ALUMINA LIME POTASSA. 47 



known as "agate," "Scotch pebble," "carnelian," &c., 

 and in the form of flint is found nearly pure, forming 

 "nodules" or rounded masses lying in layers in the chalk 

 cliffs on our coasts, the origin of these nodules of flint is 

 very obscure, but they are by many attributed to the 

 petrifaction of sponges, &c., the general form of which 

 they often retain as well as their internal structure, which 

 may be seen when viewed in thin slices by means of the 

 microscope. Pure silica is a white insoluble powder, and is 

 much used by the makers of porcelain to mix with their 

 other ingredients. 



Alumina consists of 41'1 parts of aluminium in combina- 

 tion with 24 parts of oxygen, it is the earth which forms 

 the basis of all clays, loams, and slates, it is in its pure state 

 perfectly white but in most clays is coloured brown or 

 blueish by oxide of iron, many of our most beautiful gems, 

 as the ruby, sapphire, and emerald, consist chiefly of 

 crystallised alumina coloured by oxide of iron, chromium, 

 &c., this earth like silica enters largely into the composition 

 of the primitive rocks. 



Lime is another of the earths which form a great part 

 of all soils ; it consists of 20 parts calcium with 8 of 

 oxygen. Unlike the two preceding earths, it exists only in 

 union with some acid. When united to carbonic acid it 

 forms limestone, chalk, marble, &c., and when in a crystal- 

 lised state " Iceland spar." The various kinds of limestone 

 are largely used as building-stones, and any of them when 

 burnt yields lime or " quick-lime " as it is generally called, 

 which is capable of uniting with a certain portion of water, 

 giving out a considerable amount of heat during the com- 

 bination. Lime is often found in union with sulphuric acid, 

 forming gypsum or Plaster of Paris, in combination with phos- 

 phoric acid it forms the basis or earthy part of the bones of 

 animals, and is found in some of the tissues of plants. 



Potassa consists of 39*2 parts potassium in union with 

 8 oxygen, it constitutes about 12 per cent, of the granites 

 and basalts (primitive rocks) in union with silica and 

 alumina, it is also found in the fluids of many vegetables in 

 union with carbonic, oxalic, and other acids. 



