30 THE PLANT. 



4. Combined with chlorine or sulphuric acid as 

 chloride or sulphate of lime it is a good fixer of 

 fertilizing gases. 



In nature it exists most largely in the form of car- 

 bonate of lime ; that is, as marble, limestone, and 

 chalk these all being of the same composition. In 

 manufacturing caustic (or quick) lime, the carbonate 

 of lime is burned in a kiln ; by this means the car- 

 bonic acid is driven off into the atmosphere and the 

 lime remains in a pure or caustic state. 



MAGNESIA. 



Magnesia is the remaining alkali of vegetable 

 ashes. It is well known as a medicine, both in the 

 form of calcined magnesia, and, when mixed with 

 sulphuric acid, as epsom salts. 



Although magnesia is a necessary constituent of 

 plants, it is not an element of which fertile soils are 

 likely to become exhausted, and it does not receive 

 attention in special manuring ; the amount returned 

 to the soil in farm-yard manure, and that supplied" 

 by the decay of roots, being sufficient for the growth 

 of the most luxuriant crops. 



> 



ACID S. 

 PHOSPHORIC ACID. 



Phosphoric acid is a constituent of the ashes of 

 plants which is of the greatest value to the farmer ; 

 it is composed of phosphorus and oxygen. Being an 



