n. III. OR POISONED BY VEGETABLE GROWTH? 137 



ration of carbonic acid from the humus in the soil ; and 

 Priestley and Senebier have shown that from carbonic 

 acid plants assimilate their carbon (which, loosely 

 speaking, the whole of the tree may be said to consist 

 of), by decomposiug the carbonic acid, and giving off 

 the oxygen. I should consider this as the cause why 

 roots keep within the reach of atmospheric aeration, 

 since the main article of the food of trees is found in 

 that district ; though Liebig, and a host of modern 

 physiologists, follow Priestley, Senebier, and De Saus- 

 sure in thinking that after the first infancy of the 

 plant, that is, after the development of leaves, it is 

 indebted to the atmosphere only for the supply of car- 

 bonic acid. But can we doubt that the chief growth of 

 plants is from constituents absorbed from the soil, not 

 from the atmosphere, when we see the perpetual 

 difference of growth of the same plants in the different 

 soils of the same parish ; that is, in the same at- 

 mosphere ? 



Liebig supposes plants to assimilate their nitrogen 

 by decomposing ammonia, stored in soils from rain 

 water, manure, and humus, and giving off the hydro- 

 gen ; their hydrogen, by decomposing water and giving 

 off the oxygen. Carbon, nitrogen, hydrogen, and 

 oxygen, with certain peculiar inorganic or incombus- 

 tible matters, are the sole constituents of plants. In- 

 deed, all organic existences, that is, the endless varieties 

 of the animal and vegetable kingdoms, are composed of 

 these four elements alone. These four elements are 



