Illustration: (‡ Illustrations of:)
1. EMERALD. 3. FORTIFICATION AGATE. 5. DIAMOND.
2. GARNET. 4. RUBY. 6. ROCK CRYSTAL.
All earths are metals in combination with oxygen: that is to say,
they can all be separated into a metal and oxygen. The chief earths
used are Alumina, found as clay or slate; Lime, found as chalk or
limestone; and Silica, found as sand, flint, or rock-crystal. These,
in various proportions, combined with some few other matters, form,
by far, the greatest portion of the earth we dwell on. The earths,
when pure, are all white substances, not very heavy, and having
scarcely any of the properties of the metals from which they are
derived. The next frequent of the earths are Baryta and Strontia;
but these may be said to be useless when compared with the three
former. Alumina, in its various forms of clay, is used for brick and
tile-making, and for pottery; hardened into the form of slate, it is
much used for roofing and for making cisterns. Lime, in the form of
carbonate, constitutes the best building stones, including marble,
so valuable for ornamental carving and decoration, and when burnt,
to separate the carbonic acid, forms lime itself, which is invaluable
as a cement when mixed with sand; lime, in union with another acid
(sulphuric) forms plaster of Paris, also a most useful article
in the arts, &c. Silica, in the form of sand, is very extensively
used for glass making, and, in the form of flint, it is ground and
used for pottery: all such stones as quartz, rock-crystal, Scotch
pebble, agate, cornelian, &c. are but various forms of Silica, either
crystallized or deposited in layers. Most of the precious stones
are composed of the earths in a crystalline state and colored by
some foreign ingredient, such are the emerald, ruby, garnet, &c.
The diamond is not an earth but composed of pure carbon.