_Of Solid Substances._
All solid substances are capable of becoming fluid by heat, and most of
them may thereby be reduced into a state of vapour, or air; and in
passing from a fluid into a solid state their component parts assume a
particular mode of arrangement, called _crystallization_, which differs
according to the nature of the substance; so that all solids, especially
if they be suffered to concrete slowly, may be called _crystals_.
Exclusive of _salts_, which have been considered already, as formed by
the union of acids and alkalis, solids in general have obtained the
names of _earths_, or _stones_, which differ only in their texture; and
they are distinguished into those that are _metallizable_, or those that
are not; the former being called _ores_, and the latter simply _earths_;
the principal of which are the _calcareous_, _siliceous_,
_argillaceous_, _magnesia_, _terra ponderosa_, and a few others which
have been discovered lately, but have not been much examined.
_Of Calcareous Earth._
Calcareous earth is found in the shells of fishes, the bones of animals,
chalk, lime-stone, marble, and gypsum: but all calcareous earth is
supposed to be of animal origin; and beds of chalk, lime-stone, or
marble, are thought to have been beds of shells formed in the sea, in
some pristine state of the earth.
The calcareous earth which is found in shells, lime-stone, and marble,
is combined with fixed air, discovered by effervescing with acids. To
obtain it perfectly pure, the earth must be pounded and washed with
water, in order to free it from any saline substance which may be
contained in it, then dissolved in distilled vinegar, and precipitated
by mild alkalies. Lime-stone exposed to heat loses about half its
weight, in fixed air and water, and the remainder, called _quick-lime_,
attracts water very powerfully, and their union is attended with much
heat, after which it dissolves into a fine powder called _slaked lime_.
If it be left exposed to the atmosphere, it will of itself, by gradually
imbibing moisture, fall into the state of powder.
Water dissolves about one seven hundredth part of its weight of
quick-lime, and is then called _lime-water_. Exposed to the air, a crust
will be formed on its surface, which is found to consist of calcareous
earth and fixed air.
Lime and water mixed with sand make _mortar_, by which means different
stones may be made to cohere as one mass, which is the most valuable
use of this kind of earth.
Calcareous earth, united with vitriolic acid, makes _gypsum_; and this
substance pounded and exposed to heat, parts with its water, and is then
called _plaister of Paris_. In this state, by imbibing water again, it
becomes a firm substance, and thus is useful in making moulds, &c.
The earth of animal bones is calcareous united to the phosphoric acid.
_Of Siliceous Earth._
Siliceous earth seems to be formed by nature from chalk, perhaps by the
introduction of some unknown acid, which the vitriolic acid is not able
to dislodge. It abounds in most substances which are hard enough to
strike fire with steel, as _flint_, _rock crystal_, and most _precious
stones_. It is not acted upon by any acid except the fluor and
phosphoric, but especially the former: but it is soluble in alkalies;
and being then dissolved in water, makes _liquor silicum_, from which
the purest siliceous earth may be precipitated by acids. For this
purpose about four times the weight of alkali must be made use of. With
about equal weights of alkali and siliceous sand is made _glass_, of so
great use in admitting light and excluding the weather from our houses,
as well as for making various useful utensils. To make glass perfectly
colourless, and at the same time more dense, commonly called _flint
glass_, manufacturers use a certain proportion of calx of lead and
manganese.
Siliceous earth is not affected by the strongest heat, except by means
of a burning lens, or dephlogisticated air.