Silica is composed of 21.3 parts of silicon with 24 of oxygen, it is that earth which constitutes a great part of almost every soil, and forms sand, sandstone, and flint, it enters largely also into the formation of granite and the other primitive rocks; when pure it is called "quartz" or "rock-crystal" and is often found in large and beautiful masses. Silica, in different forms, contaminated and coloured by different metallic oxides, &c., forms that class of stones 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 combination 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 crystallised 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 combination. Lime is often found in union with sulphuric acid, forming gypsum or Plaster of Paris, in combination with phosphoric 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.