_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:
[Illustration]
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.