212 CONSERVATIVE ORGANS. [LECT. V. 



cultivated on it ; and thus that poisonous plants 

 extract something, on which their hurtful proper- 

 ties depend, which is not taken up by wholesome 

 plants ; or, that the secretions of plants do not 

 vary in their qualities on account of the difference 

 in the action of the vessels which secrete them ; 

 but owing to their components being already pre- 

 sent in the soil. That this is not the case, how- 

 ever, has been clearly proved by the discoveries 

 of modern chymistry, which have enabled us to 

 analyse both the soil and the vegetables that grow 

 upon it. By its assistance the mode of investigat- 

 ing the subject has been simplified, and more sa- 

 tisfactory results obtained. 



The ultimate components of all the various 

 substances produced by vegetables have been found 

 the same, differing only in the quantity and the 

 mode of their combination ; and the parts of 

 a soil which supply these have been found to be 

 much fewer than was previously supposed. As we 

 formerly asserted, when noticing the nature of 

 sap, if this juice could be obtained near enough 

 to the extremities of roots, or in the fibrils by 

 which the soluble part of the soil is absorbed, 

 then we should be able, by a careful analysis, to 

 ascertain the real nature of the substances ab- 

 sorbed; and, by looking for these in soils, know 

 how to supply their deficiency, or to diminish their 

 superabundance. But as an accurate knowledge 



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