Dr. Hugo Mohl, on Liebig s Organic Chemistry. 85 



sistency of Liebig, if in all other parts of his work the decomposition 

 of carbonic acid is considered as a self-evident fact (p. 121); and 

 leaves are said to possess powers of decomposition, stronger than 

 that of the most powerful chemical agencies, because they can de- 

 compose carbonic acid, which resists the strongest galvanic battery ! 

 L. states further, that the formation of acids, of ethereal oils, (having 

 no oxygen,) and of caoutchouc, may be considered as combinations of 

 carbonic acid with water ; all, or the greater part, of the oxygen 

 having been eliminated. This may be true in a chemical point of 

 view, but it remains to be proved that these combinations are really 

 formed by water and carbonic acid, and are not the result of other 

 organic combinations. But if the latter be the case, — if ethereal oils 

 are formed by the mutual combination of organic substances, — if 

 they exhibit certain determined stages of vegetable metamorphosis, 

 then the decomposition of water and carbonic acid cannot be taken 

 into account, because these do not exist as such in organic combina- 

 tions ; V and then" (concludes Dr. M.) " the process to which the 

 above substances owe their origin is a far different one, and the 

 explanation of Liebig is anything but a formula explaining their 

 origin," but is quite as erroneous as the assertion would be that 

 sugar consists of carbonic acid and spirits of wine. 



In the fourth chapter, " On the Origin and the Assimilation of 

 Nitrogen," L. starts from the correct assertion, that even in a soil 

 richest in humns, no vegetation can take place without the co-opera- 

 tion of some nitrogenous substances ; and that (as it has been proved 

 by Boussingault) their rlitorgen is derived from the atmosphere. But 

 L. subsequently rejects the opinion (p. 65) that plants assimilate 

 the nitrogen of the air in a direct manner, and derive their nitrogen 

 from the ammonia contained in the rain-water (a discovery made by 

 himself), adducing in proof that nitrogen is conveyed to plants in 

 the form of ammonia, the analysis of the sap of Acorns, Birches, &c, 

 in which ammoniacal salts have been found. This idea is certainly the 

 most valuable in the whole of L.'s work. But here, as elsewhere, he 

 has been satisfied with generalities, without looking to details of 

 great importance in vegetable physiology. Considering the am- 

 monia of rain-water sufficient for explaining the amount of nitrogen 

 contained in plants, he has entirely neglected the nitric salts, and 



