(see Lamps) is an instrument comprising three articles which demand our attention, viz., the oil, the wick, and the supply of air. It is required that the oil should be readily inflammable, without containing any fetid substance which may prove offensive, or mucilage, or other matter, to obstruct the channels of the wick. Mr Nicholson says*, that he knows of no process by which oils can be meliorated for this purpose, except that of washing with water containing acid or alkali. Either of these is said to render the mucilage of animal oils more soluble in water; but acid is to be preferred, because... Lamp because it is less disposed to combine with the oil itself.

Perhaps oil might be deprived of all fetid smell in burning, by being made to pass through Collier's filtering apparatus, described under the word FILTER in this Suppl.

The office of the wick appears to be chiefly, if not solely, to convey the oil by capillary attraction to the place of combustion. As the oil is consumed and flies off, other oil succeeds, and in this way a continued current of oil and maintenance of the flame are effected. But as the wicks of lamps are commonly formed of combustible matter, it appears to be of some consequence what the nature and structure of this material may be. It is certain that the flame afforded by a wick of rush differs very considerably from that afforded by cotton; though perhaps this difference may, in a great measure, depend on the relative dimensions of each. And if we may judge from the different odour in blowing out a candle of each sort, there is some reason to suspect that the decomposition of the oil is not effected precisely in the same manner in each. We have also some obscure accounts of prepared wicks for lamps, which are stated to possess the property of facilitating the combustion of very impure oils, so that they shall burn for many hours without smoke or smell.

The economical wicks of M. Leger, concerning which a report was presented to the Academy at Paris in 1782 by Condorcet, Lavoisier, and De Milly, were composed of cotton of different sizes and forms, namely, round and flat, according to the use they were intended to serve. They were covered with a fat substance, of a smell not disagreeable, but feebly aromatic. From the trials of these commemoratives it was ascertained: 1. That they afforded a clearer flame, with less undulation. 2. That they consumed somewhat less oil; and, 3. That they possessed the remarkable property of affording neither smell nor smoke, however common the oil made use of. When using a lamp with a flat wick, we have ourselves found a piece of clean cotton flocking answer the purpose better than the cotton wicks which are sold in the shops.

The access of air is of the last 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 colour; if the wick be larger, the combustion is less perfect, and the flame is brown; a still larger wick not only exhibits a brown flame, but the lower internal part appears dark, and is occupied by a portion of volatile 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, described in the Encyclopaedia.

Much has been said of this lamp, and great praise lavished on the inventor. It cannot indeed be denied that it was a very pretty invention, nor have we the slightest wish to detract from the merit of M. Argand; but truth compels us to say, that the same thought had occurred to others as early as to him, and that lamps had been constructed on his principles long before he had published an account of his lamp to the world (A).

Many ingenious men have endeavoured to determine the most economical method of lighting up large halls and workhouses by means of different lamps and candles; and when the expense of tallow and oil is considered, it will be admitted that they could not employ their time in a manner more beneficial to the poor and the industrious. Among others, Count Rumford and M. Haffefratz, have turned their attention to this subject; and the results of their investigations are worthy of notice. To the Count, a method occurred for measuring the relative quantities of light emitted by lamps of different contrivances, which is at once simple and accurate. It is as follows:

Let the two burning lamps, or other lights to be compared, be called A and B; and let them be placed at equal heights upon two light tables, or moveable stands, in a darkened room; let a sheet of clean white paper be equally spread out, and fastened upon the window, or side of the room, at the same height from the floor as the lights; and let the lights be placed over against this sheet of paper, at the distance of six or eight feet from it, and six or eight feet from each other, in such a manner, that a line drawn from the centre of the paper, perpendicular to its surface, shall bisect the angle formed by lines drawn from the lights to that centre; in which case, considering the sheet of paper as a plane speculum, the one light will be precisely in the line of reflection of the other.

This may be easily performed, by actually laying a piece of a looking-glass, six or eight inches square, flat upon the paper, in the middle of it; and observing, by means of it, the real lines of reflection of the lights from that plane, removing it afterwards, as soon as the lights are properly arranged. When this is done, a small cylinder of wood, about 4th of an inch in diameter, and six inches long, must be held in a vertical position, about two or three inches before the centre of the sheet of paper, and in such a manner, that the two shadows of the cylinder, corresponding to the two lights, may be distinctly seen upon the paper.

If these shadows should be found to be of unequal densities, which will almost always be the case, then that light whose corresponding shadow is the densest must be removed farther off, or the other must be brought nearer to the paper, till the densities of the shadows appear to be exactly equal; or, in other words, till the densities of the rays from the two lights are equal at the surface of the paper; when, the distances of the lights from

(A) One of these was employed in the college of Glasgow, by the lecturer on chemistry, so long ago as 1766. from the centre of the paper being measured, the squares of those distances will be to each other as the real intensities of the lights in question at their sources.

If, for example, the weaker light being placed at the distance of four feet from the centre of the paper, it should be found necessary, in order that the shadows may be of the same density, to remove the stronger light to the distance of eight feet from that centre, in that case, the real intensity of the stronger light will be to that of the weaker as $8^2$ to $4^2$; or as $64$ to $16$; or $4$ to $1$; and so for any other distances.

It is well known, that when any quality proceeds from a centre in straight lines in all directions, like the light emitted by a luminous body, its intensity at any given distance from that centre will be as the square of that distance inversely; and hence it is clear, that the intensities of the lights in question, at their sources, must be to each other as the squares of their distances from that given point where their rays uniting are found to be of equal density. For, putting $x = \text{the intensity of } B$, if $P$ represents the point where the rays from $A$ and from $B$ meeting are found to be of equal density or strength, and if the distance of $A$ from $P$ be $m$, and the distance of $B$ from the same point $P = n$; then, as

$$\frac{x}{m} = \frac{y}{n},$$

and

$$\frac{x}{m^2} = \frac{y}{n^2},$$

by the supposition, it will be $x:y:m^2:n^2$.

That the shadows being of equal density at any given point, the intensities of the illuminating rays must of necessity be equal at that point also, is hence evident, that the total absence of light being perfect blackness, and the shadow corresponding to one of the lights in question being deeper or fainter, according as it is more or less enlightened by the other, when the shadows are equal, the intensities of the illuminating rays must be equal likewise.

In removing the lights, in order to bring the shadows to be of the same density, care must be taken to recede from, or advance towards, the centre of the paper in a straight line, so that the one light may always be found exactly in the line of reflection of the other; otherwise the rays from the different lights falling upon the paper, and consequently upon the shadows, at different angles, will render the experiment fallacious.

When the intensity of one strong light is compared with the intensities of several smaller lights taken together, the smaller lights should be placed in a line perpendicular to a line drawn to the centre of the paper, and as near to each other as possible; and it is likewise necessary to place them at a greater distance from the paper than when only single lights are compared.

In all cases, it is absolutely necessary to take the greatest care that the lights compared be properly trimmed, and that they burn clear and equally, otherwise the results of the experiments will be extremely irregular and inconclusive. It is astonishing what a difference there is in the quantities of light emitted by the same candle, when it burns with its greatest brilliancy, and when it has grown dim for want of snuffing. But as this diminution of light is progressive, and as the eye insensibly conforms to the quantity of light actually present, it is not always taken notice of by the spectators; it is nevertheless very considerable, in fact, as will be apparent to any one who will take the trouble to make the experiment; and so great is the fluctuation in the quantity of light emitted by burning bodies, lamps, or candles, in all cases, even under the most favourable circumstances, that this is the source of the greatest difficulties which our author met with in determining the relative intensities of lights by the method here proposed.

To ascertain by this method the comparative densities, or intensities, of the light of the moon and of that of a candle, the moon's direct rays must be received upon a plane white surface, at an angle of incidence of about $60^\circ$, and the candle placed in the line of the reflection of the moon's rays from this surface; when the shadows of the cylinder, corresponding to the moon's light and to that of the candle, being brought to be of equal density, by removing the candle farther off, or bringing it nearer to the centre of the white plane, as the occasion may require, the intensity of the moon's light will be equal to that of the candle at the given distance of the candle from the plane.

To ascertain the intensity of the light of the heavens, by day or by night, this light must be let into a darkened room through a long tube blackened on the inside, when its intensity may be compared with that of a candle or lamp by the method above described.

The Count, however, has contrived an apparatus for ascertaining the intensity of the sun's light, compared with the light emitted by any artificial illuminator, with much greater accuracy than it can be done by this simple method. That apparatus we shall describe under the title PHOTOMETER in this Supplement; and in the meantime we proceed to lay before our readers the results of his experiments as they relate to economy in the production of artificial light.

The brilliancy of Argand's lamp is not only unrivalled, but the invention is in the highest degree ingenious, native quaint, and the instrument useful for many purposes; but still, it is difficult to judge of its real merits as an illuminator, it was necessary, and of light, to know whether it gives more light than another lamp in proportion to the oil consumed. This point he determined in the following manner:

Having placed an Argand's lamp, well trimmed, and burning with its greatest brilliancy, before his photometer, and over against it a very excellent common lamp, with a ribbon wick about an inch wide, and which burnt strongly, with a clear, bright flame, without the least appearance of smoke, he found the intensities of the light emitted by the two lamps to be to each other as $17956$ to $9063$; the densities of the shadows being equal when the Argand's being placed at the distance of $134$ inches, the common lamp was placed at the distance of $95$ inches, from the field of the photometer.

Both lamps having been very exactly weighed when they were lighted, they were now (without being removed from their places before the photometer) caused to burn with the same brilliancy just $30$ minutes; they were then extinguished and weighed again, and were found to have consumed of oil, the Argand's lamp $\frac{1}{11}$, and the common lamp $\frac{1}{11}$, of a Bavarian pound.

Now, as the quantity of light produced by the Argand's grand's lamp, in this experiment, is to the quantity produced by the common lamp as 17966 to 9065, or as 187 to 100, while the quantity of oil consumed by the former is to that consumed by the latter only in the ratio of 233 to 163, or as 155 to 100; it is evident that the quantity of light produced by the combustion of a given quantity of oil in an Argand's lamp is greater than that produced by burning the same quantity in a common lamp, in the ratio of 187 to 155, or as 100 to 85.

The saving, therefore, of oil which arises from making use of an Argand's lamp instead of a common lamp, in the production of light, is evident; and it appears, from this experiment, that that saving cannot amount to less than fifteen per cent. How far the advantage of this saving may, under certain circumstances, be counterbalanced by inconveniences that may attend the making use of this improved lamp, our author does not pretend to determine.

The Count made a considerable number of experiments to determine the relative quantities of light emitted by an Argand's lamp and a common wax candle; and the general result of them is, that a common Argand's lamp, burning with its usual brightness, gives about as much light as nine good wax candles; but the sizes and qualities of candles are so various, and the light produced by the same candle so fluctuating, that it is very difficult to ascertain, with any kind of precision, what a common wax-candle is, or how much light it ought to give. He once found that his Argand's lamp, when it was burning with its greatest brilliancy, gave twelve times as much light as a good wax candle 4½ inches in diameter, but never more.

To determine to what the ordinary variations in the quantity of light emitted by a common wax-candle might amount, he took such a candle, and, lighting it, placed it before the photometer, and over against it an Argand's lamp, which was burning with a very steady flame; and measuring the intensity of the light emitted by the candle from time to time, during an hour, the candle being occasionally snuffed when it appeared to stand in need of it, its light was found to vary from 100 to about 60. The light of a wax-candle of an inferior quality was still more unequal; but even this was but trifling, compared to the inequalities of the light of a tallow-candle.

An ordinary tallow-candle, of rather an inferior quality, having been just snuffed, and burning with its greatest brilliancy, its light was as 100; in eleven minutes it was but 39; after eight minutes more had elapsed, its light was reduced to 23; and in ten minutes more, or twenty-nine minutes after it had been last snuffed, its light was reduced to 16. Upon being again snuffed, it recovered its original brilliancy, 100.

In order to ascertain the relative quantities of beeswax and of olive-oil consumed, in the production of light, the Count proceeded in the following manner: Having provided an end of a wax-candle of the best quality, 568 of an inch in diameter, and about four inches in length, and a lamp with five small wicks, which he had found upon trial to give the same quantity of light as the candle, he weighed very exactly the candle and the lamp filled with oil, and then, placing them at equal distances (forty inches) before the field of the photometer, he lighted them both at the same time; and, after having caused them to burn with precisely the same degree of brightness just one complete hour, he extinguished them both, and, weighing them a second time, he found that 100 parts of wax and 129 parts of oil had been consumed.

Hence it appears, that the consumption of beeswax is to the consumption of olive-oil, in the production of the same given quantity of light, as 100 is to 129.

In this experiment no circumstance was neglected that could tend to render the result of it conclusive; care was taken to snuff the candle very often with a pair of sharp scissors, in order to make it burn constantly with the same degree of brilliancy; and the light of the lamp was, during the whole time, kept in the most exact equilibrium with the light of the candle, which was easily done by occasionally drawing out, a little more or less, one or more of its five equal wicks. These wicks, which were placed in a right line, perpendicular to a line drawn from the middle wick to the middle of the field of the photometer, were about ¼ of an inch in diameter each, and ¼ of an inch from each other; and, when they were lighted, their flames united into one broad, thin, and very clear, white flame, without the least appearance of smoke.

In order to ascertain the relative consumption of olive-oil and rape-oil, in the production of light, two lamps, like that just described, were made use of; and, the experiment being made with all possible care, the consumption of olive-oil appeared to be to that of rape-oil, in the production of the same quantity of light, as 129 is to 125.

The experiment being afterwards repeated with olive-oil and very pure linseed-oil, the consumption of olive-oil appeared to be to that of linseed-oil as 129 to 120.

The experiment being twice made with olive-oil and with a tallow candle; once when the candle, by being often snuffed, was made to burn constantly with the greatest possible brilliancy, and once when it was suffered to burn the whole time with a very dim light, owing to the want of snuffing; the results of these experiments were very remarkable.

When the candle burnt with a clear, bright flame, the consumption of the olive oil was to the consumption of the tallow as 129 is to 101; but when the candle burnt with a dim light, the consumption of the olive oil was to the consumption of the tallow as 129 is to 229. So that it appeared, from this last experiment, that the tallow, instead of being nearly as productive of light in its combustion as beeswax, as it appeared to be when the candle was kept constantly well snuffed, was now, when the candle was suffered to burn with a dim light, by far less so than oil.

But this is not all; what is still more extraordinary is, that the very same candle, burning with a long wick, and a dim light, actually consumed more tallow than when, being properly snuffed, it burnt with a clear, bright flame, and gave near three times as much light.

To be enabled to judge of the relative quantities of light actually produced by the candle in the two experiments, it will suffice to know, that in order to counterbalance this light at the field of the photometer, it required, in the former experiment, the consumption of 141 parts, but in the latter only the consumption of 64 parts, of olive-oil. But in the former experiment 110, and in the latter 114 parts of tallow were actually found to be consumed. These parts were 8192ths of a Bavarian pound.

From the results of all the foregoing experiments, it appears that the relative expense of the undermentioned inflammable substances, in the production of light, is as follows:

| Equal Parts in Weight | |-----------------------| | Bees wax. A good wax-candle, kept well snuffed, and burning with a clear, bright flame, | 100 | | Tallow. A good tallow candle, kept well snuffed, and burning with a bright flame, | 101 | | The same tallow-candle, burning very dim for want of snuffing, | 229 | | Olive-oil. Burnt in an Argand's lamp, | 110 | | The same burnt in a common lamp, with a clear, bright flame, without smoke, | 129 | | Rape-oil. Burnt in the same manner, | 125 | | Linseed-oil. Likewise burnt in the same manner, | 120 |

With the foregoing table, and the prices current of the therein mentioned articles, the relative prices of light produced by those different materials may very readily be computed.

In the year 1795, Mr J. H. Hassenfratz was employed by the French government to make a series of experiments to determine the most economical method of procuring light from the different combustible substances usually employed for that purpose. The materials of his experiments were, wax, spermaceti, and tallow candles, fish-oil, oil of coleseed, and of poppy seeds. In using these oils, both the Argand and common lamps were employed. The wicks of the latter were round, containing thirty-six cotton threads. The tallow and spermaceti candles were mould, six to the pound. The wax candles five to the pound. Mr Hassenfratz used the same method with Count Rumford for determining the comparative intensity of the lights.

Count Rumford, as we have seen, used the Argand lamp as a standard for comparison; but as the intensity of its light varies according to the height of the wick, Mr Hassenfratz preferred a wax-candle, making use of it soon after it was lighted. When two luminous bodies, of different intensities, are put in comparison with each other, the shadows are of two colours. That from the weakest light is blue, and from the strongest, red. When the lights of two different combustible bodies are compared, they are either red or blue in a compound ratio of the colour and intensity. Thus in comparing the shadows from different luminous bodies, they will be red or blue respectively, in the following order:

- Light of the sun. - of the moon. - of Argand lamps. - of tallow-candles. - of wax ditto. - of spermaceti ditto. - of common lamps.

That is to say, when a body is illuminated by the sun and by any other luminous substance, the shadow of the former is red, and of the latter, blue. In like manner, the shadow from an Argand lamp is red, when placed by that of a tallow-candle, which is blue.

The following table will show, according to Mr Hassenfratz, the proportional distance that different luminous bodies should be placed at to produce an equally intense shadow from the same object. The second column gives the proportional intensity of each light, which is known to be in proportion to the squares of the distances of luminous bodies giving the same depth of shadow. The third column shows the quantity of combustible matter consumed in the hour by each mode of giving light, which Mr Hassenfratz calculates from the average of many repeated experiments.

| Distance | Intensity | Quantity consumed per hour | |----------|-----------|---------------------------| | Argand lamps | Oil of poppy seed | 10,000 | 23 | | - | - of fishes | 10,000 | 23.77 | | - | - of cole-seed | 9,246 | 8,549 | | Common lamps | Oil of cole-seed | 6,774 | 4,588 | | - | - of fishes | 6,524 | 4,550 | | - | - of poppy-seed | 5,917 | 3,501 | | Spermaceti candle | 5,917 | 3,501 | | Old tallow-candle | 5,473 | 2,993 | | New ditto | 5,473 | 2,993 | | Wax candle | 4,275 | 1,871 |

The relative quantity of combustible matter required to produce equal lights at equal distances, may be obtained by a simple rule of proportion from the above data. Thus, if a given intensity of light, expressed by 3,501, has been produced by a consumption of 9,23 of spermaceti in the hour, the same luminous body will produce a light of 10,000, by consuming in the same time a quantity of spermaceti = \(\frac{10,000 \times 9.23}{3.501} = 26.37\).

Therefore we may add to the table a fourth column, expressing the quantity of combustible which each body must consume to produce a light of 10,000.

From what has been laid down, it will also appear that the number of lights required to produce a given light, will be as follows: To produce a light equal to 100 Argand lamps, burning poppy-seed oil, it will require

- 100 Argand lamps with fish-oil. - 117 Ditto do. with cole-seed oil - 218 Common lamps with cole-seed oil - 219 Ditto do. with fish oil - 285 Ditto do. with poppy-seed oil - 285 Spermaceti candles - 333 Tallow ditto - 546 Wax ditto.

Mr Hassenfratz next takes notice of the comparative price of these articles; by which he finds, that in Paris the most expensive light is that produced from wax-candles; and the most economical, that from oil of cole-seed, burned in Argand lamps.

The chief difference between the Argand and com- Lancashire lamp is, that in the latter much of the oil is volatilized without combustion, and hence the unpleasant smell which it produces; whereas in the former, the heat is so great at the top of the wick, that all the oil is decomposed in passing through, the disposition of the wick allowing the free access of air to assist combustion.

It should therefore follow, that the Argand lamp consumes less fuel to produce a given light than the common lamp, and this, as we have seen, is the opinion of Count Rumford. Yet (Mr Hafenfratz observes) there are two circumstances that prevent the full effect of the complete combustion in the Argand lamp. The one is, that the glass cylinder absorbs a part of the rays of light as they pass through; the other, that the column of light proceeding from the inner surface of the wick, is, in part, lost, by being obliged to pass through that from the outer surface. Count Rumford allows the first cause of diminution of light, and estimates it at 1854, but not the latter. The author of this memoir, in repeating Count Rumford's experiments, affirms, that when two candles are placed so that the light of the one is obliged to pass through that of the other, the sum of the light so produced is not so strong as when they are placed side by side; for in the first case, a part of the hindmost light is absorbed by the foremost.