466 



plant. In some cases, however, one kind may be substituted for an- 

 other. 



The author discusses at large the doctrine of humus, humin, ulmin, 

 humic acid, apotheme, geine, all referring to one substance, as the food 

 of plants. This matter is generally understood to be a certain brown 

 or cai'bonaceous substance resulting from vegetable decomposition. 

 Some portions of it are soluble in water or alkalies ; other portions are 

 insoluble but by extraoi'dinary means. The common opinion has been 

 that it constitutes directly the food of plants, and requires only to be 

 dissolved to be taken up by the roots of the plants and assimilated by 

 them. Others have maintained that it requires to be dissolved by the 

 application of alkalies, and combining with them in the form of an acid, 

 it becomes then prepared for the food of plants. Our author wholly 

 denies these positions by showing that so far from humus being ex- 

 tracted from the soil, it is in fact increased by cultivation, as in the case 

 of a forest, the more abundant the growth of wood upon it, the greater 

 the amount of humus in the soil, where the debris of the wood is suf- 

 fered to remain upon the land, 



" A certain quantity of carbon is taken every year from the forest 

 or meadow in the form of wood or hay ; and, in spite of this, the quan- 

 tity of carbon in the soil augments; it becomes I'icher in humus." — p. 

 68. 



" The opinion that the substance called humus is extracted from the 

 soil by the roots of plants, and that the carbon entering into its compo- 

 sition serves in some form or other to nourish their tissues, is so general 

 and so firmly established, that hitherto any new argument in its favor 

 has been considered unnecessary ; the obvious difference in the growth 

 of plants according to the known abundance or scarcity of humus in 

 the soil, seemed to alford incontestable proof of its correctness. Yet 

 this position, when admitted to a strict examination, is found to be un- 

 tenable ; and it becomes evident that humus in the form in which it ex- 

 ists in the soil does not yield the smallest nourishment to plants." — p. 

 61. 



He attempts to prove his position, that the carbon of the plant can- 

 not be derived from the soil, by a calculation in weights and measures. 

 Humic acid, or the humus of the soil, can only be absorbed by the 

 plant in combination with some inorganic bases or metallic oxide. We 

 do not think it important here to give any thing inore than the results 

 of some of his calculations. He supposes that upon an average 40,000 



