174 WHAT DOES MAN LIVE UPON? 



also are suggested, fruitful for the completion and improve- 

 ment, the simplification and insuring of Agriculture, which 

 will be the prize of the next following time ; and we now 

 find it natural that England, where agriculture stood at so 

 high a pitch, according to the former standard, should 

 applaud and overwhelm with fetes and demonstrations of 

 honour of all kinds, him, the founder of a rational, in oppo- 

 sition to the former purely empirical, culture of plants, in a 

 manner which scarcely any man, and certainly no foreigner, 

 has experienced in that country. 



When we examine the ashes of plants, we find in par- 

 ticular the four following constituents, giving them their 

 characteristics : readily soluble alkaline salts ; earths, espe- 

 cially lime and magnesia ; phosphoric acid, and silicic acid 

 or silex. Sometimes one, sometimes two of these sub- 

 stances predominate in the ashes of the plant. 



According to this, Liebig divides the cultivated vege- 

 tables into : 



1 . Alkali plants ; to which belong Potatoes and Beets. 



2. Lime plants ; Clover, Peas, &c. 



3. Silex plants ; the Grasses. 



4. Phosphorus plants ; comprehending Rye and Wheat. 

 But besides these, plants contain many other substances, 



the quantity and importance of which we do not under- 

 stand so well at present. With the progress of science, 

 however, those divisions of Liebig will assume a much more 

 complete form. 



All those substances are met with in the various rocks 

 of the firm crust of the earth, but almost all in a perfectly 

 insoluble and sometimes crystalline condition, therefore 

 altogether unavailable to the plant. Geognosy alone can 

 inform us how these substances are rendered soluble, 

 and how they become gradually converted into soil. 



