DEFINITIONS AND DESCRIPTIONS. 167 



Lime is composed of the white metal, calcium 20.5 parts, 

 and 8 parts of oxygen. It is a protoxide of calcium, and is 

 thus represented, CaO.=28.5. Lime is a grayish white 

 solid, caustic, acrid and alkaline to the taste. It has a strong 

 affinity for water, with whicli it combines, attended with the 

 evolution of much light and heat, and forms a bulky hydrate, 

 called slacked lime. It has a strong affinity also for several acids, 

 with which it combines. The carbonate oj lime is the common 

 limestone and marble. Sulphate of lime is gypsum, or plaster 

 of Paris. Phosphate of lime is the substance which forms the 

 bones of animals, and exists in the mineral apatite. 



Alumina is composed of 27.4 parts of aluminium, and 

 24 parts of oxygen. Its composition is thus represented. 

 Al'^03 51.4. It is an inodorous, tasteless substance, insoluble in 

 water, possessing the properties, both of an acid, and of an alkali. 

 When moistened, it forms a ductile mass, and, when combined 

 with silicic acid, forms clay. It is the base of all kinds of pot- 

 tery. 



Oxides of iron. There are at least two oxides of iron. The 

 protoxide is composed of twenty-eight parts of iron and eight of 

 oxygen, and is represented by FeO=36. It has a dark blue 

 color, and is magnetic. It is so combustible as to take fire, 

 sometimes, in the open air, by which it becomes converted in- 

 to the 



Peroxide of iron which may be represented by Fe'^O^^BO. 

 This is the red hemetite of mineralogists. It is a brownish- 

 red substance, easily thrown down from a solution of its salts, 

 by ])ure alkalies. Both of the oxides combine with several 

 acids, and form a numerous class of salts. The sulphate 

 of the protoxide is known as copperas. The carbonate of the 

 X)rotoxide exists in most chalybeate mineral waters. 



Oxides of manganese. There are several oxides of manga- 

 nese. The principal one is the 



Peroxide of manganese, which is composed of 27.7 parts of 

 manganese (Mn) and 16 parts of oxygen. The symbol is 

 jMn02=43.7. This oxide occurs in black earthy masses, and 

 is not affected by ex|)osure to the air or water. It combines 

 with several acids and forms salts. 



Silicic acid is composed of 22.5 parts of silicon and 24 parts 

 of oxygen (Si03=46.5). It is best known in the form of sand, 

 rock-crystal, quartz and flint. It is a tasteless, very infusible 

 and insoluble substance; and although it is not acid by the or- 

 dinary chemical tests, it is the most powerful of acids, forming 



