10 THE HUMAN BODY. 



and inorganic, and the former be subdivided into those- 

 which contain nitrogen and those which do not. 



Nitrogenous or Azotized Organic Compounds. These 

 fall into several main groups: proteids, peptones, albu- 

 minoids, crystalline substances, and coloring matters. 



Proteids are by far the most characteristic substances ob- 

 tained from the Body, since they are only known as exist- 

 ing in or derived from living things, either animals or 

 plants. The type of this class of bodies may be found in 

 the white of an egg, where it is stored tip as food for 

 the developing chick ; from this typical form, which is 

 called egg albumin, the proteids- in general are often called 

 albuminous bodies. Each of them contains carbon, hydro- 

 gen, oxygen, sulphur, and nitrogen united to form a very 

 complex molecule, and although different members of the 

 family differ from one another in minor points they all 

 agree in their broad features and have a similar percentage 

 composition. The latter in different examples appears to 

 vary within the following limits, but it is almost impossible 

 to get any one of them pure for analysis: 



Carbon 52 to 54 per cent. 



Hydrogen 7 to 7.5 " 



Oxygen 21 to 24.0 " 



Nitrogen 15 to 17.0 " 



Sulphur 0.8 to 2.0 " 



Proteids are recognized by the following characters : 1.. 

 Boiled, either in the solid state or in solution, with strong ' 

 nitric acid they give a yellow liquid which becomes orange 

 on neutralization with ammonia. This is the xantlio-proteic 

 test. 



2. Boiled w r ith a solution containing subnitrate and per- 

 nitrate of mercury they give a pink precipitate, or, if in 

 very small quantity, a pink-colored solution. This is 

 known as Mitton's test. 



3. If a solution containing a proteid be acidulated with 

 strong acetic acid and be boiled after the addition of an 

 equal bulk of a saturated watery solution of sodium sul- 

 phate, the proteid will be precipitated. 



