Chapter IV: Part 4

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).

FOOTNOTES:

[1] 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.

[2] The residue in the extractors is usually dried in steam-heated
vessels, and mixed with potassium and magnesium salts; the product is
then put on the market as fish-potash guano.

GLUTARIC ACID, or NORMAL PYROTARIC ACID, HO2C.CH2.CH2.CH2.CO2H, an organic acid prepared by the reduction of [alpha]-oxyglutaric acid with hydriodic acid, by reducing glutaconic acid, HO2C.CH2.CH:CH.CO2H, with sodium amalgam, by conversion of trimethylene bromide into the cyanide and hydrolysis of this compound, or from acetoacetic ester, which, in the form of its sodium derivative, condenses with [beta]-iodopropionic ester to form acetoglutaric ester, CH3.CO.CH(CO2C2H5).CH2.CH2.CO2C2H5, from which glutaric acid is obtained by hydrolysis. It is also obtained when sebacic, stearic and oleic acids are oxidized with nitric acid. It crystallizes in large monoclinic prisms which melt at 97.5 deg. C., and distils between 302 deg. and 304 deg. C., practically without decomposition. It is soluble in water, alcohol and ether. By long heating the acid is converted into its anhydride, which, however, is obtained more readily by heating the silver salt of the acid with acetyl chloride. By distillation of the ammonium salt glutarimide, CH2(CH2.CO)2NH, is obtained; it forms small crystals melting at 151 deg. to 152 deg. C. and sublimes unchanged.

On the alkyl glutaric acids, see C. Hell (_Ber._, 1889, 22, pp. 48,
60), C. A. Bischoff (_Ber._, 1891, 24, p. 1041), K. Auwers (_Ber._,
1891, 24, p. 1923) and W. H. Perkin, junr. (_Journ. Chem. Soc._, 1896,
69, p. 268).

GLUTEN, a tough, tenacious, ductile, somewhat elastic, nearly tasteless and greyish-yellow albuminous substance, obtained from the flour of wheat by washing in water, in which it is insoluble. Gluten, when dried, loses about two-thirds of its weight, becoming brittle and semi-transparent; when strongly heated it crackles and swells, and burns like feather or horn. It is soluble in strong acetic acid, and in caustic alkalis, which latter may be used for the purification of starch in which it is present. When treated with .1 to .2% solution of hydrochloric acid it swells up, and at length forms a liquid resembling a solution of albumin, and laevorotatory as regards polarized light. Moistened with water and exposed to the air gluten putrefies, and evolves carbon dioxide, hydrogen and sulphuretted hydrogen, and in the end is almost entirely resolved into a liquid, which contains leucin and ammonium phosphate and acetate. On analysis gluten shows a composition of about 53% of carbon, 7% of hydrogen, and nitrogen 15 to 18%, besides oxygen, and about 1% of sulphur, and a small quantity of inorganic matter. According to H. Ritthausen it is a mixture of _glutencasein_ (Liebig's vegetable fibrin), _glutenfibrin_, _gliadin_ (Pflanzenleim), _glutin_ or vegetable gelatin, and _mucedin_, which are all closely allied to one another in chemical composition. It is the gliadin which confers upon gluten its capacity of cohering to form elastic masses, and of separating readily from associated starch. In the so-called gluten of the flour of barley, rye and maize, this body is absent (H. Ritthausen and U. Kreusler). The gluten yielded by wheat which has undergone fermentation or has begun to sprout is devoid of toughness and elasticity. These qualities can be restored to it by kneading with salt, lime-water or alum. Gluten is employed in the manufacture of gluten bread and biscuits for the diabetic, and of chocolate, and also in the adulteration of tea and coffee. For making bread it must be used fresh, as otherwise it decomposes, and does not knead well. Granulated gluten is a kind of vermicelli, made in some starch manufactories by mixing fresh gluten with twice its weight of flour, and granulating by means of a cylinder and contained stirrer, each armed with spikes, and revolving in opposite directions. The process is completed by the drying and sifting of the granules.

GLUTTON, or WOLVERINE (_Gulo luscus_), a carnivorous mammal belonging to the _Mustelidae_, or weasel family, and the sole representative of its genus. The legs are short and stout, with large feet, the toes of which terminate in strong, sharp claws considerably curved. The mode of progression is semi-plantigrade. In size and form the glutton is something like the badger, measuring from 2 to 3 ft. in length, exclusive of the thick bushy tail, which is about 8 in. long. The head is broad, the eyes are small and the back arched. The fur consists of an undergrowth of short woolly hair, mixed with long straight hairs, to the abundance and length of which on the sides and tail the creature owes its shaggy appearance. The colour of the fur is blackish-brown, with a broad band of chestnut stretching from the shoulders along each side of the body, the two meeting near the root of the tail. Unlike the majority of arctic animals, the fur of the glutton in winter grows darker. Like other _Mustelidae_, the glutton is provided with anal glands, which secrete a yellowish fluid possessing a highly foetid odour. It is a boreal animal, inhabiting the northern regions of both hemispheres, but most abundant in the circumpolar area of the New World, where it occurs throughout the British provinces and Alaska, being specially numerous in the neighbourhood of the Mackenzie river, and extending southwards as far as New York and the Rocky Mountains. The wolverine is a voracious animal, and also one with an inquisitive disposition. It feeds on grouse, the smaller rodents and foxes, which it digs from their burrows during the breeding-season; but want of activity renders it dependent for most of its food on dead carcases, which it frequently obtains by methods that have made it peculiarly obnoxious to the hunter and trapper. Should the hunter, after succeeding in killing his game, leave the carcase insufficiently protected for more than a single night, the glutton, whose fear of snares is sufficient to prevent him from touching it during the first night, will, if possible, get at and devour what he can on the second, hiding the remainder beneath the snow. It annoys the trapper by following up his lines of marten-traps, often extending to a length of 40 to 50 m., each of which it enters from behind, extracting the bait, pulling up the traps, and devouring or concealing the entrapped martens. So persistent is the glutton in this practice, when once it discovers a line of traps, that its extermination along the trapper's route is a necessary preliminary to the success of his business. This is no easy task, as the glutton is too cunning to be caught by the methods successfully employed on the other members of the weasel family. The trap generally used for this purpose is made to resemble a cache, or hidden store of food, such as the Indians and hunters are in the habit of forming, the discovery and rifling of which is one of the glutton's most congenial occupations--the bait, instead of being paraded as in most traps, being carefully concealed, to lull the knowing beast's suspicions. One of the most prominent characteristics of the wolverine is its propensity to steal and hide things, not merely food which it might afterwards need, or traps which it regards as enemies, but articles which cannot possibly have any interest except that of curiosity. The following instance of this is quoted by Dr E. Coues in his work on the _Fur-bearing Animals of North America_: "A hunter and his family having left their lodge unguarded during their absence, on their return found it completely gutted--the walls were there, but nothing else. Blankets, guns, kettles, axes, cans, knives and all the other paraphernalia of a trapper's tent had vanished, and the tracks left by the beast showed who had been the thief. The family set to work, and, by carefully following up all his paths, recovered, with some trifling exceptions, the whole of the lost property." The cunning displayed by the glutton in unravelling the snares set for it forms at once the admiration and despair of every trapper, while its great strength and ferocity render it a dangerous antagonist to animals larger than itself, occasionally including man. The rutting-season occurs in March, and the female, secure in her burrow, produces her young--four or five at a birth--in June or July. In defence of these, she is exceedingly bold, and the Indians, according to Dr Coues, "have been heard to say that they would sooner encounter a she-bear with her cubs than a carcajou (the Indian name of the glutton) under the same circumstances." On catching sight of its enemy, man, the wolverine before finally determining on flight, is said to sit on its haunches, and, in order to get a clearer view of the danger, shade its eyes with one of its fore-paws. When pressed for food it becomes fearless, and has been known to come on board an ice-bound vessel, and in presence of the crew seize a can of meat. The glutton is valuable for its fur, which, when several skins are sewn together, forms elegant hearth and carriage rugs. (R. L.*)

GLYCAS, MICHAEL, Byzantine historian (according to some a Sicilian, according to others a Corfiote), flourished during the 12th century A.D. His chief work is his _Chronicle_ of events from the creation of the world to the death of Alexius I. Comnenus(1118). It is extremely brief and written in a popular style, but too much space is devoted to theological and scientific matters. Glycas was also the author of a theological treatise and a number of letters on theological questions. A poem of some 600 "political" verses, written during his imprisonment on a charge of slandering a neighbour and containing an appeal to the emperor Manuel, is still extant. The exact nature of his offence is not known, but the answer to his appeal was that he was deprived of his eyesight by the emperor's orders.

Editions: "Chronicle and Letters," in J. P. Migne, _Patrologia
Graeca_, clviii.; poem in E. Legrand, _Bibliotheque grecque vulgaire_,
i.; see also F. Hirsch, _Byzantinische Studien_ (1876); C. Krumbacher
in _Sitzungsberichte bayer. Acad._, 1894; C. F. Bahr in Ersch and
Gruber's _Allgemeine Encyklopadie_.

GLYCERIN, GLYCERINE or GLYCEROL (in pharmacy _Glycerinum_) (from Gr. [Greek: glykys], sweet), a trihydric alcohol, trihydroxypropane, C3H5(OH)3. It is obtainable from most natural fatty bodies by the action of alkalis and similar reagents, whereby the fats are decomposed, water being taken up, and glycerin being formed together with the alkaline salt of some particular acid (varying with the nature of the fat). Owing to their possession of this common property, these natural fatty bodies and various artificial derivatives of glycerin, which behave in the same way when treated with alkalis, are known as glycerides. In the ordinary process of soap-making the glycerin remains dissolved in the aqueous liquors from which the soap is separated.

Glycerin was discovered in 1779 by K. W. Scheele and named _Olsuss_ (_principe doux des huiles_--sweet principle of oils), and more fully investigated subsequently by M. E. Chevreul, who named it glycerin, M. P. E. Berthelot, and many other chemists, from whose researches it results that glycerin is a trihydric alcohol indicated by the formula C3H5(OH)3, the natural fats and oils, and the glycerides generally, being substances of the nature of compound esters formed from glycerin by the replacement of the hydrogen of the OH groups by the radicals of certain acids, called for that reason "fatty acids." The relationship of these glycerides to glycerin is shown by the series of bodies formed from glycerin by replacement of hydrogen by "stearyl" (C18H35O), the radical of stearic acid (C18H35O.OH):--

Glycerin. Monostearin. Distearin. Tristearin.

CH2.OH CH2.O(C18H35O) CH2.O(C18H35O) CH2.O(C18H35O)
| | | |
CH.OH CH.OH CH.O(C18H35O) CH.O(C18H35O)
| | | |
CH2.OH CH2.OH CH2.OH CH2.O(C18H35O)

The process of saponification may be viewed as the gradual progressive transformation of tristearin, or some analogously constituted substance, into distearin, monostearin and glycerin, or as the similar transformation of a substance analogous to distearin or to monostearin into glycerin. If the reaction is brought about in presence of an alkali, the acid set free becomes transformed into the corresponding alkaline salt; but if the decomposition is effected without the presence of an alkali (i.e. by means of water alone or by an acid), the acid set free and the glycerin are obtained together in a form which usually admits of their ready separation. It is noticeable that with few exceptions the fatty and oily matters occurring in nature are substances analogous to tristearin, i.e. they are trebly replaced glycerins. Amongst these glycerides may be mentioned the following:

_Tristearin_--C3H5(O.C18H35O)3. The chief constituent of hard animal
fats, such as beef and mutton tallow, &c.; also contained in many
vegetable fats in smaller quantity.

_Triolein_--C3H5(O.C18H33O)3. Largely present in olive oil and other
saponifiable vegetable oils and soft fats; also present in animal
fats, especially hog's lard.

_Tripalmitin_--C3H5(O.C16H31O)3. The chief constituent of palm oil;
also contained in greater or less quantities in human fat, olive oil,
and other animal and vegetable fats.

_Triricinolein_--C3H5(O.C18H33O2)3. The main constituent of castor
oil.

Other analogous glycerides are apparently contained in greater or smaller quantity in certain other oils. Thus in cows' butter, _tributyrin_, C3H5(O.C4H7O)3, and the analogous glycerides of other readily volatile acids closely resembling butyric acid, are present in small quantity; the production of these acids on saponification and distillation with dilute sulphuric acid is utilized as a test of a purity of butter as sold. _Triacetin_, C3H5(O.C2H3O)3, is apparently contained in cod-liver oil. Some other glycerides isolated from natural sources are analogous in composition to tristearin, but with this difference, that the three radicals which replace hydrogen in glycerin are not all identical; thus kephalin, myelin and lecithin are glycerides in which two hydrogens are replaced by fatty acid radicals, and the third by a complex phosphoric acid derivative.

Glycerin is also a product of certain kinds of fermentation, especially of the alcoholic fermentation of sugar; consequently it is a constituent of many wines and other fermented liquors. According to Louis Pasteur, about 1/30th of the sugar transformed under ordinary conditions in the fermentation of grape juice and similar saccharine liquids into alcohol and other products become converted into glycerin. In certain natural fatty substances, e.g. palm oil, it exists in the free state, so that it can be separated by washing with boiling water, which dissolves the glycerin but not the fatty glycerides.

_Properties._--Glycerin is a viscid, colourless liquid of sp. gr. 1.265 at 15 deg. C., possessing a somewhat sweet taste; below 0 deg. C. it solidifies to a white crystalline mass, which melts at 17 deg. C. When heated alone it partially volatilizes, but the greater part decomposes; under a pressure of 12 mm. of mercury it boils at 170 deg. C. In an atmosphere of steam it distils without decomposition under ordinary barometric pressure. It dissolves readily in water and alcohol in all proportions, but is insoluble in ether. It possesses considerable solvent powers, whence it is employed for numerous purposes in pharmacy and the arts. Its viscid character, and its non-liability to dry and harden by exposure to air, also fit it for various other uses, such as lubrication, &c., whilst its peculiar physical characters, enabling it to blend with either aqueous or oily matters under certain circumstances, render it a useful ingredient in a large number of products of varied kinds.

_Manufacture._--The simplest modes of preparing pure glycerin are
based on the saponification of fats, either by alkalis or by
superheated steam, and on the circumstance that, although glycerin
cannot be distilled by itself under the ordinary pressure without
decomposition, it can be readily volatilized in a current of
superheated steam. Commercial glycerin is mostly obtained from the
"spent lyes" of the soap-maker. In the van Ruymbeke process the spent
lyes are allowed to settle, and then treated with "persulphate of
iron," the exact composition of which is a trade secret, but it is
possibly a mixture of ferric and ferrous sulphates. Ferric hydrate,
iron soaps and all insoluble impurities are precipitated. The liquid
is filter-pressed, and any excess of iron in the filtrate is
precipitated by the careful addition of caustic soda and then removed.
The liquid is then evaporated under a vacuum of 27 to 28 in. of
mercury, and, when of specific gravity 1.295 (corresponding to about
80% of glycerin), it is distilled under a vacuum of 28 to 29 in. In
the Glatz process the lye is treated with a little milk of lime, the
liquid then neutralized with hydrochloric acid, and the liquid
filtered. Evaporation and subsequent distillation under a high vacuum
gives crude glycerin. The impure glycerin obtained as above is
purified by redistillation in steam and evaporation in vacuum pans.

_Technical Uses._--Besides its use as a starting-point in the
production of "nitroglycerin" (q.v.) and other chemical products,
glycerin is largely employed for a number of purposes in the arts, its
application thereto being due to its peculiar physical properties.
Thus its non-liability to freeze (when not absolutely anhydrous, which
it practically never is when freely exposed to the air) and its
non-volatility at ordinary temperatures, combined with its power of
always keeping fluid and not drying up and hardening, render it
valuable as a lubricating agent for clockwork, watches, &c., as a
substitute for water in wet gas-meters, and as an ingredient in
cataplasms, plasters, modelling clay, pasty colouring matters, dyeing
materials, moist colours for artists, and numerous other analogous
substances which are required to be kept in a permanently soft
condition. Glycerin acts as a preservative against decomposition,
owing to its antiseptic qualities, which also led to its being
employed to preserve untanned leather (especially during transit when
exported, the hides being, moreover, kept soft and supple); to make
solutions of gelatin, albumen, gum, paste, cements, &c. which will
keep without decomposition; to preserve meat and other edibles; to
mount anatomical preparations; to preserve vaccine lymph unchanged;
and for many similar purposes. Its solvent power is also utilized in
the production of various colouring fluids, where the colouring matter
would not dissolve in water alone; thus aniline violet, the tinctorial
constituents of madder, and various allied colouring matters dissolve
in glycerin, forming liquids which remain coloured even when diluted
with water, the colouring matters being either retained in suspension
or dissolved by the glycerin present in the diluted fluid. Glycerin is
also employed in the manufacture of formic acid (q.v.). Certain kinds
of copying inks are greatly improved by the substitution of glycerin,
in part or entirely, for the sugar or honey usually added.

In its medicinal use glycerin is an excellent solvent for such
substances as iodine, alkaloids, alkalis, &c., and is therefore used
for applying them to diseased surfaces, especially as it aids in their
absorption. It does not evaporate or turn rancid, whilst its marked
hygroscopic action ensures the moistness and softness of any surface
that it covers. Given by the mouth glycerin produces purging if large
doses are administered, and has the same action if only a small
quantity be introduced into the rectum. For this purpose it is very
largely used either as a suppository or in the fluid form (one or two
drachms). The result is prompt, safe and painless. Glycerin is useless
as a food and is not in any sense a substitute for cod-liver oil. Very
large doses in animals cause lethargy, collapse and death.

GLYCOLS, in organic chemistry, the generic name given to the aliphatic dihydric alcohols. These compounds may be obtained by heating the alkylen iodides or bromides (e.g. ethylene dibromide) with silver acetate or with potassium acetate and alcohol, the esters so produced being then hydrolysed with caustic alkalis, thus:

C2H4Br2 + 2 C2H3O2.Ag --> C2H4(O.C2H3O)2 --> C2H4(OH)2 + 2 K.C2H3O2;

by the direct union of water with the alkylen oxides; by oxidation of the olefines with cold potassium permanganate solution (G. Wagner, _Ber._, 1888, 21, p. 1231), or by the action of nitrous acid on the diamines.

Glycols may be classified as _primary_, containing two -CH2OH groups; _primary-secondary_, containing the grouping -CH(OH).CH2OH; _secondary_, with the grouping -CH(OH).CH(OH)-; and _tertiary_, with the grouping >C(OH).(OH)C<. The secondary glycols are prepared by the action of alcoholic potash on aldehydes, thus:

3(CH3)2CH.CHO + KHO = (CH3)2CHCO2K + (CH3)2CH.CH(OH).CH(OH).CH(CH3)2.

The tertiary glycols are known as _pinacones_ and are formed on the reduction of ketones with sodium amalgam.

The glycols are somewhat thick liquids, of high boiling point, the pinacones only being crystalline solids; they are readily soluble in water and alcohol, but are insoluble in ether. By the action of dehydrating agents they are converted into aldehydes or ketones. In their general behaviour towards oxidizing agents the primary glycols behave very similarly to the ordinary primary alcohols (q.v.), but the secondary and tertiary glycols break down, yielding compounds with a smaller carbon content.

Ethylene glycol, C2H4(OH)2, was first prepared by A. Wurtz (_Ann.
chim._, 1859 [3], 55, p. 400) from ethylene dibromide and silver
acetate. It is a somewhat pleasant smelling liquid, boiling at 197
deg. to 197.5 deg. C. and having a specific gravity of 1.125 (0 deg.).
On fusion with solid potash at 250 deg. C. it completely decomposes,
giving potassium oxalate and hydrogen,

C2H6O2 + 2 KHO = K2C2O4 + 4H2.