The action of sulphuric acid on gelatin has been studied by 13raconnot, When one part of glue and two of sulphuric acid are mixed, they form in twenty-four hours a clear fluid, which, when diluted with eight parts of water, boiled for eight hours, (the loss by evaporation being replaced by fresh portions of water,) and then neutralised by chalk, filtered, evaporated to the consistency of syrup, and set aside for a month, yields a crystalline crust of a peculiar saccharine substance, which is insoluble in alcohol and ether, unsusceptible of vinous fermentation, and gives ammonia by destructive distillation. It combines and forms a peculiar crystallisable compound with nitric acid, which he calls the nitro-saccharic acid, and which combines with the salifiable bases and forms distinct salts, the properties of which closely resemble the carbazotates.
Dilute nitric acid dissolves gelatin without the evolution of nitrous gas, and forms a yellow solution, which, by evaporation, (or the ad dition of an alkali,) becomes darker, and at last evolves nitrous gas, and passes (often with ignition) into a spongy coal.1- Ry concen trated nitric acid gelatin is converted into malic and oxalic acids, a fatty substance, and artificial tam! Acetic acid dissolves gelatin and the solution does not gelatinise, but upon drying, the ad hesive power of the gelatin is unimpaired: the dilute acids do not generally prevent ge latinisation.
Neither the dilute caustic alkalis nor am monia prevent the concretion of a solution of gelatin, but they render it turbid by precipi tating its phosphate of lime. Gelatin is soluble in strong caustic potash, with the exception of a residue of phosphate of lime. The solu tion, when neutralised by acetic acid, does not gelatinise, and yields on evaporation a compound of gelatine with acetate of potash, which is soluble in alcohol. Sulphuric acid precipitates sulphate of potash from this acetic solution, in combination with gelatin ; and this compound precipitate, dissolved in water, crystallizes by spontaneous evaporation to the last drop.§ Hydrate of lime does not affect a solution of gelatin, but much lime is dissolved by it : it also takes up a considerable quantity of recently precipitated phosphate of lime.
Gelatin is not precipitated by solution of alum, but when an alkali is added the alumine falls in combination with gelatin. The alu minous solution of gelatin is used for sizing paper, and for communicating to woollen cloth a certain degree of impenetrability to water.
The acetates of lead do not precipitate pure gelatin ; by corrosive sublimate its solution is rendered at first turbid, and when excess is added a white adhesive compound falls: nitrate and per-nitrate of mercury and chloride of tin occasion nearly similar changes. But the me tallic salt which is the most decided precipitant of gelatin, and which does not affect albumen, is sulphate of platinum ; it throws it down even from very dilute solutions, in the form of brown flocculi, which, when collected and dried, become black and brittle, and which, according to Mr. Edmund Davy, to whom we owe this effective test, consist of about 76 per cent. of sulphate of platinum and 24 per cent.
of gelatin and water.
We now come to the most important and characteristic property of gelatin, which is, that of combining with tannin, and upon which the art of tanning, or the conversion of skin into leather, essentially depends, for the true skin (cutis) of animals consists of a condensed and fibrous form of organised gelatin, and, when properly prepared and im mersed in a solution of vegetable astringent matter or tannin, it becomes gradually pene trated by and combined with it, and when dried is rendered insoluble and durable. The tannin of the gall-nut is perhaps that which forms the most insoluble precipitate in gelati nous solutions, and is therefore the most de licate test of the presence of gelatin ; but, as albumen is also thrown down by it, the absence of the latter must have been previously ascer tained. (See Ausu za ex.) A strong infusion of galls occasions a precipitate in water holding less than a five-thousandth part of gelatin in solution, and, if added to a strong solution of gelatin, it throws it down in the form of a curdy precipitate, more or less dense and coloured according to the greater or less excess of the precipitant. The precipitated compound is insoluble in water, dilute acids, and alcohol, and when dried becomes hard and brittle, but again softens and acquires its former appear ance when soaked in water : it may be termed tanno-gelatin. When tannin is added to a solution of gelatin, the latter being in excess, and especially if it be warm, no precipitate is immediately formed, for tanno-gelatin, when recently precipitated, is to a certain extent soluble in liquid gelatin. Tanno-gelatin does not appear to be a definite compound ; at least it is difficult to obtain it as such : the preci pitate by infusion of galls consists, when care fully dried, of about 40 per cent. of tan and 60 of gelatin. When obtained by other astringents, such as oak-bark, catechu, and kino, it differs in the relative proportion of its components and in its other characters, and often contains extractive matter. According to Sir H. Davy,* 100 parts of calf-skin thoroughly tanned by infusion of galls increase in weight 64 parts ; by strong infusion of oak-bark 34, and by weak 17; by concentrated infusion of willow bark 34, and by dilute 15 ; and by infusion of catechu 19.
Mr. Hatchett's researches have shewn that gelatin is also precipitated by the varieties of artificial tan, and that the compound thrown down resembles in its leading characters the tanno-gelatin of natural tan. The ultimate composition of gelatin (pure isinglass) has been quantitatively determined by Gay Lussac and Thenard, with the following results :— Atoms. Equiv. Theory. Experiment.
Nitrogen . 1 14 16.09 16.998 Carbon . 7 42 48.28 47.881 Ilydrogen . 7 7 8.04 7.914 Oxygen . 3 24 27.59 27.207 1 87 100.00 100.000As the combining proportion of gelatin has not been accurately ascertained, its equivalent number, as above given, is open to doubt, but it is probably correct, and the theoretical and experimental results closely correspond.