LECT. V.J THE ROOT. SOILS. 231 



these two elements can be obtained from the de- 

 composition of this fluid by the powers of the 

 plant; but, without the presence of any soil, 

 whence is the carbon, which is required for the 

 formation of the additional vegetable matter, ob- 

 tained ? It has been suggested that the air is ca- 

 pable of supplying this, and that the quantity of 

 carbonic acid gas it contains, although small, is 

 never wanting. It is, however, possible that it 

 may have been afforded by the water ; for although 

 it was distilled, yet, as Sir H. Davy's experiments 

 have proved, distilled water may hold many sub- 

 stances in solution, and these never can be com- 

 pletely separated from it. But, if we even allow 

 that water and air are the only sources from 

 which the vegetable matter thus gained could be 

 derived, we also know that many plants cannot 

 be supported in this manner, and yet a direct 

 supply of nutritious matter is indispensable to 

 their growth and existence. Every farmer knows 

 the fact, that many plants will grow only in cer- 

 tain soils ; and his art consists in supplying to the 

 natural soils that part which is most essentially 

 necessary for their support. As we have proved 

 that the components of all vegetable matter are 

 carbon, hydrogen, and oxygen, we must look for 

 the supply of these ingredients in the soil: and 

 it is from water and decaying organic matter 

 that they are undoubtedly obtained. From this 



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