DOMAIN V. CALCAREOUS. 377 



It may however be fused when joined with 

 silex or clay. 



Limestone is composed of lime and car- 

 bonic acid. Heat separates the latter, and 

 the lime is left pure. This acid is a spe- 

 cies of gas, formerly called fixed air, and 

 discovered by Dr. Black in 1756; an event 

 which formed a revolution in the history 

 of chemistry. Atmospheric air is composed 

 of about seventy-four parts in the hundred 

 of nitrogen, and twenty-six of oxygen: 

 but the latter varies; and there is com- 

 monly one in the hundred of carbonic acid 

 gas. Hence lime exposed to the air ab- 

 sorbs the carbonic acid, and may again 

 become a carbonate, or limestone. 



In architecture, mortar is composed of 

 quick lime and sand; and when mixed 

 with a proportion of iron, or manganese, it 

 becomes extremely hard, even under water. 



When combined with sulphuric acid, the 

 calcareous earth forms gypsum, or selenite, 

 which being burnt produces what is called 

 plaister of Paris. The alabaster of the mo- 

 derns commonly belongs to the same com- 



