PART XIV.

Illuminating power of Coal Gas, and quantity of Gas consumed in a given time, by different kinds of Burners, and Gas Lamps.

The illuminating power of coal gas, differs according to the nature of the coal from which it is obtained, and the manner in which it is purified, together with the quantity of naptha or essential oil chemically combined, or mechanically suspended in the gas. For if the gas be strongly agitated with water, its illuminating power is diminished. Coal gas, which abounds in olifiant gas or supercarburetted hydrogen possesses the greatest illuminating power, and hence carburetted hydrogen obtained from the decomposition of coal tar possesses a greater illuminating power than the gas obtained from the coals which produced the tar. The illuminating power of carburetted hydrogen obtained from coal tar when compared to the gas obtained from the best Newcastle coal is in the proportion as six to five. In fact the intensity of light evolved during the combustion of gazeous bodies composed of carbon, hydrogen, and oxigen, is always in the ratio of the quantity of carbon contained in equal quantities of the gazeous compound, and hence the gas from animal oil which is chiefly composed of supercarburetted hydrogen or olifiant gas, surpasses in illuminating power the gas obtained from coal.

Half a cubic foot of coal gas, obtained in the ordinary way of manufacturing coal gas, from Newcastle coal, is equal in illuminating power and duration of time, to the light produced by a tallow candle six in the pound, burning for one hour, and as such a tallow candle lasts five hours, therefore fifteen cubic feet of coal gas, are equal in value with regard to illuminating power to one pound of candles. And as 112 pounds of Newcastle coal produce by the new method of manufacturing coal gas, at least 550 cubic feet of gas, therefore the quantity of gas produced from a chaldron of Newcastle or Sunderland coal, (the minimum weight of which is 27 cwt.) is equal in illuminating power to 1000 pounds of tallow candles.

The illuminating power of coal gas may readily be ascertained. Though the eye is not fitted to judge of the proportional power of different lights, it can distinguish in many cases with sufficient precision where two similar surfaces are equally illuminated. As the lucid particles emitted from luminous bodies are darted in right lines, they must spread uniformly, and hence their density diminishes in the duplicate ratio of their distance. From the respective situations, therefore, of the centres of divergency, when the contrasted and illuminated surfaces become equally bright, we are enabled to compute their relative degrees of intensity. And for this purpose it is assumed as a principle, that the same quantity of light, diverging in all 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 object, 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 the two sources of light to such distances from an object that they may illuminate it in equal degrees, we are authorized to 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 opaque 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 the greater light, will be illuminated by the smaller light only; and reversely, the other shadow will be illuminated by the greater light; that is to say, the stronger light will be attended with the deeper shadow.

Now it is easy by removing the stronger light to a greater distance, to make the shadow which it produces equal to that afforded by the less light. Experiments of this kind may be made in the following manner:

Fasten a sheet of white paper against the wall of a room, and place the two lights intended to be compared, so that the rays of light from each 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 shadows may be made to appear as in this figure:

where A represents the surface illuminated by one of the lights only; B, the surface illuminated by the other light; C, the perfect shadow from which both lights are excluded. It will easily be understood that the lights about D and E, near the angle F, will fall with equal incidences when the double shadow is made to occupy the middle of the paper; and consequently, if one or both of the lights be removed directly towards or from the paper, as the appearances may require, until the two shadows at E and D have the same intensity, the quantities of light emitted by each, will be as the squares of the distances from the paper.

By experiments of this kind, many useful particulars may be shewn; for, since the cost and duration of candles, and the consumption of coal gas, or oil in lamps, are easily ascertainable, it may be shewn whether more or less light is obtained at the same expense during a given time, by burning a number of small lights, instead of one or more of greater intensities. And thus we may compare the power of different kinds of lamps or candles, with gas lights of different intensities, 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 light be required, and the candle kept 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 or chemical constitution of the coal gas. The bulk of the gas consumed, and the weight of tallow or oil used by weighing the candle or oil before and after the experiment furnish the data for calculating the relative cost of tallow, or oil and gas, when compared with each other.

The following statement exhibits the quantity of coal gas consumed in a given time, by different kinds of argand lamps. An argand burner which measures in the upper rim half an inch in diameter, between the holes from which the gas issues, when furnished with five apertures ¹⁄₂₅ part of an inch in diameter, consumes two cubic feet of gas in an hour, when the gas flame is one and a half inch high. The illuminating power produced by this burner is equal to three tallow candles eight in the pound.

An argand burner three quarters of an inch in diameter between the holes in the upper rim, and perforated with holes, ¹⁄₃₀ of an inch in diameter, consumes three cubic feet of gas in an hour, when the flame is two and a quarter inches high, and produces a light equal in intensity to four tallow candles, eight in a pound.

An argand burner seven-eighths of an inch in diameter, perforated with eighteen holes ¹⁄₃₂ of an inch in diameter, consumes when the flame of the gas is three inches high, four cubic feet of gas in an hour, and produces a light equal in intensity to six tallow candles, eight in the pound.

When the flame obtained by these kind of burners rises to a greater height, than what has been stated, the combustion of the gas is imperfect, the intensity of the light becomes diminished, and there is a waste of gas. The same holds good with regard to the size of the holes from which the gas issues; if the holes be made larger than ¹⁄₂₅ part of an inch in these kind of burners, the gas is not completely burnt, and its illuminating power decreases.

The height of the glass which surrounds the flame, should never be less than five inches, and the interval for the current of air within and without the flame, ought to bear the usual proportion adopted for the combustion of oil in the common argand lamps of similar diameters.

Ventilation of Apartments lighted by Coal Gas.

Before means had been devised for the effectual purification of coal gas, a disagreeable odour was found to attend its combustion in an impure state, and hence an opinion became prevalent, that the benefit of this new species of illumination must be confined to open places, and that it could not with any regard to pleasure or salubrity, be adapted to private dwellings.

The art of purifying coal gas, has at length however, been carried to such a perfection, that every possibility of a disagreeable odour arising from its combustion has been wholly removed, in all cases where attention is paid to the perfect combustion of the gas, by keeping the flame of the same of a proper magnitude.

And since this improvement, the use of coal gas, as a means of illumination has become as general, and has been found attended with as superior advantages within doors as without, and hence a vast number of dwelling houses are now lighted throughout with gas.

Although there is no occasion therefore, to make provision for ventilating apartments where gas light is employed, on account of any odour which it can produce when honestly used, so that the combustion is perfect, yet on other accounts such means of ventilations are very salutary and necessary.

The flame of coal gas produces a degree of heat,[52] which in some places, such as large public offices, and warehouses of dry goods, is a strong additional recommendation in favour of its use, ([page 15],) while in others, on the contrary, such as small rooms numerously frequented, and shops containing commodities requiring to be kept cool, it can only be used beneficially when means are provided for conveying away the heated air.

[52] Mr. Dalton’s method of ascertaining the comparative effect of heat evolved during the combustion of inflammable gases, and other substances capable of burning with flame, (Dalton’s System of Chemistry, vol. I. p. 76,) is simple, easy, and accurate. It is as follows:

Take a bladder of any size, (let us suppose for the sake of illustration, the bladder to hold 30,000 grains of water,) and having furnished it with a stop-cock and small jet pipe, fill it with the combustible gas the heating power of which is to be tried. Take also a tinned iron vessel with a concave bottom of the same capacity, pour into it as much water as will make the vessel and water together equal to the bulk of the water in the bladder, viz. 30,000 grains. Then set fire to the gas at the orifice of the pipe, bring the point of the flame under the bottom of the tinned vessel, and suffer it to burn there, by squeezing the bladder till the whole of the gas is consumed. The increase of temperature of the water in the tinned vessel before and after the experiment, expresses very accurately the heating power of the given bulk of the inflammable gas. It was thus proved that—

Olifiant gas raises an equal volume of water	14	deg.
Carburetted hydrogen, or coal gas	10
Carbonic oxid gas	4
Hydrogen gas	5
Spermaceti oil, 10 grains burnt in a lamp raised 30,000 grains of water	5
Tallow	5
Wax	5,75
Oil of turpentine	3
Spirit of wine	2

The best method for this purpose is to make an aperture of about two or three inches in diameter into the chimney near the ceiling, and inserting into it a tube bending upwards into the interior of the chimney. A complete ventilation of the room will thus be established, by producing an extra vent which will be amply sufficient for carrying off the heated air. The aperture can easily be masked with some ornamental open work, corresponding with the style of the room.

If there happens to be no chimney in the apartment, the ventilator may be made in the ceiling, and the tube may be carried between the ceiling and the floor above, into the open air. The mode of ventilation now suggested, has been uniformly found most efficient, and has, under existing circumstances, a decided superiority over another method, which we see in some instances adopted. This method consists in enclosing the gas burner in a bell-shaped glass, from the upper part of which a large copper tube proceeds, and leads out into the open air. It is certain that by this means not only the heated air is carried off, and the possibility of any waste gas escaping into the apartment is also completely prevented. But at the same time, by taking away all occasion for a prudent limitation in the use of the gas, it exposes it to a degree of improvident waste, in the hands of dishonest and careless individuals, which must prove ruinous to the manufacturer. The mode of regulating the light of the flames by means of the governor, of which a description has been given, [page 232], indeed provides a check against such waste, and there can be no doubt that in proportion as this instrument gets into general use, the objection on this score must of course fall to the ground; but under any circumstances the inelegance of the contrivance of such an object in a chamber, as the large branching tube, must always induce a preference, for the more simple, and for all necessary purposes, equally efficient method, of the ventilator before described.