LAW OF ISOMORPHISM. 57 



earthy ingredients, always select a due proportion of 

 each, according to their functions ; yet, if to such 

 soil, an excess of either of the alkalies, or of the al- 

 kaline earths is given, an excess of potash, soda, lime, 

 magnesia, may be taken up by the plants, to the ex- 

 clusion of the usual proportion of another; hence, 

 it may be established, as the seventh principle in 

 Agricultural Chemistry, one base may be substi- 

 tuted FOR ANOTHER, IN AN ISOMORPHOUS PROPOR- 

 TION. 



96. This is a very important law, in the agricul- 

 tural relations of the inorganic parts of soil. What- 

 ever may be the office, performed by these, in the 

 living structure, none is higher value than this, that 

 they may be thus substituted, the one for the other. 

 It is a fact, of the highest practical value. Its val- 

 ue will be perceived, when it is considered, that if 

 soil, containing originally all the elements, essential 

 to a crop, becomes exhausted of one, yet another 

 may be substituted, which combining with the organ- 

 ic acid of the plant, enables this to perform, and 

 perfect all its functions. If a crop fails, this is oft- 

 en charged upon the deficiency of lime in the soil. 

 It has been already shown, that this is quite impos- 

 sible, yet granting it true, so long as the law of isom- 



