CHONDRIN-PREPARATION OF glue. 847 condition gelatin soon becomes acid on exposure to air, and then loses its gelatinous character. Gelatin is capable of dissolving a considerable quantity of calcic phosphate. By the frequent boiling of its aqueous solution, gelatin is converted into an isomeric compound which does not possess the faculty of gelatinizing as the liquid cools. Chlorine precipitates gelatin completely, forming with it a peculiar white elastic compound which appears to consist of gelatin in combination with hypochlorous acid; this compound when treated with ammonia yields nitrogen, ammonia hydrochlorate, and unchanged gelatin. Solutions of gelatin are not rendered turbid by the addition of acids. Concentrated sulphuric acid dissolves gelatin without change of colour, and gradually converts it into glycocine, leucine, and some other bodies. Gerhardt found that when isinglass was boiled for some days with dilute sulphuric acid, ammonium sulphate was formed, and a considerable quantity of a saccharine body which was susceptible of the alcoholic fermentation. Nitric acid produces oxalic acid from it amongst other substances. A solution of gelatin to which the alkalies have been added in small quantity loses its faculty of gelatinizing, but the liquid on being neutralized gives no protein. Gelatin is decomposed by strong alkaline solutions; at a boiling temperature ammonia is evolved, and leucine and glycocine are amongst the products. (1662) CHONDRIN (from xóvopos, cartilage) differs from gelatin in composition: it is also coagulated by the vegetable acids, such as acetic acid, as well as by alum, and by the normal and the basic acetates of lead, none of which yield precipitates with gelatin. Most of these precipitates are soluble in an excess of the precipitant. Chondrin is obtained by boiling the cornea of the eye, or any of the permanent cartilages, with water for some hours; it may also be obtained from the temporary cartilages prior to ossification. Its solution gelatinizes on cooling. It is remarkable that when a permanent cartilage becomes ossified, it yields not chondrin but gelatin by boiling with water. Recent researches appear to show that by oxidation chondrin becomes converted into gelatin. Preparation of Glue and Size. (1663) The preparation of gelatin on a large scale gives rise to a branch of industry of considerable importance, viz., the manufacture of glue and size. The strongest glue is obtained from the parings of ox-hides, 848 PREPARATION OF Glue. and from the ears and refuse trimmings of thick hides in general, which yield from 45 to 50 per cent. of glue. These clippings are first steeped for some days in lime water, to remove the hair, the blood, and other impurities; they are then washed in cold water, and are placed to drain upon a sloping pavement, in layers of two or three inches thick, and are turned over once or twice a day; the quicklime thus absorbs carbonic acid from the air, and becomes converted into chalk; so that the injurious effects of lime at a boiling heat upon the gelatin are prevented. The washed clippings are next introduced into a copper which is filled to two-thirds of its capacity with rain water, and furnished with a perforated false bottom; frequently the animal matters are put first of all into a coarse rope cloth, and the whole is placed in the cauldron, a precaution which prevents the portions of skin, when they begin to gelatinize, from adhering to the bottom and sides of the boiler, and so becoming burned; fresh portions of clippings are added as the animal matters gradually sink down into the liquid which is kept gently boiling. A still better method consists in boiling the liquid by the injection of steam under moderate pressure, by which means the direct application of fire to the boiler is avoided. As soon as a portion of the liquid sets into a firm jelly on cooling, the fire is checked, and the solution is allowed to run into a deep vessel or settling-back, where it is kept warm for some hours to allow the impurities to subside. The undissolved matters contained in the cloth are again boiled with water, and yield an inferior description of glue; if boiled a third time they give a still weaker gelatinous product or size: the refuse, after this treatment, may still be advantageously applied as manure. From the settling-back, the solution is transferred to long wooden boxes or coolers, in which it gelatinizes; when sufficiently solid it is removed, cut into blocks, and each block is split by a wire frame into slices. The slices of moist glue are then laid upon nets placed in a wooden frame, and are so arranged in lofts as to allow free circulation of air. These cakes of glue are turned over two or three times daily, and the marks which are seen upon the glue exposed for sale are those left by the threads of the nets. This operation of drying is the most delicate part of the manufacture, since at this stage much of the success of the operation depends upon the state of the weather. A sudden rise of temperature might liquefy the whole; the occurrence of a fog might render it mouldy, or a frost might split all the cakes into fragments. Spring and autumn are found to be the most favourable seasons for the manufacture of glue. After the drying has pro SIZE-PREPARATION OF LEATHER. 849 ceeded to a certain extent in the open air, the process may advantageously be shortened by the use of artificial heat from a stove. When glue is properly made, it should be of a pale brown colour, hard, brittle, and should break with a smooth, glassy fracture. Cold water does not dissolve glue, but merely softens it, and causes it to swell up. Size is a less tenacious kind of glue, and is chiefly made from parings of parchment, and of the thinner kind of skins. It is almost entirely used in the gelatinous condition, so that the precarious process of drying is dispensed with. Much of the gelatin at present sold in fine shreds for use in confectionery is obtained from clippings of hides, which are boiled with water till they form a very liquid size, which is allowed to become clear by standing, and is then dried in very thin sheets, and afterwards cut up into shreds. Preparation of Leather. (1664) The hides of animals, like the soft parts in general, are prone to putrefaction and decomposition, if kept in a flexible and moist condition, and when allowed to dry, they become too rigid and brittle to allow of their employment for a number of purposes, to which they would otherwise be admirably adapted. The operations of the tanner have in view the preservation of the hide from decay, and the preparation of a material possessed of the requisite suppleness, tenacity, and power of resisting the action of moisture. The hides of different animals vary very much in thickness. The hippopotamus is furnished with a hide which is in some parts between one and two inches (from 25 to 50 millimetres) in thickness, whilst the leather produced from the skins of the lamb and of the kid does not equal ordinary pasteboard in thickness. The thicker kinds of leather receive the general name of hides; and the strongest parts of these hides, which cover the back of the animal, constitute the butts. Ox-hides, the most valuable of which are those imported from the plains of South America, and from the Cape of Good Hope, yield the principal portion of this leather, which is largely consumed for the soles of boots and shoes, for harness, and for articles requiring much strength and durability. The upper-leathers of boots and shoes are chiefly made of calves' and seals' skins; whilst for book-binding, for aprons, and for ordinary dyed leathers, the leather furnished by sheep-skins constitutes the material principally used. The skin of most animals is highly elastic; it consists of two 850 PREPARATION OF LEATHER, portions which are quite distinct in chemical composition; an outer thin superficial layer, the epidermis, which dips down and lines the tubes containing the roots of the hair, and which rises. in blisters after a scald. This is composed of a pellicle which resembles horny matter in composition; it is stained yellow permanently by the action of concentrated nitric acid: beneath this is the cellular layer, termed the rete mucosum, in which the colouring matter resides, but which is now generally regarded as a less compact portion of the epidermis: and below this is the thick sensitive vascular integument, or dermis, which constitutes the true skin. When the dermis is boiled with water for some hours, it becomes dissolved, leaving the vascular parts, which can be removed by straining through linen or flannel; the clear liquid is nearly colourless; it gelatinizes on cooling, and consists mainly of gelatin. It gives with tincture of galls the well-known insoluble precipitate by which such solutions are characterized. Excepting in those cases where it is desired to preserve the hair, as in the different species of furs, the preliminary steps in the preparation of all skins are pretty much the same, whatever the use to which the leather is applied, but greater care is requisite in the thinner kinds. After the hair has been removed, the preservative process differs with the nature of the skin to be operated Four principal processes are in use for the preparation of leather. They consist of— on. 1. Tanning. This is employed for the thicker kinds of leather; it is essentially a process for combining the astringent principle of vegetables with the hide. 2. Sumaching. This operation is similar in its results to tanning, but is less laborious and tedious; it is performed upon the thinner leathers or skins, which are often subsequently dyed. 3. Tawing. This process is followed in preparing white and black kid, principally for the glover: in this operation alum and common salt are worked with some oily matter into the skin. 4. Shamoying. This process is used in the preparation of wash-leather. It consists essentially in combining some suitable fatty matter with the texture of the softer part of the hide. Liming, Unhairing, and Raising the Skin.-In all cases the first thing to be done is to cleanse the skin thoroughly from blood, from the salt used in preserving it, and from other extraneous matters. This is effected, in the case of recent hides, simply by washing, or, in those which have been salted, by soaking them for some days in pits in which the water is occasionally changed. It is frequently found advisable, in order to LIMING, UNHAIRING, RAISING. 851 expedite this cleansing, to place the hides in the fulling-mill, where they are beaten with heavy hammers shod with iron, by which the skin is speedily rendered supple. The ears and projecting useless pieces having been removed, and disposed of to the glue-maker, the hides are generally transferred to pits containing lime-water, with an excess of lime, which gradually dissolves the sheath of the hairs, and combines with the fat; here they remain. for three weeks or a month, and are transferred gradually from pits containing a weak lime-water to those in which the proportion of lime is much more considerable. In some of the French tanneries a weak solution of caustic soda has been substituted for lime-water, with decided advantage; the skin is more rapidly prepared for unhairing, and the excess of caustic soda is afterwards much more readily removed than the excess of lime. Sometimes, instead of plunging the hides into lime-water, they are placed in a warm room till a slight degree of putrefaction commences; during this incipient putrescence ammonia is disengaged, and the hair becomes loosened as readily as when lime is used. By In order to remove the loosened hair, the hides are placed upon a sloping semi-cylindrical table, with a rounded end, termed the beam, and upon this, with a two-handled knife, the hide is scraped on the hair side to remove the epidermis and hair. this operation the upper surface of the cuticle marked by the insertions of the hair is exposed; it is of a denser structure than the layers of cuticle below, and it forms what is called the grain of the skin; great care is taken not to injure this grain, as such injury impairs the quality of the leather, and much reduces its value. The hair having been removed, the hide is once more rinsed, and scraped upon the beam on the flesh side, to remove any irregularities and adhering loose cellular tissue. The cleansed hide is then immersed for twelve hours in a bath of weak sulphuric acid and water (1 part of acid and 1000 of water), which raises the skin, that is to say, makes it swell, opens its pores, removes adhering lime, and fits it for the actual operation of tanning. Care must be taken to avoid an excess of sulphuric acid, as it is liable to injure the texture of the hide. The removal of lime from the skin is a process of great importance, since, owing to the circumstance of the astringent matter being of an acid character, it would combine with the lime, instead of with the gelatin, and would form a harsh, insoluble compound within the pores of the skin, which would impair the suppleness of the leather. (1665) 1. Tanning.-Up to this point the preparation of the |