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-γ-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-γ-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-β-phenyl-trioxybenzo-γ-pyrone.
Rhamnetin, a splitting product of the glucosides of
Rhamnus, is monomethyl quercetin; fisetin, from Rhus cotinus,
is monoxyquercetin; chrysin is phenyl-dioxybenzo-γ-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° 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 ℔ 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 ℔ to start with, afterwards 5 ℔) 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°; 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
- ↑ This fat contains a small quantity of solvent, which is removed by heating with steam, when the solvent distils off. Hot water is then run in to melt the fat, which rises to the surface of the water and is floated off. Another boiling with water, and again floating off, frees the fat from dirt and mineral matter, and the product is ready for casking.
