CHAPTER XVI 



Potash and Phosphorus 



If we burn any plant the first effect of the 

 heat is to show the carbon of which it was 

 composed, in combination with other sub- 

 stances. As the combustion continues the 

 carbon is absorbed in the air as carbonic acid 

 gas, and an earthy matter remains which we 

 call ash. There are then two substances, 

 carbon and ash, which form part of every 

 plant, without exception. The plant has not 

 produced them independently : it did not 

 derive them from nothing ; for nothing can 

 come from nothing. Therefore, it must have 

 received them from some source. We know 

 the origin of the carbon. The greater part of 

 it comes from the atmosphere, whence the 

 leaves draw the carbonic acid gas, decom- 

 posing it in the sunlight, retaining the carbon 

 and rejecting the purified air. Thus] the 

 vegetation of the whole world finds its chief 

 food in the atmosphere — a store which is 



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