[1] Not, as frequently spelt, Gluck.
GLUCKSBURG, a town of Germany, in the Prussian province of
Schleswig-Holstein, romantically situated among pine woods on the
Flensburg Fjord off the Baltic, 6 m. N.E. from Flensburg by rail. Pop.
(1905) 1551. It has a Protestant church and some small manufactures and
is a favourite sea-bathing resort. The castle, which occupies the site
of a former Cistercian monastery, was, from 1622 to 1779, the residence
of the dukes of Holstein-Sonderburg-Glucksburg, passing then to the king
of Denmark and in 1866 to Prussia. King Frederick VII. of Denmark died
here on the 15th of November 1863.
GLUCKSTADT, a town of Germany, in the Prussian province of
Schleswig-Holstein, on the right bank of the Elbe, at the confluence of
the small river Rhin, and 28 m. N.W. of Altona, on the railway from
Itzehoe to Elmshorn. Pop. (1905) 6586. It has a Protestant and a Roman
Catholic church, a handsome town-hall (restored in 1873-1874), a
gymnasium, a provincial prison and a penitentiary. The inhabitants are
chiefly engaged in commerce and fishing; but the frequent losses from
inundations have greatly retarded the prosperity of the town. Gluckstadt
was founded by Christian IV. of Denmark in 1617, and fortified in 1620.
It soon became an important trading centre. In 1627-28 it was besieged
for fifteen weeks by the imperialists under Tilly, without success. In
1814 it was blockaded by the allies and capitulated, whereupon its
fortifications were demolished. In 1830 it was made a free port. It came
into the possession of Prussia together with the rest of
Schleswig-Holstein in 1866.
See Lucht, _Gluckstadt. Beitrage zur Geschichte dieser Stadt_ (Kiel,
1854).
GLUCOSE (from Gr. [Greek: glykys], sweet), a carbohydrate of the formula
C6H12O6; it may be regarded as the aldehyde of sorbite. The name is
applied in commerce to a complex mixture of carbohydrates obtained by
boiling starch with dilute mineral acids; in chemistry, it denotes, with
the prefixes d, l and d + l (or i), the dextro-rotatory, laevo-rotatory
and inactive forms of the definite chemical compound defined above. The
d modification is of the commonest occurrence, the other forms being
only known as synthetic products; for this reason it is usually termed
glucose, simply; alternative names are dextrose, grape sugar and
diabetic sugar, in allusion to its right-handed optical rotation, its
occurrence in large quantity in grapes, and in the urine of diabetic
patients respectively. In the vegetable kingdom glucose occurs, always
in admixture with fructose, in many fruits, especially grapes, cherries,
bananas, &c.; and in combination, generally with phenols and aldehydes
belonging to the aromatic series, it forms an extensive class of
compounds termed glucosides. It appears to be synthesized in the plant
tissues from carbon dioxide and water, formaldehyde being an
intermediate product; or it may be a hydrolytic product of a glucoside
or of a polysaccharose, such as cane sugar, starch, cellulose, &c. In
the plant it is freely converted into more complex sugars,
poly-saccharoses and also proteids. In the animal kingdom, also, it is
very widely distributed, being sometimes a normal and sometimes a
pathological constituent of the fluids and tissues; in particular, it is
present in large amount in the urine of those suffering from diabetes,
and may be present in nearly all the body fluids. It also occurs in
honey, the white appearance of candied honey being due to its
separation.
Pure d-glucose, which may be obtained synthetically (see SUGAR) or by
adding crystallized cane sugar to a mixture of 80% alcohol and 1/15
volume of fuming hydrochloric acid so long as it dissolves on shaking,
crystallizes from water or alcohol at ordinary temperatures in nodular
masses, composed of minute six-sided plates, and containing one molecule
of water of crystallization. This product melts at 86 deg. C., and
becomes anhydrous when heated to 110 deg. C. The anhydrous compound can
also be prepared, as hard crusts melting at 146 deg., by crystallizing
concentrated aqueous solutions at 30 deg. to 35 deg. It is very soluble
in water, but only slightly soluble in strong alcohol. Its taste is
somewhat sweet, its sweetening power being estimated at from 1/2 to 3/5
that of cane sugar. When heated to above 200 deg. it turns brown and
produces caramel, a substance possessing a bitter taste, and used, in
its aqueous solution or otherwise, under various trade names, for
colouring confectionery, spirits, &c. The specific rotation of the plane
of polarized light by glucose solutions is characteristic. The specific
rotation of a freshly prepared solution is 105 deg., but this value
gradually diminishes to 52.5 deg., 24 hours sufficing for the transition
in the cold, and a few minutes when the solution is boiled. This
phenomenon has been called mutarotation by T. M. Lowry. The specific
rotation also varies with the concentration; this is due to the
dissociation of complex molecules into simpler ones, a view confirmed by
cryoscopic measurements.
Glucose may be estimated by means of the polarimeter, i.e. by
determining the rotation of the plane of polarization of a solution, or,
chemically, by taking advantage of its property of reducing alkaline
copper solutions. If a glucose solution be added to copper sulphate and
much alkali added, a yellowish-red precipitate of cuprous hydrate
separates, slowly in the cold, but immediately when the liquid is
heated; this precipitate rapidly turns red owing to the formation of
cuprous oxide. In 1846 L. C. A. Barreswil found that a strongly alkaline
solution of copper sulphate and potassium sodium tartrate (Rochelle
salt) remained unchanged on boiling, but yielded an immediate
precipitate of red cuprous oxide when a solution of glucose was added.
He suggested that the method was applicable for quantitatively
estimating glucose, but its acceptance only followed after H. von
Fehling's investigation. "Fehling's solution" is prepared by dissolving
separately 34.639 grammes of copper sulphate, 173 grammes of Rochelle
salt, and 71 grammes of caustic soda in water, mixing and making up to
1000 ccs.; 10 ccs. of this solution is completely reduced by 0.05
grammes of hexose. Volumetric methods are used, but the uncertainty of
the end of the reaction has led to the suggestion of special indicators,
or of determining the amount of cuprous oxide gravimetrically.
_Chemistry._--In its chemical properties glucose is a typical
oxyaldehyde or aldose. The aldehyde group reacts with hydrocyanic acid
to produce two stereo-isomeric cyanhydrins; this isomerism is due to
the conversion of an originally non-asymmetric carbon atom into an
asymmetric one. The cyanhydrin is hydrolysable to an acid, the lactone
of which may be reduced by sodium amalgam to a glucoheptose, a
non-fermentable sugar containing seven carbon atoms. By repeating the
process a non-fermentable gluco-octose and a fermentable glucononose
may be prepared. The aldehyde group also reacts with phenyl hydrazine
to form two phenylhydrazones; under certain conditions a hydroxyl
group adjacent to the aldehyde group is oxidized and glucosazone is
produced; this glucosazone is decomposed by hydrochloric acid into
phenyl hydrazine and the keto-aldehyde glucosone. These
transformations are fully discussed in the article SUGAR. On reduction
glucose appears to yield the hexahydric alcohol _d_-sorbite, and on
oxidation _d_-gluconic and _d_-saccharic acids. Alkalis partially
convert it into _d_-mannose and _d_-fructose. Baryta and lime yield
saccharates, e.g. C6H12O6.BaO, precipitable by alcohol.
CH2OH CH2OH
. .
CH.OH CH.OH
. .
CH CH
/ /
O< O<
\ . \ .
(CH.OH)2 (CH.OH2
. .
HC.OH HO.CH
[alpha]-glucose [beta]-glucose
The constitution of glucose was established by H. Kiliani in
1885-1887, who showed it to be CH2OH.(CH.OH)4.CHO. The subject was
taken up by Emil Fischer, who succeeded in synthesizing glucose, and
also several of its stereo-isomers, there being 16 according to the Le
Bel-van't Hoff theory (see Stereo-Isomerism and Sugar). This open
chain structure is challenged in the views put forward by T. M. Lowry
and E. F. Armstrong. In 1895 C. Tanret showed that glucose existed in
more than one form, and he isolated [alpha], [beta] and [gamma]
varieties with specific rotations of 105 deg., 52.5 deg. and 22 deg.
It is now agreed that the [beta] variety is a mixture of the [alpha]
and [gamma]. This discovery explained the mutarotation of glucose. In
a fresh solution [alpha]-glucose only exists, but on standing it is
slowly transformed into [gamma]-glucose, equilibrium being reached
when the [alpha] and [gamma] forms are present in the ratio
0.368:0.632 (Tanret, _Zeit. physikal. Chem._, 1905, 53, p. 692). It is
convenient to refer to these two forms as [alpha] and [beta]. Lowry
and Armstrong represent these compounds by the following spatial
formulae which postulate a [gamma]-oxidic structure, and 5 asymmetric
carbon atoms, i.e. one more than in the Fischer formulae. These
formulae are supported by many considerations, especially by the
selective action of enzymes, which follows similar lines with the
[alpha]- and [beta]-glucosides, i.e. the compounds formed by the
interaction of glucose with substances generally containing hydroxyl
groups (see GLUCOSIDE).
_Fermentation of Glucose._--Glucose is readily fermentable. Of the
greatest importance is the alcoholic fermentation brought about by
yeast cells (_Saccharomyces cerevisiae seu vini_); this follows the
equation C6H12O6 = 2C2H6O + 2CO2, Pasteur considering 94 to 95% of the
sugar to be so changed. This character is the base of the plan of
adding glucose to wine and beer wort before fermenting, the alcohol
content of the liquid after fermentation being increased. Some fusel
oil, glycerin and succinic acid appear to be formed simultaneously,
but in small amount. Glucose also undergoes fermentation into lactic
acid (q.v.) in the presence of the lactic acid bacillus, and into
butyric acid if the action of the preceding ferment be continued, or
by other bacilli. It also yields, by the so-called mucous
fermentation, a mucous, gummy mass, mixed with mannitol and lactic
acid.
We may here notice the frequent production of glucose by the action of
enzymes upon other carbohydrates. Of especial note is the
transformation of maltose by maltase into glucose, and of cane sugar
by invertase into a mixture of glucose and fructose (invert sugar);
other instances are: lactose by lactase into galactose and glucose;
trehalose by trehalase into glucose; melibiose by melibiase into
galactose and glucose; and of melizitose by melizitase into touranose
and glucose, touranose yielding glucose also when acted upon by the
enzyme touranase.
_Commercial Glucose._--The glucose of commerce, which may be regarded
as a mixture of grape sugar, maltose and dextrins, is prepared by
hydrolysing starch by boiling with a dilute mineral acid. In Europe,
potato starch is generally employed; in America, corn starch. The acid
employed may be hydrochloric, which gives the best results, or
sulphuric, which is used in Germany; sulphuric acid is more readily
separated from the product than hydrochloric, since the addition of
powdered chalk precipitates it as calcium sulphate, which may be
removed by a filter press. The processes of manufacture have much in
common, although varying in detail. The following is an outline of the
process when hydrochloric acid is used: Starch ("green" starch in
America) is made into a "milk" with water, and the milk pumped into
boiling dilute acid contained in a closed "converter," generally made
of copper or cast iron; steam is led in at the same time, and the
pressure is kept up to about 25 lb. to the sq. in. When the converter
is full the pressure is raised somewhat, and the heating continued
until the conversion is complete. The liquid is now run into
neutralizing tanks containing sodium carbonate, and, after settling,
the supernatant liquid, termed "light liquor," is run through bag
filters and then on to bone-char filters, which have been previously
used for the "heavy liquor." The colourless or amber-coloured filtrate
is concentrated to 27 deg. to 28 deg. B., when it forms the "heavy
liquor," just mentioned. This is filtered through fresh bone-char
filters, from which it is discharged as a practically colourless
liquid. This liquid is concentrated in vacuum pans to a specific
gravity of 40 deg. to 44 deg. B., a small quantity of sodium
bisulphite solution being added to bleach it, to prevent fermentation,
and to inhibit browning. "Syrup glucose" is the commercial name of the
product; by continuing the concentration further solid glucose or
grape sugar is obtained.
Several brands are recognized: "Mixing glucose" is used by syrup and
molasses manufacturers, "jelly glucose" by makers of jellies,
"confectioners' glucose" in confectionery, "brewers' glucose" in
brewing, &c.
GLUCOSIDE, in chemistry, the generic name of an extensive group of
substances characterized by the property of yielding a sugar, more
commonly glucose, when hydrolysed by purely chemical means, or
decomposed by a ferment or enzyme. The name was originally given to
vegetable products of this nature, in which the other part of the
molecule was, in the greater number of cases, an aromatic aldehydic or
phenolic compound (exceptions are sinigrin and jalapin or scammonin). It
has now been extended to include synthetic ethers, such as those
obtained by acting on alcoholic glucose solutions with hydrochloric
acid, and also the polysaccharoses, e.g. cane sugar, which appear to be
ethers also. Although glucose is the commonest sugar present in
glucosides, many are known which yield rhamnose or iso-dulcite; these
may be termed pentosides. Much attention has been given to the non-sugar
parts of the molecules; the constitutions of many have been determined,
and the compounds synthesized; and in some cases the preparation of the
synthetic glucoside effected.
The simplest glucosides are the alkyl esters which E. Fischer (_Ber._,
28, pp. 1151, 3081) obtained by acting with hydrochloric acid on
alcoholic glucose solutions. A better method of preparation is due to E.
F. Armstrong and S. L. Courtauld (_Proc. Phys. Soc._, 1905, July 1),
who dissolve solid anhydrous glucose in methyl alcohol containing
hydrochloric acid. A mixture of [alpha]- and [beta]-glucose result,
which are then etherified, and if the solution be neutralized before the
[beta]-form isomerizes and the solvent removed, a mixture of the
[alpha]- and [beta]-methyl ethers is obtained. These may be separated by
the action of suitable ferments. Fischer found that these ethers did not
reduce Fehling's solution, neither did they combine with phenyl
hydrazine at 100 deg.; they appear to be stereo-isomeric [gamma]-oxidic
compounds of the formulae I., II.: The difference between the [alpha]-
and [beta]-forms is best shown by the selective action of enzymes.
Fischer found that maltase, an enzyme occurring in yeast cells,
hydrolysed [alpha]-glucosides but not the [beta]; while emulsin, an
enzyme occurring in bitter almonds, hydrolyses the [beta] but not the
[alpha]. The ethers of non-fermentable sugars are themselves
non-fermentable. By acting with these enzymes on the natural glucosides,
it is found that the majority are of the [beta]-form; e.g. emulsin
hydrolyses salicin, helicin, aesculin, coniferin, syringin, &c.
CH2OH CH2OH
. .
CHOH CHOH
. .
/ CH / CH
O < . O < .
\(CHOH)2 \(CHOH)2
. .
H.C.OCH3 CH3O.C.H
I. [alpha]-methyl II. [beta]-methyl
_d_-glucoside _d_-glucoside
Classification of the glucosides is a matter of some difficulty. One
based on the chemical constitution of the non-glucose part of the
molecules has been proposed by Umney, who framed four groups: (1)
ethylene derivatives, (2) benzene derivatives, (3) styrolene
derivatives, (4) anthracene derivatives. A group may also be made to
include the cyanogenetic glucosides, i.e. those containing prussic acid.
J. J. L. van Rijn (_Die Glykoside_, 1900) follows a botanical
classification, which has several advantages; in particular, plants of
allied genera contain similar compounds. In this article the chemical
classification will be followed. Only the more important compounds will
be noticed, the reader being referred to van Rijn (_loc. cit._) and to
Beilstein's _Handbuch der organischen Chemie_ for further details.
1. _Ethylene Derivatives._--These are generally mustard oils, and are
characterized by a burning taste; their principal occurrence is in
mustard and _Tropaeolum seeds_. Sinigrin or the potassium salt of
myronic acid, C10H16NS2KO9.H2O, occurs in black pepper and in
horse-radish root. Hydrolysis with baryta, or decomposition by the
ferment myrosin, gives glucose, allyl mustard oil and potassium
bisulphate. Sinalbin, C30H42N2S2O15, occurs in white pepper; it
decomposes to the mustard oil HO.C6H4.CH2.NCS, glucose and sinapin, a
compound of choline and sinapinic acid. Jalapin or scammonin,
C34H56O16, occurs in scammony; it hydrolyses to glucose and
jalapinolic acid. The formulae of sinigrin, sinalbin, sinapin and
jalapinolic acid are:--
/ N.C3H5 / N.CH2.C6H4.OH
C6H11O5.S.C < C6H11O5.S.C <
\ O.SO2.OK \ O.SO2.OC16H24O5N
Sinigrin Sinalbin
(CH3O)2 \ / (CH3)3
> C6H2.CH:CH.CO.C2H4.O.N <
HO / \ OH
Sinapin
CH3 \
> CH.CH(OH).C10H20.CO2H
C2H6 /
Jalapinolic acid (Kramer)
2. _Benzene Derivatives._--These are generally oxy and oxyaldehydic
compounds. Arbutin, C12H16O7, which occurs in bearberry along with
methyl arbutin, hydrolyses to hydroquinone and glucose.
Pharmacologically it acts as a urinary antiseptic and diuretic; the
benzoyl derivative, cellotropin, has been used for tuberculosis.
Salicin, also termed "saligenin" and "glucose," C13H18O7, occurs in
the willow. The enzymes ptyalin and emulsin convert it into glucose
and saligenin, ortho-oxybenzylalcohol, HO.C6H4.CH2OH. Oxidation gives
the aldehyde helicin. Populin, C20H22O8, which occurs in the leaves
and bark of _Populus tremula_, is benzoyl salicin.
3. _Styrolene Derivatives._--This group contains a benzene and also an
ethylene group, being derived from styrolene C6H5.CH:CH2. Coniferin,
C16H22O8, occurs in the cambium of coniferous woods. Emulsin converts
it into glucose and coniferyl alcohol, while oxidation gives
glycovanillin, which yields with emulsin glucose and vanillin (see
EUGENOL and VANILLA). Syringin, which occurs in the bark of _Syringa
vulgaris_, is methoxyconiferin. Phloridzin, C21H24O10, occurs in the
root-bark of various fruit trees; it hydrolyses to glucose and
phloretin, which is the phloroglucin ester of para-oxyhydratropic acid.
It is related to the pentosides naringin, C21H26O11, which hydrolyses
to rhamnose and naringenin, the phloroglucin ester of para-oxycinnamic
acid, and hesperidin, C50H60O22(?), which hydrolyses to rhamnose and
hesperetin, C16H14O6, the phloroglucin ester of
meta-oxy-para-methoxycinnamic acid or isoferulic acid, C10H10O4. We may
here include various coumarin and benzo-[gamma]-pyrone derivatives.
Aesculin, C15H16O9, occurring in horse-chestnut, and daphnin, occurring
in _Daphne alpina_, are isomeric; the former hydrolyses to glucose and
aesculetin (4.5-dioxycoumarin), the latter to glucose and daphnetin
(3.4-dioxycoumarin). Fraxin, C16H18O10, occurring in _Fraxinus
excelsior_, and with aesculin in horse-chestnut, hydrolyses to glucose
and fraxetin, the monomethyl ester of a trioxycoumarin. Flavone or
benzo-[gamma]-pyrone derivatives are very numerous; in many cases they
(or the non-sugar part of the molecule) are vegetable dyestuffs.
_Quercitrin_, C21H22O12, is a yellow dyestuff found in _Quercus
tinctoria_; it hydrolyses to rhamnose and quercetin, a
dioxy-[beta]-phenyl-trioxybenzo-[gamma]-pyrone. Rhamnetin, a splitting
product of the glucosides of _Rhamnus_, is monomethyl quercetin;
fisetin, from _Rhus cotinus_, is monoxyquercetin; chrysin is
phenyl-dioxybenzo-[gamma]-pyrone. Saponarin, a glucoside found in
_Saponaria officinalis_, is a related compound. Strophanthin is the
name given to three different compounds, two obtained from
_Strophanthus Kombe_ and one from _S. hispidus_.
4. _Anthracene Derivatives._--These are generally substituted
anthraquinones; many have medicinal applications, being used as
purgatives, while one, ruberythric acid, yields the valuable dyestuff
madder, the base of which is alizarin (q.v.). Chrysophanic acid, a
dioxymethylanthraquinone, occurs in rhubarb, which also contains
emodin, a trioxymethylanthraquinone; this substance occurs in
combination with rhamnose in frangula bark.
The most important cyanogenetic glucoside is amygdalin, which occurs
in bitter almonds. The enzyme maltase decomposes it into glucose and
mandelic nitrile glucoside; the latter is broken down by emulsin into
glucose, benzaldehyde and prussic acid. Emulsin also decomposes
amygdalin directly into these compounds without the intermediate
formation of mandelic nitrile glucoside. Several other glucosides of
this nature have been isolated. The saponins are a group of substances
characterized by forming a lather with water; they occur in soap-bark
(q.v.). Mention may also be made of indican, the glucoside of the
indigo plant; this is hydrolysed by the indigo ferment, indimulsin, to
indoxyl and indiglucin.
GLUE (from the O. Fr. _glu_, bird-lime, from the Late Lat. _glutem_,
_glus_, glue), a valuable agglutinant, consisting of impure gelatin and
widely used as an adhesive medium for wood, leather, paper and similar
substances. Glues and gelatins merge into one another by imperceptible
degrees. The difference is conditioned by the degree of purity: the more
impure form is termed glue and is only used as an adhesive, the purer
forms, termed gelatin, have other applications, especially in culinary
operations and confectionery. Referring to the article GELATIN for a
general account of this substance, it is only necessary to state here
that gelatigenous or glue-forming tissues occur in the bones, skins and
intestines of all animals, and that by extraction with hot water these
agglutinating materials are removed, and the solution on evaporating and
cooling yields a jelly-like substance--gelatin or glue.
Glues may be most conveniently classified according to their sources:
bone glue, skin glue and fish glue; these may be regarded severally as
impure forms of bone gelatin, skin gelatin and isinglass.
_Bone Glue._--For the manufacture of glue the bones are supplied fresh
or after having been used for making soups; Indian and South American
bones are unsuitable, since, by reason of their previous treatment with
steam, both their fatty and glue-forming constituents have been already
removed (to a great extent). On the average, fresh bones contain about
50% of mineral matter, mainly calcium and magnesium phosphates, about
12% each of moisture and fat, the remainder being other organic matter.
The mineral matter reappears in commerce chiefly as artificial manure;
the fat is employed in the candle, soap and glycerin industries, while
the other organic matter supplies glue.
The separation of the fat, or "de-greasing of the bones" is effected (1)
by boiling the bones with water in open vessels; (2) by treatment with
steam under pressure; or (3) by means of solvents. The last process is
superseding the first two, which give a poor return of fat--a valuable
consideration--and also involve the loss of a certain amount of glue.
Many solvents have been proposed; the greatest commercial success
appears to attend Scottish shale oil and natural petroleum (Russian or
American) boiling at about 100 deg. C. The vessels in which the
extraction is carried out consist of upright cylindrical boilers,
provided with manholes for charging, a false bottom on which the bones
rest, and with two steam coils--one for heating only, the other for
leading in "live" steam. There is a pipe from the top of the vessel
leading to a condensing plant. The vessels are arranged in batteries. In
the actual operation the boiler is charged with bones, solvent is run
in, and the mixture gradually heated by means of the dry coil; the
spirit distils over, carrying with it the water present in the bones;
and after a time the extracted fat is run off from discharge cocks in
the bottom of the extractor.[1] A fresh charge of solvent is introduced,
and the cycle repeated; this is repeated a third and fourth time, after
which the bones contain only about 0.2% of fat, and a little of the
solvent, which is removed by blowing in live steam under 70 to 80 lb.
pressure. The de-greased bones are now cleansed from all dirt and flesh
by rotation in a horizontal cylindrical drum covered with stout wire
gauze. The attrition accompanying this motion suffices to remove the
loosely adherent matter, which falls through the meshes of the gauze;
this meal contains a certain amount of glue-forming matter, and is
generally passed through a finer mesh, the residuum being worked up in
the glue-house, and the flour which passes through being sold as a
bone-meal, or used as a manure.
The bones, which now contain 5 to 6% of glue-forming nitrogen and about
60% of calcium phosphate, are next treated for glue. The most economical
process consists in steaming the bones under pressure (15 lb. to start
with, afterwards 5 lb.) in upright cylindrical boilers fitted with false
bottoms. The glue-liquors collect beneath the false bottoms, and when of
a strength equal to about 20% dry glue they are run off to the
clarifiers. The first runnings contain about 65 to 70% of the total
glue; a second steaming extracts another 25 to 30%. For clarifying the
solutions, ordinary alum is used, one part being used for 200 parts of
dry glue. The alum is added to the hot liquors, and the temperature
raised to 100 deg.; it is then allowed to settle, and the surface scum
removed by filtering through coarse calico or fine wire filters.
The clear liquors are now concentrated to a strength of about 32% dry
glue in winter and 35% in summer. This is invariably effected in vacuum
pans--open boiling yields a dark-coloured and inferior product. Many
types of vacuum plant are in use; the Yaryan form, invented by H. T.
Yaryan, is perhaps the best, and the double effect system is the most
efficient. After concentration the liquors are bleached by blowing in
sulphur dioxide, manufactured by burning sulphur; by this means the
colour can be lightened to any desired degree. The liquors are now run
into galvanized sheet-iron troughs, 2 ft. long, 6 in. wide and 5 in.
deep, where they congeal to a firm jelly, which is subsequently removed
by cutting round the edges, or by warming with hot water, and turning
the cake out. The cake is sliced to sheets of convenient thickness,
generally by means of a wire knife, i.e. a piece of wire placed in a
frame. Mechanical slicers acting on this principle are in use. Instead
of allowing the solution to congeal in troughs, it may be "cast" on
sheets of glass, the bottoms of which are cooled by running water. After
congealing, the tremulous jelly is dried; this is an operation of great
nicety: the desiccation must be slow and is generally effected by
circulating a rapid current of air about the cakes supported on nets set
in frames; it occupies from four to five days, and the cake contains on
the average from 10 to 13% of water.
_Skin Glue._--In the preparation of skin glue the materials used are the
parings and cuttings of hides from tan-yards, the ears of oxen and
sheep, the skins of rabbits, hares, cats, dogs and other animals, the
parings of tawed leather, parchment and old gloves, and many other
miscellaneous scraps of animal matter. Much experience is needed in
order to prepare a good glue from such heterogeneous materials; one
blending may be a success and another a failure. The raw material has
been divided into three great divisions: (1) sheep pieces and fleshings
(ears, &c.); (2) ox fleshings and trimmings; (3) ox hides and pieces;
the best glue is obtained from a mixture of the hide, ear and face
clippings of the ox and calf. The raw material or "stock" is first
steeped for from two to ten weeks, according to its nature, in wooden
vats or pits with lime water, and afterwards carefully dried and stored.
The object of the lime steeping is to remove any blood and flesh which
may be attached to the skin, and to form a lime soap with the fatty
matter present. The "scrows" or glue pieces, which may be kept a long
time without undergoing change, are washed with a dilute hydrochloric
acid to remove all lime, and then very thoroughly with water; they are
now allowed to drain and dry. The skins are then placed in hemp nets and
introduced into an open boiler which has a false bottom, and a tap by
which liquid may be run off. As the boiling proceeds test quantities of
liquid are from time to time examined, and when a sample is found on
cooling to form a stiff jelly, which happens when it contains about 32%
dry glue, it is ready to draw off. The solution is then run to a
clarifier, in which a temperature sufficient to keep it fluid is
maintained, and in this way any impurity is permitted to subside. The
glue solution is then run into wooden troughs or coolers in which it
sets to a firm jelly. The cakes are removed as in the case of bone glue
(see above), and, having been placed on nets, are, in the Scottish
practice, dried by exposure to open air. This primitive method has many
disadvantages: on a hot day the cake may become unshapely, or melt and
slip through the net, or dry so rapidly as to crack; a frost may produce
fissures, while a fog or mist may precipitate moisture on the surface
and occasion a mouldy appearance. The surface of the cake, which is
generally dull after drying, is polished by washing with water. The
practice of boiling, clarification, cooling and drying, which has been
already described in the case of bone glue, has been also applied to the
separation of skin glue.
_Fish Glue._--Whereas isinglass, a very pure gelatin, is yielded by the
sounds of a limited number of fish, it is found that all fish offals
yield a glue possessing considerable adhesive properties. The
manufacture consists in thoroughly washing the offal with water, and
then discharging it into extractors with live steam. After digestion,
the liquid is run off, allowed to stand, the upper oily layer removed,
and the lower gluey solution clarified with alum. The liquid is then
filtered, concentrated in open vats, and bleached with sulphur
dioxide.[2] Fish glue is a light-brown viscous liquid which has a
distinctly disagreeable odour and an acrid taste; these disadvantages to
its use are avoided if it be boiled with a little water and 1% of sodium
phosphate, and 0.025% of saccharine added.
_Properties of Glue._--A good quality of glue should be free from all
specks and grit, have a uniform, light brownish-yellow, transparent
appearance, and should break with a glassy fracture. Steeped for some
time in cold water it softens and swells up without dissolving, and when
again dried it ought to resume its original properties. Under the
influence of heat it entirely dissolves in water, forming a thin syrupy
fluid with a not disagreeable smell. The adhesiveness of different
qualities of glue varies considerably; the best adhesive is formed by
steeping the glue, broken in small pieces, in water until they are quite
soft, and then placing them with just sufficient water to effect
solution in the glue-pot. The hotter the glue, the better the joint;
remelted glue is not so strong as the freshly prepared; and newly
manufactured glue is inferior to that which has been long in stock. It
is therefore seen that many factors enter into the determination of the
cohesive power of glue; a well-prepared joint may, under favourable
conditions, withstand a pull of about 700 lb. per sq. in. The following
table, after Kilmarsch, shows the holding power of glued joints with
various kinds of woods.
+---------+-----------------------------+
| | lb. per sq. in. |
| Wood. +-------------+---------------+
| | With grain. | Across grain. |
+---------+-------------+---------------+
| Beech | 852 | 434.5 |
| Maple | 484 | 346 |
| Oak | 704 | 302 |
| Fir | 605 | 132 |
+---------+-------------+---------------+
_Special Kinds of Glues, Cements, &c._--By virtue of the fact that the
word "glue" is frequently used to denote many adhesives, which may or
may not contain gelatin, there will now be given an account of some
special preparations. These may be conveniently divided into: (1)
liquid glues, mixtures containing gelatin which do not jelly at
ordinary temperatures but still possess adhesive properties; (2)
water-proof glues, including mixtures containing gelatin, and also the
"marine glues," which contain no glue; (3) glues or cements for
special purposes, e.g. for cementing glass, pottery, leather, &c., for
cementing dissimilar materials, such as paper or leather to iron.
_Liquid Glues._--The demand for liquid glues is mainly due to the
disadvantages--the necessity of dissolving and using while hot--of
ordinary glue. They are generally prepared by adding to a warm glue
solution some reagent which destroys the property of gelatinizing. The
reagents in common use are acetic acid; magnesium chloride, used for a
glue employed by printers; hydrochloric acid and zinc sulphate; nitric
acid and lead sulphate; and phosphoric acid and ammonium carbonate.
_Water-proof Glues._--Numerous recipes for water-proof glues have been
published; glue, having been swollen by soaking in water, dissolved in
four-fifths its weight of linseed oil, furnishes a good water-proof
adhesive; linseed oil varnish and litharge, added to a glue solution,
is also used; resin added to a hot glue solution in water, and
afterwards diluted with turpentine, is another recipe; the best glue
is said to be obtained by dissolving one part of glue in one and a
half parts of water, and then adding one-fiftieth part of potassium
bichromate. Alcoholic solutions of various gums, and also tannic acid,
confer the same property on glue solutions. The "marine glues" are
solutions of india-rubber, shellac or asphaltum, or mixtures of these
substances, in benzene or naphtha. Jeffrey's marine glue is formed by
dissolving india-rubber in four parts of benzene and adding two parts
of shellac; it is extensively used, being easily applied and drying
rapidly and hard. Another water-proof glue which contains no gelatin
is obtained by heating linseed oil with five parts of quicklime; when
cold it forms a hard mass, which melts on heating like ordinary glue.
_Special Glues._--There are innumerable recipes for adhesives
specially applicable to certain substances and under certain
conditions. For repairing glass, ivory, &c. isinglass (q.v.), which
may be replaced by fine glue, yields valuable cements; bookbinders
employ an elastic glue obtained from an ordinary glue solution and
glycerin, the water being expelled by heating; an efficient cement for
mounting photographs is obtained by dissolving glue in ten parts of
alcohol and adding one part of glycerin; portable or mouth glue--so
named because it melts in the mouth--is prepared by dissolving one
part of sugar in a solution of four parts of glue. An india-rubber
substitute is obtained by adding sodium tungstate and hydrochloric
acid to a strong glue solution; this preparation may be rolled out
when heated to 60 deg.
For further details see Thomas Lambert, _Glue, Gelatine and their
Allied Products_ (London, 1905); R. L. Fernbach, _Glues and Gelatine_
(1907); H. C. Standage, _Agglutinants of all Kinds for all Purposes_
(1907).