Liebig's Organic Chemistry applied to Agriculture. 549 



as an historical fact that which is not true. The humus of the soil 

 has indeed been identified with artificial humus, and has been con- 

 sidered as the principal source from which plants receive nourish- 

 ment. This, however, has not been done originally by the vegetable 

 physiologists, but by chemists ; as is evident from the circum- 

 stance that Dr. Liebig, in his short view of the theory of humus, 

 does not produce the name of one physiologist, but only those of 

 chemists. Further, Dr. Liebig is of opinion, that it can be shown 

 by the strictest proof, that humus, in that form in which it exists in 

 the soil, does not, in the slightest way, contribute to the nourishment 

 of plants. According to my view of the matter, we are not yet so 

 far advanced in our researches as to be authorised to make such an 

 assertion. It is easy to show that Dr. Liebig has not succeeded in 

 establishing what he promised ; but, instead of it, he has adduced a 

 fact which perhaps may give a slight probability to this view of the 

 matter. His first observation (p. 9), is, that the cold of winter and 

 the heat of summer deprive the humic acid of solubility in water ; 

 which is quite in accordance with the old experience, that drought 

 in summer and a high degree of cold without snow in winter, consi- 

 derably diminish the fertility of soil. This observation, therefore, 

 may rather be considered as supporting than as refuting the theory 

 of humus. The observation which follows, that cold water deprives 

 the good mould only of the salts of the rain water and that it 

 remains colourless, is not true, according to the experiments of 

 Berzelius (viii. 386). I myself always have obtained from good 

 mould a yellowish extract, which indeed differed considerably accord- 

 ing to the difference of the earthy matter, but which always con- 

 tained a considerable admixture of brown organic matter. By 

 these two observations, which evidently prove nothing. Dr. Liebig 

 thinks he has succeeded in completely disposing of humic acid 

 (p. 10), and passes to humic salts. Here he offers us some calcula- 

 tions by which he intends to prove that they are equally incapable 

 of supplying a plant with the amount of carbonic acid required for its 

 nourishment. All calculations beginning with such phrases as, 

 "Let us suppose," "Let us put aside for the present," &c., are 

 usually valueless to science ; and if they are made without the least 

 regard to essential points, and are based on arbitrary suppositions, as 

 those of Dr. Liebig, they are perfectly childish. His first calculation 



