122 GENERAL ZOOLOGY 



oxygen to pass through the tissue of the lungs into the blood- 

 filled capillaries and permit wastes to escape from the blood. 



In the circulating blood there is a red-colored substance 

 called haemoglobin. Oxygen combines chemically with this, 

 and the compound goes with the blood until it reaches 

 tissue cells which have some food stored in them ; then the 

 oxygen combines with the food, which may be carbohydrate, 

 fat, or protein. The chemical union of oxygen with other 

 elements is of the greatest importance in the life of any 

 plant or animal. A simple example of the result of the 

 chemical union of oxygen with another element may be ob- 

 served in the burning of coal. It is well known that coal 

 is composed chiefly of carbon. When a quantity of coal is 

 heated it begins to unite with oxygen from the air. During 

 this process four phenomena may be observed : first, the 

 quantity of oxygen in the room is reduced ; second, the 

 quantity of coal is reduced ; third, an invisible gas is 

 formed ; and fourth, heat and light are given off. 



When oxygen unites chemically with carbon two kinds of 

 gas may be formed, either carbon monoxide or carbon di- 

 oxide, or both. The union of a solid element with a gaseous 

 one to form a gaseous compound accounts for the fact that 

 so great a mass of coal disappears in burning, leaving only 

 the mineral ash behind. We call the union of oxygen with 

 another substance oxidation. 



Forms of Energy. It is important to observe that as oxi- 

 dation takes place heat and light, which are called forms of 

 energy, are given off. Energy — that is, the power to do 

 work — can be transformed from one form to another ; for 

 example, the heat derived by oxidizing coal may be trans- 

 formed into mechanical energy like that of an engine, and 

 the mechanical energy may be changed into electricity, and 

 the electricity into mechanical energy again, or into heat 

 and light. We may regard the energy which is suddenly 



