which the manufacturer strives to remove by passing the gas over lime,
milk of lime, oxide of iron, &c. Sulphuretted hydrogen, a compound of
sulphur and hydrogen, has an extremely nauseous odour resembling that of
rotten eggs. It is always formed in the distillation of coal, and if not
removed from the gas in the process of purification, it has a very
objectionable effect; for one product of its combustion is sulphurous
acid, and in a room where such gas is burnt much damage may be done by
the acid vapours; for example, the bindings of books, &c., soon become
deteriorated from this cause. The detection of sulphuretted hydrogen in
coal-gas is quite easy, for it is only necessary to hold in a current of
the gas a piece of paper dipped in a solution of the acetate of lead. If
in a few minutes the paper becomes discoloured the presence of
sulphuretted hydrogen is indicated.
But the _bête noire_ of the gas-maker is a substance called “sulphide of
carbon,” which is formed whenever sulphur and carbonaceous matters are
brought together at an elevated temperature. Sulphide of carbon is, in
the pure state, a colourless liquid, of an intensely offensive odour,
resembling the disagreeable effluvia of putrefying cabbages. The liquid
is extremely volatile, and coal-gas usually contains some of its vapour.
When too high a temperature is used in the generation of the gas, it
contains a large quantity of this deleterious ingredient, especially if
the amount of sulphur contained in the coal is at all considerable. This
sulphide of carbon vapour is very inflammable, and one product of its
combustion is a large quantity of sulphurous acid. This substance cannot
be removed from coal-gas by any process sufficiently cheap to admit of
its application on the large scale. It is said, however, that by passing
the gas over a solution of potash in methylated spirit, the sulphide of
carbon vapour can be completely got rid of. The price of these materials
renders the process available in special cases only, where the damage
done by the sulphurous acid would be serious, as in libraries, &c.
Besides the impurities we have already enumerated, many others are
present in greater or less quantity. Carbonic acid—the gas resulting
from the complete combustion of carbon—should be entirely removed by the
lime purifiers, as the presence of even a small percentage detracts
materially from the illuminating power. This gas is not inflammable and
cannot support combustion. It has decided acid properties, and readily
unites with alkaline bases forming carbonates: it is upon this behaviour
that its removal by lime depends. The illuminating power of coal-gas
containing only 1 per cent. of carbonic acid is reduced thereby by about
one-fifteenth of its whole amount.
The proper mode of burning the gas so as to obtain the maximum amount of
light it is capable of yielding requires a compliance with certain
physical and chemical conditions. The artificial production of light
depends upon the fact that by sufficiently heating any substance, it
becomes luminous, and the higher the temperature the greater the
luminosity. The light emitted by solid bodies moderately heated is at
first red in colour; as the temperature rises it becomes yellow, which
gradually changes to white when the heat becomes very intense. The
widest difference exists, however, in the temperature required to render
solids or liquids luminous, and that needed to cause gases to give off
light. In all luminous flames the light is emitted by _solid_ particles
highly heated. Every luminous gas-flame contains solid particles of
carbon, as may be easily shown by the soot deposited on any cold
body—such as a piece of metal—introduced into the flame. On the other
hand, the flame of burning hydrogen, which produces only aqueous vapour,
furnishes no light, but a heat so intense, that a piece of lime
introduced into the jet becomes luminous to a degree hardly supportable
by the eye. The conditions requisite, therefore, for burning
illuminating gas are, first, just such a supply of air as will prevent
particles of carbon from escaping unconsumed in the form of smoke, and
yet not enough to burn up the carbon before it has separated from the
hydrogen, and passed through the flame in the _solid_ state; second, the
attainment of the highest possible temperature in the flame, compatible
with the former condition. When the supply of oxygen is not in excess,
the hydrogen of the gaseous hydro-carbon appears to burn first; the
carbon is set free, and its solid particles immersed in the flame of the
burning hydrogen are there intensely heated; but ultimately reaching the
outer part of the flame, they enter into combination with the oxygen of
the air, producing carbonic acid; or if present in excessive quantity,
they are thrown off as smoke. If the purpose of burning the gas is to
obtain heating effects only, this is accomplished by supplying air in
such quantities, that the carbon enters into combination with oxygen in
the body of the flame, without a previous separation from the hydrogen
with which it is combined. In this case a higher temperature is
attained, and the flame is wholly free from smoke; so that vessels of
any kind placed over it remain perfectly clean and free from the least
deposit of soot. The last result is of great advantage in chemical
processes, especially where glass vessels require to be heated, for the
chemist retains an uninterrupted view of the actions taking place in his
flasks and retorts.
[Illustration:
FIG. 350.—_Bunsen’s Burner._
]
No better illustration of the nature of the combustion in a gas-flame
can be found than is furnished by Bunsen’s burner, Fig. 350, now
universally employed as a source of heat in chemical laboratories. In
this burner the gas issues from a small orifice at the level of _a_,
near the bottom of the tube, _b_, which is open at the top, and is in
free communication at the bottom with openings through which air enters
and mixes with the gas, as they rise together in the tube and are
ignited at the top. If the pressure of the gas be properly regulated,
the flame does not descend in the tube, but the mixture burns at the top
of the tube, producing a pale blue flame incapable of emitting light,
but much hotter than an ordinary flame, for the combustion is much
quicker. If the openings at _a_ be stopped, the supply of air to the
interior of the tube is cut off, and then the gas burns at the top of
the tube, _b_, in the ordinary manner, giving a luminous flame. Ordinary
gas-jets burning in the streets, at open stalls or shops, may be seen on
a windy night to have their light almost extinguished by the increased
supply of oxygen, carried mechanically into the body of the flame, the
white light instantly changing to pale blue. The disappearance of the
light in such cases is due, as in Bunsen’s burner, to the supply of
oxygen in sufficient quantity to combine at once with the carbon as well
as the hydrogen of the hydro-carbons.
[Illustration:
FIG. 351.—_Faraday’s Ventilating Gas-burner._
]
The burners now chiefly used for the consumption of coal-gas for
illuminating purposes are the bat’s-wing, the fish-tail, and various
forms of Argand. The bat’s-wing burner is simply a fine slit cut in an
iron nipple, and it produces a flat fan-like flame. The fish-tail is
formed by boring two holes so that two jets of gas inclined at an angle
of about 60° infringe on each other and produce a flat sheet of flame.
The Argand, in its simplest form, consists of a tubular ring perforated
with a number of small holes from which the gas issues. Many
modifications of this kind of burner have been devised, in all of which
a glass chimney is requisite to obtain a current of air sufficient to
consume the gas without smoke, and it is important that the height of
the chimney should be adapted to the amount of light required if the gas
is to be used economically. Argand burners are specially advantageous
where a concentrated light is required. Fig. 351 represents a
ventilating gas-burner, contrived by Faraday, the object being to remove
from the apartment the whole of the products of the combustion of the
gas. A is the pipe conveying the gas to the Argand burner, B, the flame
of which is enclosed in the usual cylindrical glass chimney, C C, open
at the top. This is enclosed in a wider glass cylinder closed at the top
by a double disc of talc, D D, and opening at its base into the
ventilating tube, E E. The direction of the currents produced by the
heat of the flame is shown by the arrows. The whole is entirely enclosed
by a globe of ground glass. Means are provided for regulating the
draught in the pipe, E E, which, when heated, creates of itself a strong
current of air through the apparatus.
The illuminating power of coal-gas may be measured directly by comparing
the intensity of the light emitted by a gas-flame consuming a known
quantity of gas per hour with the light yielded by some standard source.
The standard usually employed is a spermaceti candle burning at the rate
of 120 grains of sperm per hour. It is not necessary that the candle
actually used should consume exactly this amount, but the consumption of
sperm by the candle during the course of each experiment is ascertained
by the loss of weight, and the results obtained are easily reduced to
the standard of 120 grains per hour. An instrument is used for
determining the relative intensities of the illumination, called
Bunsen’s photometer. It consists of a graduated rule, or bar of wood or
metal, about 10 ft. long. At one end of this bar is placed the standard
candle, at the other is the gas-flame. A stand slides along the rule
supporting a circular paper screen at the level of the two flames, and
at right angles to the line joining them. This paper screen is made of
thin writing-paper, which has been brushed over with a solution of
spermaceti, except a spot in the centre, or, more simply, a grease-spot
is made in the middle of a piece of paper. In consequence the paper
surrounding the spot is much more transparent; yet when it is placed so
that both sides are equally illuminated, a spectator will not perceive
the spot in the centre when viewing the screen on either side. When the
screen has been placed by trial in such a position between the two
sources of light, it is only necessary to measure its distance from each
flame in order to compute the number of times the illuminating power of
the gas-flame exceeds that of the candle. This computation is based on
the fact that the intensity of the light from any source diminishes as
the square of the distance from the source. Thus, if a sheet of paper be
illuminated by a candle at 2 ft. distance, it will receive only
one-fourth of the light that would fall upon it were its distance but 1
ft., and if removed to 3 ft. distance it has only one-ninth of the
light. In the instrument used for measuring the illuminating power of
gas the rule is graduated in accordance with this law, so that the
relative intensities may be read off at once. The gas passes through a
meter for measuring accurately the quantity per minute which is consumed
by the burner, and there is also a gauge for ascertaining the pressure.
Another mode of estimating the illuminating power of coal-gas is by
determining the quantity of carbon contained in a given volume. For, in
general, the richness of the gas in carbon is a fair index of the
quantity of its luminiferous constituents. This may be readily effected
by exploding the gas with oxygen, and measuring the amount of carbonic
acid produced. Still more accurate determinations of the illuminating
value of gas may be obtained by a detailed chemical analysis.
The illuminating power of any gas is so calculated that it represents
the number of times that the light emitted by a jet of the gas, burning
at the rate of 5 cubic feet per hour, exceeds the light given off by the
standard sperm candle burning 120 grains of sperm per hour. For example,
when it is said that the illuminating power of London gas is 13, it is
meant that when the gas is burnt in an ordinary burner at the rate of 5
cubic feet per hour, the light is equal to that given by thirteen sperm
candles burning together 13 × 120 grains per hour. The quality of gas
varies very much, as it depends upon the kind of coal employed, and upon
the mode in which the manufacture is conducted. The following are the
results of experiments made to determine the illuminating power of the
gas supplied to several large towns:
Candles.
London 12·1
Paris 12·3
Birmingham 15·0
Berlin 15·5
Carlisle 16·0
Liverpool 22·0
Manchester 22·0
Glasgow 28·0
The relative quantities of tar, ammonia water, and coke yielded in
various gas manufactories also vary very considerably for the same
reasons.
In the early days of gas illumination the consumers were charged
according to the number of burners; but this arrangement proved so
unsatisfactory that the _gas-meter_ became a necessity, and already in
1817 meters had been devised, which were not essentially different from
those now in use. Although gas is used in so many houses, there are few
persons who have any notion of the mechanism of the gas-meter. Our space
will not allow full details of the construction, but the following
particulars may be mentioned. In the ordinary “wet” meter there is a
drum divided into four compartments by radiating partitions; this drum
revolves on a horizontal axis, and the lower half of the drum, or rather
more, is beneath the surface of water contained in the case, the water
being at the same level inside and outside the drum. The gas enters one
of the closed chambers formed between the surface of the water and a
partition of the drum. Its pressure tends to increase the size of the
chamber, hence the drum revolves. The preceding division of the drum
being filled with gas, this is driven into the exit pipe by the motion
of the drum, as it is included in a space comprised between the water
and a partition. Each division in turn comes into communication with the
gas-main, and as it is filled passes on towards the position in which a
passage is opened for it to the exit-pipe. Each turn of the drum,
therefore, carries forward a definite quantity of gas, and the only
thing necessary is a train of wheels, to register the number of
revolutions made by the drum. The “wet” meter is much inferior in almost
every respect to the “dry” meter, in which no water is used. The
principle of the “dry” meter is very simple. The gas pours into an
expanding chamber, partly constructed of a flexible material, and which
may be compared to the bellows of a circular accordion. The expansion is
made to compress another similar chamber, already filled with gas, which
is thus forced through the exit-pipe. When the first chamber has
expanded to a definite volume, it moves a lever, and this reverses the
communications. The expanded chamber is now opened to the exit-pipe, and
the other to the entrance-pipe, and so on alternately. A train of wheels
registers the number of movements on a set of dials.
Recent years have brought no essential changes in the methods of gas
making, although of course improvements in many minor details of the
processes and of the apparatus have been effected. These demand no
description at our hands, as they are of interest only to those
concerned with the actual technology of gas-making, nor need some of the
later forms of burners for using the gas be noticed, as these are
sufficiently familiar. They really do effect a considerable economy in
the consumption of gas, especially in cases where a more powerful light
is required. But the reader will have already learnt from a foregoing
section on Electric Lighting that the importance of gas as an illuminant
is already on the wane. Indeed, it will not be too much to say that,
before the close of the present century, every town will have its
streets, and still more certainly, all its places of public assembly,
such as theatres, concert halls, churches, libraries, &c., fitted with
installations for electric illumination, and even in shops and private
houses, it is probable that before long, gas will be superseded by the
electric light. Some of the disadvantages of burning gas have already
been referred to, and the danger attending its accidental escape into
apartments is illustrated by the yearly tale of victims to suffocation
and violent explosions. The inherent disadvantage of gas used as an
illuminant, is the enormous quantity of heat produced by its combustion,
compared with the amount of light evolved. The absolute quantity of heat
required to render a body highly luminous is really very small, for
masses of matter almost inappreciable become very luminous, provided
only that their _temperature_ be sufficiently raised. Thus, for example,
the few residual particles of gas in a Geissler’s tube (p. 431) become
incandescent by electrical discharges, while the number of them is too
small to sensibly heat the glass vessel, and the very attenuated carbon
filament in an electric glow lamp suffices by the mere contraction and
concentration of the current within it raising its temperature high
enough, to diffuse a brighter light than a large gas-flame. This
explains the fact alluded to elsewhere, that if instead of burning the
gas we use it in a gas engine, driving a dynamo connected with an
electric light installation, we shall obtain a much greater luminous
effect. As there is no combustion, the surrounding air is neither heated
nor deteriorated with gaseous products and smoke.
Without any rivalry from the electric-light, gas, as a domestic
luminant, has now met with a competitor on the ground of cheapness in
the mineral oils mentioned in the preceding article. If these could be
deprived of their unpleasant odour, and a perfectly safe lamp contrived
for burning them, it would be only under very favourable conditions that
gas could compete with them on the score of economy. But of late years
two applications of gas to other purposes than to illumination will have
been observed. First to heating, for warming, cooking, and other
domestic purposes, and also in various processes in the arts. In all the
appliances so used, the principle of Bunsen’s burner (p. 722) is
adopted, and stoves, fireplaces, and kitchen-ranges, heated by gas have
obvious advantages in their greater cleanliness and readiness. The other
new application of gas is as a motive power in the gas engine, by which
a very convenient supply of mechanical energy is afforded. There can be
little doubt that in the future, gas will be greatly used for these
purposes, and perhaps be for them consumed as largely as at present. A
singular thing in the history of gas-manufacture is the great value that
the bye-products have attained, that is to say, the ammoniacal liquor,
the coke, and especially the tar. So many valuable substances are now
derived from this last, that even if coal should cease to be
destructively distilled for gas, the operation would still be largely
carried on if only for the tar.
A jet of hydrogen gas burning in a dark room is all but invisible, yet
no gas can give so intense a heat. The lime-light, which no doubt is
perfectly familiar to everyone as an illuminant in magic lantern
projections, is simply a jet of mixed hydrogen and oxygen gases directed
on a piece of lime, which is rendered incandescent by the heat. The
flame of the Bunsen burner (p. 772) is distinguishable only by a very
pale blue colour, and it is impossible to discern objects, or to read by
its light in an otherwise dark room. But if a piece of thin platinum
wire formed into a coil, as by twisting round a pencil, be introduced
into the flame, the wire will glow with great brilliancy, and its
thickness will seem much increased. It will, in fact, emit so much light
that reading by its glow becomes easy. This shows that, as already
stated, a solid will give off light at a temperature which scarcely
suffices to make a gas visible. Thus a Bunsen burner flame can be made
to give light simply by putting into it some incombustible solid, which
itself incapable or suffering any chemical change under the conditions,
nevertheless becomes luminous by merely acquiring the temperature of the
almost invisible heated gas. The cause of the luminosity of the ordinary
gas burner, as compared with the almost invisible Bunsen burner flame,
has, indeed, been already explained on a previous page, but the
phenomenon is again, by the experiment just referred to, brought clearly
before our attention; and it becomes obvious that substances other than
the carbon of the hydro-carbon constituents of the coal gas will emit
rays of light. Chemical analysis shows that by far the larger proportion
of the constituents of ordinary coal-gas consist of gases which do not
themselves produce luminous flames, and that, taking 16 candle-gas,
about 10 candles of the illuminating power is due to compounds of which
the gas does not contain more than 4 per cent. Nearly half the bulk of
purified coal-gas is hydrogen, which itself gives no light whatever when
burnt; marsh-gas, which burns with only a slight luminosity, forms 35
per cent. of ordinary coal-gas; and there is usually present about 7 per
cent. of carbonic oxide, which in burning gives only a pale blue flame.
This shows that by far the greatest product of the combustion of
coal-gas is not light but heat. The flame of hydrogen is much the
hottest known, and as that gas enters so largely into the composition of
coal-gas, and the complete combustion of all the other constituents
takes place when the gas is previously mixed with air, as in the Bunsen
burner, we are provided with an economical means of obtaining high
temperatures. But coal-gas was in the first instance intended to provide
us with a cheap illuminant, and although for some time the gas itself
was very impure, and it was long before the crude appliances for burning
it were superseded by contrivances giving steadier and more brilliant
lights, such as the Argand and the regenerative burners. It is only
quite recently that the full illuminating possibilities of coal-gas have
been developed by the happy notion of converting the heating power of
its flame into light-giving power, by the simple plan of suspending a
suitable solid over the hot but non-luminous Bunsen burner.
The manner in which an effective method of doing this was discovered is
not a little curious. The construction of the ordinary incandescent
electric lamp, Fig. 280_h_, involves the necessity of enclosing the
carbon filament in an exhausted glass bulb; and it was when Auer von
Welsbach was engaged in attempting to find some substance that could be
brought into incandescence by the electric current, and yet be
incombustible even in the open air, that his investigations led to the
invention we have now to describe—an invention apparently destined to
give a new lease of life to coal-gas illumination.
[Illustration:
FIG. 351_a_.
]
[Illustration:
FIG. 351_b_.
]