Dr. Hugo Moldy on Liebig' s Organic Chemistry. 95 



This theory, says Dr. Mohl, is not only destitute of all reasonable 

 foundation, but is directly contradicted by the experience of Rota- 

 tion. There is no known evidence in proof of the existence of such 

 excrementitious matter. It is true, Liebig says, that such must be 

 the case, but then he adduces no proof except an ambiguous analogy 

 with the animal kingdom, and forgetting, as he so often does, 

 what he said (page 24,) " that analogy is the parent of that unfor- 

 tunate comparison between vegetable and animal functions which 

 places both on the bed of Procrustes, and is the cause of all error." 

 rt There is not," concludes Dr. Mohl, " the least necessity for assum- 

 ing a secretion from roots. If substances formed by vital processes 

 are of no further use to a plant, they are excreted in the form of gas 

 through the leaves, or deposited in the form of secretion in the glands 

 and other organs, or thrown off with decaying leaves." This theory 

 is, moreover, at variance with the experience of what takes place in 

 the shifting of crops. According to Liebig's views, the excrementi- 

 tious matter of the second class above mentioned would not only 

 injure the plants whence it is derived, but could not be assimilated 

 by any others before it is transformed into humus. But experience 

 points quite another way, because the stubble of Clover, Lucerne, or 

 Saintfoin, which is unfit for the growth of those species, will at once 

 produce excellent crops of other plants. If Liebig should attempt to 

 meet this objection by saying that such excrementitious matter can- 

 not be assimilated by the plants, whence they are derived, but may 

 be used by others, he will upset his whole doctrine of vegetable 

 nutrition, according to which not only all the organic compounds 

 which remain behind after the formation of starch, sugar, &c, but 

 even starch and sugar themselves (and thus all the organic sub- 

 stances of plants,) are absolutely deleterious to other plants. It is 

 impossible, therefore, not to arrive at conclusions entirely opposite to 

 those of Liebig, especially if we consider the phenomena of rotation 

 at greater length, The barrenness of soil for the growth of one kind 

 of plant, whilst it is still fertile for others, can only depend (says Dr. 

 Mohl) on two causes. The first generation of plants may exhaust the 

 soil of such substances as are indispensable to growth, so that the 

 second generation will be starved ; and this certainly takes place : but 

 it cannot be the main cause of the failure of crops, else manure would 



