ENERGY SUPPLY OF THE BODY 



189 



giving heat; and if the two gases are mixed in the right 

 proportions and then ignited, they explode with violence. 

 This energy was derived from the elec- 

 tricity. It was stored by decomposing 

 the water. 



Energy is stored by chemical means 

 by causing it to do work in opposition 

 to the force of chemism, or chemical 

 affinity. Instead of changing the form 

 of bodies or moving them against 

 gravity, it overcomes the force that 

 causes atoms to unite and to hold 

 together after they have united. Since 

 in most cases the atoms on separating 

 from any given combination unite at 

 once to form other combinations, we 

 may say that energy is stored wJien 

 strong chemical combinations are broken 

 ^lp and weak ones formed. Energy decomposing water with 

 stored by this means becomes active 

 when the atoms of weak combinations unite to form com- 

 binations that are strong. 1 



How Plants store the Sun's Energy. The earth's sup- 

 ply of energy comes from the sun. While much of this, 

 after warming and lighting the earth's surface, is lost by 

 radiation, a portion of it is stored up and retained. The 

 sun's energy is stored both through the force of gravity 2 



1 As the atoms of hydrogen and oxygen that make up the molecules of water 

 separate, they unite with atoms of their own kind the hydrogen with hydrogen 

 and the oxygen with oxygen atoms. Since these combinations are weaker than 

 those of the water molecules, energy is required to bring about the change. But 

 when hydrogen burns in the oxygen, the change is from a weaker to a stronger com- 

 bination. The stored-up energy is then given up or becomes active. 



2 In the evaporation of water, the energy of the sun is stored with reference to 

 the force of gravity. In evaporating, w.iter rises as a gas, or vapor, above the earth's 



FIG. 

 energy 

 means. 



82. Storing 

 by chemical 



Apparatus for 



