LAW OF SUBSTITUTION. 69 



value. Its value 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 sub- 

 stituted, which, combining with the organic acid of the 

 plant, enables this to perform and perfect all its functions. 

 If a crop fails, this is often charged upon the deficiency of 

 lime in the soil. It has been already shown that this is 

 quite impossible, yet granting it true, so long as the law of 

 substitution exists, so long may potash, soda, magnesia, that 

 is, ashes, supply the place of lime. 



Ashes are the product of combustion, or its equivalent, 

 decaying and mouldering vegetable matter. 



97. Substitutions in plants, relate chiefly to the bases 

 combined with the vegetable or organic acids. The mineral 

 or inorganic acids exist already saturated in the soil, as sul- 

 phates, phosphates, or muriates. 



It has been observed, however, in recent investigations, 

 that phosphoric acid is not always united to the same num- 

 ber of equivalents of base, in the ashes of seeds ; all the 

 leguminous seeds have three of fixed base, to one of phos- 

 phoric acid, while the cereal grains contain two of base to 

 one of acid. The quantity of oxygen contained in the bases 

 of these seeds is so nearly alike, that Drs. Will and Frese- 

 nius think that the law of substitution applies to the phos- 

 phates. 



98. In consequence of the law of substitution, the oxygen 

 in the bases of organic acid salts is a constant quantity, 

 although ashes of the same plant may, by analysis, show a 

 great diversity of composition ; this can arise only from the 

 fact that the organic acids exist probably in a definite pro- 

 portion in each family of plants. The acids are formed by 

 the essential vital functions of the plant. To the perfection 

 of this process the silicates and salts of the soil are not less 



