Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/141

Rh GELATIN time it was supposed that in each of these three cases the gelatinizing materials obtained were formed by the hydra- tion er by a physical metamorphosis of a different substance pre-existing in the respective tissues, to which the names collagen-, ossein, and clzomlrogen were given respectively—— the two former yielding gelatin, and the last chondrin. Further experiments have made it more probable that gelatin and chondrin do not differ essentially from their parent tissues, analyses of tendons and of gelatin or isinglass (a very ﬁne form of gelatin obtainable from the sturgeon) agreeing within the range of experimental error. At the same time, as Foster observes in the case of chondrin, the fact that its extraction from cartilage requires an amount of boiling with water, nu1cl1 more than would be necessary to dissolve the same amount of dried product, points rather the other wa_y. Most probably the change which occurs is at a purely physical character. True gclatigenous tissue occurs in all mature vertebrates, with the single exception, according to Hoppe-Seyler, of that in other respects anomalous vertebrate, Amp}u'o.-ms lanceul«.ztas. In the embryo it does not appear till late in fcetal life, chondrin being found instead; and the change l.Vlli'll brings gelatin into the place of chondrin is effected, not by a metamorphosis of the latter, but by its removal, ancl the independent formation of gelatin. The tissue in question was believed to be peculiar to Vertebrata until lIoppe—Seyler discovered it in the bodies of Octopus and Sc7)inlu.. By boiling these cephalopods with water he obtained large quantities of gelatin free from chondrin, but ._.—.._j in an extension of his experiments to other invertebrates, I as cnckchafers and Anodon and Unio, no such tissue could be detected. Gelatin, as such, is not met with in any of the normal ﬂuids of tl1e body, but occurs in the blood in cases of loul‘/ujcznict. Various qualities of impure gelatin are prepared on the large scale by boiling up the hides of oxen, skins of calves, and spongy parts of horns; fron1 any of the crude gelatius the pure substance may be obtained by bleaching with sulplmrous acid and steeping repeatedly in warm water, when in the state of soft jelly. I Pure gelatin is an amorphous, brittle, nearly transparent substance, faintly yellow, tasteless, and inodorous, neutral to vegetable colours, and unaltered by exposure to dry air. 131 alone, as the total obtainable by the successive actions of the two reagents. N ow, as there appear to be good grounds for believing the molecule of albuminoids to con- tain one or more urea—residues, and as urea, and presumably therefore a urea-residue, would yield its ammonia to potash alone, Wanklyn concludes that gelatin differs in constitution from albuminoids by containing no urea. On the other hand, as Foster observes, the behaviour of gelatin as a. food (see below), in diminishing the amount of fat used by an animal fed partly on it, as well as the quantity of nitro- gen abstracted from other sources, is readily intelligible on the hypothesis that it splits into a urea and a fat moiety. Although gelatin in a dry state is unalterable by exposure to air, its solution exhibits, like all the proteids, a remark- able tendcncy to putrefaction ; but a characteristic feature of this process in the case of gelatin is that the solution assumes a transient acid reaction. The ultimate products of this decomposition are the same as are produced by pro- longed boiling with acid (see below). It has been found that oxalic acid, over and above the action common to all dilute acids of preventing the solidiﬁcation of gelatin solu- tions, has the further property of preventing in a large measure this tendency to putrefy when the gelatin is treated with hot solutions of this acid, and then freed from adhering acid by means of carbonate of lime. Gelatin so treated has been called mctaf/clat-1'72. Strange to say, in spite of the marked tendency of gelatin solutions to develop ferment-organisms, and under- go putrcfaction, the stability of the substance in the dry state is such that it l1as even been used, and with some success, as a means of preserving perishable foods. The process, invented by Dr Campbell Morfil, consists in im- pregnating the foods with gelatin, and then drying them till about 10 per cent. or less of water is present. Milk gelatinized in this way is superior in several respects to the products of the ordinary condensation process, more especially in the retention of a 1n11ch larger proportion of albuminoids. Gelatin has a marked affinity for water, abstracting it from admixture with alcohol, for example. Solid gelatin steeped for some hours in water absorbs a certain amount and swells up, in which condition a gentle heat, as that of the water-bath, serves to convert it into a liquid ; or this Submitted to analysis it exhibits an elementary composi- I may be readily produced by the addition of a trace of alkali tinn agreeing closely with that of chondrin, containing in round numbers C 50, H 7, N 18, O + S 24 per cent. ; whilst chondrin contains about 3 per cent. less nitrogen and more oxygen. Nothing is known with any certainty as to its chemical constitution, or of the mode in which it is formed from allnuuinoids. Besides a similarity in elementary constitu- cuts, it exhibits in a general way a connexion with that large and important class of animal substances called proteids, being, like them, amorphous, soluble in acids and alkalies, and giving in solution a left-handed rotation of the plane of polarization. Nevertheless, the ordinary well-recognized reactions for proteids are but faintly observed in the ease of gelatin, and the only substances which at once and freely precipitate it from solution are corrosive sub- limate, strong alcohol, and tannic acid. According to Wanklyn, gelatin is distinctly differentiated from such substances as casein and albumin by a marked difference in behaviour‘ when treated successively with boil- ing potash and alkaline permanganate. All nitrogenous organic substances yield large quantities of ammonia when decomposed by boiling with these solutions; but whereas albuminoids give up their ammonia at two successive stages, one of which is achieved by the action of potash alone, the other on tl1e subsequent addition of permanganate, gelatin yields the same amount after the action of permanganate or mineral acid, or by strong acetic acid. In the last case, however, or if we use the mineral acids in a more con- centrated form, the solution obtained has lost its power of solidifying, though not that of acting as a glue. By pro- longed boiling of strong aqueous solutions at a high, or of weak solutions at a lower temperature, the characteristic properties of gelatin are impaired and ultimately destroyed. After this treatment it acts less powerfully as a glue, loses its tendency to solidify, and becomes increasingly soluble in cold water; nevertheless the solutions yield on precipitation with alcohol asubstance identical in composi- tion with gelatin. By prolonged boiling in contact with hydrolytic agents, such as sulphuric acid or caustic alkali, it yields quantities of lcucin and glycocoll (so—called “sugar of gelatin,” this being the method by which glycocoll was ﬁrst prepared), but no t_z/rosin. In this last respect it agrees with its near allies, chondrin and elastin, and differs from the great body of proteids, the characteristic solid products of the decom- position of which are leucin and tyrosin. At the same time the formation of glycocoll differentiates it from chondrin, from which, moreover, it can be readily distinguished by its non-precipitability by acetate of lead. When it is mixed with copper sulphate a bright green liquid is formed, from which the copper cannot be thrown down free of organic matter. Addition of potash to the