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



this substance influences the nutritive process of plants, was the pro- 

 per question for Liebig to examine, but which he has neglected 

 to enter into. Saussure has, however, made some experiments on 

 the absorption of humate of potash, and the extract of vegetable 

 mould by the roots of plants (Bibl. Univ., vol. xxxvi. p. 340). Al- 

 though these experiments do not strictly prove that this is really the 

 case, under all circumstances ; still they go so far as to prove, that it 

 will be the case as long as the roots are healthy, and do not, by their 

 putrescence, yield humous substances to the water. L., in a subse- 

 quent refutation of these experiments, ascribes their telling against 

 him to an error of calculation ! In another part of this chapter 

 (p. 109), L. says, that humus, if not properly exposed to the influence 

 of atmospheric air, will form with water a solution of a brown 

 colour ; but that no plants can grow in such soil, for the humus will 

 consume all the oxygen contained in the air. It is (says Dr. Mohl), 

 quite inconceivable how a man can write such things, when the in- 

 spection of any peat moss will show the fallacy of such assertions. 



The second argument which L. brings forth to support his 

 favourite assertion of plants not feeding on humus, is based on 

 physiological grounds. He first announces as a general principle, 

 that, under the appellation of food, such substances, only can be 

 included, as being derived from external sources, can maintain all the 

 vital functions, and which the organs of a plants can use for the 

 formation of the substances peculiar to them. This ambiguous 

 definition includes (says Dr. Mohl) some anomalies, and does not 

 apply to several reputed kinds of food. Starch, for instance, is 

 certainly one of the substances on which man feeds ; yet he could not 

 live on it alone. In farther explantion of his views, Liebig adduces 

 the example of a grain of Wheat, which contains the necessary 

 ingredients of the germ, and the first fibres of roots ; and he adds 

 that we have to suppose that these ingredients are mixed just in 

 the proportion that is required for the development of those organs. 

 If one of the ingredients, say starch or gluten, were superabundant, 

 they would not serve either for the formation of leaves or otherwise 

 Carbon, also, as well as ammonia and water, are always combined 

 in plants with an azotised matter ; and it is for this reason that 

 substances, containing no nitrogen, like gum, sugar, &c, and con- 



