MATERIALS EXISTING IN THE SOIL. 45 



fergus readily splits into thin layers, which are as tran- 

 sparent as glass. Gypsum is slightly soluble in water: 500 

 parts of cold water dissolving one part of it. When burned, 

 it parts with the water combined with it, and can be readily 

 reduced to a fine powder. In this dry state it exhibits a 

 remarkable character, which has rendered it of gi'eat value 

 in the arts, for when mixed with water, to the consistence 

 of a paste, it hardens into a compact mass. Unburned 

 gypsmn usually contains about 2 1 per cent of water. 



47. Magnesia is also a familiar substance. It is the calcined 

 magnesia of the apothecary — and like the bodies just described, 

 is a compound of oxygen with a metal, being an oxide of 

 magnesium. It exists abundantly in the waters of the 

 ocean, and in various parts of the world is found in com- 

 bination with carbonic acid forming rocks which also contain 

 lime, and are termed magnesian limestones. These rocks 

 occupy a considerable extent in England, but in Ireland 

 appear in only a few situations. They may be observed at 

 Cultra, in the neighbourhood of the pleasant little village of 

 Holywood, county of Down. Carbonate of Magnesia when 

 exposed to heat parts with its acid more readily than cai- 

 bonate of lime. The caustic magnesia produced is not so 

 soluble in water as quicklime: one part of it requirmg 5,142 

 times its weight of water for its solution. Rain water, how- 

 ever, charged with carbonic acid, is found to dissolve it more 

 readily than lime. 



48. Oxide of Iron The well-known metal. Iron at once 



the most abundant and the most important metallic substance 

 found in nature, exists in every part of the world, combmed 

 with oxygen, and is also a constituent of almost all our rocks. 

 It has already been explained that when a piece of iron is ex- 

 posed to the air, it is gradually covered with a reddish brown rust, 

 which is a compound of the metal with oxygen (42), taken 

 from the air. This rust is termed by the chemist peroxide of 

 iron, as there is another compound of iroa which contains a 

 smaller proportion of oxygen, and is named protoxide^ or first 

 oxide of iron.* Both of these oxides exist in the soils of 

 this country. The first oxide has a great disposition to 



Iron. Oxygen. 



* The first, or protoxide of iron, consists in the 100 



parts of 77.23 22.77 



The second, or peroxide of irop, of 69..S4: 30.66 



