MAGNESIA. BAKYTES. 89 



stance called plaster of Paris, so much employed 

 in making casts of statues, and in plastering 

 rooms. 



Filiate of lime, or lime united to the fluoric acid, 

 is the substance so well known by the name of Der- 

 byshire spar, and which is much used for vases and 

 other ornaments. 



Nitrate of lime is a very soluble salt ; its taste is 

 acrid and bitter. It is often found efflorescing on 

 old plaster walls. 



MAGNESIA. 



This earth, when pure, is white, nearly destitute of 

 taste and has no smell. It is insoluble in water, but 

 changes vegetable blues to green, and unites to the 

 acids. It is conjectured to be composed of a me- 

 tallic base, magnesiumy united to oxygen ; but the 

 metal has not yet been distinctly obtained. 



Native magnesia is a rare substance, but it en- 

 ters as a constituent in many rocks, as serpentine, 

 steatite, &c. 



Carbonate of magnesia is extensively employed 

 as a medicine. When a red heat is applied to it 

 it loses its carbonic acid, and becomes calcined 

 magnesia. 



Sulphate of 7nagnesia is known by the name of 

 Epsom salt, because formerly procured from the 

 springs of Epsom, in Surrey. 



BARYTES. 



This earth was formerly called terra ponderosa, 

 from its great specific gravity. It has strong alka- 

 line properties, a caustic taste, and changes vege- 

 table blues to green. 



