FORCE. 379 



earth; but we can wind up the weight and prepare it 

 for another fall, and so these atoms can be wound up 

 and thus enabled to repeat the process of combination. 

 In the building of plants carbonic acid is the material 

 from which the carbon of the plant is derived; and 

 the solar beam is the agent which tears the atoms 

 asunder, setting the oxygen free, and allowing the 

 carbon to aggregate in woody fibre. Let the solar rays 

 fall upon a surface of sand; the sand is heated, and 

 finally radiates away as much heat as it receives; let 

 the same beams fall upon a forest, the quantity of heat 

 given back is less than the forest receives; for the 

 energy of a portion of the sunbeams is invested in 

 building the trees. Without the sun the reduction of 

 the carbonic acid cannot be effected, and an amount of 

 sunlight is consumed exactly equivalent to the molec- 

 ular work done. Thus trees are formed; thus the 

 cotton on which Mr. Bazley discoursed last Friday is 

 produced. I ignite this cotton, and it flames; the oxy- 

 gen again unites with the carbon; but an amount of 

 heat equal to that produced by its combustion was 

 sacrificed by the sun to form that bit of cotton. 



We cannot, however, stop at vegetable life, for it 

 is the source, mediate or immediate, of all animal life. 

 The sun severs the carbon from its oxygen and builds 

 the vegetable; the animal consumes the vegetable 

 thus formed, a reunion of the severed elements takes 

 place, producing animal heat. The process of building 

 a vegetable is one of winding up; the process of build- 

 ing an animal is one of running down. The warmth 

 of our bodies, and every mechanical energy which we 

 exert, trace their lineage directly to the sun. The 

 fight of a pair of pugilists, the motion of an army, or 

 the lifting of his own body by an Alpine climber up 

 a mountain slope, are all cases of mechanical energy 



