24 PHYSIOLOGICAL CHEMISTRY 



compounds. Nitrogen is a constituent of all proteins. Many 

 proteins contain sulphur and phosphorus as well. Nitrogen 

 occurs in various other compounds throughout the body, and 

 the body fluids contain gaseous nitrogen in solution, as is to be 

 expected since nitrogen is somewhat soluble in an aqueous 

 liquid, and in the lungs the blood comes in contact with the air 

 which is made up about four-fifths of nitrogen. So far as is 

 known this dissolved nitrogen has no influence on the body's 

 activities. Nitrogen is excreted mainly as urea, uric acid and 

 other products formed by the decomposition of proteins in 

 the body. If heated with soda lime, an organic substance of 

 this type gives off its nitrogen as ammonia. Sulphur and phos- 

 phorus are present in various compounds other than the pro- 

 teins. Of these the phosphatids are perhaps most interesting. 

 This group will be considered later. Calcium phosphate makes 

 up about 85% of the ash of bone. Sodium phosphate is found 

 in the blood and tissues. Sulphur artd phosphorus are excreted 

 mainly in the urine as sulphates and phosphates, but also in the 

 feces. Unoxidized sulphur may be split off from organic com- 

 pounds by boiling with alkali. On adding lead acetate, a dark 

 precipitate of lead sulphide will form. This test may be con- 

 firmed by adding an acid. Hydrogen sulphide will be given 

 off and may be identified by a paper moistened with lead acetate 

 on which lead sulphide will form as a shiny dark precipitate. 

 Sulphates are detected by precipitating as barium sulphate. 

 Phosphates are detected by precipitating as ammonium phospho- 

 molybdate. 



Sodium, Potassium, Calcium, Magnesium and Iron. These 

 metals make up only a small part of the body, but they are none 

 the less important. They are distributed widely throughout the 

 body tissues and fluids. There usually is more sodium than 

 potassium in body fluids, and more potassium than sodium in 

 the solid tissues. These metals are present as chlorides, sul- 

 phates or phosphates. Sodium chloride in the blood is interest- 

 ing since it furnishes chlorine for the hydrochloric acid of the 

 gastric juice, whereas sodium bicarbonate is very important in 



