True gelatigenous tissue occurs in all mature vertebrates, with the single exception, according to Hoppe-Seyler, of that in other respects anomalous vertebrate, Amphioxus lanceolatus. In the embryo it does not appear till late in foetal life, chondrin being found instead; and the change which brings gelatin into the place of chondrin is effected, not by a metamorphosis of the latter, but by its removal, and the independent formation of gelatin. The tissue in question was believed to be peculiar to Vertebrata until Hoppe-Seyler discovered it in the bodies of Octopus and Sepiola. By boiling these cephalopods with water he obtained large quantities of gelatin free from chondrin, but in an extension of his experiments to other invertebrates, as cockchafers and Aηodon and Uηio, no such tissue could be detected. Gelatin, as such, is not met with in any of the normal fluids of the body, but occurs in the blood in cases of leukhoemia.

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; from any of the crude gelatins the pure substance may be obtained by bleaching with sulphurous acid and steeping repeatedly in warm water, wheu in the state of soft jelly.

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. Submitted to analysis it exhibits an elementary composition 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 albuminoids. Besides a similarity in elementary constituents, 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 case of gelatin, and the only substances which at once and freely precipitate it from solution are corrosive sublimate, 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 boiling 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 the subsequent addition of permanganate, gelatin yields the same amount after the action of permanganate alone, as the total obtainable by the successive actions of the two reagents. Now, as there appear to be good grounds for believing the molecule of albuminoids to contain 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 nitrogen 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 remarkable tendency 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 prolonged 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 solutions, 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 metagelatin.

Strange to say, in spite of the marked tendency of gelatin solutions to develop ferment-organisms, and undergo putrefaction, the stability of the substance in the dry state is such that it has even been used, and with some success, as a means of preserving perishable foods. The process, invented by Dr Campbell Morfit, consists in impregnating 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 much 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 may be readily produced by the addition of a trace of alkali or mineral acid, or by strong acetic acid. In the last case, however, or if we use the mineral acids in a more concentrated form, the solution obtained has lost its power of solidifying, though not that of acting as a glue. By prolonged 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 a substance identical in composition with gelatin.

By prolonged boiling in contact with hydrolytic agents, such as sulphuric acid or caustic alkali, it yields quantities of leucin and glycocoll (so-called “sugar of gelatin,” this being the method by which glycocoll was first prepared), but no tyrosin. 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 decomposition of which are leuciu 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 liquid merely changes the colour from green to violet, which by boiling is further transformed into a pale red, but without any precipitation of hydrate. Hence the inapplicability of Trommer’s sugar test in presence of gelatin, the cuprous oxide being soluble in gelatin solutions.

Treated with strong oxidizing agents, such as a mixture of sulphuric acid and bichromate of potash, or binoxide of manganese, it exhibits a close resemblance in behaviour to casein, formic and valerianic acids being the principal products, along with a small quantity of benzoic aldehyde. When solution of gelatin is mixed with chromate of potash alone, it forms a medium very sensitive to light, which converts it into an insoluble yellow mass.

As bones are capable of yielding one-third of their weight of solid gelatin, it follows that, if gelatin had a value equivalent to albuminoids, the bones of an animal would contain one-fifth of the total nutritive material in its body. Accordingly, at a time when gelatin was in high esteem for its food-value, recourse was had largely to this source, more especially in France, for a cheap nutritive soup for soldiers, pauper establishments, and hospitals. To prepare such a soup the bones may be either simply boiled in water under pressure, as in a Papin’s digester, or without pressure, or they may be previously freed from salts of calcium by treatment with dilute hydrochloric acid. On the large scale the crushed bones are submitted to the combined action of steam at high pressure and a current of water percolating through the fragments. The bones, preferably in a fresh condition, or preserved by thorough drying or by antiseptic agents such as brine, are crushed by passing them between solid iron cylinders grooved longitudinally and kept revolving. They are then packed into a cylindrical cage, which can be lowered into a cylindrical jacket of rather larger diameter than itself, the whole closing with a well-fitting lid. A pipe for the entrance of water, regulated by a stopcock, projects from the top of the outer cylinder, and is connected before the lid is put on with an adjustable nozzle, through which the water trickles down among the caged bones. Another pipe is connected with the bottom of the apparatus for the passage of high-pressure steam. The gelatin solution may be removed at intervals by means of a stopcock at the bottom. The quantity of water percolating through the bones is carefully regulated in accordance with the varying pressure of the steam, so as to produce a soup of nearly uniform consistence.

r As to the nutritive value of such a soup very different opinions have been entertained at different times. It was at the time of the first French Revolution, when the question of the improvement of the diet of soldiers and people was much discussed, that attention began to be directed to gelatin as a cheap and useful food; and at that time such men as Proust and D’Arcet were trying improved methods of extracting it from bone. The discovery of nitrogen as a constituent of foods generally led to its being regarded as the special criterion of food-value, and, as this element was found to exist in large proportion in gelatin, the percentage of gelatin extractable from any substance was held as determining its worth as food.

In 1802 a commission appointed by the Academy to investigate the question reported that, though it might to a certain extent replace flesh in soups, yet it could not be taken as the measure of food-value. Meanwhile experiments on men and dogs, especially by Donné, Gannal, Edwards, and Balzac, along with the results of hospital rations at St Antoine and St Louis, showed the impossibility of feeding upon gelatin alone, and in general its unsatisfactory character as a food. Accordingly, a second commission was appointed by the Academy in 1841, who reported very strongly against the use of gelatin at all as an article of diet, alleging that, besides being valueless itself, it actually diminishes the value of otherwise nutritious food; but this latter part of the indictment was overturned by the Netherlands’ commission (Compt. Rend., 1844). It ended by the Academy in 1850 declaring that gelatin was positively injurious to the digestive organs; and the natural result of this extreme reaction was of course a complete cessation of its use as food.

In Germany, Liebig had declared, in his Thierchemie (1843), that gelatin, being a product of the decomposition of albumen, could not take the place of albumen as food, though it might be conceived to be useful for the growth of gelatinous tissue. Boussingault’s experiments on ducks (Ann. Chem. Phys., 1846) showed that, contrary to what should happen if the report of the French Academy were true, gelatin did not pass unaltered into their faeces, but that a large increase of uric acid was found in their urine, a result which was confirmed by Frerichs and Bischoff, who found in the urine of dogs fed on gelatin large amounts of urea—uric acid in birds and urea in mammals being the characteristic forms in which nitrogen is eliminated from the system of these animals. The conclusion they arrived at was that the use of gelatin as a food was limited to its power of undergoing decomposition in the body, like the carbohydrates, to yield heat, but that it cannot replace the other nitrogenous constituents of the body. In 1853 Dr Donders of Utrecht published a treatise on foods, in which he dealt with gelatin, and expressed opinions that have pretty much held their ground since, and only been confirmed in detail by subsequent investigators. Large quantities of gelatin, he says, are detrimental to digestion. In moderate quantity it gets decomposed in the body, and acts as a food probably by diminishing the otherwise necessary amount of albumen, the sole use of which, he remarks, is not merely to form tissues. In 1860 Bischoff and Voit published the result of their experiments on the subject, which completely established the fact that gelatin can take the place of albumen to a limited extent, in a way that fat cannot, so that the body-weight maintains itself on a smaller supply of albumen, and that gelatin has a function therefore of a higher character than a mere heat-producer like starch and sugar. In a more recent memoir by Voit, from which the previous historical sketch is mostly borrowed (Zeitschrift fur Biologic, viii., 1872), the results of an extensive series of careful experiments are given, in which the same conclusion comes out. He finds, moreover, that the saving of albumen is even more marked when a moderate amount of fat accompanies the gelatin, but that no combination of fat and gelatin can replace albumen or prevent the animal from losing flesh; but, on the contrary, when a dog was fed on equal parts of gelatin and fat it lost more flesh than when fed on gelatin alone. Fed on gelatin alone, it after a time evinced such a repugnance to the food that it would rather starve than feed; and, if it was induced to eat, vomiting and diarrhoea were the results. The time which gelatin takes for its complete metamorphosis in the body is far less than in the case of albumen, never exceeding 24 hours, in the course of which time all its nitrogen may be found in the urine and faeces.

A parallel series of experiments to determine how far gelatin could replace fats or carbohydrates in food showed that, though it could not be substituted for them to any large extent, it does somewhat diminish the amount of fat used up. As Voit puts it at the end of his paper, gelatin cannot, any more than fats or carbohydrates, take the place of that moiety of albumen which he calls the organic albumen,—the part which goes to build the organs and tissues; it cannot produce new blood-corpuscles to replace those that are worn out, or form muscles or any tissues, not even the gelatigenous. What it is capable of doing is to act as a substitute to some extent for that other and far larger part of the albumen of food which, never at any time forming part of any organ, circulates in the blood, and is carried to all the tissues, undergoing continual metamorphoses.

A later series of experiments by Etzinger, a pupil of Voit, was undertaken in order to elucidate the action of the digestive fluids on gelatin or gelatigenous tissue. Direct experiments showed that these substances are scarcely altered by prolonged contact with a dilute (0’3 per cent.) solution of hydrochloric acid at the ordinary temperature of the body. But when gelatin or tissues yielding it, such as ligamentum nuchoe, tendons, and bones were treated at the same temperature with an artificial gastric juice made by acidifying with acid of the above strength glycerin extract of pigs’ stomach, a large quantity of these substances speedily disappeared to foιm a solution which did not gelatinize. The solution thus obtained exhibits physical and chemical characters so analogous to those of the peptones formed by a similar process from albuminoids that it has been called by some authors gelatin-peptone. ·

In a quite recent research by Hofmeister (Zeitschrift fur Physiol. Chem., ii. [5] 299, 1878) an attempt has been made to study the product formed in this digestion transformation. Taking the soluble gelatin obtained by prolonged boiling of gelatin in water to be the same material as is produced by the action of gastric juice, the author found that from the solutions so obtained two distinct substances could be separated, one precipitable by pel chloride of platinum, which he calls semiglutin, and the other not so precipitable, and also more soluble in alcohol, which he calls hemicollin. Semiglutin forms definite salts with platinum and copper, analyses of which agree pretty well with the formula C 65 H 8 5N 17 O 22 as the simplest expression for the substance. Similarly the copper-salt of hemicollin gave results indicating for hemicollin a formula C 47 H 70 Ni 4 O 19 , Both of these substances yield leucin and glycocoll when treated by boiling with hydrochloric acid and stannous 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 hemicollin by three molecules of water, so that a probable empirical formula for gelatin would be C 10? H 151 N 31 O 39 , agreeing pretty fairly with the percentage numbers given in an earlier part of this article.

See Hoppe-Seyler, Medicinisch-Chemische Untersuchungen, 1866 and 1871, and his Physiologische Chemie, just being published; Grnelin's Handbook, vol. xviii., 1871; watts's Dictionary of Chemistry, vol. ii. For the digestion of gelatin, see Cari Voit, Zeitschrift für Biologie, viii. 297, 1872; Etzinger, same work, X. 84, 1874; and for constitution of collagen, Hofmeister, Zeitsch. für Physiol. Chemie, ii. [5] 299, 1878. (D. C. R.)

Industrial Delations Of Gelatin. Glue.— 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 known 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 substances and isinglass or fish glue, and therefore the preparation and industrial applications of these three varieties of commercial gelatin—glue, 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, and hoofs. Chondrin, the substance yielded by cartilaginous 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 material, it is first steeped for some weeks in a pit with lime water, and afterwards...