_Hair, Epidermis, and Glands._--These 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 keratins 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 gelatinise 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 hydrochloric 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 leucin, tyrosin, ammonia, &c. The alkaline solution of keratin (hair, horns, &c.) is precipitated by acids, and, mixed with oil and baryta sulphate, 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. Eitner attempted to use it for the same purpose with bark-tanned leather, but without much success. Putz has also proposed to precipitate the material after working its solution into the pores of the leather. CHAPTER III. COMMERCIAL TANNING MATERIALS.