AGENCY OF GROWING PLANTS. 191 



The vegetable acids are capable of combining with the lime, 

 soda, ammonia, potash, magnesia, oxide of iron and manga- 

 nese. These bases are thus withdrawn from the rocks, and 

 the latter crumble to pieces, and salts are formed, which are 

 useful in the ^^nourishment of future generations of plants. 

 During decay, large quantities of carbonic acid are formed. 

 This acid is not only direct food for plants, but is capable of 

 combining with the potash in the feldspar of granitic rocks, 

 and of facilitating their decomposition. This acid is the 

 most powerful agent in its action upon the alkalies, even de- 

 composing the silicates and forming soluble salts. 



5. Groicing plants exert the most powerful agency in de- 

 composing the rocks. Not only do the lichens, mosses and 

 other plants insert their roots into the crevices of the rocks, 

 and by keeping them moist, favor the chemical action of air 

 and water, but the living plant forms with the rock or soil a 

 galvanic battery, of immense power ; by this means the 

 plant is enabled to obtain from the soil those ingredients 

 which its wants may require. This is proved by the fact, 

 that plants, growing in glass vessels, will decompose the glass 

 to obtain the potash, of which the glass is in part composed. 

 It is highly probable, that a greater amount of decomposition 

 is produced in this way than by all other causes together. 

 Similar to this influence, if not identical with it, is what has 

 been called " catalysis of life ^ The living plant acts by its 

 presence to decompose the rocks, and to effect rapid changes, 

 which not only convert them into the state of soil, but form 

 the elements into different substances. 



The above process will serve to illustrate the chemical and 

 mechanical agencies which are constantly at work tq crumble 

 down the solid rocks, and bring them into a state fit for the 

 support of the vegetable kingdom. These agents are con- 

 stantly active. The great effect of stirring the soil, is to fa- 

 cilitate the decomposition of the rocks, and of the vegetable 

 bodies which are always present in the soil. But for this 



