Observations on Liehig's " Orga7iic Chemistry.''^ 99 



principal requisites ; together with a soil to furnish the inorganic 

 matters, as alkalies or metallic oxides, which are likewise 

 essential. 



On the subject of the Assimilation of Carbon, or the manner 

 in which carbon is added to plants, the views of this writer ai'e 

 difFereat from those of most preceding writers. At p. 19. he goes 

 so far as to say that the carbon of plants is derived exclusively 

 from the atmosphere; though this must be qualified by his sub- 

 sequent statements, when considering the action of humus, &c., 

 where he allows of its being partly taken up by the roots. He 

 says, the idea of its being principally taken up by the roots is 

 chiefly maintained by vegetable physiologists : but the observa- 

 tions of Sir Humphry Davy, in his Agricultural Chemistry, would 

 lead us to believe he was of that opinion. More lately, Dr. 

 Madden, in his valuable papers on the Action of Manures, in- 

 serted last year in the Qimrterly Journal of Agriculturey seems 

 also to imagine that it is principally by the roots. The opinions 

 of Dr. Lindley, and other celebrated physiologists, are not ex- 

 clusively for the absoiption by the roots ; they admit that it is 

 also imbibed by the leaves, but principally by the roots. Amongst 

 these conflicting opinions, it will be well for practical men to 

 have recourse to the work itself for a full account of Dr. Liebig's 

 proofs, which no limited essay can furnish, and carefully weigh 

 them in their minds, and put them to the test of experiment. 



He begins by stating the opinion prevailing at present, that 

 humin, or coal of humus, and humic acid, are the sources of car- 

 bon to the roots. The humic acid is sparingly soluble in alkalies, 

 or by itself, 1 part in 2,500; the humin insoluble. In the state of 

 humin or humus, the form, he says, in which it exists in the 

 soil, it does not yield the smallest nourishment to plants ; and 

 adherence to the above incorrect opinion has been the source 

 of much error. He quotes the opinion of Sprengel, who says that 

 vegetable physiologists agree in supposing humus to be absorbed 

 by roots, by the aid of water; while chemists have observed that 

 humic acid is only soluble in alkalies when newly precipitated, 

 and becomes insoluble when dried, or exposed to cold at freezing 

 temperature. He does not say whether it regains its solubility 

 by heat and moisture; I think it would. These facts, he says, 

 have not escaped the observation of physiologists ; and they 

 have assumed that lime, and the alkalies found in the ashes of 

 vegetables, render the humic acid soluble, and fit it for the 

 process of assimilation. Taking this for granted, he states the 

 quantity of ashes and alkalies found in dry fir wood by M. 

 Berthier, and the quantity of dry fir wood stated to be produced 

 annually on a stated quantity of ground by Dr. Heyer; also the 

 estimates of Malagutti and Sprengel, of the quantities of humic 

 acid which combine with the alkalies. From all these data he 



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