; which, with the candle in this position, became an insuperable bar to the use of it.

“For the prevention of this inconvenience, I have had a wire skeleton-shade adapted to a rod bearing the same inclination as the candle, and which at bottom joins the candlestick in an horizontal line of about two inches, terminatingin a nozzle fitting that of the candlestick.—The distance of this rod from the candlestick, or, which is the same thing, the length of the foot or horizontal line, is of course to be determined by the distance between the two circles which form the upper and lower apertures of the shade.—It may serve, perhaps, more familiarly to describe this part of the apparatus, to state, that it bears a perfect resemblance to the two first strokes of the written figure 4; and the third stroke, if carried up as high as the first, and made sloping instead of upright, will very well represent the situation of the candle.

“When a strong light, for the purposes of reading or writing, be required, a white silk or paper may be used, as is common, over the skeleton; but when it be required that the light should be dispersed over the room, a glass of a similar shape may be adopted, for the purpose of preventing the flame from being influenced by any agitation of the air of the room. If the upper circle of the shade be four inches in diameter, the apex of the flame will be within it during more than half the time of the complete consumption of the candle; the shadewill not, therefore, require adjusting for the purpose of preventing injury to the silk, or whatever else may be used over the skeleton, more than once during that time.

“Being myself much averse to the interruptions which a candle used in a vertical position occasions, and which, though short, may, under some circumstances, be highly vexatious, I wish to extend to others a benefit which I prize rather highly.”

LordStanhope[9]has published a simple method of manufacturing candles, which, according to his Lordship’s statement, is superior to the method usually employed. The principles upon which the process depends are the following:—First, the wick of the candle is to have only three-fourths of the usual number of cotton threads, if the candle be of wax or spermaceti; and only two-thirds of the usual number, if the candle be of tallow. Secondly, it is required that the wick in all cases be perfectly free from moisture, a circumstance seldom attended to in the manufacturing of candles; and thirdly, to deprive the wick of waxcandles, of all the air which is entangled in its fibres, and this may conveniently be done, by boiling it in melted wax, till no more air bubbles, or froth appear on the surface of the fluid.

[9]Repository of Arts, Vol. I, p. 86.

If these circumstances be attended to, three candles of any size thus prepared, last as long as four of the same size manufactured in the common way. The light which they afford is superior and more steady than the light of common candles; and lastly, candles made in this manner, whether of wax, spermaceti, or tallow, do not require to be snuffed as often. Besides all this, they flame much less, and are consequently better for writing, reading, working and drawing, than candles made by the common method.

The following observations will enable any person who is willing to try the candles manufactured according to Lord Stanhope’s plan, to ascertain the real value of the improvements suggested by his Lordship. It shews also the result of some experiments, made to ascertain the expence of burning oil in lamps with wicks of various sizes.

A taper lamp, with eight threads of cotton, will consume in one hour225⁄1000oz. of spermaceti oil: at six shillings per gallon, the expence of burning twelve hours is 13.71 farthings.

At seven shillings, it is 15.995 farthings.

At eight shillings, it is 18.280 farthings.

N. B. This gives as good a light as tallow candles of eight and ten in the pound. This lamp seldom wants snuffing, and casts a steady and strong light.

A taper, chamber, or watch lamp, with four ordinary threads of cotton in the wick, consumes 1.664 oz. of spermaceti oil in one hour: the oil at seven shillings per gallon, the expence of burning twelve hours, 7.02 farthings.

At eight shillings, it is 8.022 farthings.

At nine shillings, it is 9.024 farthings.

TABLE,

Exhibiting a series of experiments, made with a view to determine the real and comparative expence of burning candles of different sorts and sizes.

The time each candle lasted, was taken from an average of several trials on each size.

It has been suggested by Dr.Franklin, that the flame of two candles joined, gives a much stronger light than both of them separately. The same, has been observed by Mr.Warren, to be the case with flames of gas-lights, which,when combined, give a much stronger light than they would afford, when in a separate state.

Indeed, in all cases, where flames for producing light are placed near to each other, it is always beneficial to preserve the heat of the flame as much as possible. One of the most simple methods of doing this, is no doubt, the placing of the several flames together, and as near as possible to each other without touching, in order that they may mutually cover and defend each other against the powerful cooling influence of the surrounding cold bodies. This principle is now employed in the Liverpool lamp, which acts by several flat or ribband wicks placed in the form of a cylinder. The power of illumination of this lamp is superior in effect and more economical than any other lamp in use—and as flame is perfectly transparent to the light of another flame which passes through it, there is no danger of loss of light on account of the flames covering each other.

A new art of procuring artificial light, which consists in burning the gazeous fluid obtained by distillation from common pit-coal, has of late engaged the attention of the public, under the name ofgas-light.

The encouragement that has been given for some years past by the legislature to this system of lighting, has induced certain individuals to apply the coal-gas light for the illumination of streets, houses, roads, and public edifices. And it is sufficiently known that a company has been incorporated by charter under the name of the “Gas Light and Coke Company,” to apply this new art of procuring light, byway of experiment, on a large scale, in lighting the streets of the metropolis.[10]

[10]An Act for granting certain powers and authorities to a company to be incorporated by charter, called the “Gas Light and Coke Company,” for making inflammable air for the lighting of the streets of the metropolis, &c.—Session 1810, 50th Geo. III.

The power and authorities granted to this corporate body are very restricted and moderate. The individuals composing it have no exclusive privilege; their charter does not prevent other persons from entering into competition with them. Their operations are confined to the metropolis where they are bound to furnish not only a stronger and better light to such streets and parishes as chuse to be lighted with gas, but also at a cheaper price than shall be paid for lighting the said streets with oil in the usual manner. The corporation is not permitted to traffic in machinery for manufacturing or conveying the gas into private houses, their capital or joint stock is limited to 200,000l.and His Majesty has the power of declaring the gas-light charter void, if the company fail to fulfil the terms of it.

Pit-coal exists in this island in strata, which, as far as concerns many hundred generations after us, may be pronounced inexhaustible; and is so admirably adapted, both for domestic purposes and the uses of the arts, that it is justly regarded as a most essential constituent of our national wealth. Like all other bituminous substances, it is composed of a fixed carbonaceous base or bitumen, united to more or less earthy and saline matter constituting the ashes left behind when this substance is burnt. The proportions of these parts differ considerably, in different kinds of coal; and according to the prevalency of one or other of them, so the coal is more or less combustible, and possesses the characters ofperfect pit-coal; and by various shades, passes from the most inflammable canel-coal, into blind, Kilkenny, or stone-coal; and, lastly, into a variety of earthy or stony substances; which, although they are inflammable, do not merit the appellation of coal.

Every body knows that when pit-coals are burning in our grates, a flame more or less luminous issues from them, and that they frequently emit beautiful streams of flame remarkably bright. But besides the flame, which is a peculiar gas in the state of combustion, heat expels from coal an aqueous vapour, loaded with several kinds of ammoniacal salts, a thick viscid fluid resembling tar, and some gases that are not of a combustible nature. The consequence of which is, that the flame of a coal-fire is continually wavering and changing, both in shape, as well as brilliance and in colour, so that what one moment gave a beautiful bright light, in the next, perhaps, is obscured by a stream of thick smoke.

But if coals, instead of being suffered to burn in this way, are submitted to distillation in close vessels, all its immediate constituent parts may be collected. The bituminous partis melted out in the form of tar. There is disengaged at the same time, a large quantity of an aqueous fluid, contaminated with a portion of oil, and various ammoniacal salts. A large quantity of carburetted hidrogen, and other uninflammable gases, make their appearance, and the fixed base of the coal remains behind in the distillatory apparatus in the form of a carbonaceous substance, called coke.

All these products may be separately collected in different vessels. The carburetted hidrogen, or coal-gas, may be freed from the non-inflammable gases, and afterwards forced in streams out of small appertures, which, when lighted, may serve as the flame of a candle to illuminate a room or any other place. It is thus, that from pit-coal a native production of this country, we may procure a pure, lasting, and copious light; which, in other cases, must be derived from expensive materials, in part imported from abroad.

It is chiefly upon the power of collecting the products afforded by coal, with convenience and cheapness, that the promoters of the gas-light illumination found their claims to public encouragement. They conceive that the flamewhich pit-coal yields, as it is now consumed, is turned to very little advantage: it is not only confined to one place, where a red heat is more wanted than a brilliant flame, but it is obscured, and sometimes entirely smothered, by the quantity of incombustible materials that ascend along with it and pollute the atmosphere.

That much inflammable matter is thus lost, is evident from facts that come under our daily observation. We often see a flame suddenly burst from the densest smoke, and as suddenly disappear; and if a light be applied to the little jets that issue from the bituminous parts of the coal, they will catch fire, and burn with a bright flame. A considerable quantity of a gazeous fluid, capable of affording light and heat continually escapes up the chimney, whilst another part is occasionally ignited, and exhibits the phenomena of the flame and light of the fire.

The theory of the production of gas-light is therefore analogous to the action of a lamp or candle. The wick of a candle being surrounded by the flame, is in the same situation of the pit-coal exposed to distillation. The office of the wick is chiefly to convey tallow, by capillaryattraction, to the place of combustion. As it is decomposed into carburetted hidrogen gas it is consumed and flies off, another portion succeeds; and in this way a continued current of tallow and maintenance of flame are effected. Seepage 15.

The combustion of oil by means of a lamp depends on similar circumstances. The tubes formed by the wick serve the same office as a retort placed in a heated furnace through which the inflammable liquid is transmitted. The oil is drawn up into these ignited tubes, and is decomposed into carburetted hidrogen gas, and from the combustion of this gas the illumination proceeds. Seep. 15. What then does the gas-light system attempt? Nothing more than to generate, by means of sufficient furnaces and a reservoir of sufficient capacity, desired quantities of the gas, which is the same material of the flame of candles or lamps; and then by passing it through pipes to any desired distance, to exhibit it there at the mouths of the conducting tubes, so that it may be ignited for any desired purpose. The only difference between this process and that of an ordinary candle or lamp, consists in having the furnaceat the manufactory, instead of its being in the wick of the candle or lamp—in having the inflammable material distilled at the station, instead of its present exhibitions in oil, wax, or tallow, and then in transmitting the gas to any required distance, and igniting it at the orifice of the conducting pipe instead of igniting it at the apex of the wick. The principle is rational, and justified by the universal mode in which all light is produced. Indeed, this discovery ranks among the numerous recent applications of chemical science to the purposes of life, which promise to be of the most general utility.

It is evident from the outline here given of the production and application of coal-gas, that all the uses of pit-coal are not exhausted; it will be sufficient to observe, that the complete analysis of coal, which has been hitherto confined to the laboratory of the chemist, requiring skill and nicety in the operator, and attended with great trouble and expence, is now so far simplified, that many chaldrons of coals may be decomposed by one gas-light apparatus in the space of six hours, and all the component parts produced in their most usefulshape, at an expence out of all proportion below the value of the products.

To assist the reader in comprehending the nature and object of substituting coal-gas for tallow or oil, for the purpose of obtaining light, it may be proper to touch slightly upon the successive discoveries that have been made as to the decomposition of coal, and the application of its different ingredients. Such a sketch will add to the many examples that occur in the history of science and art, showing the slow progress of mankind in following up known principles, or extracting from acknowledged facts every possible advantage.

In the Philosophical Transactions of theRoyal Society, V. XLI. so long ago as the year 1739, is recorded a paper, exhibiting an account of some experiments made by Dr. James Clayton, from which it appears that the inflammable nature of coal-gas was then already known. Dr. Clayton having distilled Newcastle coal, obtained, as products of the process, an aqueous fluid, a black oil, and an inflammable gas, which he caught in bladders, and by pricking these he was enabled to inflame the gas at pleasure.

It is further known, that in the beginning of the last century, Dr. Hales[11]on submitting pit-coal to a chemical examination, found, that during the ignition of this fossil in close vessels, nearly one-third of the coal became volatilized in the form of an inflammable vapour. Hence the discovery of the inflammable nature of coal-gas can no longer be claimed by any person now living.

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