78 Dr. Hugo Mohl t on Liebig' s Organic Chemistry, 



write, nor has fully considered them. The manner in which Liebig 

 attributes erroneous views, entertained perhaps by individual bota- 

 nists, to " vegetable physiologists" and " botanists" in general, is 

 objectionable and liable to mislead. Thus he says (p. 6) that " ve- 

 getable physiologists" consider humus as the principal food of plants. 

 Now this is not true : vegetable physiologists have no sacred books 

 in which their code of laws is contained, and if any individuals have 

 maintained such a view, the great body has not. In fact, Ingen- 

 housz, Senebier, Curt Sprengel, Link, and De Candolle, have all 

 either denied it or taken other views. The doctrine of humus is 

 altogether a chemical one, and has only been supported by chemists. 

 Again, Liebig says (p. 24) that " all botanists and vegetable physi- 

 ologists have doubted the assimilation of the carbon of the atmos- 

 phere by plants." Yet all books on vegetable physiology contradict 

 such a statement ; and the absorption of carbonic acid from the at- 

 mosphere is so generally admitted that Adolphe Brongniart, in the 

 13th volume of the " Annales des Sciences," has even proposed to 

 account for the excessive vegetation of the primitive world upon the 

 supposition, that the atmosphere at the period those plants were 

 growing contained a larger amount of carbonic acid in its composi- 

 tion than it now does. This might have been considered misrepre- 

 sentation, had not Liebig in many other instances displayed an equal 

 amount of ignorance of botanical literature and facts. As, for ex- 

 ample, when he says (p. 91) that the woody fibre of lichens may be 

 replaced by oxalate of lime, and that in Equisetum and the Bamboo 

 silica assumes the form and functions of the woody bundles, and 

 (p. 36) that a leaf secreting oil of lemons or oil of turpentine has a 

 different structure from one secreting oxalic acid. 



An instance of Liebig's misrepresentation of facts occurs in his 

 rejecting the theory of the respiration of plants. It is well known 

 that plants absorb oxygen in the dark, and give out carbonic acid ; and 

 this has been attributed by botanists to a true process of respiration. 

 This, Liebig thinks, betrays great ignorance on the part of botanists. 

 He believes the giving out of the carbonic acid to be merely a me- 

 chanical process, and the absorption of oxygen to be a chemical one. 

 He says all leaves, dead or living, absorb oxygen, and the more oil 

 or tannic acid they possess, the more oxygen they absorb. He en- 



