FORMS OF ENERGY 113 



power is present, but that it is not at work. It is energy 

 of this sort that is stored in seeds. In fact, all food con- 

 tains energy of this sort. It is not at work, but it may be 

 set at work. After food has been digested, its energy is 

 soon set at work. All food when it is dry will burn. Every- 

 thing which will burn contains energy. The act of burn- 

 ing releases this latent energy. Heat is a form of energy. 

 We see its power to work exhibited in the steam engine. 

 It is heat which expands the water into steam ; it is this 

 tendency to expand, directly caused by heat, which en- 

 ables the engine to drive machinery. 



Heat and motion, then, are two forms of energy, and one 

 may be transformed into the other. When a steam engine 

 pulls a train, heat is transformed into motion. If ^ the 

 train gets a " hot box," we have a case of motion being 

 transformed into heat. , 



Light is another form of energy. When things burn 

 latent energy is transformed into both light and heat; 

 the energy which has been stored up is released in both of 

 these forms. 



Man by means of his inventions has " harnessed " to 

 do his work heat and motion and that other form of energy 

 we call electricity. But he has not harnessed light to do 

 his work. He produces artificial light by transforming 

 into Ught the energy which is stored in kerosene, or gas, or 

 in an electric current,, but he has no invention for storing 

 up again the energy of light or for transforming light into 

 heat or motion. 



Though man has no such invention, nature has. She long 

 ago established factories in which the energy of light is 

 stored up to serve her many purposes. She proved a far 

 more ingenious inventor than any man. The heat and 



