SOLID PARTS OF ANIMALS. 



The liquid principles of the cutis may be extracted by water. 

 If we dry the skin after this treatment it becomes yellowish, trans- 

 lucent, and stiff but flexible and tough. Ether extracts from 

 it a good deal of fat. When macerated in water it recovers its 

 original softness. At the common temperature of the atmo- 

 sphere it is insoluble in water. When boiled in water it contracts, 

 becomes convex on the outside, thickens, becomes stiff and elas- 

 tic. But if the boiling be long continued it softens, becomes 

 mucous and translucent, and finally dissolves. The solution is 

 muddy from minute blood-vessels suspended in it. On cooling 

 the solution concretes into a jelly. Thus the cutis, by long 

 boiling, is converted into collin. The rapidity with which the 

 skins of different animals dissolve in boiling water is very differ- 

 ent The stronger, and larger, and older the animal is from 

 which the skins were obtained, the longer do they take to dis- 

 solve, but the stronger and stiffer is the glue into which they 

 are converted. The skins of fishes, of little birds a of the small 

 mammalia dissolve readily. It is only necessary to keep them 

 in water of the temperature of 77, to be converted into a kind 

 of jelly, which solidifies with difficulty, or remains half-liquid. 



Skins are not dissolved by alcohol, ether, nor by the fixed or 

 volatile oils, whether hot or cold. But alkalies and acids diluted 

 to a certain point convert them into glue, even at the ordinary 

 temperature of the atmosphere. Thus, if we steep a skin in con- 

 centrated acetic acid, it swells into a jelly, which is soluble in 

 water. When a softened skin is steeped in persulphate of iron 

 or corrosive sublimate, it gradually combines with the metalline 

 salt, becomes more dense, harder, and incapable of putrefying. 

 A similar combination takes place when they are steeped in in- 

 fusion of oak-bark, or of any substance containing tannin. 



It is from the skin or cutis of animals that leather is formed ; 

 and the goodness of the leather, or at least its strength, depends 

 in some measure on the toughness of the hides. Those easily 

 soluble, as seal-skins, afford a weaker leather than those which 

 are more difficultly soluble in water. The process by which the 

 skins of animals are converted into leather is called tanning. It 

 seems to have been known and practised in the earliest ages ; but 

 its nature was unknown till after the discovery of the tanning 

 principle by Seguin. That chemist ascertained that leather is a 

 compound of tannin and skin ; that it is to the tannin that lea- 



