AND AGRICULTURE. 71 



guano, and other manures, they make a very 

 useful compost. 



Limestone consists of calcium a"nd oxygen ; it 

 is an oxide of the metal, calcium. In fifty 

 pounds of limestone we find twenty-eight pounds 

 of lime and twenty-two pounds of carbonic acid. 

 Limestone is hence called the carbonate of lime, 

 and is found in several varieties. It is soft, as 

 chalk; hard, as our common limetone; some- 

 times yellow, as in magnesian limestones ; pure 

 and white, as seen in statuary marbles; black, 

 as the Derbyshire black marble, besides other 

 varieties. 



Marl is a carbonate of lime, often in the form 

 of a fine powder, and usually mixed with earthy 

 matter. Shell-sand, or broken sea-shells, is car- 

 bonate of lime mixed with a small quantity of 

 organic matter. 



All these varieties of limestone, or carbonate 

 of lime, may be profitably applied to land, either 

 :is a top-dressing, or ploughed or harrowed into 

 arable fields. They are most useful on sour, 



Questions. What is limestone ? What are the propor- 

 tions of carbonic acid and lime ? What is limestone called ? 

 Name some of the varieties of carbonate of lime. How may 

 carbonate of lime be applied to soils ? 



