GABRONITE, is a yellowish stony substance, of a greasy lustre and spec.
gr. = 2·74; affording no water by calcination; fusible at the blowpipe
into an opaque glass; soluble in muriatic acid; solution affords hardly
any precipitate by oxalate of ammonia. This mineral is distinguished by
the large quantity of soda which it contains; its constituents
being,--silica, 54; alumina, 24; soda, 17·25; magnesia, 1·5; oxide of
iron, 1·25; water, 2. It belongs to the species Nepheline.
GADOLINITE; called also Yttrite and Ytterbite; is a mineral of a black,
brownish, or yellowish colour, granular, or compactly vitreous, and
conchoidal fracture; of spec. grav. 4·23? readily scratching glass;
fusible at the blowpipe into an opaque glass, sometimes with
intumescence. It affords, with acids, a solution that lets fall, with
caustic soda, a precipitate partly re-soluble in carbonate of ammonia.
It is remarkable for containing from 45 to 55 per cent. of the earth
Yttria; its remaining constituents being silica, 25·8; oxide of cerium,
17·92; oxide of iron, 11·43. This mineral is very rare, having been
hitherto found only in the neighbourhood of Fahlun and Ytterby, in
Sweden; its peculiar constituent was discovered by Professor Gadolin.
GALACTOMETER, or LACTOMETER, is an instrument to ascertain the quality
of milk; an article often sophisticated in various ways. Fresh milk,
rich in cream, has a less specific gravity, than the same milk after it
has been skimmed; and milk diluted with water becomes proportionably
lighter. Hence, when our purpose is to determine the quantity of cream,
the galactometer may consist merely of a long graduated glass tube
standing upright upon a sole. Having filled 100 measures with the recent
milk, we shall see, by the measures of cream thrown up, its value in
this respect. A delicate long-ranged glass hydrometer, graduated from
1·000 up to 1·060, affords the most convenient means of detecting the
degree of watery dilution, provided the absence of thickening materials
has been previously ascertained by filtration. Good fresh milk indicates
from 1·030 to 1·032; when the cream is removed, 1·035 to 1·037. When its
density is less than 1·028, we may infer it has been thinned with water.
GALBANUM, is a gum-resin, which occurs sometimes in yellow, shining
tears, easily agglutinated; of a strong durable smell; an acrid and
bitter taste; at other times in lumps. It exudes either spontaneously or
from incisions made into the stem of the _bubon galbanum_, a plant of
the family of _umbelliferæ_, which grows in Africa, particularly in
Ethiopia. It contains 67 of resin; 19·3 of gum; 6·4 of volatile oil and
water; 7·5 of woody fibres and other impurities; with traces of acid
malate of lime.
GALENA; (_Plomb sulfuré_, Fr.; _Bleiglanz_, Germ.;) is a metallic
looking substance of a lead-gray colour, which crystallizes in the
cubical system, and is susceptible of cleavages parallel to the faces of
the cube; spec. gr. 7·7592; cannot be cut; fusible at the blowpipe with
exhalation of sulphureous vapours; is easily reduced to metallic lead.
Nitric acid first dissolves it, and then throws down sulphate of lead in
a white precipitate; the solution affording with plates of zinc,
brilliant laminæ of lead (arbor Saturni.) It consists of sulphur, 13;
lead, 85; with a little iron, and sometimes a minute quantity of silver.
This is the richest ore of lead, and it occurs in almost every
geological formation, in veins, in masses, or in beds. It is almost
always accompanied by sulphuret of zinc, different salts of lead, heavy
spar, fluor spar, &c. Galena in powder, called Alquifoux, is employed as
a glaze for coarse stoneware.
GALIPOT, is a name of a white semi-solid viscid rosin found on
fir-trees; or an inferior sort of turpentine, poor in oil.
GALLATES; salts consisting of gallic acid combined with bases; the most
important being that with oxide of iron, constituting a principal part
of the black dye.
GALLIC ACID, is the peculiar acid extracted from gall-nuts; which see.
GALLIPOLI OIL, is a coarse olive oil, containing more or less mucilage;
imported from a sea port so named, of the province of Otranto, in the
kingdom of Naples.
GALL-NUTS, or GALLS; (_Noix de Galle_, Fr.; _Galläpfel_, Germ.;) are
excrescences found upon the loaves and leaf-stalks of a species of oak,
called _Quercus infectoria_, which grows in the Levant. They are
produced in consequence of the puncture of the female of the gall wasp,
(Cynips folii quercus), made in order to deposit her eggs; round which
the juice of the tree exudes, and dries in concentric portions. When the
insect gets fully formed, it eats through the nut, and flies off.
The Levant galls are of two different appearances and qualities; the
first are heavy, compact, imperforated, the insect having not been
sufficiently advanced to eat its way through the shell; prickly on the
surface; of a blackish or bluish green hue; about the size of a musket
ball. These are called _black_, blue, or Aleppo galls. The second are
light, spongy, pierced with one or more holes; smooth upon the surface,
of a pale grayish or reddish yellow colour, generally larger than the
first, and are called _white_ galls. Besides the galls of the Levant,
others come from Dalmatia, Illyria, Calabria, &c.; but they are of
inferior quality, being found upon the _Quercus Cerris_; they are
smaller, of a brownish colour, and of inferior value. The further south
the galls are grown, they are reckoned the better.
Galls consist principally of three substances; tannin or tannic acid;
yellow extractive; and gallic acid. Their decoction has a very
astringent and unpleasant bitter taste. The following are their
habitudes with various reagents:--
Litmus paper is powerfully reddened.
Stannous chloride (_protomuriate of tin_), produces an isabel yellow
precipitate.
Alum; a yellowish gray precipitate.
Acetate of lead; a thick yellowish white precipitate.
Acetate of copper; a chocolate brown precipitate.
Ferric sulphate (red sulphate of iron); a blue precipitate.
Sulphuric acid; a dirty yellowish precipitate.
Acetic acid brightens the muddy decoction.
The galls of the _Quercus Cerris_ and common oak (_Galles à l’épine_,
Fr.; _Knoppern_, Germ.) are of a dark-brown colour, prickly on the
surface, and irregular in shape and size. They are used chiefly for
tanning in Hungary, Dalmatia, and the southern provinces of the Austrian
states, where they abound.
Tannin or tannic acid is prepared as follows: Into a long narrow glass
adopter tube shut at its lower orifice with a cotton wick, a quantity of
pounded galls are put, and slightly pressed down. The tapering end of
the tube being inserted into a matrass or bottle, the vacant upper half
of the tube is filled with sulphuric ether, and then closed with a
ground-glass stopper. Next day there will be found in the bottle a
liquid in two distinct strata; of which the more limpid occupies the
upper part, and the other, of a syrupy consistence and amber colour, the
lower. More ether must be filtered through the galls, till the thicker
liquid ceases to augment. Both are now poured into a funnel, closed with
the finger, and after the dense liquor is settled at the bottom, it is
steadily run off into a capsule. This, after being washed repeatedly
with ether, is to be transferred into a stove chamber, or placed under
the receiver of an air pump to be evaporated. The residuary matter
swells up in a spongy crystalline form of considerable brilliancy,
sometimes colourless, but more frequently of a faintly yellowish hue.
This is pure tannin, which exists in galls to the amount of from 40 to
45 per cent. It is indispensable that the ether employed in the
preceding process be previously agitated with water, or that it contain
some water, because by using anhydrous ether, not a particle of tannin
will be obtained.
Tannic acid is a white or yellowish solid, inodorous, extremely
astringent, very soluble in water and alcohol, much less so in sulphuric
ether, and uncrystallizable. Its watery solution, out of contact of air,
undergoes no change; but if, in a very dilute state, it be left exposed
to the atmosphere, it loses gradually its transparency, and lets fall a
slightly grayish crystalline matter, consisting almost entirely of
gallic acid. For procuring this acid in a perfectly pure state, it is
merely necessary to treat that solution thus changed with animal
charcoal, and to filter it in a boiling state, through paper previously
washed with dilute muriatic acid. The gallic acid will fall down in
crystals as the liquid cools.
If the preceding experiment be made in a graduated glass tube containing
oxygen over mercury, this gas will be absorbed, and a corresponding
volume of carbonic acid gas will be disengaged. In this case the liquor
will appear in the course of a few weeks as if traversed with numerous
crystalline colourless needles of gallic acid.
Tannin or tannic acid consists of carbon 51·56; hydrogen 4·20; oxygen
44·24.
From the above facts it is obvious that gallic acid does not exist ready
formed in gall nuts, but that it is produced by the reaction of
atmospheric oxygen upon the tannin of these concretions.
Gallic acid is a solid, feebly acidulous and styptic to the taste,
inodorous, crystallizing in silky needles of the greatest whiteness;
soluble in about 100 times its weight of cold, and in a much smaller
quantity of boiling water; more soluble in alcohol than in water, but
little so in sulphuric ether.
Gallic acid does not decompose the salts of protoxide of iron, but it
forms, with the sulphate of the peroxide, a dark blue precipitate, much
less insoluble than the tannate of iron. Gallic acid takes the oxide
from the acetate and nitrate of lead, and throws down a white gallate
unchangeable in the air, when it is mixed with that acetate and nitrate.
It occasions no precipitate in solutions of gelatine (isinglass or
glue), by which criterion its freedom from tannin is verified.
Gallic acid occurs but seldom in nature; and always united to brucine,
veratrine, or lime. Its constituents are, carbon 49·89; hydrogen 3·49;
oxygen 46·62. In the crystalline state it contains one atom of water,
which it loses by drying.
Scheele obtained gallic acid by infusing pounded galls for 3 or 4 days
in 8 times their weight of water, and exposing the infusion to the air,
in a vessel covered loosely with paper. At the end of two months, the
liquor had almost all evaporated, leaving some mouldiness mixed with a
crystalline precipitate. The former being removed, the deposit was
squeezed in a linen cloth, and then treated with boiling water. The
solution being gradually evaporated, yielded crystals of gallic acid,
granular or star-like, of a grayish colour. These crystals might be
whitened by boiling their solution along with a little animal charcoal.
About one fifth of gallic acid may be obtained by Scheele’s process from
good gall-nuts.
From a decoction of 500 parts of galls, Sir H. Davy obtained 185 parts
of solid extract; which consisted of 130 parts of tannin; 31 parts of
gallic acid with extractive; 13 parts of mucilage; 12 parts of lime and
salts. Hence gall-nuts would seem to contain, by this statement, more
than two-thirds of their weight of tannin. This result is now seen, from
the above experiments of Pelouze, to have been incorrect, in consequence
of the admixture of yellow extractive in Davy’s tannin.
The uses of galls in many processes of dyeing, and in making black ink,
are detailed under their respective heads.
GALL OF ANIMALS, or OX-GALL, _purification of_. Painters in water
colours, scourers of clothes, and many others employ ox-gall or bile,
but when it is not purified, it is apt to do harm from the greenness of
its own tint. It becomes therefore an important object to clarify it,
and to make it limpid and transparent like water. The following process
has been given for that purpose. Take the gall of newly killed oxen, and
after having allowed it to settle for 12 or 15 hours in a basin, pour
the supernatant liquor off the sediment into an evaporating dish of
stone ware, and expose it to a boiling heat in a water bath, till it is
somewhat thick. Then spread it upon a dish, and place it before a fire
till it becomes nearly dry. In this state it may be kept for years in
jelly pots covered with paper, without undergoing any alteration. When
it is to be used, a piece of it of the size of a pea is to be dissolved
in a table spoonful of water.
Another and probably a better mode of purifying ox-gall is the
following. To a pint of the gall boiled and skimmed, add one ounce of
fine alum in powder, and leave the mixture on the fire till the alum be
dissolved. When cooled, pour into a bottle, which is to be loosely
corked. Now take a like quantity of gall also boiled and skimmed, add an
ounce of common salt to it, and dissolve with heat; put it when cold
into a bottle, which is likewise to be loosely corked. Either of these
preparations may be kept for several years without their emitting a bad
smell. After remaining three months, at a moderate temperature, they
deposit a thick sediment, and become clearer, and fit for ordinary uses,
but not for artists in water colours and miniatures, on account of their
yellowish-green colour. To obviate this inconvenience, each of the above
liquors is to be decanted apart, after they have become perfectly
settled, and the clear portion of both mixed together in equal parts.
The yellow colouring matter still retained by the mixture coagulates
immediately and precipitates, leaving the ox-gall perfectly purified and
colourless. If wished to be still finer, it may be passed through
filtering paper; but it becomes clearer with age, and never acquires a
disagreeable smell, nor loses any of its good qualities.
Clarified ox-gall combines readily with colouring matters or pigments,
and gives them solidity either by being mixed with or passed over them
upon paper. It increases the brilliancy and the durability of
ultramarine, carmine, green, and in general of all delicate colours,
whilst it contributes to make them spread more evenly upon the paper,
ivory, &c. When mixed with gum Arabic, it thickens the colours without
communicating to them a disagreeable glistering appearance; it prevents
the gum from cracking, and fixes the colours so well that others may be
applied over them without degradation. Along with lamp black and gum, it
forms a good imitation of China ink. When a coat of ox-gall is put upon
drawings made with black lead or crayons, the lines can no longer be
effaced, but may be painted over safely with a variety of colours
previously mixed up with the same ox-gall.
Miniature painters find a great advantage in employing it; by passing it
over ivory, it removes completely the unctuous matter from its surface;
and when ground with the colours, it makes them spread with the greatest
ease, and renders them fast.
It serves also for transparencies. It is first passed over the varnished
or oiled paper, and is allowed to dry. The colours mixed with the gall
are then applied, and cannot afterwards be removed by any means.
It is adapted finally for taking out spots of grease and oil.
GALL OF GLASS, called also sandiver, is the neutral salt skimmed off the
surface of melted crown glass; which, if allowed to remain too long, is
apt to be reabsorbed in part, and to injure the quality of the _metal_,
as the workmen call it.
GALVANIZED IRON, is the somewhat fantastic name newly given in France to
iron tinned by a peculiar patent process, whereby it resists the rusting
influence of damp air, and even moisture, much longer than ordinary tin
plate. The following is the prescribed process. Clean the surface of the
iron perfectly by the joint action of dilute acid and friction, plunge
it into a bath of melted zinc, and stir it about till it be alloyed
superficially with this metal; then take it out, and immerse it in a
bath of tin, such as is used for making tin plate. The tin forms an
exterior coat of alloy. When the metal thus prepared is exposed to
humidity, the zinc is said to oxidize slowly by a galvanic action, and
to protect the iron from rusting within it, whereby the outer tinned
surface remains for a long period perfectly white, in circumstances
under which iron tinned in the usual way would have been superficially
browned and corroded with rust.
GAMBOGE; (_Gomme Gutte_, Fr.; _Gutti_, Germ.) is a gum resin, concreted
in the air, from the milky juice which exudes from several trees. The
_gambogia gutta_, a tree which grows wild upon the coasts of Ceylon and
Malabar, produces the coarsest kind of gamboge; the _guttaefera vera_
(_Stalagmites cambogioides_) of Ceylon and Siam affords the best. It
comes to us in cylindrical lumps, which are outwardly brown yellow, but
reddish yellow within, as also in cakes; it is opaque, easily reducible
to powder, of specific gravity 1·207, scentless, and nearly devoid of
taste, but leaves an acrid feeling in the throat. Its powder and watery
emulsion are yellow. It consists of 80 parts of a hyacinth red resin,
soluble in alcohol; and 20 parts of gum; but by another analysis, of 89
of resin, and 10·5 of gum. Gamboge is used as a pigment, and in
miniature painting, to tinge gold varnish; in medicine as a powerful
purge. It should never be employed by confectioners to colour their
_liqueurs_, as they sometimes do.
GANGUE. A word derived from the German _gang_, a vein or channel. It
signifies the mineral substance which either encloses or usually
accompanies any metallic ore in the vein. Quartz, lamellar carbonate of
lime, sulphate of baryta, sulphate and fluate of lime, generally form
the gangues; but a great many other substances become such when they
predominate in a vein. In metallurgic works the first thing is to break
the mixed ore into small pieces, in order to separate the valuable from
the useless parts, by processes called stamping, picking, sorting. See
METALLURGY and MINES.
GARNET (_Grenat_, Fr.; _Granat_, Germ.); is a vitreous mineral of the
cubic system, of which the predominating forms are the rhomboidal
dodecahedron and the trapœzohedron; specific gravity varying from 3·35
to 4·24; fusible at the blowpipe. Its constituents are, silica, 42;
alumina, 20·0; lime, 34·0; protoxide of iron, 4. Garnets are usually
disseminated, and occur in all the primitive strata from gneiss to clay
slate. The finer varieties, noble garnet or Almandine, and the reddish
varieties of Grossulaire (Essonite), are employed in jewellery; the
first are called the Syrian or oriental; the others, hyacinth. In some
parts of Germany garnets are so abundant as to be used as fluxes to some
iron ores; in others, the garnet gravel is washed, pounded, and employed
as a substitute for emery. The garnets of Pegu are most highly valued.
Factitious garnets may be made by the following composition:--Purest
white glass, 2 ounces; glass of antimony, 1 ounce; powder of cassius, 1
grain; manganese, 1 grain.
GAS (Eng. and Fr.; _Gaz_, Germ.); is the generic name of all those
elastic fluids which are permanent under a considerable pressure, and at
the temperature of zero of Fahrenheit. In many of them, however, by the
joint influence of excessive cold and pressure, the repulsive state of
the particles may be balanced or subverted, so as to transform the
elastic gas into a liquid or a solid. For this most interesting
discovery, we are indebted to the fine genius of Mr. Faraday.
The following table exhibits the temperatures and pressures at which
certain gases are liquefied.
+---------------------+--------------------------+-------------------+
| | Becomes liquid | Calculated |
| Name of the gas. +------+-------------------+ boiling point; |
| | At |Under a pressure of|Barom. = 30 inches.|
+---------------------+------+-------------------+-------------------+
|Sulphurous acid |59° F.| 3 atmospheres. | - 4° Fahr. |
|Chlorine |60 | 4 | - 22 |
|Ammonia |50 | 6·5 | - 64 |
|Sulphuretted hydrogen|50 | 17 | - 142 |
|Carbonic acid |32 | 36 | - 229 |
|Hydrochloric or | | | |
|muriatic acid |50 | 50 | - 249 |
|Deutoxide of azote |45 | 50 | - 254 |
+---------------------+------+-------------------+-------------------+
Liquid carbonic acid becomes solidified, into a snowy-looking substance,
by its own rapid evaporation. Oxygen, hydrogen, and azote, have hitherto
resisted all attempts to divest them of their elastic form. For this
purpose, it is probable that a condensing force equal to that of 650
atmospheres, will be required.
The volume of any gas is, generally speaking, inversely as the pressure
to which it is exposed; thus, under a double pressure its bulk becomes
one-half; under a triple pressure, one-third; and so on. For the change
of volume in gaseous bodies by heat, see EXPANSION.
Ammonia, carbonic acid, carburetted hydrogen, chlorine, muriatic acid,
sulphurous acid, sulphuretted hydrogen, are the gases of most direct
interest in the arts and manufactures. Their detailed examination
belongs to a work on chemistry.
GAS-LIGHT. (_Eclairage par gas_, Fr.; _Gaslicht_, Germ.) Dr. Clayton
demonstrated, by numerous experiments in 1737 and 1738, that bituminous
pit-coal, subjected to a red heat in close vessels, afforded a great
deal of an air similar to the fire-damp of mines, but which burned with
a brighter flame. It does not appear that this species of factitious air
was ever produced from pit-coal for the purpose of artificial
illumination till 1792, when Mr. William Murdoch, engineer to Messrs.
Bolton and Watt, employed coal gas for lighting his house and offices,
at Redruth in Cornwall. The gas was generated in an iron retort, whence
it was received in a gasometer, distributed in different situations by
pipes, and finally burned at small apertures which could be opened and
stopped at pleasure. He moreover made this light movable, by confining
the gas in portable tin-plate vessels, and burning it wherever he
pleased. Between this period and 1802, Mr. Murdoch continued at
intervals to make similar experiments; and upon occasion of the national
illumination in the spring of the latter year, at the peace of Amiens,
he lighted up part of the Soho manufactory with a public display of gas
lights.
The earliest application of this artificial light, on a large systematic
scale, was made at Manchester; where an apparatus for lighting the great
cotton mills of Messrs. Philips and Lee, was fitted up in 1804 and 1805,
under the direction of Mr. Murdoch. A quantity of light, nearly equal to
3000 candles, was produced and distributed in this building. This
splendid pattern has been since followed very generally in Great
Britain, and more or less in many parts of the continents of Europe and
America. By the year 1822, gas-lighting in London had become the
business of many public companies. At the Peter-street station, for
example, 300 retorts had been erected, supplying 15 gasometers, having
each an average capacity of 20·626 cubic feet, but, being never quite
filled, their total contents in gas might be estimated at 309,385 cubic
feet. The extent of main pipes of distribution belonging to this station
was then about 57 miles, with two separate mains in some of the streets.
The product of gas was from 10,000 to 12,000 cubic feet from a chaldron
of coals. The annual consumption of coals was therefore altogether 9282
chaldrons, affording 11,384,000 cubic feet of gas, allowing 153 retorts
to be in constant daily action, upon an average of the year; and
illuminating 10,660 private lamps, 2248 street lamps, and 3894 theatre
lamps.
At the Brick-lane works, 371 retorts were fixed in 1822, 133 being
worked on an average of summer and winter. There were 12 gasometers,
charged with an average quantity of gas amounting to 197,214 cubic feet.
Of coals, 8060 chaldrons were annually consumed; 96,720,000 cubic feet
of gas were generated; for the supply of 1978 public lamps, and 7366
private ones, connected with main pipes 40 miles long.
At the Curtain-road gas establishment, there were 240 retorts; but the
greatest number worked in 1821 was only 80, and the lowest 21. The six
gasometers had an average contents of 90,467 cubic feet. Of coals, 3336
chaldrons were annually consumed, yielding 40,040,000 cubic feet of gas,
that supplied 3860 private lamps, and 629 public ones, by means of mains
25 miles long. The above three stations belonged to the London Gas Light
and Coke Company.
The City of London Gas Light Company, Dorset-street, had built up 230
retorts, and 6 gasometers, while two were preparing; having a total
capacity of 181,282 cubic feet. Of private lamps 5423 were lighted, and
2413 public ones, from mains extending 50 miles. The quantity of coals
carbonized amounted to 8840 chaldrons; producing 106,080,000 cubic feet
of gas.
The South London Gas Light and Coke Company had mounted at Bankside 143
retorts, with 3 gasometers; the contents of the whole being 41,110 cubic
feet, connected with mains from 30 to 40 miles long. At their other
station, in Wellington-street, 3 large gasometers were then erecting,
with a capacity of 73,565 cubic feet, which were to be supplied with gas
from Bankside, till retorts were mounted for them.
The Imperial Gas Light and Coke Company had at that time 6 gasometers in
progress at their Hackney station.
In 1822 there were thus four great companies, having in all 47
gasometers at work, capable of containing 917,940 cubic feet of gas,
supplied by 1315 retorts, which generated per annum upwards of
397,000,000 cubic feet of gas, by which 61,203 private lamps, and 7268
public or street lamps, were lighted in the metropolis. Besides these
public companies, there were likewise several private ones.
1. _Of the generation of illuminating gases._--Pure hydrogen gas burns
with too feeble a flame to be employed for illumination. But carburetted
hydrogen having the property of precipitating its carbon in the act of
burning, its solid particles become incandescent, and diffuse a vivid
light. The more carbon it contains, the more brightly does it burn. This
gas exists in two distinct states of combination. In the first, two
measures of hydrogen gas are combined with one measure of the vapour of
carbon, forming together one measure whose specific gravity is of course
the sum of the weights of the constituents, or 0·559; atmospherical air
being 1·000. This is the gas which is found in mines, and is also
evolved in ditches from decomposing vegetable matter. In the second, two
measures of hydrogen gas are combined with two of gaseous carbon,
forming also one volume or measure whose weight or specific gravity is
0·985. This was at one time called the olefiant gas, because when mixed
with chlorine an oily looking compound was produced. It may be called as
well oil gas, because it is generated in considerable quantities by the
igneous decomposition of oil. Thus the olefiant gas contains in the same
volume double the quantity of carbon of common carburetted hydrogen, and
it burns with a proportionably brighter flame. The gaseous oxide of
carbon, as well as sulphuretted hydrogen gas, burns with a feeble blue
light, but the latter produces in combustion sulphurous acid, an
offensive and noxious gas.
By dry distillation or carbonization in close vessels, all bodies of
vegetable and animal origin disengage carburetted hydrogen gas; even
charcoal when placed in ignition in contact with steam, by decomposing
the water, produces abundance of carbonic acid, carburetted hydrogen,
hydrogen, and carbonic oxide. After separating the carbonic acid with
lime water, that mixed gas contains in 100 measures, 20 of carburetted
hydrogen; the rest being hydrogen and carbonic oxide, so that the
gaseous mixture cannot be used for illumination. The best substances for
furnishing a gas rich in luminifereous materials are, pitcoal,
especially the cannel coal, resin, oil, fats of all kinds, tar, wax, &c.
In some cases the gases evolved during the igneous decomposition of
bones and other animal matters for the production of ammonia, may be
employed for procuring light, but they are apt to emit a fetid odour.
When coals are heated in a cast-iron retort to ignition, the progress of
decomposition is as follows. First, and before the retort becomes red
hot, steam issues along with the atmospheric air. When the retort begins
to redden, tar distils in considerable quantity with some combustible
gas, of which hydrogen mixed with ammoniacal gas forms a part. The
evolution of gas increases as the retort becomes hotter, with a
continual production of tar and ammoniacal liquor as well as sulphurous
acid from the pyrites of the coal, which unites with the ammonia. When
the retort has come to a bright cherry red heat, the disengagement of
gas is most active. By and bye the gaseous production diminishes, and
eventually ceases entirely, although the heat be increased. In the
retort a quantity of carbonized coal or coke remains, while tar is found
at the bottom of the receiver, covered with the ammoniacal liquor, and
combined with carbonic and sulphurous acids, and sulphuretted hydrogen.
If during this distillation, the combustible gas be collected and
examined at the several stages of the process, it is found to differ
extremely in its luminiferous powers. That which comes off before the
retort has acquired its proper temperature, gives a feeble light, and
resembles the gas obtained by the ignition of moist charcoal, consisting
chiefly of hydrogen. That evolved when the retort has just acquired
throughout a vivid red heat, is the best of all, consisting chiefly of
bicarburetted hydrogen or olefiant gas. From good coal, it consists, for
example in 100 measures, of 13 of olefiant gas, 82·5 of carburetted
hydrogen, 3·2 carbonic oxide, 1·3 azote; the mixture having a specific
gravity of 0·650. At a later period, as after 5 hours, it contains 7
measures of olefiant gas, 56 of carburetted hydrogen, 11 of carbonic
oxide, 21·3 of hydrogen, 4·7 of azote; the specific gravity of the whole
being 0·500. Towards the end of the operation, as after 10 hours, it
contains twenty measures of carburetted hydrogen, 10 of carbonic oxide,
60 of hydrogen, 10 of azote, with a specific gravity of only 0·345. The
hydrogen becomes sulphuretted hydrogen, if there be much pyritous matter
in the coal. The larger proportion of the gas is disengaged during the
first hour, amounting to about one fifth of the whole; in the three
following hours the disengagement is tolerably uniform, constituting in
all fifty-four hundredths; in the sixth hour, it is one tenth; in the
seventh and eighth hours, sixteen hundredths.
From these observations are derived the rules for the production of a
good light gas from coals. They show that the distillation should
commence with a retort previously heated to a cherry red, since thereby
good gas is immediately produced, and a portion of the tar is also
converted into gas, instead of being simply distilled over into the
condenser pit; that this heat should be steadily continued during the
whole operation, from 5 to 8 hours; that it should not be increased,
especially towards the end, for fear of generating carbonic oxide and
hydrogen gases, as well as of injuring the retort when the cooling
agency of gasefication has become feeble; and that the operation should
be stopped some time before gas ceases to come over, lest gases with
feeble illuminating power should impoverish the contents of the
gasometer. Upon the average, a pound of good coal affords four cubic
feet of gas, or a chaldron = 26 cwt. London measure, affords from 12,000
to 15,000 cubic feet, according to the form of the retort, and the
manner of firing it.
When oil, fats, rosin, tar, &c. are employed for the production of a
light gas, it is not sufficient to introduce these substances into the
retorts, and to heat them, as is done with coals. In this case, the
greater part of them would distil over in the state of volatile oils,
and very little gas be generated, only as much as corresponded to the
quantity of fat, &c. in immediate contact with the retort. It becomes
therefore necessary to fill the retorts with pieces of brick or coke;
and to keep them in ignition, while the oil, &c. is slowly introduced
into their interior. The fats instantly assume the vaporous state, and
thus coming into contact upon an extensive surface with the ignited
bricks, are decomposed into combustible gases. A small portion of
carbonaceous matter remains in the retort, while much olefiant gas is
formed, possessing a superior illuminating power to common coal gas, and
entirely free from sulphureous impregnation. The best oil gas is
generated at a dull red, a heat much below what is requisite for the
decomposition of coal. A more intense heat would indeed produce a
greater volume of gas, but of a poorer quality, because the olefiant gas
thereby deposits one half of its carbon, and is converted into common
carburetted hydrogen. Oil affords at a lively red heat, gases which
contain in 100 measures, 19 of olefiant gas, 32·4 of carburetted
hydrogen, 12·2 of carbonic oxide gas, 32·4 of hydrogen, and 4 of azote;
the mean specific gravity being only 0·590. At a more moderate
temperature it yields 22·5 of the olefiant, 50·3 carburetted hydrogen,
15·5 carbonic oxide, 7·7 hydrogen, and 4 azote, with a specific gravity
of 0·758. It contains when generated by dull ignition, as is usual in
works on the manufacturing scale, in 100 parts, from 38 to 40 of
olefiant gas, and besides the carburetted hydrogen, a few per cents. of
carbonic oxide and azote, with a specific gravity of 0·900, and even
upwards. One pound of oil or fluid fat affords 15 cubic feet of gas; of
tar affords about 12 cubic feet; of rosin or pitch, 10 cubic feet.
When the oil gas is compressed by a force of from 15 to 20 atmospheres,
as was the practice of the Portable Gas Company, about one fifth of the
volume of the gas becomes liquefied into an oily, very volatile fluid,
having the specific gravity 0·821. It is a mixture of three fluids
(consisting of carburetted hydrogen), of different degrees of
volatility. The most volatile of these boils even under 32° F. Some of
the vapour of this gas-oil is mixed with the olefiant gas in the general
products of decomposition; in consequence of which they are sometimes
richer in carbon than even olefiant gas, and have a higher illuminating
power. Oil gas contains about 22 per cent. and coal gas about 3-1/4 per
cent. of this oily vapour. In the estimations of the composition of the
gases given above, this vapour is included under olefiant gas. This
vapour combines readily with sulphuric acid, and is thus precipitated
from the gaseous mixture. The amount of olefiant gas is shown, by adding
to the gas, contained over water, one half of its volume of chlorine,
which, in the course of an hour or two, condenses the olefiant gas into
an oily looking liquid (chloride of hydrocarbon.) After the mixture, the
gases must be screened from the light, otherwise the common carburetted
hydrogen would also combine with the chlorine, while water and carbonic
acid would make their appearance.
The oil employed for affording gas is the crudest and cheapest that can
be bought; even the blubber and sediment of whale oil are employed with
advantage. After all, however, coal is so much cheaper, and the gas
produced from it is now so well purified, that oil and rosin are very
little used in gas apparatus.
_Apparatus for Coal Gas._--Coal gas, as it issues from the retort,
cannot be directly employed for illumination; for it contains vapours of
tar and coal oil, as also steam impregnated with the carbonate,
sulphite, and hydrosulphuret of ammonia. These vapours would readily
condense in the pipes through which the gas must be distributed, and
would produce obstructions; they must therefore be so far removed by
previous cooling, as to be liable to occasion no troublesome
condensation at ordinary temperatures. The crude coal gas contains
moreover sulphuretted hydrogen, whose combustion for light would exhale
an offensive sulphureous odour, that ought to be got rid of as much as
possible. Carbonic acid and carbonic oxide gases, generated at first
from the decomposition of the steam by the ignited coal, enfeeble the
illuminating power of the gas, and should be removed. The disengagement
of gas in the retorts is never uniform, but varies with the degree of
heat to which they are exposed; for which reason the gas must be
received in a gasometer, where it may experience uniform pressure, and
be discharged uniformly into the pipes of distribution, in order to
ensure a steady discharge of gas, and uniform intensity of light in the
burners. A coal gas apparatus ought therefore to be so constructed as
not only to generate the gas itself, but to fulfil the above
conditions.
In _fig._ 482., such an apparatus is represented, where the various
parts are shown connected with each other, in section.
[Illustration: 482]
A is the furnace with its set of cylindrical or elliptical retorts, five
in number. From each of these retorts, a tube _b_ proceeds
perpendicularly upwards, and then by a curve or saddle-tube, it turns
downwards, where it enters a long horizontal cylinder under B, shut at
each end with a screw cap, and descends to beneath its middle, so as to
dip about an inch into the water contained in it. From one end of this
cylinder the tube _d_ passes downwards, to connect itself with a
horizontal tube which enters into the _tar pit_ or _cistern_ C, by means
of the vertical branch _f_. This branch reaches to near the bottom of
the cylindrical vessel, which sits on the sole of the tar cistern. From
the other side of the vertical branch _f_, the main pipe proceeds to the
condenser D, and thence by the pipe _l_, into the purifier E; from which
the gas is immediately transmitted by the pipe _p_ into the gasometer F.
The operation proceeds in the following way:--As soon as gas begins to
be disengaged from the ignited retort, tar and ammoniacal liquor are
deposited in the cylindrical receiver B, and fill it up till the
superfluity runs over by the pipe _d_, the level being constantly
preserved at the line shown in the figure. By the same tarry liquid, the
orifices of the several pipes _b_, issuing from the retorts, are closed;
whereby the gas in the pipe _d_ has its communication cut off with the
gas in the retorts. Hence if one of the retorts be opened and emptied,
it remains shut off from the rest of the apparatus. This insulation of
the several retorts is the function of the pipe under B, and therefore
the recurved tube _b_ must be dipped as far under the surface of the
tarry liquid, as to be in equilibrio with the pressure of the gas upon
the water in the purifier. The tube _b_ is closed at top with a screw
cap, which can be taken off at pleasure, to permit the interior to be
cleansed.
Both by the overflow from the receiver-pipe B, and by subsequent
condensation in the tube _d_, tar and ammoniacal liquor collect
progressively in the cistern or pit under C, by which mingled liquids
the lower orifice of the vertical tube _f_ is closed, so that the gas
cannot escape into the empty space of this cistern. These liquids flow
over the edges of the inner vessel when it is full, and may, from time
to time, be drawn off by the stopcock at the bottom of the cistern.
Though the gas has, in its progress hitherto, deposited a good deal of
its tarry and ammoniacal vapours, yet, in consequence of its high
temperature, it still retains a considerable portion of them, which must
be immediately abstracted, otherwise the tar would pollute the lime in
the vessel E, and interfere with its purification. On this account the
gas should, at this period of the process, be cooled as much as
possible, in order to condense these vapours, and to favour the action
of the lime in the purifier E, upon the sulphuretted hydrogen, which is
more energetic the lower the temperature of the gas. The coal gas
passes, therefore, from the tube _f_ into the tube _h_ of the condenser
D, which is placed in an iron chest _g_ filled with water, and it
deposits more tar and ammoniacal liquor in the under part of the cistern
at _t_, _t_. When these liquids have risen to a certain level, they
overflow into the tar-pit, as shown in the figure, to be drawn off by
the stopcock as occasion may require.
The refrigerated gas is now conducted into the purifier E, which is
filled with milk of lime, made by mixing one part of slaked lime with 25
parts of water. The gas, as it enters by the pipe _l_, depresses the
water in the wide cylinder _n_, thence passes under the perforated disc
in the under part of that cylinder, and rising up through innumerable
small holes is distributed throughout the lime liquid in the vessel _m_.
By contact with the lime on this extended surface, the gas is stripped
of its sulphuretted hydrogen and carbonic acid, which are condensed into
the hydro-sulphuret and carbonate of lime; it now enters the gasometer F
in a purified state, through the pipe _p t_, and occupies the space _q_.
The gasometer, pressing with a small unbalanced force over the
counterweight _s_, expels it through the main _u u_, in communication
with the pipes of distribution through the buildings or streets to be
illuminated.
The parts A B C D E and F, of which this apparatus consists, are
essential constituents of every good coal-gas work. Their construction
rests upon peculiar principles, is susceptible of certain modifications,
and therefore deserves to be considered in detail.
_The Retorts._--These are generally made of cast iron, though they have
occasionally been made of baked clay, like common earthenware retorts.
The original form was a cylinder, which was changed to an ellipse, with
the long axis in a horizontal direction, then into the shape of the
letter D with the straight line undermost, and lastly into a
semi-cylinder, with its horizontal diameter 22 inches, and its vertical
varying from 9 to 12. The kidney form was at one time preferred, but it
has been little used of late.
[Illustration: 483]
The form of retort represented in _fig._ 483. has been found to yield
the largest quantity of good gas in the shortest time, and with the
least quantity of firing. The length is 7-1/2, and the transverse area,
from one foot to a foot and a half square. The arrows show the direction
of the flame and draught in this excellent bench of retorts, as mounted
by Messrs. Barlow.
The charge of coals is most conveniently introduced in a tray of sheet
iron, made somewhat like a grocer’s scoop, adapted to the size of the
retort, which is pushed home to its further end, inverted so as to turn
out the contents, and then immediately withdrawn.
The duration of the process, or the time of completing a distillation,
depends upon the nature of the coal and the form of the retort. With
cylindrical retorts it cannot be finished in less than 6 hours, but with
elliptical and semi-cylindrical retorts, it may be completed in 4 or 5
hours. If the distillation be continued in the former for 8 hours, and
in the latter for 6, gas will continue to be obtained, but during the
latter period of the operation, of indifferent quality.
[Illustration: 484]
_The Receiver._--If the furnace contains only 2 or 3 retorts, a simple
cylindrical vessel standing on the ground half filled with water, may
serve as a receiver; into which the tube from the retort may be plunged.
It should be provided with an overflow pipe for the tar and ammoniacal
liquor. For a range of several retorts, a long horizontal cylinder is
preferable, like that represented at B in _fig._ 484. Its diameter is
from 10 to 15 inches. This cylinder may be so constructed as to separate
the tar from the ammoniacal liquor, by means of a syphon attached to one
of its ends.
_The Condenser._--The condenser, represented in _fig._ 482., consists of
a square chest, _g_, made of wrought iron plates open at top, but having
its bottom pierced with a row of holes, to receive a series of tubes. To
these holes the upright four-inch tubes _h h_ are secured by flanges and
screws, and they are connected in pairs at top by the curved or saddle
tubes. The said bottom forms the cover of the chest _t_, _t_, which is
divided by vertical iron partitions, into half as many compartments as
there are tubes.
These partition plates are left open at bottom, so as to place the
liquids of each compartment in communication. Thereby the gas passes up
and down the series of tubes, in proceeding from one compartment to
another. The condensed liquids descend into the box _t_, _t_, and flow
over into the tar cistern, when they rise above the level _t_, _t_. The
tar may be drawn off from time to time by the stopcock. Through the tube
_k_, cold water flows into the condenser chest, and the warm water
passes away by a pipe at its upper edge.
The extent of surface which the gas requires for its refrigeration
before it is admitted into the washing-lime apparatus, depends upon the
temperature of the milk of lime, and the quantity of gas generated in a
certain time.
It may be assumed as a determination sufficiently exact, that 10 square
feet of surface of the condenser can cool a cubic foot of gas per minute
to the temperature of the cooling water. For example, suppose a furnace
or arch with 5 retorts of 150 pounds of coal each, to produce in 5 hours
3000 cubic feet of gas, or 10 cubic feet per minute, there would be
required, for the cooling surface of the condenser, 100 square feet = 10
× 10. Suppose 100,000 cubic feet of gas to be produced in 24 hours, for
which 8 or 9 such arches must be employed, the condensing surface must
contain from 800 to 900 square feet.
_The Purifier._--The apparatus represented in the preceding figure is
composed of a cylindrical iron vessel, with an air-tight cover screwed
upon it, through which the cylinder _n_ is also fixed air-tight. The
bottom of this cylinder spreads out like the brim of a hat, forming a
horizontal circular partition, which is pierced with holes. Through a
stuffing box, in the cover of this interior cylinder, the vertical axis
of the agitator passes, which is turned by wheel and pinion work, in
order to stir up the lime from the bottom of the water in the purifier.
The vessel _o_ serves for introducing fresh milk of lime, as also for
letting it off by a stopcock when it has become too foul for further
use.
The quantity of lime should be proportioned to the quantity of
sulphuretted hydrogen and carbonic acid contained in the gas. Supposing
that in good coal gas there is 5 per cent. of these gases, about one
pound and a half of lime will be requisite for every hundred cubic feet
of coal gas generated, which amounts to nearly one-sixteenth of the
weight of coal subjected to decomposition. This quantity of lime mixed
with the proper quantity of water will form about a cubic foot of milk
of lime. Consequently, the capacity of the purifier, that is, of the
interior space filled with liquid, may be taken at four-sevenths of a
cubic foot for every hundred cubic feet of gas passing through it in one
operation; or for 175 cubic feet of gas, one cubic foot of liquor. After
every operation, that is, after every five or six hours, the purifier
must be filled afresh. Suppose that in the course of one operation
20,000 cubic feet of gas pass through the machine, this should be able
to contain 20,000/175 = 114 cubic feet of milk of lime; whence its
diameter should be seven feet, and the height of the liquid three feet.
If the capacity of the vessel be less, the lime milk must be more
frequently changed.
In some of the large gas works of London the purifier has the following
construction, whereby an uninterrupted influx and efflux of milk of lime
takes place. Three single purifiers are so connected together, that the
second vessel stands higher than the first, and the third than the
second; so that the discharge tube of the superior vessel, placed
somewhat below its cover, enters into the upper part of the next lower
vessel; consequently, should the milk of lime in the third and uppermost
vessel rise above its ordinary level, it will flow over into the second,
and thence in the same way into the first; from which it is let off by
the eduction pipe. A tube introduces the gas from the condenser into the
first vessel, another tube does the same thing for the second vessel,
&c., and the tube of the third vessel conducts the gas into the
gasometer. Into the third vessel, milk of lime is constantly made to
flow from a cistern upon a higher level. By this arrangement, the gas
passing through the several vessels in proportion as it is purified,
comes progressively into contact with purer milk of lime, whereby its
purification becomes more complete. The agitator _c_, provided with two
stirring paddles, is kept in continual rotation. The pressure which the
gas has here to overcome is naturally three times as great as with a
single purifier of like depth.
[Illustration: 485]
_Fig._ 485. is a simple form of purifier, which has been found to answer
well in practice. Through the cover of the vessel A B, the wide cylinder
_e d_ is inserted, having its lower end pierced with numerous holes.
Concentric with that cylinder is the narrower one _s z_, bound above
with the flange _a b_, but open at top and bottom. The under edge _g h_
of this cylinder descends a few inches below the end _c d_ of the outer
one. About the middle of the vessel the perforated shelf _m n_ is
placed. The shaft of the agitator _l_, passes through a stuffing box
upon the top of the vessel. The gas-pipe _g_, proceeding from the
condenser, enters through the flange _a b_ in the outer cylinder, while
the gas-pipe _h_ goes from the cover to the gasometer. A stopcock upon
the side, whose orifice of discharge is somewhat higher than the under
edge of the outer cylinder, serves to draw off the milk of lime. As the
gas enters through the pipe _g_ into the space between the two
cylinders, it displaces the liquor till it arrives at the holes in the
under edge of the outer cylinder, through which, as well as under the
edge, it flows, and then passes up through the apertures of the shelf _m
n_ into the milk of lime chamber; the level of which is shown by the
dotted line. The stirrer, _l_, should be turned by wheel work, though it
is here shown as put in motion by a winch handle.
In order to judge of the degree of purity of the gas after its
transmission through the lime machine, a slender syphon tube provided
with a stopcock may have the one end inserted in its cover, and the
other dipped into a vessel containing a solution of acetate of lead.
Whenever the solution has been rendered turbid by the precipitation of
sulphuret of lead, it should be renewed. The saturated and fetid milk of
lime is evaporated in oblong cast-iron troughs placed in the ash-pit of
the furnaces, and the dried lime is partly employed for luting the
apparatus, and partly disposed of for a mortar or manure.
By this purifier, and others of similar construction, the gas in the
preceding parts of the apparatus, as in the retorts and the condenser,
suffers a pressure equal to a column of water about two feet high; and
in the last described purifier even a greater pressure. This pressure is
not disadvantageous, but is of use in two respects; 1. it shows by a
brisk jet of gas when the apparatus is not air-tight, and it prevents
common air from entering into the retorts; 2. this compression of the
gas favours the condensation of the tar and ammoniacal liquor. The
effect of such a degree of pressure in expanding the metal of the
ignited retorts is quite inconsiderable, and may be neglected. Two
contrivances have, however, been proposed for taking off this pressure
in the purifier.
[Illustration: 486]
In _fig._ 486., _m m_ are two similar vessels of a round or rectangular
form, furnished at their upper border with a groove filled with water,
into which the under edge of the cover fits, so as to make the vessel
air-tight. The cover is suspended by a cord or chain, which goes over a
pulley, and may be raised or lowered at pleasure. The vessels themselves
have perforated bottoms, _r r´_, covered with wetted moss or hay
sprinkled over with slaked and sifted quicklime. The gas passes through
the loosely compacted matter of the first vessel, by entering between
its two bottoms, rises into the upper space _t_, thence it proceeds to
the second vessel, and, lastly, through the pipe _u_ into the gasometer.
This method, however, requires twice as much lime as the former, without
increasing the purity of the gas.
[Illustration: 487]
The second method consists in compressing the gas by the action of an
Archimedes screw, to such a degree, before it is admitted into the
purifier, as that it may overcome the pressure of the column of water in
that vessel. _Fig._ 487. exhibits this apparatus in section. D D is the
Archimedes worm, the axis of which revolves at bottom upon the gudgeon
_e_; it possesses a three-fold spiral, and is turned in the opposite
direction to that in which it scoops the water. The cistern which
contains it has an air-tight cover. The gas to be purified passes
through the pipe C into the space D, over the water level _d_; the upper
cells of the worm, scoop in the gas at this point, and carry it
downwards, where it enters at _g_ into the cavity E of a second cistern.
In order that the gas, after it escapes from the bottom of the worm, may
not partially return through _g_ into the cavity D, an annular plate _g
h_ is attached to its under edge, so as to turn over it. The compressed
gas is conducted from the cavity E through the pipe G into the purifying
machine; _a_ is a manometer, to indicate the elastic tension of the gas
in D. On the top of the worm a mechanism is fitted for keeping it in
constant rotation.
A perfect purification of light-gas from sulphuretted hydrogen, either
by milk of lime or a solution of the green sulphate of iron, is attended
with some difficulty, when carried so far as to cause no precipitation
of sulphuret in acetate of lead, because such a degree of washing is
required as is apt to diminish its illuminating power, by abstracting
the vapour of the rich oily hydrocarburet which it contains. Moreover,
the coal gas obtained towards the end of the distillation contains some
sulphuret of carbon, which affords sulphurous acid on being burned, and
can be removed by no easy method hitherto known. The lime in the
purifier disengages from the carbonate and hydrosulphuret of ammonia
carried over with the gas, especially when it has been imperfectly
cooled in the condenser, a portion of ammoniacal gas, which, however, is
not injurious to its illuminating power. The best agent for purifying
gas would be the pyrolignite of lead, were it not rather expensive,
because it would save the trouble of stirring, and require a smaller and
simpler apparatus.
_The Gasometer._--The gasometer serves not merely as a magazine for
receiving the gas when it is purified, and keeping it in store for use,
but also for communicating to the gas in the act of burning such an
uniform pressure as may secure a steady unflickering flame. It consists
of two essential parts; 1. of an under cistern, open at top and filled
with water; and 2. of the upper floating cylinder or chest, which is a
similar cistern inverted, and of somewhat smaller dimensions, called the
gas-holder: see F, _fig._ 482. The best form of this vessel is the round
or cylindrical; both because under equal capacity it requires least
surface of metal, and it is least liable to be warped by its own weight
or accidents. Since a cylindrical body has the greatest capacity with a
given surface when its height is equal to its semi-diameter, its
dimensions ought to be such that when elevated to the highest point in
the water, the height may be equal to the radius of the base. For
example, let the capacity of the gas-holder in cubic feet be _k_, the
semi-diameter of its base be _x_, the height out of the water be _h_;
_h_ is = _x_ = ∛(k/3·14). This height may be increased by one or two
feet, according to its magnitude, to prevent the chance of any gas
escaping beneath its under edge, when it is raised to its highest
elevation in the water.
The size of the gasometer should be proportional to the quantity of gas
to be consumed in a certain time. If 120,000 cubic feet be required, for
instance, in 10 hours for street illumination, and if the gas retorts be
charged four times in 24 hours, 30,000 cubic feet of gas will be
generated in 6 hours. Hence the gasometer should have a capacity of at
least 70,000 cubic feet, supposing the remaining 50,000 cubic feet to be
produced during the period of consumption. If the gasometer has a
smaller capacity, it must be supplied from a greater number of retorts
during the lighting period, which is not advantageous, as the first
heating of the supernumerary retorts is wasteful of fuel. Some engineers
consider that a capacity of 30,000 cubic feet is the largest which can
with propriety be given to a gasometer; in which case, they make its
diameter 42 feet, and its height 23. When the dimensions are greater,
the sheet iron must be thicker and more expensive; and the hollow
cylinder must be fortified by strong internal cross braces.
The water cistern is usually constructed in this country with cast-iron
plates bolted together, and made tight with rust-cement.
[Illustration: 488]
In cases where the weight of water required to fill such a cistern might
be inconvenient to sustain, it may be made in the form represented in
_fig._ 488.; which, however, will cost nearly twice as much. Parallel
with the side of the cistern, a second cylinder C, of the same shape but
somewhat smaller, is fixed in an inverted position to the bottom of the
first, so as to leave an annular space B B between them, which is filled
with water, and in which the floating gasometer A plays up and down. The
water must stand above the cover of the inverted cylinder. _a_ and _b_
are the pipes for leading the gas in and out. Through an opening in the
masonry upon which the gasometer apparatus rests, the space C may be
entered, in order to make any requisite repairs.
The water cistern may also be sunk in the ground, and the sides made
tight with hydraulic mortar, as is shown in _fig._ 489., and to make it
answer with less water, a concentric cylindrical mass of masonry may be
built at a distance of 2 or 3 inches within it.
Every large gasometer must be strengthened interiorly with cross iron
rods, to stiffen both its top and bottom. The top is supported by rods
stretching obliquely down to the sides, and to the under edge an iron
ring is attached, consisting of curved cast-iron bars bolted together;
with which the oblique rods are connected by perpendicular ones. Other
vertical rods stretch directly from the top to the bottom edge. Upon the
periphery of the top, at the end of the rods, several rings are made
fast, to which the gas-holder is suspended, by means of a common chain
which runs over a pulley at the centre. Upon the other end of the chain
there is a counterpoise, which takes off the greater part of the weight
of the gas-holder, leaving only so much as is requisite for the
expulsion of the gas. The inner and outer surfaces of the gas-holder
should be a few times rubbed over with hot tar, at a few days’ interval
between each application. The pulley must be made fast to a strong
frame.
[Illustration: 489]
If the water cistern be formed with masonry, the suspension of the
gas-holder may be made in the following way. A A, _fig._ 489., is a
hollow cylinder of cast iron, standing up through the middle of the
gasometer, and which is provided at either end with another small hollow
cylinder G, open at both ends and passing through the top, with its axis
placed in the axis of the gas-holder. In the hollow cylinder G, the
counterweight moves up and down, with its chain passing over the three
pulleys B, B, B, as shown in _fig._ 489. E F are the gas pipes made fast
to a vertical iron rod. Should the gasometer be made to work without a
counterweight, as we shall presently see, the central cylinder A A,
serves as a vertical guide.
In proportion as the gas-holder sinks in the water of the cistern, it
loses so much of its weight, as is equal to the weight of the water
displaced by the sides of the sinking vessel; so that the gas-holder
when entirely immersed, exercises the least pressure upon the gas, and
when entirely out of the water, it exercises the greatest pressure. In
order to counteract this inequality of pressure, which would occasion an
unequal velocity in the efflux of the gas, and of course an unequal
intensity of light in its flame, the weight of the chain upon which the
gas-holder hangs is so adjusted as to be equal, throughout the length of
its motion, to one half of the weight which the gas-holder loses by
immersion. In this case, the weight which it loses by sinking into the
water, is replaced by the portion of the chain which passing the pulley,
and hanging over, balances so much of the chain upon the side of the
counterweight; and the weight which it gains by rising out of the water,
is counterpoised by the links of the chain which passing over the
pulley, add to the amount of the counterweight. The pressure which the
gas-holder exercises upon the gas, or that with which it forces it
through the first main pipe, is usually so regulated as to sustain a
column of from one to two inches of water; so that the water will stand
in the cistern from one to two inches higher within, than without the
gas-holder. The following computation will place these particulars in a
clear light.
Let the semi-diameter of the gas-holder, equal to the vertical extent of
its motion into and out of the water, = _x_; let the weight of a foot
square of the side of the gas-holder, including that of the
strengthening bars and ring, which remain plunged under the water, be =
_p_; then