504 THE SKIN AND ITS SECRETIONS. 



going decay. Upon this depends the process of tanning, 

 or the conversion of skins into leather (cf. p. 424). 



The horny tissues, viz., the epidermis, the nails, daws, 

 talons, hoofs, horns, whalebone, wool, feathers, tortoise-shell, 

 and similar continuations and coverings of the skin, 

 are formations composed of various substances, the 

 principal mass of which, however, appears to consist 

 of one and the same body (keratin), a substance con- 

 taining sulphur and nitrogen, and closely allied to the 

 protein compounds. All three formations are soluble 

 in caustic potassa with the aid of heat, evolving at the 

 same time a great deal of ammonia, and forming potas- 

 sium^sulphide. Acids precipitate from the solution a 

 gelatinous, nitrogenous substance. Nitric acid turns 

 them yellow and destroys them ; when boiled with 

 dilute sulphuric acid, they form leucine (p. 98), and 

 ty rosin (p. 350); subjected to dry distillation, they 

 yield a large quantity of nitrogenous products. The 

 epidermis contains 0.74 per cent., the nails 2.8 per 

 cent., the horse s hoof 4.2 per cent., whalebone 3.6 

 per cent, of sulphur. They also contain small quanti 

 ties of calcium phosphate, iron, and silicic acid, which 

 latter is contained as a constant ingredient in larger 

 quantity In the vane of bird-feathers.* 



Human hair contains as principal ingredient a pro 

 tein-like body, that contains over 5 per cent, of sul 

 phur. The presence of this large amount of sulphur 

 is the cause of the turning black of light hair by 

 means of metallic salts. In addition to some calcium 

 phosphate, and small quantities of other salts, hair 

 also contains iron oxide and silicic acid. The cause of 

 the different colors of hair is unknown ; it appears, 

 however, that according as the color of the hair differs, 

 the composition also varies. 



The sebaceous matter of the human skin contains a 



* Cliitin, a substance, that forms the real skeleton, the testa and 

 coverings^ the wings of all insects, is entirely different in composition 

 and chemical properties from these formations. Its composition is pro 

 bably C 9 Hi5N0 6 . It is not dissolved even by the most concentrated 

 potassa, and carbonizes without fusing, when heated. When boiled with 

 sulphuric acid, it yields grape-sugar and ammonia. 



