Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/142

Rh 132 G E 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 euprous 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 pro- ducts, along with a small quantity of benzoic aldehyde. When solution of gelatin is mixed with ehromate of potash alone, it forms a medium very sensitive to light, which con- verts 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 hydro- chloric 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 en- trance 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 pas- sage 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 nmch 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 cnerally led to its being regarded as the special criterion of foot?-value, and, as this element was found to exist in large proportion in gelatin, the percentage of gelatin extfra_.c3able from any substance was held as determining its worth as oo . 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 impossi- bility 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 nutri- tious food; but this latter part of the indictment was overturned by the Netherlands’ commission (Compt. I€cml., 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 Thicrclzcmic (1843), that LATIN 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. lloussingaulfs experiments on ducks (Jun. (‘Izem.. 1’h_1/9., 1846) showed that, con- trary to what should happen if the report of the French Academy were true, gelatin did not pass unaltered into their fa.-ces, but that a large increase of uric acid was found in their urine, a result which was confirmed by l"rerichs and l’»ischoll', who found in the urine of dogs fed on gelatin large amounts of nrea—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 carbo- hydrates, to yield heat, but that it cannot replace the other nitro- genous constitucnts of the body. In 1853 Dr Ilondcrs of l'trccht published a treatise on foods, in which he dealt with gtlatin, 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, aml 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 llisehoff 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 Yoit, from which th-. previous historical sketch is mostly borrowed (Zcz'tscI¢rzﬂ fit." .1,-‘1'olo_r/ic, viii., 1872), the results of an extensive series of careful experiments are given, in which the same conclusion comes out. I le 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 I1'(-'(-Ilt 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, nc'c1' exceeding ‘24 hours, in the course of which time all its nitrogen may be found in the urine and freees. 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 usel 11p. As '0it puts it at the end of his paper, gelatin cannot, any more than fats or carbohydratcs, take the place of that moiety of albumen which he calls the organic albn1ncn,——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 gclatigcnons. What it is capable of doing is to act as a substi- tute to some extent for that other and far larger part of the albmnen of food which, never at any time forming part of any organ_. cir- cnlates in the blood, and is carried to all the tissues, undergoing continual mctamorphoses. A later series of experiments by Etzinger, a pupil of 'oit, was undertaken in order to elucidate the action of the digestive fluids on g-latin or gclatigcnous tissue. Direct experiments showed that these substances are scarcely altered by prolongedcontact with a diiute (0 '3 per cent.) solution of hydrochloric acid at the ordinary temperature of the body. But when gelatin or tissues yielding it, such as li_qamc7ztum ’)llICh(L', tendons, and bom-s 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 fonn a solution which did not gclatinize. The solution thus obtained exhibits physical and chemical characters so analogous to those of the pcptoncs formed by a similar process from albummoids that it has been called by some authors gclat2'n.-gzcptonc. -. In a quite recent research by Hofmeister (Zcz'lsr7n'2_'ft fiir 1'lzys[uI. Chem,’ ii, [5] 299, 1878) an attempt has been made to study the pro- duct formcd 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 sub- stances could be separated, one preeipitablc by pciehloride of platinum, which he calls scm2'glu(z'n., and the other not so precipit- able, and also more soluble in alcohol, which he calls Izc7m'coIlz'71. Semiglntin forms definite salts with platinum and copper, analyses of which agree pretty well wit.l1 the formula t‘5_.,Il,35I',,t),., as the simplest expression for the substance. Similarly the copper-salt of hemicollin gave results indicating for hemicollin a formula ,,7If,.0I',,,O,,,. Both of these substances yield leucin and glyeocoll when treated by boiling with hydrochloric acid and stannous