294 THE ANIMALS AND MAN 



ment. The gas formed by oxidized carbon is carbon dioxide. 

 It is colorless but may be readily detected on account of its 

 power to turn lime water milky. 



Carbon dioxide is constantly formed in our bodies and 

 given off in the breath. If we breathe through a straw or 

 glass tube into a jar of lime water, the water in the jar will 

 soon take on a milky appearance due to the union of the 

 carbon and lime. In the formation of carbon dioxide, as in 

 other oxidations (or formation of other oxides), heat is gen- 

 erated. 



This principle is made use of in the cells of the body for 

 the generation of heat and energy. 



Oxidation in the body. — The oxygen of the air is taken 

 into the lungs and passed into the blood. The blood carries 

 it to the cells. The carbon of the living cell derived from 

 the food takes up oxygen, that is, becomes oxidized. Heat 

 is produced, and the energy, by means of which the work of 

 the body is done, is liberated. 



Phosphorus. — Phosphorus in its pure state is a yellowish 

 waxy substance. With calcium and oxygen it forms a large 

 part of our bones. The affinity of pure phosphorus for 

 oxygen is so great that it must be kept under water or else 

 combustion will take place. 



Sulphur. — Pure yellow sulphur is familiar to all of us. 

 When sulphur is oxidized it gives off suffocating fumes. 

 Muscle is largely a compound of sulphur and other elements. 

 The disagreeable odor given off by decaying flesh is caused 

 chiefly, by the sulphur fumes. In eating an egg with a 

 silver spoon the sulphur of the egg forms with the silver a 

 blackish compound. 



FOODS AND NUTRITION 



Food nutrients. — Those substances needed by the cells 

 for their metabolism and their growth are called food nu- 

 trients. The nutrient or food value of any substance there- 



