88 FOOD OF THE VEGETATING PLANT. CHAP. II. 



aliment, and with regard to the augmentation of 

 carbon in the plant. The opinion of Senebier, there- 

 fore, may still be correct. 



Conjee- It must be acknowledged, however, that the subject 

 Thomson. ^ s not 7 et altogether satisfactorily cleared up ; and 

 that carbon may certainly enter the plant in some 

 state different from that, either of charcoal in solution, 

 or of carbonic acid gas. Is not the carbonic acid of 

 the soil decomposed before entering the plant ? This 

 is a conjecture of Dr. Thomson's, founded upon the 

 following facts : The green oxide of iron is capable 

 of decomposing carbonic acid ; and many soils con- 

 tain that oxide. Most soils indeed contain iron, 

 either in the state of the brown or green oxide, and 

 it has been found that oils convert the brown oxide 

 into green.* But dung and rich soils contain a 

 quantity of oily substance. One effect of manures, 

 therefore, may be that of reducing the brown oxide 

 of iron to the green, thus rendering it capable of 

 decomposing carbonic acid gas, so as to prepare it 

 for some new combination, in which it may serve as 

 an aliment for plants. All this, however, is but a 

 conjecture ; and it is more probable that the carbonic 

 acid of the soil enters the root in combination with 

 some other substance, and is afterwards decomposed 

 within the plant itself. 



* Thomson's Chemistry, vol. iv. p. 394-. 



