ALBUMINOUS SUBSTANCES. 21 



transitions from the solid to the liquid state by the alter- 

 nate abstraction and addition of heat, may be repeated 

 several times -, but at length the gelatin is so far altered, 

 and, apparently, oxydized by the process, that it no longer 

 becomes solid on cooling. Gelatin in solutions too weak 

 to solidify when cold, is distinguished by being precipitable 

 with alcohol, ether, tannic acid, and bichloride of mercury, 

 and not precipitable with the ferrocyanide of potassium. 

 The most delicate and striking of these tests is the tannic 

 acid, which is conveniently supplied in an infusion of oak- 

 bark or gall-nuts ; it will detect one part of gelatin in 

 5,000 of water; and if the solution of gelatin be strong 

 it forms a singularly dense and heavy precipitate, which 

 has been named tanno-gelatin, and is completely insoluble 

 in water. 



Chondrin. The kind of gelatin obtained from carti- 

 lages agrees with gelatin in most of its characters, but its 

 solution solidifies on cooling much less firmly, and, unlike 

 gelatin, it is precipitable with acetic and the mineral and 

 other acids, and with alum, persulphate of iron and acetate 

 of lead. 



The albuminous substances, or ^ro^m-compounds, as 

 they are sometimes called, are more different from inorganic 

 bodies than are any of the substances yet considered, or 

 perhaps, any in the body. The chief among them are, 

 albumen, fibrin, and casein ; the last is found almost 

 exclusively in milk. Principles essentially similar to them 

 all are found also in vegetables, especially in the sap and 

 fruits. And substances much resembling, though not 

 classed with, the albuminous, are horny matter and ex- 

 tractive matter. 



Albumen exists in some of the tissues of the body, espe- 

 cially the nervous, in the lymph, chyle, and blood, and in 

 many morbid fluids, as the serous secretions of dropsy, 

 pus, and others. In the human body it is most abundant, 

 and most nearly pure, in the serum of the blood. In all 



