LIMESTONE. GYPSUM. 



43 



forms extensive beds, and is the repository of the 

 chrome iron, from which is manufactured the paint 

 called chromate of lead, or chrome yellow. 



Limestone. — When pure, lim.estone is composed 

 of about 57 parts of lime and 43 carbonic acid ; but 

 there is often intermixed with it magnesia, alumine, 

 silex, or iron. This mineral may be scraped with a 

 knife, and effervesces by the application of strong 

 acids, and can be entirely dissolved in nitre or mu- 

 riatic acid. Common limestone, then, is carbonate 

 of lime, while gypsum is a sulphate of lime. Lime- 

 stone occurs of every variety of colour; and its 

 texture varies from the most compact and solid 

 marble, to a fine earthy powder like marl or chalk. 

 It crystallizes in a variety of forms, some writers 

 say as many as seven hundred, though, on splitting 

 them, the same primitive ^orm, a rhomboid, 

 is obtained from them all. This is a 

 solid, having, as in the adjoining figure, 

 all its faces equal to each other, but the 

 angles not right angles. When carbo- 

 nate of lime is crystallized, it is called 

 Fig. 5. calcareous spar, possessing the property 

 ^T~^ double refraction, and of becoming 

 electric by friction. Another form in 

 which it crystaUizes is the six-sided 

 prism, as in fig 5. Limestone is a very 

 common mineral in almost every coun- 

 try, forming extensive beds and even 

 mountains. The granular and compact 

 varieties are used for marble or for ma- 

 king lime ; chalk is used for marking, making lime, 

 and whiting, and marl is highly useful as a ma- 

 nure. 



Gypsum, or sulphate of lime, is a compound of 

 oil of vitriol, or sulphuric acid and lime, with a little 

 water. It is less abundant than the carbonate, but 

 forms beds of considerable thickness and extent. 

 It is usually white, though sometimes tinged with 



