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No soil is fertile that contains as much as 19 parts 

 out of 2O of any of the constituents that have been 

 mentioned. 



It will be asked, are the pure earths in the soil 

 merely active as mechanical or indirect chemical 

 agents, or do they actually afford food to the plant? 

 This is an important question; and not difficult of sol- 

 ution. 



The earths consist, as I have before stated, of 

 metals united to oxygene; and these metals have not 

 been decomposed; there is consequently no reason to 

 suppose that the earths are convertible into the ele- 

 ments of organized compounds, into carbon, hydro- 

 gene, and azote. 



Plants have been made to grow in given quanti- 

 ties of earth. They consume very small portions only; 

 and what is lost may be accounted for by the quantities 

 found in their ashes; that is to say, it has not been 

 converted into any new products. 



The carbonic acid united to lime or magnesia, if 

 any stronger acid happens to be formed in the soil 

 during the fermentation of vegetable matter which will 

 disengage it from the earths, may be decomposed: 

 but the earths themselves cannot be supposed convert- 

 ible into other substances, by any process taking place 

 in the soil. 



In all cases the ashes of plants contain some of 

 the earths of the soil in which they grow; but these 

 earths, as may be seen from the table of the ashes af- 

 forded by different plants given in the last Lecture, 

 never equal more than ?V of the weight of the plant 

 consumed. 



