AGENCY OF AFFINITY. 101 



chemical affinity. All the decompositions and recomposi- 

 tions, which take place in its mineral ingredients, and in the 

 vegetable and animal manures are due to this power. Hence, 

 it is active in dissolving the rocks, in forming saline com- 

 pounds, and in converting manures into vegetable food. The 

 nutriment, it is supposed by some, is held in solution in water 

 by this same force. The sap itself is prepared for the pro- 

 cess of assimilation, and even in this process, that is, in form- 

 ing the various vegetable products, affinity exerts a constant 

 agency, although controlled by the vital power, and made 

 subservient to it. 



Most of the substances which are added to the soil owe 

 their utility to the changes which this power produces, espe- 

 cially the application of saline manures, such as lime, potash, 

 ashes, etc. by which woody fibre is decomposed, metallic 

 salts and acids neutralized, and their injurious properties de- 

 stroyed and converted into nutriment. 



So many are the changes in the process of vegetation which 

 are purely chemical, or due to affinity, that some chemists 

 have attempted to solve the mystery of life itself by this force, 

 and hence have discarded the idea of any other vital power. 

 It doubtless ranks next in importance to the vital principle it- 

 self, and is employed by it in nearly all the organic changes, 

 through which the plant passes from the seed to mature growth. 



IV. Caloric. The influence of heat in vegetation is well 

 known, but the precise manner in which it accomplishes its 

 beneficial or injurious effects, may need some further illus- 

 tration. 



Heat or caloric exists in two states, sensible, or the heat of 

 temperature, and insensible, or in a state incapable of affect- 

 ing the senses. The tendency of caloric to pass from one of 

 these states to the other, as the forms of matter change, is 

 one of the most important properties to consider, in its rela- 

 tions to vegetation. A second property is its tendency to ex- 



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