Liehigs Organic Chemistry applied to Agriculture. 55S 



of plants from the air, decomposed into its constituents, and after the 

 carbon has been fixed in the plant, the oxygen is emitted." This 

 question, however, cannot be decided in so hasty a way, if it were 

 only because the answer expresses much more than the ques- 

 tion implies. The question. What becomes of the carbonic acid } 

 and the answer. It remains fixed in the plants, have nothing to 

 do with the other questions, by what organ is carbonic acid in- 

 troduced into a plant } and is it there decomposed, or only fixed } 

 That carbonic acid is the matter from which the carbon of plants is 

 derived, is a fact which has been stated long ago, and which, up 

 to this day, has been asserted as true, by a great number of phy- 

 siologists. That under certain circumstances plants absorb carbonic 

 acid, and emit oxygen, by means of their leaves, is likewise a fact, 

 which has been acknowledged since the times of Senebier, Priestley, 

 and Saussure. But that it is certain, as Dr. Liebig thinks, that 

 carbonic acid is dissolved in the leaves of plants, has not in any way 

 been proved, and he himself thinks it very improbable in another 

 part of his book. Lastly, that the leaves absorb all the carbonic 

 acid which is required for the maintenance and growth of the plants 

 from the atmosphere; that the plants, when they are perfectly 

 formed, are not in need of the carbonic acid of the soil ; and that 

 want of moisture and complete dryness of the soil do not impede the 

 completion of their development (p. 46) ; — all these propositions are 

 mere fictions, and have evidently been written without consideration. 

 For common experience shews, that plants must die if the soil loses 

 its moisture entirely, and thus refutes the statement of the author in 

 a manner which cannot be questioned. In conceiving this unfound- 

 ed theory, he evidently has been influenced by a solitary instance 

 mentioned in his Appendix (p. 181.) I do not call in doubt the cre- 

 dibility of Mr. W. Macnab, though many important difficulties have 

 risen in my mind on reading his account ; but I must observe, 

 that Ficus australis is a plant whose roots grow in the air, and that 

 it appears to me more than probable that such plants are, more than 

 others, possessed of qualities which enable them to condense the 

 moisture of the atmosphere. But even if this fact is admitted in 

 all its force, it proves nothing more, than that Ficus australis con- 

 stitutes an exception to this general rule. Experiments by which 



