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 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 (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 + 2C2H3O2·Ag → C2H4(O·C2H3O)2 → C2H4(OH)2 + 2K·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 (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° to 197.5° C. and having a specific gravity of 1.125 (0°). On fusion with solid potash at 250° C. it completely decomposes, giving potassium oxalate and hydrogen,

C2H6O2 + 2KHO = K2C2O4 + 4H2.

Two propylene glycols, C3H8O2, are known, viz. -propylene glycol, CH3·CH(OH)·CH2OH, a liquid boiling at 188° to 189°, and obtained by heating glycerin with sodium hydroxide and distilling the mixture; and trimethylene glycol, CH2OH·CH2·CH2OH, a liquid boiling at 214° C. and prepared by boiling trimethylene bromide with potash solution (A. Zander, Ann., 1882, 214, p. 178).

GLYCONIC (from Glycon, a Greek lyric poet), a form of verse, best known in Catullus and Horace (usually in the catalectic variety _ – ◡ ◡ – ◡ ), with three feet—a spondee and two dactyls; or four—three trochees and a dactyl, or a dactyl and three chorees. Sir R. Jebb pointed out that the last form might be varied by placing the dactyl second or third, and according to its place this verse was called a First, Second or Third Glyconic.

GLYPH (from Gr. , to carve), in architecture, a vertical channel in a frieze (see ).

GLYPTODON (Greek for “fluted-tooth”), a name applied by Sir R. Owen to the typical representative of a group of gigantic, armadillo-like, South American, extinct Edentata, characterized by having the carapace composed of a solid piece (formed by the union of a multitude of bony dermal plates) without any movable rings. The facial portion of the skull is very short; a long process of the maxillary bone descends from the anterior part of the zygomatic arch; and the ascending ramus of the mandible is remarkably high. The teeth, in the later species, are much alike, having two deep grooves or flutings on each side, so as to divide them into three distinct lobes (fig.).

They are very tall and grew throughout life. The vertebral column is almost entirely welded into a solid tube, but there is a complex joint at the base of the neck, to allow the head being retracted within the carapace. The limbs are very strong, and the feet short and broad, resembling externally those of an elephant or tortoise.

Glyptodonts constitute a family, the Glyptodontidae, whose position is next to the armadillos (Dasypodidae); the group being represented by a number of generic types. The Pleistocene forms, whose remains occur abundantly in the silt of the Buenos Aires pampas, are by far the largest, the skull and tail-sheath in some instances having a length of from 12 to 16 ft. In Glyptodon (with which Schistopleurum is identical) the tail-sheath consists of a series of coronet-like rings, gradually diminishing in diameter from base to tip. Daedicurus, in which the tail-sheath is in the form of a huge solid club, is the largest member of the family; in Panochthus and Sclerocalyptus (Hoplophorus) the tail-sheath consists basally of a small number of smooth rings, and terminally of a tube. In some specimens of these genera the horny shields covering the bony scutes of the carapace have been preserved, and since the foramina, which often pierce the latter, stop short of the former, it is evident that these were for the passage of blood-vessels and not receptacles for bristles. In the early Pleistocene epoch, when South America became connected with North America, some of the glyptodonts found their way into the latter continent. Among these northern forms some from Texas and Florida have been referred to Glyptodon. One large species from Texas has, however, been made the type of a separate genus, under the name of Glyptotherium texanum. In some respects it shows affinity with Panochthus, although in the simple structure of the tail-sheath it recalls the undermentioned Propalaeohoplophorus. All the above are of Pleistocene and perhaps Pliocene age, but in the Santa Cruz beds of Patagonia there occur the two curious genera Propalaeohoplophorus and Peltephilus, the former of which is a primitive and generalized type of glyptodont, while the latter seems to come nearer to the armadillos. Both are represented by species of comparatively small size. In Propalaeohoplophorus the scutes of the carapace, which are less deeply sculptured than in the larger glyptodonts, are arranged in distinct transverse rows, in three of which they partially overlap near the border of the carapace after the fashion of the armadillos. The skull and limb-bones exhibit several features met with in the latter, and the vertebrae of the back are not welded into a continuous tube. There are eight pairs of teeth, the first four of which are simpler than the rest, and may perhaps therefore be regarded as premolars. More remarkable is Peltephilus, on account of the fact that the teeth, which are simple, with a chevron-shaped section, form a continuous series from the front of the jaw backwards, the number of pairs being seven. Accordingly, a modification of the character, even of the true Edentata, as given in the earlier article, is rendered necessary. The head bears a pair of horn-like scutes, and the scutes of the carapace and tail, which are loosely opposed or slightly overlapping, form a number of transverse rows.

.—R. Lydekker, “The Extinct Edentates of Argentina,” ''An. Mus. La Plata—Pal. Argent.'' vol. iii. p. 2 (1904); H. F. Osborn, “&#8202;‘Glyptotherium texanum,’ a Glyptodont from the Lower Pleistocene of Texas,” ''Bull. Amer. Mus.'', vol. xvii. p. 491 (1903); W. B. Scott, “Mammalia of the Santa Cruz Beds—Edentata,” ''Rep. Princeton Exped. to Patagonia'', vol. v. (1903–1904).

GLYPTOTHEK (from Gr. , carved, and  , a place of storage), an architectural term given to a gallery for the exhibition of sculpture, and first employed at Munich, where it was built to exhibit the sculptures from the temple of Aegina.