194 ANIMALS 



the carbohydrates and fats. We also know that these sub- 

 stances may be readily made to yield energy by heating them 

 in the presence of oxygen. The carbohydrates are then decom- 

 posed and their constituents unite with oxygen to form carbon- 

 dioxide and water. This union produces heat, which is a form 

 of energy. 



448. The wood in the firebox of an engine is a case where 

 C6Hio06+602 = 6C02 + 5H20. The heat liberated in this 

 process is taken up by the water, which, because of the added 

 energy, expands into stem. The energy of the enclosed steam 

 is evident in the pressure which it exerts, and by means of the 

 mechanism of the engine, the pressure energy is transformed 

 into the motion of piston, crankshaft and wheels. Thus the 

 oxidation of the fuel releases the energy which causes the 

 engine to move and do work. Fats and oils, 0^11^(0) when 

 burned yield CO2 and H2O and energy is also set free in the 

 same way. 



449. That in these processes it is a question of liberation 

 rather than the creation of energy becomes clear if we consider 

 the origin of the substances. The carbohydrates, we will recall, 

 were formed in the leaf from CO2 and H2O, through the agency 

 of chlorophyll and light. But light is a form of energy whose 

 source is the sun. In some way the energy of the ether vibra- 

 tions breaks up the CO2 molecule and the carbon atom unites 

 with the H2O molecule, producing, as a final result, starch 

 (Ce Hio Os), sugar, cellulose, oil, proteid or other carbon com- 

 pound. These substances remain stable at ordinary tempera- 

 ture, but when slightly heated they break down, and the C then 

 unites with the and the energy which was stored up in photo- 

 synthesis is again set free. In physical terms, the energy of 

 the Ught becomes potential energy in the chemical compound, 

 and again kinetic energy of heat in combustion. 



450. The processes by which the food yields energy to the 

 animal are closely analogous to the case of the fuel in the 



