182 GEOLOGY AND CHEMISTRY OF SOILS. 



they remain unaffected, unless substances are presented capa- 

 ble of decomposing them, and of forming soluble compounds. 



2. Carbonates are a class of compounds formed by the 

 union of carbonic acid with the bases above mentioned. All, 

 excepting the carbonate of lime, or marble, are found in 

 small quantities in the rocks. 



Carbonate of lime is very abundant, forming nearly ^ part 

 of the crust of the globe. 



Carbonate of potassa is the common potash, pearlash, etc. 

 and is usually obtained from ashes. 



Carbonate of soda is the well known substance used for 

 soda powders. 



Carbonate of magnesia is a white powder used in medi- 

 cine,"under the name of" calcined magnesia." 



Carbonate of iron is more widely diffused among the rocks, 

 but the quantity is small. 



Carbonate of ammonia is a powerful stimulant to animal 

 and vegetable organs, and is found in the fermentation of ani- 

 mal manures, and exists, according to Liebig, in the atmos- 

 phere. It is, as we have seen, one of the substances from 

 which plants derive their nitrogen. All the carbonates are 

 easily decomposed, and have an important influence on vege- 

 tation, especially by means of their action upon the silicates 

 from which alkali is obtained for the use of the vegetable. 



3. Sulphates. The sulphates are formed by the union of 

 sulphuric acid with potassa, alumina, soda, magnesia, am- 

 monia, lime, oxide of iron and of manganese. Most of the sul- 

 phates are well known substances. The sulphate of potassa 

 and alumina is known by the common name, alum. Sul- 

 phate of soda is glauber's salts; of magnesia, epsom salts ; of" 

 lime, plaster of Paris ; of oxide of iron, copperas, etc. The 

 most abundant sulphate is that of lime or plaster, which 

 forms extensive mountain ranges and is widely disseminated 

 among the rocks. 



4. Nitrates. The nitrates are compounds of nitric acid 



