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Gelatin

glue water acid solution boiling food time obtained isinglass substances

GELATIN. When intercellular connective tissue, ti,s met with in skin, tendons, ligaments, and the fasche of the muscles, of which it forms the basis, is treated with water, preferably hot, or in presence of dilute acids, for sonic time, a solution is obtained which in cooling solidifies to a jelly. The.dissolved substance bears the name of Gelatin or Glutin.

The same substance is obtained when the matrix of bones is submitted to similar treatment, after previous removal of the lime salts by means of mineral acids. Again, when unossified cartilage, as for instance the bone-cartilages of the vertebrate foetus, is treated with water or dilute acids, a solution is obtained which also gelatinizes on cooling. The coagulation in this case, however, is due, not to gelatin, but to a closely allied substance called chondrin. At one.

time it was supposed that in each of these three cases the gelatinizing materials obtained were formed by the hydration or by a physical metamorphosis of a different substance pre-existing in the respective tissues, to which the names collagen, ossein, and clsondrogen 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, much more than would be necessary to dissolve the same amount of dried product, points rather the other way. Most probably the change which occurs is Of a purely physical character.

True gelatigenous tissue occurs in all mature vertebrates, with the single exception, according to Hoppe-Seyler, of that in other respects anomalous vertebrate, Ansp4ioxus lanccolatus. 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, :rot 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 Vertebrates 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 Anodon and Unio, 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 leukkcemia.

Various qualities of impure gelatin are prepared on the large scale by boiling up the hides of oxen, skins of calves, and spongy parts or horns ; front any of the crude gelatins the pure substance may be obtained by bleaching with sulphurous acid and steeping repeatedly in warm water, when 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, 0 + 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 age 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 proteicls, 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 leu.cin 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 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 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.

° 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'Areet 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 fo'od.

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 Donne, 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 Thierehemie (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. lioussiugault'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 frees, 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 mu•ea - uric acid in birds and urea iu 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 Pr Ponders 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 Op-previous historical sketch is mostly borrowed (Zcitschrift file Biologic, viii., 1872), the results of an extensive series of careful experiments are given, in which the same conclusion comes out. lie 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 lie found in the urine and faces.

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 use I 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 W0111 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 Veit, 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 nuchce, 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 form 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 (Zeitsehrift fur 17hysiol. 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 lie 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 peichloride of platinum, which he calls semighttin, and the other not so precipitable, and also more soluble in alcohol, which he calls hem/ea/in. Serniglutin forms definite salts with platinum and copper, analyses of which agree pretty well with the formula C„H„N,7O,2z as the simplest expression for the substance. Similarly the copper-salt of hemicollin gave results indicating for hemicollin a formula C471-170N,40,,. 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 C1o11-1151N30391 agreeing pretty fairly with the percentage numbers given in an earlier part of this article.

See lloppe-Seyler, Medicitsisch-G7temische Untersuchunyen,1855 and 1871, and his Physiologische Chernie, just being published; Gmelin s handbook, vol.

1671; Watts Dictionary of Chemistry, vol.ii. For the digestion of gelatin, sec Carl Volt, Zeitschrift far Biolovie, viii. 297, 1870; Etzinger, same work, x. aa, 18'71; and for constitution of collagen, Ilofmeister, Zeitsch. far Physiot. Chemie, 0. L01'000, 1878. (D. C.11.) 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 fur 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 tawecl 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 carefully dried and stored. The object of the lime steeping is to remove any blood and flesh which may be attached to the skin, and to form a lime soap with the fatty matter it contains. So prepared the " scrows " or glue pieces, as they are termed, may be kept a long time without undergoing change. Before being boiled, 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 ran off. The boiler is heated by direct firing, a series of boilers being arranged in the manner 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 exhausted after six separate boiliugs, 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 permitted to subside. The glue solution is then run into wooden troughs or coolers about 6 feet long by 2 feet broad 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 appearance, 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 in 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 considerably; and there are several methods of measuring the comparative value of commercial samples, the most reliable of which are based on 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 submitted 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.

Commercial 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 sparkling gelatin of Messrs Cox of Gorgie, near Edinburgh, the following is the process followed, according to their patent obtained in 1841. The shoulders and cheeks 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 1600 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, mow to avoid the injury caused by long subjection to heat ; but it may also be dried on a steam-heated 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 solidifies, when it is cut up and washed to free it from all traces of acid. It is again redissolved at the lowest possible temperature, then resolidified and dried in thin sheets on nets. Heuze of Berlin prepares a pure transparent gelatin, having a fine meaty flavour, from very- impure materials, by ' • • coloured gelatinous material a mixture of wood charcoal and animal charcoal, leaving the whole together for some hours, then redissolving and straining off the clarified gelatin.

Isinglass. - Isinglass or fish glue, in its raw state, is the swimming-bladder or sound of various species of fish. The sounds undergo no other preparation than careful drying, but in the drying they are variously treated and made up, so that the isinglass comes into commerce under the names of "leaf," "staple," "book," "pipe," "lump," "honeycomb," and other designations, according to its form. The finest isinglass, which comes from Russia, is prepared by cutting open the sounds, steeping them in water till the outer membrane separates from the inner, then washing the latter and exposing it to dry in the air. Russian isinglass is obtained from several species of sturgeon (ilcipenser), found in the Volga and other tributaries of the Caspian Sea, in the Black Sea, and in the Arctic Ocean. Brazilian isinglass, obtained from Brazil and Guiana, is the produce of a large fish, Silurus parkeri4, and probably some other species; and Manila and East Indian isinglass are yielded by species of fish not yet satisfactorily determined. The sounds of the common cod, the hake, and other Gadidw are also used as a kind of isinglass. The principal uses to which isinglass is applied are for jellies and confections, and as a clarifying or filtering medium for wine, beer, and other liquids. When used for culinary and confectionery purposes, isinglass is rolled into thin sheets and cut into fine shreds to facilitate its solution. For clarifying liquids its fibrous structure is of great value, as it forms a fine network in the liquid in which it is disseminated, and thereby mechanically carries down all the minute particles which render the liquid thick and turbid. Isinglass dissolved in strong acetic acid forms a powerful cement, much used for repairing glass, pottery, and similar small objects.

Uses of Gelatin. - The gelatin derivable from bones enters very largely into human food, in the stock for soups, &c., and as prepared gelatin, "calves foot jelly," and isinglass. In addition to the uses already alluded to, gelatin has many other applications in the arts. It is employed as a sizing agent in paper-making, and by painters it is also used for sizing or priming, and for preparing tempera colours. Further, it is used in the preparation of elastic moulds of undercut work, and in the manufacture of inking rollers for printing. Gelatin treated with bichromate of potash, under the influence of light, undergoes a remarkable chemical and physical change, whereby it is rendered entirely inebsorbent of and insoluble by water. The change is due to the 'oxidizing effect of the bichromate ; and the circumstance has given rise to the numerous so-called carbon-processes introduced into photography by Swan, Johnson, Woodbury, Albert, Edwards, and others, in all of which an image is produced in gelatin oxidized by chromium compounds. An insoluble glue may be prepared by adding to dissolved glue, just before using, a proportion of a solution of bichromate of potash, and such a preparation forms a useful waterproofing medium. Glue may he kept liquid at ordinary temperatures by the addition of concentrated acetic acid or of weak nitric acid. Dumoulin's liquid glue, which possesses powerful adhesive properties, is composed of glue in the proportion of 2 11, dissolved in 1 quart of water with 7 oz. of nitric acid (sp. gr. 1.335) added. Mouth or lip glue is prepared by adding lb or thereby of sugar to each pound of dissolved glue. It forms solid but easily dissolved cakes, and as it can be sufficiently softened by the tongue, it is for many purposes extremely convenient. Transparent gelatin, brightly coloured by dyeing substances, and cast in excessively thin sheets, is largely used for ornamental wrappings for bon-bons, &c.

Various adhesive but non-gelatinous substances are, on account of their Properties. known commercially as [due. and are used as substitutes for ordinary glue. Thus marine glue, employed in shipbuilding and for other purposes, is a compound of india-rubber and shell-lac dissolved in coal-tar naphtha. Clue substitutes are also prepared from the albuminoicls casein and gluten, but they are not likely to become substances of any considerable commercial importance. (J. e.k.)

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