56 LAW OF ISOMORPHISM. 



tionate to the size, age, and part of the plants; the 

 acid being constant, the bases to saturate them, will 

 be equally constant. 



94. A curious and beautiful chemical law governs 

 this saturation, of the vegetable acids. It is the law 

 of Isomorphism, or the law of similar forms. In 

 minerals which are crystallized, it was formerly 

 thought that similarity of external form, indicated 

 identity of chemical composition. Later observa- 

 tion has established the fact, that minerals and salts 

 exist, with perfect similarity of external form, yet of 

 totally different chemical constitution. For exam- 

 ple, the alumina in alum, may be replaced by iron. 

 The form will not be changed ; but all its chemical 

 properties and relations are destroyed. This is cal- 

 led an Isomorphous substitution, 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 quantity of oxygen. 



95. The relation between agriculture and this law 

 is so wisely and beneficially ordained, that it might 

 well be called, a law of compensation, by the Nat- 

 ural Theologian. It is a well established fact, that 

 plants, growing on soil, containing a due mixture of 



