68 LAW OF SUBSTITUTION. 



established the fact, that minerals and salts exist, with per- 

 fect similarity of external form, yet of totally different 

 chemical constitution. Tor exannple, the alumina in alum 

 may be replaced by oxide of iron. The form will not be 

 changed, but all its chemical properties and relations are 

 destroyed. This is called an isomorphous replacement of 

 one element for another, which produces a like form. The 

 law of this substitution is, that the body replacing another, 

 must be, not an equal, but an equivalent proportion (56) ; 

 that is, replaced by a proportion containing the same quan- 

 tity of oxygen. Replacement retains the form, not the 

 properties of the displaced body. Substitution may retain 

 the form, but always the properties and functions of the ele- 

 ment whose place is thus occupied. Like replacement, sub- 

 stitution occurs in equivalent proportions. 



95. The I'elation between agriculture and this law is so 

 wisely and beneficially ordained, that it might well be called 

 a law of compensation, by the Natural Theologian. It is a 

 well-established fact, that plants growing on soil containing 

 a due mixture of 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 alka- 

 line earths is given, an excess of potash, soda, lime, magne- 

 sia, may be taken up by the plants, to the exclusion of the 

 usual proportion of another ; hence, it may be established 

 as the seventh principle in Agricultural Chemistry, one base 



MAY BE SUBSTITUTED FOR ANOTHER, IN AN EQUIVALENT PRO- 

 PORTION. 



96-. This is a very important law in the agricultural rela- 

 tions of the inorganic parts of soil. Whatever may be the 

 office performed by these, in the living structure, none is 

 of higher value than this, that they may be thus substituted, 

 the one for the other. It is a fact of the highest practical 



