Purified glue prepared by boiling gelatinous animal tissues in water, evaporating and drying the product in the air. Carefully selected fresh bones are preferred.
SYNONYMS: Gelatine, Artificial isinglass, Glutin.
Source and History.—Gelatin is found in abundance in various animal substances, especially in the skin, cartilages, tendons, membranes and bones. The common gelatin of commerce, called glue, is made from trimmings and scraps of skins, ears, bones and hoofs of animals. It maybe obtained by boiling these animal parts in water, straining the decoction, and evaporating it until it forms a jelly on cooling. This is divided into thin slices of various sizes, which are allowed to dry in the open air. The purest variety of gelatin is obtained from the air bladder of fishes—e. g., the sturgeon and codfish-and is named isinglass (see Ichthyocolla). The gelatin used for culinary and pharmaceutical purposes, in photography, etc., usually called gelatin proper, is carefully prepared from the bones of animals from which the fat is previously removed and the earthy matter dissolved out by means of hydrochloric acid. (For a detailed description of the processes of manufacture involved, see Prof. S. P. Sadtler, Ind. Org. Chem., 1895, p. 334.) Gelatin may also be obtained from the vegetable kingdom, viz.: From certain species of sea weeds in Asiatic waters (see, for example, Agar Agar).
Description and Chemical Composition.—GLUE (Colla). Glue of good quality is firm and friable, not easily pulverized, of a light-brown color, and translucent. On the addition of water it becomes soft and swells up, but does not dissolve except the water be hot or boiling. When dissolved in hot water, it is much in use for uniting wood and various other substances together, but is too impure for internal employment or for a chemical test. Addition of acetic acid, or boiling with dilute nitric acid, has the effect of destroying the gelatinizing power of gelatin, while its adhesive properties are fully retained. A cement or liquid glue is thus obtained, which does not require the aid of heat to render it fit for use. A strong, liquid glue, very convenient for a number of objects, and even for porcelain, glass, and pearl, and which is preferable to that made with vinegar or nitric acid, is prepared as follows: To 3 parts of strong glue well bruised add 8 parts of water, and allow them to remain in contact for several hours; then add ½ a part of hydrochloric acid, and ¾ of a part of sulphate of zinc. Expose the whole for 10 or 12 hours to a temperature of 80° to 90° C. (176° to 194° F.).
GELATIN appears in commerce in thin, rectangular, transparent sheets, variously marked by impressions received from the nets upon which the moist jelly is spread in order to dry. It also occurs in smooth, transparent pieces, or in thicker, opaque, porous pieces. It is not so thick as pieces of glue. Gelatin comes also in shreds and is often artificially colored. After digestion in hot water it should develop no odor nor should it change color. Dried gelatin, when dissolved in 100 parts of hot water, solidifies in the form of a tremulous jelly upon cooling. Prolonged boiling of the aqueous solution causes it to lose its gelatinizing properties.
Gelatin differs from albuminous bodies in not coagulating in aqueous solution on boiling, nor being precipitated by nitric acid or potassium ferrocyanide. Its aqueous solution is precipitated, however, by alcohol and by tannic acid. Upon the latter reaction depends the conversion of hide into leather in the process of tanning. Two proximate principles may be distinguished in various forms of gelatin: Glutin, or gelatin proper, which is the gelatinous principle of tendons, hides, and the larger bones; and chondrin, which occurs mostly in the cartilages of the ribs and joints and the young bones while yet soft (S. P. Sadder). Glutin has all the aforenamed properties of gelatin, and has a greater adhesive power than chondrin, swelling up in cold and dissolving in hot water, forming a jelly upon cooling. When boiled with diluted sulphuric acid or alkali, glycocoll (C2H5NO2) and leucin (C6H13NO2) are chiefly produced. The former substance is not formed with chondrin. Dry distillation yields bases of the fatty and the pyridine series. Chondrin is precipitated by alum, lead acetates and metallic salts, not by corrosive sublimate, while glutin is precipitated by corrosive sublimate, but not by lead acetates, nor by alum or ferric chloride T.S.
Action and Medical Uses.—Gelatin probably does not affect the growth of the bodily structures. In the form of jellies it as been used during convalescence, but the nutrition derived from these preparations is believed to be due to the sugar, etc., usually employed in preparing them. Gelatin may act as a protective in rectal enemas, in the treatment of skin affections, and in cases of poisoning by corrosive substances. Medicated gelatin (see Gelanthum) is now used to some extent in the treatment of skin diseases, particularly those of an eczematous type, and in the treatment of catarrhal affections of the nasal passages.
Gelatin has been introduced here, in consequence of its application in pharmacy, for the purpose of promoting certain useful indications. Several remedial agents of a valuable character, are unfortunately so repulsive to the palate as to produce nausea and vomiting whenever swallowed, and, as in many instances, it is almost impossible to dispense with them, an important object is to prepare them so that they may reach the stomach without offending the organs of taste. This has been effected by inclosing the medicine in a case or cover of gelatin, forming what are called gelatin capsules, invented in France by M. Mothe. There are several methods at the present day for making these capsules; thus the end of an iron rod is made bulbous or egg-shaped, and is highly polished; being slightly oiled it is dipped into a hot, concentrated solution of 3 parts of pure gelatin, ½ part of sugar, and 6 parts of water. A number of rods are generally used.
The rods are then rotated to spread the solution evenly over the mold or bulb, and placed, bulb upward, on a board perforated for the purpose; when cool and dry they may be removed by giving to the capsule or bulb a pulling and gently twisting motion. These are then filled with the medicine, and the orifice closed over with more of the gelatin solution. Sometimes animal membrane, or fine skin, distended with mercury, is used instead of the iron bulb. (For a detailed method of preparing gelatin capsules, see standard works on pharmacy and Amer. Jour. Pharm., Vol. IX, p. 20). In this way capsules may be made to contain from 10 to 20 grains of liquid. Since the foregoing appeared in former editions of this Dispensatory, commercial empty capsules of all sizes have become a standard article of commerce. When received into the stomach the gelatin is dissolved, allowing the medicine to accomplish its therapeutical influences. If soft capsules are demanded a little glycerin added to the gelatin will make the product elastic. Capsules are now largely employed for dispensing quinine and similar medicines of unpleasant taste. These capsules are oblong, rounded and closed at one end, and cut off and open at the other end. It is only necessary to introduce the powder and slip a second capsule over the open end of the filled one. Folding or devorative capsules are thin films of gelatin designed to be used like powder-papers, except that after folding upon the powder the edges are made to adhere by moistening them. When ready to be taken the whole capsule (and powder) is dipped in water until softened, and then swallowed. Medicinal pearls of gelatin, combined with sugar, acacia and honey, are also employed to enclose ether and similar fluids. Gelatin (3 parts) and glycerin (7 parts) is sometimes used as a basis for bougies and rectal and vaginal medicated suppositories. Gelatin is also used for making court-plaster, hectographs, for coating pills, and for estimating the amount of tannin contained in a drug or preparation.
A good paste is made by dissolving best white glue, 3 ounces (av.); refined sugar, 1 ½ ounces; water, 10 fluid ounces, or a sufficient quantity, together by the aid of a water-bath, and, while warm, apply it by means of a suitable brush to the reverse side of the labels while uncut or in sheets. After being dried and moderately pressed they are ready for cutting. Thick paper and not sized will require less water than when thin and well sized, and in all cases it should be quickly and evenly applied. It can only be used while warm. It does not penetrate the paper and disfigure the labels, is very adhesive, never loosens from glass and leaves no disagreeable impression in the mouth after being moistened with saliva.