94 Dr. Hugo Mohl, on Liebig'' s Organic Chemistry. 



In a subsequent part of this chapter (p. 124) Dr. Liebig expresses 

 his surprise, that in all the works of Agronomists and Physiologists, 

 one looks in vain for the leading principles of cultivation ; neverthe- 

 less, at the end of this part of his work, he states that cultivation 

 supplies every plant with that sort of food which it requires for the 

 development of such organs or substances as are most available to 

 man. He further dwells on the means of arriving at that end, viz., 

 the chemical analysis of the inorganic ingredients of soil. But these 

 latter facts, says Dr. Mohl, were known long before Liebig, Charles 

 Sprengel having written a series of memoirs, to demonstrate the 

 importance of the inorganic ingredients of the soil, both for the ge- 

 neral growth of crops, and for that of certain organs in particular. 

 Under this head, Liebig certainly ought to have mentioned the name 

 of Sprengel, and although he has not done so (concludes Dr. M.,) 

 the history of science will amply repay the omission. 



In the last chapter, which is headed " Rotation and Manures," L. 

 opens the difficult question, why several crops of the same plant will 

 not succeed on the same soil in an uninterrupted succession, and 

 why, therefore, farmers resort to rotation. He thinks De Candolle's 

 theory the best explanation of this, forgetting, it seems, that that 

 coarse excrementitious theory has no better foundation than bad and 

 injudicious experiments of Macaire Prinsep, the same man who 

 misled De Candolle on other occasions also. Liebig, however, 

 (says Dr. Mohl,) who has no idea that these experiments are fallaci- 

 ous and controverted by all succeeding ones of the same kind, works 

 out this theory in its most minute details, and proves, a priori (p. 

 149,) that plants must have excrements. He divides the latter into 

 two classes : those ' namely, which have been absorbed by the roots, 

 but not being adapted for the nourishment of plants, are again re- 

 turned to the soil ; and secondly, such substances as having been 

 transformed in the vegetable organism by the process of nutrition, 

 are the result of the formation of starch, woody fibre, gluten, &c. 

 Excrementitious matter of the first class may serve as food for other 

 plants ; nay, they may even be essential for that purpose. Those of 

 the second, however, cannot be used by other plants in the formation 

 of woody fibre, &c. until changed into humus, and decomposed into 

 ammonia, carbonic acid &c. 



