ON FORCE 399 



river in the world as it rolls toward the ocean is drawn 

 from the heat of the sun. No streamlet glides to a lower 

 level without having been first lifted to the elevation from 

 which it springs by the power of the sun. The energy 

 of winds is also due entirely to the same power. 



But there is still another work which the sun performs, 

 and its connection with which is not so obvious. Trees 

 and vegetables grow upon the earth, and when burned 

 they give rise to heat, and hence to mechanical energy. 

 Whence is this power derived? You see this oxide of 

 iron, produced by the falling together of the atoms of iron 

 and oxygen; you cannot see this transparent carbonic acid 

 gas, formed by the falling together of carbon and oxygen. 

 The atoms thus in close union resemble our lead weight 

 while resting on the 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 de- 

 rived; 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 can- 

 not be effected, and an amount of sunlight is consumed 

 exactly equivalent to the molecular work done. / Thus 

 trees are formed; thus the cotton on which Mr. Bazley 

 discoursed last Friday is produced. I ignite this cotton, 



