Matter and Energy. 17 



shall have to consider later on, to build up a complex 

 substance known as grape-sugar. Grape-sugar is a store 

 of potential energy, for by the expenditure of kinetic 

 energy in its formation, the atoms of carbon, hydrogen, 

 and oxygen have obtained potential energy, because their 

 chemical affinities are only partially satisfied (p. 20). It is 

 found that in a molecule of grape-sugar there are six atoms 

 of carbon, twelve of hydrogen, and six of oxygen, which 

 chemists tell us are probably related to one another in the 

 following manner : 



H H H H 



till 



H-C-C-C-C-C-C-H 



I ! I I I I 



o o o o o 



1 I I I I I 



H H H H H H 



the atoms of carbon being linked to each other, and to 

 the hydrogen atoms. But the affinity of the carbon and 

 hydrogen for oxygen is far greater than that of carbon for 

 carbon, or carbon for hydrogen ; therefore, when grape- 

 sugar is burnt or decomposed in the living body, every 

 atom of carbon and of hydrogen immediately quits its 

 former connection and links itself firmly to oxygen, produc- 

 ing in consequence a series of molecules of much simpler 

 composition, known as carbonic acid and water, the excess 

 of oxygen required being obtained from the atmosphere. 

 The potential energy of the grape-sugar becomes transformed 

 mainly into heat 



B. DIRECTLY AVAILABLE SOURCES OF ENERGY 

 (KINETIC). 



From a purely biological point of view these sources are 

 not of very great importance. 



i. Tides. Tides are sources of potential energy four 

 times in the twenty-four hours, becoming kinetic at ebb and 



c 



