180 GEOLOGY AND CHEMISTRY OF SOILS. 



malleable, and fusible at a red heat. It combines with oxy- 

 gen and forms magnesia, which possesses alkaline properties, 

 and is the basis of the common magnesia (the carbonate) and 

 epsom salts. It exists in serpentine rocks. 



Aluminium is a grey powder, slightly cohesive. It gen- 

 erally exists in small scales or spangles of a metallic lustre. 

 When combined with oxygen, it forms alumina the basis of 

 all clays, and is a: constituent of most rocks, especially of the 

 primitive and tertiary formations. 



Calcium is a white metal, and combines with oxygen and 

 forms lime, the basis of all lime rocks, shells, marble, chalk, 

 plaster of Paris, etc. 



Iron is a well known metal, existing in all the primitive 

 rocks, in combination with oxygen and sulphur. 



Manganese is a more rare metal, of a greyish-white color, 

 existing in primitive rocks in combination with oxygen, and 

 is called oxide of manganese, or peroxide of manganese. 



Chlorine, iodine and hromine are found in sea-weeds ; jlu- 

 orine and lithium exist in some plants, but in small quantities. 



Sect. 2. Compounds formed hy the fourteen simple Bodies. 



I. Primary compounds, or bodies composed of two simple 

 substances. 



The combinations of these fourteen simple substances, 

 above described, form three distinct classes of primary com- 

 pounds. 1. Acids. 2. Alkalies, or alkaline earths and me- 

 tallic oxides. 3. Urets. 



1. Acids. Of this class, only five or six enter into the 

 composition of rocks; viz. silicic, carbonic, sulphuric, phos- 

 phoric, nitric and hydrochloric or muriatic acids. These 

 acids have been described pp. 48, 168. The silicic is most 

 abundant in the rocks, constituting about forty per cent, of 

 the crust of the globe. Carbonic acid ranks next in quan- 

 tity. Then follow the others, in the order above named. 



