288 CHEMICAL CAUSE 



Liebig states that, though this change takes 

 place in nature during the vital action of plants, 

 it is nevertheless a purely chemical process. It 

 occurs equally in the dead as in the living plant : 

 in the dark as on exposure to solar light. * "The 

 green resinous principle of the leaf," says this able 

 chemist, " diminishes in quantity while oxygen is 

 absorbed, when fruits are ripened in the dark : red 

 yellow colouring matters are formed; tartaric, 

 citric, and tannic acids disappear, and are replaced 

 by sugar, amylin or starch, or gum."f Again — 

 " When the leaves of the poplar, the beech, the oak, 

 or the holly, are dried under the air-pump, with 

 exclusion of light, then moistened with water, and 

 placed under a glass globe filled with oxygen, they 

 are found to absorb that gas in proportion as they 

 change colour. The chemical nature of this process 

 is thus established. The oxygen unites with the 

 substances already in the state of oxides, or to the 

 oxidation of the hydrogen in those vegetable com- 

 pounds which contain it in excess. The fallen 

 brown or yellow leaves of the oak contain no longer 

 tannin, and those of the poplar no balsamic consti- 

 tuents." J 



Here, then, it appears to me, that the true 



* Liebig's Chemistry of Agriculture, translation, 2d edit, 

 pp. 30, 31. 



f lb. p. 67. t lb. p. 33. 



