SOME GENERAL BIOLOGIC PROCESSES 527 



up in the food substance by photosynthesis, and has to be released 

 or set free by oxidation. Except as it is oxidized, the energy in 

 foods or fuels cannot be released. Hence the importance of oxi- 

 dation as the key which unlocks the store houses of solar energy, 

 and makes it available to support life. We do not know how the 

 energy, thus released by oxidation, produces what we call " life," 

 but we do know, that without it, no life exists and that, when 

 oxidation ceases, life ceases too. 



Outside of living energy there are two other general sources which 

 man has learned to use, the power derived from fire and that ob- 

 tained from water. In the case of heat energy we burn (oxidize) 

 various fuels such as wood, coal, gas, or oil. All these fuels are 

 originally derived from plant life. The energy which we set free 

 from them, therefore, came originally from the sun. Someone has 

 called coal " petrified sunshine"; this is almost true. When we 

 warm our hands at the open grate, or heat our house with coal, 

 or cook with gas, or light our rooms with electricity, we are setting 

 free in various forms, the energy absorbed from the sun by plants. 



But suppose the mill is run or the electricity used is generated 

 by water power. Here, again the sun is the final source because its 

 heat has evaporated the water, which has risen as clouds, fallen 

 on the hills as rain, and, flowing down again to the sea, turns the 

 water wheels. To be sure there is no oxidation involved in this 

 process, but it shows how the sun, either by its light or its heat, 

 is the source of all our energy, both living and mechanical. 



Circles in Nature. It might seem, since food is oxidized or fuel 

 is burned to release its energy, that the supply would be exhausted 

 and all life come to an end. Nature, however, works in circles, 

 reclaims all waste, and aided by the sun, recombines them into 

 useful compounds again. 



The Carbon Circle. Carbon is one of the most necessary ele- 

 ments for all living things. Animals obtain it from plants and 

 plants get it from the carbon dioxide of the air. Plants take this 

 carbon dioxide from the air, combine it with water from the soil, 

 and lock up within the starch which is formed the energy of the 

 sun which formed it. 



