THE CHEMISTRY OF ALBUMINS 209 



the red colouring matter of the blood, and consists of an 

 albumin compound with a prosthetic group, which in this 

 case gives rise to colouring matter and is therefore called a 

 chroma togenic group.) 



3. Glycoproteid sand mucins occurring in mucus. In 

 this case the prosthetic group is a residue of a carbohydrate. 



Group IV 



Albuminoids. These are many of them rather ill-defined 

 bodies which form part of the skeletal structure of the animal 

 or plant organism. The classification is mainly anatomical. 



1. Collagin, gelatine. (The sub-stratum of bone and 

 cartilage consists of collagin ; on boiling with water it yields 

 gelatine or glue.) 



2. Keratin. (The chief constituents of the horny sub- 

 stances of mammals and birds.) 



3. Elastin. (Occurs in certain fibrous animal tissue.) 



4. Fibroin. (Occurs in raw silk.) 



5. Spongin. (Forms the frame- work of the bath-sponge.) 



6. Amyloid. (A pathological product, sometimes found in 

 the brain, liver, etc.) 



7. Albumoids. (Sundry substances found in various 

 animals, membranes, etc., difficult to classify.) 



8. Colouring matters derived from albumins, e.g., mela- 

 nin, the pigment substance of the skin of dark-skinned 

 races. 



It may be useful shortly to summarise the information in 

 the foregoing chapter as follows : 



Albumins or Proteins are complex nitrogenous colloidal 

 substances occurring in animal and vegetable protoplasm, etc., 

 and capable of being separated by their varying solubility in 

 solutions of certain salts (pp. 204-207). 



