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{hall indicate by what manures each particular fort of foil is 

 brought into a fertile ftate, which is the beneficial effed expeded 

 from them, and how in each particular cafe they contribute to 

 the due adminiftration of the vegetable food which is the caufe 

 of their beneficial effed. 



SECTION I. 



Of the Food of Plants. 



To difcover the food of plants, particularly of thofe which 

 form the objed of our prefent inquiry, we muft examine the 

 nature and proportion of the fubftances in which they grow, 

 and of thofe which they themfelves contain; thus we fhall be 

 enabled to fee which of the latter are derived from the former. 



First, All plants (except the fubaqueous) grow in a mixed 

 earth moiftened with rain and dew, and expofed to the atmof- 

 phere ; if this earth be chemically examined it will be found 

 to confift of filiceous, calcareous and argillaceous particles, often 

 alfo of magnefia, in various proportions, a very confiderable 

 quantity of water, and fome fixed air. The moft fertile alfo 

 contain a fmall proportion of oil, roots of decayed vegetables, 

 a coaly fubflance arifing from putrefadion, fome traces of marine 

 acid and gypfum *." On the other hand, if vegetables be 



T 2 analyzed, 



• Home, 15 Mem. D' Agriculture, Par. 1790. Encycloped. Vegetation, p. 277- 



