The Kinds of Work That Are Done by Plants 93 



none of either is ever created anew or obliterated, though they 

 may change their forms multifariously. Where, then, and in 

 what form was the energy in food before it was released by respir- 

 ation? The answer is easy, though its comprehension is not. 

 It was where the energy was in the coal before it was released 

 as heat in combustion: where the energy was in the storage bat- 

 tery before it turned the wheels of the electric automobile : where 

 the energy was in the coiled spring or the wound-up weight of 

 the clock before it turned the wheels to move the hands: where 

 the energy was in the full millpond before it drove the looms of the 

 water-power mill : where the energy was in the gunpowder before 

 it started the flying bullet. The fact of the matter is this, that 

 energy exists in Nature in two different forms, not only in the 

 familiar active or kinetic form which produces motion and does 

 work, but also in a resting, latent, or potential form, when its 

 power to produce motion is held in suspension. Whenever, in 

 Nature, kinetic energy is exerted to force apart bodies whose 

 attractions, whether through gravitation, magnetism, cohesion, 

 or chemical affinity, tend to bring them together, the energy goes 

 into the potential form for so long as those bodies are kept apart, 

 and it becomes again manifest in kinetic form when the bodies 

 are allowed to re-unite. All unsatisfied attractions in Nature are 

 latent energy. When a small boy draws back the powerful elastic 

 of his favorite sling-shot, he is exerting kinetic energy against 

 the tension of the elastic; while he holds the elastic stretched to 

 take aim, that energy is latent as energy of tension; and when he 

 lets go of the string the energy becomes kinetic again as it drives 

 the stone in delightful swiftness of flight. So, kinetic energy can 

 raise a weight, go into the latent form as energy of position while 

 it is suspended, and come out again in kinetic form, as it does 

 when it turns the wheels of an old-fashioned clock. Kinetic 

 energy can charge a storage battery, become latent for a time, 

 and come out once more as kinetic energy driving an electric 

 automobile. The storage battery, indeed, is typical of all cases 



