LIMESTONE. — GYPSUM. 
43 
Fig. 4. 
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, limestone 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 a 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 pri mitive form, 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 
calcareous spar, possessing the property 
of 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, j-hough sometimes tinged with 
Fig. 5. 
