204 GENERAL ZOOLOGY 



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

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

 other element 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 transformed 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 released upon the 

 oxidation of the carbon in the coal as having been stored there 

 by the sun millions of years ago, when the coal was the grow- 

 ing tissue of a tree. We may borrow from physics two other 

 terms which will help us in getting the notion of the states in 

 which energy may exist. Energy at rest as, for example, 

 chemical affinity (that is, the readiness of the carbon to com- 

 bine with oxygen) is called potential energy ; energy in action, 

 as heat, light, electricity, and motion, is called kinetic energy. 

 As we have already seen, potential energy may become kinetic 

 energy, and kinetic energy may become potential energy. 



When the earthworm swallows a bit of leaf which passes 

 through all stages of digestion, absorption, circulation, and 

 food-storing, or assimilation, the form of the bit of leaf is 

 completely lost, and the chemical composition is also changed ; 

 but oxidation has not taken place, and hence the potential 

 energy transformed from the kinetic energy of the sun (heat 

 and light) when the leaf grew, remains unchanged until the 

 all-important phenomenon of oxidation occurs. 



Carbohydrates, and especially fats, are capable of combin- 

 ing with a relatively large amount of oxygen, because of the 

 small proportion of that element in those compounds, and the 

 large proportion of carbon. Carbohydrates, chiefly glycogen, 



