98 Obseivations on Liebig's " Organic Chemistry^' 



decomposition. The analysis of the minerals the debris of which 

 constitute the soil in which plants grow has produced various 

 results. Mica, the remains of which enter into most soils, is by 

 some mineralogists said to contain as high as 10 per cent of pot- 

 ash, a very important substance in soils ; while others allow it 

 very little, if any at all. Feldspar, the most generally allowed 

 source of potash, is also exceedingly variable. The analysis of 

 oro^anic substances, from their complicated nature, is much more 

 difficult. Very different statements have been given of the results 

 of experiments, from the circumstances under which they have 

 been conducted ; and in the analysis of manures we are still very 

 much at a loss. The analysis of the urine of the cow, by Brande, 

 in Sir H. Davy's Agricultural Chemistry, gives 40 parts of urea, 

 its most essential constituent, in the 1000; while that of the 

 horse, (both herbivorous animals,) by Fourcroy and Vauquelin, 

 <xives only 7. It becomes practical men, therefore, while they 

 receive the statements of such high authorities with the defe- 

 rence which is undoubtedly their due, to put them to the test of 

 reiterated experiment, in tiifferent soils, seasons, and situations ; 

 and fully to investigate every doubt, before receiving them as 

 jyrinciples in the art of cultivation ; they may be truths, yet 

 other truths may prevent their being Rules. 



The first part of the work before us is devoted to the exami- 

 nation of the matters which supply the nutriment of plants, and 

 the changes which they undergo in the living organism ; the 

 second treats of the chemical processes after death, of fermen- 

 tation, putrefaction, and decay. The different parts of plants are 

 shown to consist principally of carbon, hydrogen, and oxygen. 

 Woody fibre, starch, sugar, and mucilage, which form the prin- 

 cipal and most abundant proportions of all plants, contain the 

 oxygen and hydrogen in the proportions necessary to form water ; 

 the vegetable acids, as the carbonic, citric, tartaric, &c., contain 

 an excess of oxygen ; and the vegetable oils, wax, resins, &c., 

 contain an excess of hydrogen, though seme of them, as tannin, 

 &c., have acid characters. The juices contain acids united to 

 the metallic oxides or alkalies, as carbonates of potash, soda, &c. 

 Nitrogen, or azote, is an element of albumen and gluten; it is 

 also, he says, a constituent of the acids, and of what he terms 

 the indifferent substances of plants, and organic bases. Esti- 

 mated by its proportional weight, he says, nitrogen forms only a 

 very small part of plants, but is never entirely absent fiom any 

 portion of them ; even when it does not enter into the composition 

 of a particular organ, it is always to be found in the fluids which 

 pervade it. The essentials to vegetable life are, therefore, oxygen, 

 hydrogen, carbon, and nitrogen. The food must consist of 

 substances yielding these: and, as watci: furnishes the first two, 

 the carbon and nitrogen, or substances yielding these, are the 



