32 ZOOLOGY 



Forms of 3- Why should free oxygen be so necessary? It is 



energy needed for the process known as oxidation, in the course 

 of which energy is released or made manifest. Stu- 

 dents of physics speak of potential or latent energy, 

 and kinetic or active energy, states which may alter- 

 nate, while the sum total of energy remains the same. 

 When energy becomes potential, it is just as though 

 matter were to disappear into a fourth dimension of 

 space ; it exists, but cannot be appreciated. The 

 conception of potential energy is thus in a sense meta- 

 physical, but the ordinary experience of mankind 

 makes it commonplace when we recall the lifted weight, 

 the bent spring. We know very well that when we 

 lift a lo-pound weight a foot, we expend or use a 

 definite amount of energy ; and that if we set the 

 weight upon a shelf it will stay there, ready to liberate 

 the same amount of energy at any time by falling on 

 our toes or otherwise. The agent which disturbs the 

 weight knocks it off the shelf does not make the 

 energy, but only sets it free. 



Work 4. The oxygen unites with carbon, forming the very 



stable or static compound known as carbon dioxid, 

 with the chemical formula CO 2 . This is a. gas, heavier 

 than air, and is given off through the respiratory ap- 

 paratus. Carbon and oxygen have a chemical affinity 

 for each other, and from the standpoint of the present 

 discussion may be compared to the weight and the 

 earth, attracted together by gravitation. The falling 

 weight and the uniting atoms display kinetic energy, 

 and work results. The word " work," in biological dis- 

 cussions, is used in the broadest sense, to describe all 

 the life activities; so that even a sleeping person is 

 said to be doing work, or at any rate the organs of his 

 body are. The beating of the heart is only an obvious 



