52 APPLIED BIOLOGY 



in a jar of pure oxygen. But in the oxygen of the air, diluted 

 as it is with nitrogen, oxidation of iron is the very slow process 

 of "rusting." Another example : A piece of magnesium wire 

 may be burned quickly at a high temperature ( 10) ; but 

 if we place a piece of magnesium wire in water kept at the 

 ordinary temperature, it will slowly oxidize (turn to white 

 powder) on the surface. Now, the slow oxidation of the 

 magnesium wire in water, or the familiar rusting of iron, 

 illustrates the slow chemical changes which are continually 

 taking place in every living cell of animals and plants. We 

 emphasize the word "continually," for oxidation of the cell- 

 substances is invariably associated with the activities which 

 we call " living " ; and as long as there is life in a cell, oxida- 

 tion is going on and particle by particle the cell-substance 

 and the food brought by the blood are being burned ; that 

 is, they are being combined with oxygen to form new sub- 

 stances. 



Heat. This slow burning (oxidation) results in heat, 

 and this is how the human body is kept warm. In frogs 

 and other lower animals the heat thus generated is lost 

 rapidly, because the surface of the body is not covered with 

 hair or other structures to prevent loss of heat, and so the 

 frog is never perceptibly warmer than the water which touches 

 its skin. We call it " cold-blooded," because it is usually 

 colder than the human body. 



48. Oxygen Required. We have just learned that 

 oxidation is a process necessary to the life of the cells in the 

 frog's body. Oxidation requires oxygen ; and therefore 

 there must be in the cells of the frog a continual supply of 

 that gas. In some very small aquatic animals oxygen is 

 absorbed from the surrounding water by all the cells, and 

 there is no need of any special organs for supplying oxygen. 

 But in an animal as large as a frog the cells of the internal 

 organs are so far from the external air and water that some 

 method of distributing oxygen is required, just as it is neces- 



