+ + = Hide Albumen.—The fresh hide, besides this coriin (which, very possibly, is only evolved by the action of the lime), contains a portion of actual albumen, viz, that of the blood serum and of the lymph, which is not only contained in the abundant blood-vessels, but saturates the fibrous connective tissue, of which it forms the nourishment. This albumen is mostly removed by the liming and working on the beam, which is preparatory to tanning. Probably for sole-leather, the albumen itself would be rather advantageous if left in the hide, as it combines with tannin, and would assist in giving firmness and weight to the leather. It is, however, for reasons which will be seen hereafter, absolutely necessary to get rid of any lime which may be in combination with it. The blood also must be thoroughly cleansed from the hide before tanning, as its colouring matter contains iron, and, in combination with the tannin, would give a bad colour.
The reactions of blood and lymph albumen are very similar to those of ordinary white of egg. It is precipitated by strong mineral acids, especially nitric, and also by boiling. The precipitate produced by strong hydrochloric acid re-dissolves by the aid of heat to a blue or purple solution. Tribaaic phosphoric, tartaric, acetic, and most other organic acids, do not precipitate moderately dilute solutions of albumen, but convert it into a sort of jelly, which, like gelatine, does not coagulate, but liquefies on heating. It is precipitated by neutral salts of the alkali metals. Albumen slightly acidified (with acetic acid) is precipitated by potassium ferrocyanide.
Elastic Fibres.—The elastic or yellow fibres of the bide are of a very stable character. They are not completely dissolved even by prolonged boiling, and acetic acid and hot solutions of caustic alkalies scarcely attack them. Probably they do not combine with tannin, and are very little changed in the tanning process.
The hair, epidermis, and glands are, as has been seen, all derived from the epithelial layer, and hence, as might be inferred, have much in common in their chemical constitution. They are all
classed by chemists under one name, " keratin," or horny tissue, and their ultimate analysis shows that in elementary composition they nearly agree. It is evident, however, that the horny tissues are rather a class than a single compound.
The keratina are gradually loosened by prolonged soaking in water, and, by continued boiling in a Papin's digester, are dissolved to an extract which does not gelatinize on cooling. Keratin is dissolved by caustic alkalies ; the epidermis and the softer horny tissues are easily attacked, while hair and horn require strong solutions and the aid of heat to effect complete solution. The caustic alkaline earths act in the same manner as dilute alkaline solutions ; hence lime easily attacks the epidermis, and loosens the hair, but does not readily destroy the latter. Alkaline sulphides, on the other hand, seem to attack the harder tissues with at least the same facility as the soft ones, the hair being often completely disintegrated, while the epidermis is still almost intact ; hence their applicability to unhairing by destruction of the hair. Keratins are dissolved by fuming hydro chloric acid, with the production of a blue or violet coloration, like the albuminoids. They also resemble albumen, in the fact that their solution in sulphuric acid is precipitated by potassium ferrocyanide. By fusion with potash, or prolonged boiling with dilute sulphuric acid, keratin is decomposed, yielding leucine, tyrosine, ammonia, &c. The alkaline solution of keratin (hair, horns, &c.) is precipitated by acids, and, mixed with oil and sulphate of baryta, is employed under Dr. Putz's patent as a filling material for leather, for which purpose it acts in the same way as the egg-yolks and meal used in kid-leather manufacture. Either has also proposed its use for the same purposes with bark-tanned leather.