MAGNESIA. 115 



and sulphuric acid is frequently called super-phos- 

 phate of lime, and is much used as a manure. 



246. Magnesia is an earth somewhat similar to 

 lime, but far less common and abundant ; like lime, 

 it is the oxide of a very combustible metal, which, 

 exposed to the air, soon takes oxygen and forms the 

 earth. The base of magnesia is called magnesium 

 (202). Magnesia occurs usually in connection with 

 lime, forming what is called magnesian limestone, 

 which is a mixture of carbonate of lime and carbonate 

 of magnesia. 



247. The commonest forms in which magnesia is 

 found are the carbonate, the sulphate, the muriate, 

 and the phosphate. The carbonate is sometimes found 

 native, pure, but by far more frequently it is mixed 

 with carbonate of lime or limestone, the properties of 

 which it considerably modifies. When carbonate of 

 magnesia is strongly heated, it parts with its car- 

 bonic acid, and the pure earth magnesia is obtained. 



248. This earth is almost insoluble in water, and 

 far less strongly caustic than lime; it also has less 

 affinity for carbonic acid, so that when left exposed 

 to the air, it does not, like lime, rapidly become con- 

 verted into carbonate, by absorbing carbonic acid 

 from the air, but remains for a long time caustic; 

 and hence, when a magnesian limestone is burnt in a 

 limekiln, the quicklime obtained of course contains 

 caustic magnesia, which, if exposed to the air, will 

 remain caustic long after the lime has become recon- 

 verted into carbonate; it is for this reason that lime- 



