86 Dr. Hugo Mohl, on Liebig a Organic Chemistry, 



asserts (without adducing any proof) that Borage, the Chenopodia, 

 and the Jerusalem Artichoke, owe their nitric salts merely to 

 ammonia — an assertion by no means confirmed by the experiments 

 of John. 



In a chapter like that "On the Origin and Assimilation of 

 Nitrogen," it was to be presumed that Liebig would have examined 

 the form under which ammonia is conveyed to the plants. As 

 almost all plants grow in black mould, the relation existing between the 

 atmospheric ammonia and the humus was to be examined — a topic 

 interesting not only in a theoretical but practical point of view. 

 But L. treats the matter very slightly, merely stating (p. 83) that 

 humus stands in the same relation to ammonia as powdered char- 

 coal, viz., condensing the ammonia, but we have seen already that 

 humus can not be supplied by charcoal, and although the latter 

 substance absorbs ammonia even more forcibly than humus, 

 yet plants will not prosper in it. This, therefore, does not 

 corroborate L's assertion. The province of a true chemist, in this 

 case, instead of being satisfied with the trivial fact that rotten wood 

 absorbs ammonia, would have been rather to inquire whether these 

 two substances will combine, and what combinations they will form. 

 C. Sprengel and Mulder have lately asserted that humus and am- 

 monia will form combinations soluble in water ; Saussure also found, 

 in all sorts of humus, a soluble nitrogenous extract, by which he 

 explains the conveyance of nitrogen to plants. Instead of making 

 experiments, and without even taking notice of those of such men as 

 the above, Liebig despatches the whole question with the asser- 

 tion that the humic substances contained in black mould (Damm 

 Ere) are entirely insoluble in water. 



The explanation of the action of gypsum on plants is con- 

 nected by L. with the existence of carbonate of ammonia in the 

 atmosphere. He assumes that gypsum is decomposed by the car- 

 bonate of ammonia of the air ; and he considers it, therefore, a means 

 of fixing ammonia, and conveying it to plants ; and he adds 

 that — " This is obvious from the evident action of gypsum on 

 the growth of grasses, and by the increased luxuriance and fer- 

 tility of meadows manured with gypsum." This explanation (replies 

 Dr. Mohl) is only true in a chemical, but not in a physiological point 



