Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/143

Rh chloride. Further, this author states that, according to his analyses, collagen differs from gelatin by one molecule of water, and from the sum of the molecules of semiglutin and hemieollin by three molecules of water, so that a probable empirical formula for gelatin would be C10.,H15,_N31O39, agreeing pretty fairly with the percentage numbers given in an earlier part of tlus article. See Iinppc-Scylcr, Ifladicinisch-0/cenzische Untersuchungcn, 1S66 and ]S7],and his I’/:_r/.cinln_qiscI:e Cliemie, just being published; Gmc1in's Handbook, vol. xviii., 1.571; 'atts‘s I)i«-tiona:-_u of Chemi.m~y, vol. ii. For the digestion of gelatin, see Carl 'oif, Zeitscln-11ft fiir Biologic, viii. 297, 1872; Etzingcr, same work, 5: st, 1:471: and for constitution of collagen, llofmcister, Zcitsch. fiir Ph_u.<1'ol. (‘In-mic, ii. [-3] 299, 1878. (D. C. R.) I n(lu.ctrz'al Ifelations of Gelatin. 0lue.—Glue is a form of gelatin, which, on account of its impure condition, is employed only as an adhesive medium for wood, leather, paper, and like substances. There is, however, no absolute distinction between glue and gelatin, as they merge into each other by imperceptible degrees; and although the dark-coloured varieties of gelatin which are k11ow11 as ordinary glue are in no case treated as food, yet for several purposes the ﬁne transparent kinds, prepared chiefly for culinary use, are employed also as adhesive agents. Neither again, except in respect of its source, is there any chemical or physical distinction between these two sub- stances aml isinglass or fish glue, and therefore the prepara- tion and industrial applications of these three varieties of commercial gclatin—gluc, gelatin, and isinglass—will be here noticed. The gelatin-yielding substances in the animal kingdom are very numerous, comprising the skins of all animals, tendons, intestines, bladders and ﬁsh sounds, bones, horns, aml hoofs. Chondrin, the substance yielded by carti- laginous tissue, which is simply an impure variety of gelatin (see above), has greatly inferior power of adhesion. In the preparation of ordinary 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. Taking tan-yard refuse to be the principal miterial, it is first steeped for some weeks in a pit with lime water, and afterwards carefully dried and stored. The object of the lime steeping is to remove any blood and ﬂesh which maybe attached to the skin, and to form a lime soap with the fatty matter it contains. S0 prepared the "‘ serows ” or glue pieces, as they are termed, may be kept a long time without undergoing change. Before being hailed, the glue pieces are thoroughly washed. They 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. The boiler is heated by direct ﬁring, a series of boilers being arranged in the 111anner best ﬁtted to obtain the greatest possible heating effect from one ﬁre. 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, it is ready to draw off. Usually the ﬁrst boiling occupies about eight hours, and when the liquid has been drawn off, more water is added and the boiling process repeated. In this way the gelatinous matter is only ex- hausted after six separate boilings, occupying about two days, the last boiling yielding a darker-coloured glue than the first. It is essential that the boiling out of a charge should not be continued longer than is necessary for yielding a sufficiently stiff gelatinous solution, as it is found that, when the liquid is long exposed to a heat at or above boiling point, the gelatin loses its power of congealing. From the boiler the sufficiently concentrated solution is run to a tank or “setting back,” in which a temperature sufficient to keep it fluid is maintained, and in this way any impurity is per- mitted to subside. The glue solution is then run into wooden troughs or coolers about 6 feet long by 2 feet broad I‘ ' 11 1"‘ lIr‘] 1 $lIL .1,..... Z... ..-L!_1. _'L __L_ 1, GELATIN 133 set, a little water is run over its surface, and with knives of suitable form it is detached from the sides and bottom, cut into uniform slices about an inch thick, and squares of these are placed on nets stretched between upright wooden frames or hurdles for drying. The drying operation, which requires very special care, is best done in the open air; the plastic masses must, however, be protected from rain. Frost and strong dry heat are equally injurious, and the best results are obtained in spring and autumn weather, when the glue dries in from twelve to eighteen days. When the pieces have become quite hard and sonorous, they are washed to remove dust from their surface, and to give them a glazed or polished appearance. A good quality of glue should be free from all specks and grit, and ought to have a uniform, light brownish-yellow, transparent appear- ance, and it 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 i11 water, forming a thin syrupy ﬂuid with a not disagreeable smell. The adhesiveness of different qualities of glue, on which quality its value depends, differs con- siderably; and there are several methods of measuring the comparative value of commercial samples, the most reliable of which are based 011 actual experiment. Glue is also made from bones by ﬁrst boiling them to remove the fatty matter they contain, and then treating them with strong hydrochloric acid till they become quite soft and translucent. In this condition, after they are washed and the acid neutralized, they are enclosed in a covered vessel and sub- mitted to the action of steam, by which a concentrated gelatinous solution is ﬁrst obtained. At a subsequent stage the whole mass is boiled by direct heat, and a further quantity of glue is so procured. The glue yielded by bones has a milky hue, owing to the phosphate of lime it carries with it. C'ommerc1'al Gelatin.—Gelatin, as a commercial product, is prepared in a manner similar to that followed in the manufacture of glue; but the materials used are selected with great attention to purity, and the various operations are carried out with the most scrupulous care and cleanliness. In the manufacture of the well—known spark- ling gelatin of Messrs Cox of Gorgie, near Edinburgh, the following is the process followed, according to their patent obtained in 1844. The shoulders and checks of ox-hides are preferred, but other parts may be used. The hide and skin pieces are cleansed in water, cut in small pieces by a machine, and reduced to pulp in a pulp mill. The pulp is pressed between rollers, mixed with water, and then subjected to heat varying from 150° to 212° F., whereby gelatin is produced. When a very pure quality is required, liquid gelatin is mixed with a small quantity of ox blood at a temperature not exceeding 160’ or 170°, and further heated. The albumen of the blood becomes coagulated, and rises as a scum; the heat is then withdrawn, after which the scum is removed and the purer liquor allowed to settle, and afterwards it is run into coolers to congeal and dry. The gelatin is evaporated in zvacuo to avoid the injury caused by long subjection to heat ; but it may also be dried on a stea1n—l1eatcd surface. In Nelson’s process the gelatin is extracted by steam heat from hide pieces which have been submitted to the bleaching action of sulphurous acid The strained and purified product is spread in a thin layer on a marble slab till it partly solidi- ﬁes, when it is cut up and washed to free it fro1n all traces of acid. It is again redissolved at the lowest possible tem- perature, then resolidiﬁed and dried in thin sheets on nets. Hcuze of Berlin prepares a pure transparent gelatin, having a fine meaty ﬂavour, from very impure materials, by inti- . I ' ' ‘LL 1...; Any-lr_ L1. - ....1.-L:...-. AC I.rII1vIr