74 



is, in the same atmosphere ? The cactus 

 is quoted by Liebig, Richard, &c. as thriv- 

 ing on the driest soils, by deriving its 

 nourishment from the air, independent of 

 its root. Yet Lucas informs us, that " a 

 cactus planted in a mixture of equal parts 

 of charcoal and earth throve progressively, 

 and attained double its former size in the 

 space of a few weeks." This must have 

 been from the soil, not from the atmo- 

 sphere. 



Liebig supposes plants to assimilate 

 their nitrogen by decomposing ammonia, 

 stored in soils from rain water, manure 

 and humus, and giving off the hydrogen ; 

 their hydrogen by decomposing water and 

 giving off the oxygen. Carbon, nitrogen, 

 hydrogen, and oxygen, with certain pecu- 

 liar inorganic matters absorbed from the 

 soil, are the sole constituents of plants. 

 Indeed all organic existences, that is, 

 the endless varieties of the animal and 

 vegetable kingdoms, are composed of 



