Agricultural and Horticultural Chemistry. 323 



earth, through which the roots of such a tree spread, never 

 contains carbon enough to supply the very large quantity 

 of that element required by the tree. They therefore con- 

 ceived that the carbon was derived from carbonic acid; 

 partly from that existing in the air, and partly from gas 

 generated in the soil by the gradual oxidation of carbona- 

 ceous matters. More recently a theory was started, which 

 derived considerable importance from the names of the 

 eminent chemists who supported it; it was supposed that 

 the brown carbonaceous matters of the soil were the prin- 

 cipal source of the carbon of plants, and that these sub- 

 stances, being rendered soluble in water by the presence of 

 alkaline and other inorganic compounds in the soil, were 

 thus brought into a fit state to be absorbed by the roots of 

 plants. These two theories have been well contrasted to- 

 gether by Liebig, who has shown the many great objections 

 to the latter view, and very completely reestablished the 

 old theory, that plants derive their carbon from the decom- 

 position of carbonic acid gas, and not from the absorption 

 of solid carbonaceous matters existing in the soil. 



"The chief use. then, of the various organic substances 

 added to land, as far as they supply carbon, consists in 

 their furnishing a continual source of carbonic acid, a gas 

 which is evolved by all organic substances whilst undergo- 

 ing decay. 



" The importance of nitrogen in the growth of plants 

 has in former times been greatly overlooked, and by some 

 observers entirely forgotten ; it is true that Priestley and 

 his contemporaries thought it probable that this gas must 

 have some influence on vegetation, but their experiments 

 were imperfect, and the conclusion to which they appear 

 to have arrived at was, that nitrogen was of very little 

 importance compared to carbon and oxygen. More re- 

 cently, as chemistry avdanced, the assimilation of nitro- 

 gen began to be thought more necessary to the growth of 

 plants, but still very indefinite ideas were entertained re- 

 specting the mode in which it was absorbed ; and although 

 the agricultural cbomists, at the commencement of this 

 century, were aware that nitrates and salts of ammonia 

 had strong influence on the growth of plants, and consid- 

 ered that they were valuable chiefly as sources of nitrogen, 

 yet they did not take into consideration the fact, that nitro- 

 gen can only be assimilated in a state of combination, and 



