20 



CHEMICAL COMPOSITION OP LIVING MATTER 



makes use of oxidation to maintain the body temperature, 

 wliile the necessity for a continuous oxidation explains 

 the presence of carbon in the body and the continuous 

 entrance of oxygen and exit of carbon dioxide in breathing. 

 Nitrogen. — Experiments with pure oxygen show that 

 combustion takes place more rapidly in it than in air. 

 This implies that air must contain something besides 

 oxygen, which dilutes it and makes oxidation slower. 

 What is this substance? A simple experiment answers 

 this question. (See Ex. IX.). Place a piece of phos- 

 phorus the size of a pea in an evaporating dish that is 

 floating on water. Light the phosphorus and thus raise 

 its temperature to the combining point. Then cover the 

 dish quickly with a bell jar (Fig. 3). The phosphorus 



will at once begin to combine 

 with the oxygen in the in- 

 closed air, and when it stops 

 burning it will be because there 

 is no more oxygen to combine 

 with it to form the white 

 fumes of phosphorus oxide. 

 If we let the fumes settle they 

 will dissolve in the water and 

 there will be left in the jar 

 only what was in the air before 

 the oxygen was taken out. 

 Further, since the water rises in the jar to take the place 

 of the consumed oxygen the volume of gas left in the 

 jar subtracted from its original volume will give us the 

 proportion of oxygen present in a given volume of air; 

 roughly one fifth of its volume. 

 What is this gas that is left? If we examine it by 



Fig. 3 — Apparatus arranged for 

 obtaining nitrogen from air. 



