Dr. Hugo Mohl, on Liebig s Organic Chemistry. 93 



to make plants absorb organic substances, and to observe the pheno- 

 mena which their growth exhibits subsequently. It has been shown 

 already that certain plants live only when they absorb organic sub- 

 stances ; our cultivated plants prove the same thing, as well as the 

 detailed experiments of Davy and Saussure, which have not been 

 hitherto disproved. The theory of Liebig, therefore, is untenable, and 

 is equally unsupported by experiment, or by exact reasoning on the 

 nutritive processes of plants. 



Liebig has himself felt that arguments founded on fact are requir- 

 ed for the support of his theory, and he has pitched upon Mount 

 Vesuvius to supply it. He dwells, therefore (page 131), on the lux- 

 uriant crops in the environs of this mountain, in a " soil, which, ac- 

 cording to its origin, does not contain the least trace of organic mat- 

 ter, and still is considered as the very type of fertility." We pos- 

 sess, however, no chemical analysis of this soil, derived as it is from 

 volcanic cinders, neither does L. say anything on the subject, but 

 merely appeals to its general volcanic origin. But it would be a 

 surprising thing (says Dr. Mohl), if a soil, on which, for many cen- 

 turies past, most extensive farming operations (Dreisch-wirthschaft ?) 

 have been carried on, should be destitute of humus. This could be 

 only the case, if, after every few years, it was again covered so deep- 

 ly with the ashes of constantly new eruptions, that the ancient soil 

 and all its organic matter should be placed beyond the reach of new 

 crops. This, however, we know is not the case, and even such 

 eruptions as do take place at intervals could not effect this. The 

 heaviest fall of ashes (since the destruction of Pompeii) was that of 

 1822, which amounted on the slope of the mountain to 3 inches, and 

 in the plain from 15 to 18 inches; but this was (according to 

 Humboldt) the treble of any previous fall of ashes. We know, 

 moreover, that even the slightest fall of volcanic cinders kills vegeta- 

 tion over an extensive area, so much so that one crop amongst eight 

 near Mount Vesuvius is always lost through such calamities. It is, 

 therefore, these very falls of ashes which cause the formation of a 

 vegetable stratum on a large scale, and such must contain humus. 

 This corresponds entirely with what Mr. Lyell states (" Princ, of 

 Geol." ii., 148), that he found, near Pompeii, under the volcanic cin- 

 ders of 1 822, a layer of vegetable mould of the thickness of three inches. 



