For further information, the reader is referred to microscopic manuals, such as Schaefer's Practical Histology.' Some important researches on the structure of hide, and its modification in tanning, have been made by F. Kathreiner, of Worms, who has invented refined and convenient methods of microscopic research, specially adapted to the purpose. Particulars of these are in course of publication.
Chemical Composition of Hide.—The chemical composition of skin is very imperfectly understood. The bulk of the skin is, as has long been known, converted by boiling into gelatine or glue. The yellow fibres and cellular tissue remain undissolved. Mentz, who made some interesting researches on the subject, found that completely dried hide per cent. of cellular tissue insoluble in hot water, 1.058 of fat, of mineral matter, and 95.395 of matters soluble in hot water. Mentz counts the whole of the tissue soluble in hot water as converted into glue ; but this is not strictly the case. Gelatine is not identical with the fibre of the hide, which is only converted into it by boiling. The nature of the change is not well understood ; but it is either simply molecular, or depends on the addition of one or more molecules of water. Raw hide, unhaired and purified, contains, according to Miintz—carbon, 51.43 per cent. ; hydrogen, ; nitrogen, ; oxygen, ; ash, 0.71 ; while gelatine has, according to Mulder—carbon, per oent. ; hydrogen, 6.6 ; nitrogen, 18-3. Probably, however, neither substance was quite pure.
Gelatine is insoluble in alcohol, ether, and cold water, but swells in the last. It is soluble in hot water, but is reprecipitated on the addition of a sufficient quantity of alcohol. This reaction is common to gum, dextrine, and other substances. Moist gelatine exposed to the air rapidly putrefies. It first becomes very acid, from formation of butyric (and perhaps other) acids, but afterwards alkaline, from evolution of ammonia. Boiled with concentrated potash, it yields leucine, glyeocine (sugar of gelatine), and other substances.
The same products are obtained by boiling with sulphuric acid, and probably also more gradually, and in greater or less proportions, by the prolonged action of lime or barium hydrato, by putrefaction, and by any other influence which tends to resolve the gelatine molecule into its simpler parts. Gelatine is precipitated by all tannins, even from very dilute solution. A solution containing parts is rendered turbid by infusion of gall-nuts or gallotannic acid. The precipitate is soluble in excess of gelatine. Solution of gelatine dissolves considerable quantities of phosphate of lime, hence this is always largely present in common glue. It is not precipitated by ferrocyanide of potassium, by which it is distinguished from albuminoids, and it differs from albumen iu not being coagulated by heat. On the contrary, by prolonged boiling, glue loses the property of gelatinizing, but is not altered in composition.
The connective-tissue fibres are partially converted into gelatine by the action of strong acids and alkalies, as well as by heat. By weak acids, they are swollen and gradually dissolved, and Reimer has found that the fibrous material may be reprecipitated by lime-water. It forms an
irregular fibrous mass, which has not the sticky feel of gelatine, but is at once converted into the latter by boiling. Rollet has demonstrated that when hide and other forms of connective tissue are soaked in lime- or baryta-wator, the fibres become split up into finer fibrils, and as the action proceeds, these again separate into still finer, till the ultimate fibrils are as fine as can he dis tinguished under a powerful microscope. At the same time, the alkaline solution dissolves the substance which cemented the fibres together, and this may be recovered by neutralizing the solution with acetic acid, when it comes down as a flocculent precipitate. This was considered by Rollet as an albuminoid substance ; but Reimer has shown thilt it is much more closely allied to the gelatigenous fibres, if indeed it is not actually produced from them by the action of the alkaline solution. Reimer used limed calf-skin for his experiments, and subjected it to prolonged cleansing with distilled water, so that all soluble parts must have been pretty thoroughly removed beforehand. He then digested it in closed glasses with lime-water for 7-8 days, and precipitated the clear solution with dilute acetic acid. He found that the same portion of hide might be used again and again, without becoming exhausted, which strongly supports the supposition that it is merely a product of the partial decomposition of the hide fibre. The substance, which he called "coriin," was purified by repeated solution in lime-water, and reprecipitation by acetic acid. It was readily soluble by alkalies, but insoluble in dilute acids, though in some cases it became so swollen and finely divided through the latter as to appear almost as if dissolved. It was, however, very soluble in common salt solution of about 10 per cent., though it was precipitated both by the addition of much water, and by saturating the solution with salt. Reimer found that a 10 per cent. salt solution was equally effective with lime-water in extracting it from the hide, and that it was partially precipitated on the addition of acid, and completely on saturating the acidified solution with salt. Other salts of the alkalies and alkaline earths acted in a similar manner, so that Reimer was at first deceived when experimenting with baryta-water, because, being more concentrated than Jime-water, the coriin remained dissolved in the baryta salt formed on neutralizing with acid, and it was necessary to dilute before a precipitate could be obtained. The slightly acid solution of coriin gave no precipitate with potassium ferrocyanide, nor was it precipitated by boiling, being thus distinguished from albuminoids. The neutral or alkaline solution was not precipitated by iron or mercuric chloride, copper sulphate, nor by neutral acetate of lead ; but was precipitated by basic lead acetate, basic sulphate of iron, and excess of tannin. Its elementary composition is— carbon, hydrogen, ; nitrogen,17-82 ; oxygen, 29.60 ; and Reimer proposes the follow ing equation as representing its relation to hide fibre :— Hide fibre. Water.