1 8 THE LIVING WORLD. 



the energy thus within their reach, they are able to 

 build complicated chemical substances out of such 

 simple compounds as water, carbonic acid, and simple 



^nitrogenous compounds. It is a well-known principle 

 of physics that to build a complicated chemical body 

 out of simple ones requires the exertion of energy, 

 just as it does to place a number of bricks one on top 



^of another. All of the energy thus used is rendered 

 latent, but it can all be obtained again in active con- 

 dition by pulling the structure to pieces. Every com- 

 plex chemical compound may therefore be looked 

 upon as a store of energy. Plants, then, growing in 



/The sunlight are continually making use of the sun's 

 rays to enable them to build up complex compounds, 

 and they are therefore storing up the sun's energy 

 in the form of chemical energy. The energy of their 

 life therefore, consists in transformed sunlight. Now 

 animals use as food the chemical compounds thus 

 built by plants. Animals, unlike plants, are not able 

 to make use of the sun's rays directly, but they can 

 make use of the store of energy provided by the 

 plants. They therefore derive all the energy of their 

 life by breaking to pieces these products of the 

 plant's constructive power. Just as the steam-engine, 

 by breaking to pieces the coal which forms its fuel, 

 makes use of the energy thus liberated, so the body 

 by similarly breaking to pieces its food makes use of 



ithe energy thus liberated. The steam-engine con- 

 erts the energy of chemical composition contained 

 in its coal into motion and heat ; the body also con. 

 verts the energy of chemical composition contained 

 in its foods into motion and heat. All of this is 



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