THE BIOLOGY OF DAILY LIFE. 13 



Thus the grand " Law of Interchange " is justified 

 in its followers. 



For man in maturity and health that food is best 



influences of the sunbeam, is one of the great mysteries of 

 Nature. We believe that the effect is in some way connected 

 with the molecular structure of matter ; but our theories are, as 

 yet, unable to cope with the subject. That the power comes 

 from the sun, we know ; and, moreover, we are able to put our 

 finger on the exact spot where the mysterious action takes place, 

 and where the energy is stored ; and that spot, singular as it 

 may appear, is the delicate leaf of a plant. 



" This same carbonic dioxide, on which we are experimenting, 

 is the food of the plant, and indeed the chief article of its diet. 

 The plant absorbs the gas from the air, into which it is con- 

 stantly being poured from our chimneys and lungs, and the sun's 

 rays acting upon the green parts of the leaf, decompose it. The 

 oxygen it contains is restored to the atmosphere, while the carbon 

 remains in the leaf to form the structure of the growing plant. 

 This change may be represented thus : 



C O 2 = C + O=O 

 Carbonic Dioxide. Carbon. Oxygen. 



"Now to tear apart the oxygen atoms from the carbon, 

 requires the expenditure of a great amount of energy, and that 

 energy remains latent until the wood is burned ; and then, when 

 the carbon atoms again unite with oxygen, the energy reappears 

 undiminished in the heat and light, which radiate from the 

 glowing embers. Just as when a clock is wound up, the energy 

 which is expended in raising the weight reappears when the 

 weight falls ; so the energy, which is expended by the sun in 

 pulling apart the oxygen and carbon atoms, reappears when 

 those atoms again unite. . . . 



"It is one of the greatest achievements of modern science, 

 that it has been able to measure this energy in the terms of our 

 common mechanical unit, the foot-pound ; and we know that 

 the energy exerted by the sun, and rendered latent in each 

 pound of carbon, which is laid away in the growing wood, would 

 be adequate to raise a weight of five thousand tons one foot." 

 " The New Chemistry" (Internat Scientific Series, vol. ix., p. 156). 



