colour, sometimes striped or spotted, and of an earthy texture._
_It feels harsh and dry to the touch; is soft, scarcely adheres to the
tongue, and will not take a polish from the nail._
This substance obtained its name from having formerly been imported into
Europe from Tripoli, on the north coast of Africa. It is, however, now
found in several parts of Germany; and a granulated kind has been
discovered in England.
Tripoli is used for the polishing of metals and stones. For this
purpose, it is mixed with sulphur, in the proportion of two parts of
tripoli to one of sulphur. These are well rubbed together on a marble
slab, and are applied to the stone or metal with a piece of leather.
When tripoli is combined with red ironstone, it is used for the
polishing of optical glasses. It is sometimes made into moulds, in which
small metallic or glass figures and, medallions are cast; and a kind of
tripoli is found near Burgos, in Spain, which is used as an ingredient
in the manufacture of porcelain.
In Derbyshire,, and several parts of Staffordshire, is dug a kind of
tripoli which has the name of _rotten stone_. This is considered to be a
produce of limestone shale, which has undergone a decomposition by
exposure to the air and moisture. It is used for most of the same
purposes as tripoli.
CLAY SLATE FAMILY.
120. _CLAY SLATE, or ROOFING SLATE, is a kind of stone of foliated
texture, and greyish, black, brown, green, or bluish colour._
_It breaks into splinters, does not adhere to the tongue, yields
generally a clear sound when struck, and is nearly thrice as heavy as
water._
Vast and extensive beds of slate occur in different parts of the world;
and this mineral sometimes constitutes even a principal portion of
mountains. In our own country there are many important quarries of it,
particularly in Westmoreland, Yorkshire, Wales, and Derbyshire.
The uses of slate are numerous and important; but its principal use is
for the roofing of houses. For this purpose it is split into thin plates
or laminæ. These are fastened to the rafters by pegs driven through
them; and are made to lap over each other at the edges, in such manner
as to exclude the rain and other moisture. The kinds which are preferred
for this purpose are such as have the smoothest surface, and split into
the thinnest plates. It is requisite that slates should be damp when
they are split, otherwise this cannot be done without difficulty. Hence
it is generally customary to split the masses as soon as possible after
they have been separated from the rock.
Slate should not be porous. If it be so, rain and snow water will pass
through it, and destroy the wood─work of the house on which it is
placed. Porous slate is also liable to have moss and lichens grow upon
and cover it. These plants retain moisture long, and keep the surface,
and even the interior of the slate, moist; so that, during the winter
season, by the freezing of the moisture, the slate is apt to split and
fall into pieces. To ascertain whether the slate be of requisite
compactness, it should be completely dried, then weighed, and afterwards
soaked for some time in water. When taken out it is to be wiped with a
cloth, and again weighed. If it have not acquired any considerable
increase of weight, it is a proof of its being sufficiently compact. If,
on the contrary, it have absorbed much of the water, and have become
considerably heavier by the immersion, it is shown to be of a porous
texture. Slates that are brittle are bad. If they emit a tolerably clear
sound, when struck with a hammer, it is considered a proof that they are
not too brittle: if, on the contrary, the sound be dull, they are soft
and shattery. A good slate ought also to resist the action of a
considerable degree of heat.
The slates that are principally used in London are brought from North
Wales, from quarries that are worked near Bangor. There are also
extensive slate quarries near Kendal, in Westmoreland; and the Kendal
slates, which are of a bluish green colour, are more highly esteemed
than those from Wales. They are not of large size, but they possess
great durability, and give a peculiarly neat appearance to the roofs on
which they are placed. The slate quarries near Easdale, in Scotland, are
so extensive as to furnish annually more than 5,000,000 in number, and
to give employment to upwards of 300 men.
_French slates_ were much used in London about seventy years ago; but
they have been found too small, thin, and light, to resist the winds and
storms of this changeable climate.
Dark─coloured, compact, and solid slates are manufactured into _writing
slates_, or _table slates_, as they are sometimes called. In the
preparation of these, the slate, after it is split of proper thickness,
is smoothed with an iron instrument. It is then ground with sandstone,
and slightly polished with tripoli (119), and, lastly, rubbed with
charcoal powder. It is cut into the requisite shape, set in a wooden
frame, and is then ready for use.
For writing on these slates, pencils are used which are also made of
slate. These, which are called _slate pencils_, are made of a particular
kind of slate, that, on splitting, falls into long splintery fragments.
It is necessary that the pencils should be considerably softer than the
slate to be written upon, so that they may leave a whitish streak on its
surface, without scratching it. Such is the shivery nature of the slate
of which they are made, that, if it be exposed for some time to the
action of the sun or frost, it is rendered useless. Hence, workmen are
careful to cover it up and sprinkle it with water, as soon as it is
taken from the quarry, and to preserve it in damp cellars. The pieces
are afterwards split by a particular instrument, and then wrought into
the requisite shape.
In some of the quarries in Derbyshire and Wales the slate is so thick as
to admit of being split into large and tabular pieces. These are used
for gravestones, and for slabs for dairies and cellars. Paving stones
and mile─stones are also formed of them; and vessels for the salting of
meat, and setting of milk in dairies. For the latter use slate is
peculiarly well adapted, on account of its resistance of greasy or oily
substances. But this property renders it unfit for any purpose for which
it is requisite to be painted; as, the oil not entering the stone, the
paint soon peels off, and leaves the stone as black as it was at first.
Cut into narrow strips, slate has also been applied, in the
neighbourhood of Bangor, North Wales, for the formation of fences.
When sufficiently solid for the purpose, slate is cut into inkstands,
and turned into vases, and fancy articles of various kinds. And a
singular circumstance has been remarked, that, if a window or door be
suddenly opened, in an apartment where the workmen are turning these,
they will sometimes fly in pieces; though, after the work is finished,
they may be exposed to the usual changes of temperature without injury.
Pounded slate is advantageously used for cleaning iron and other works
in metal. When well ground, and mixed with a certain proportion of loam,
slate is made into moulds for the casting of metals in; and, when burned
and coarsely ground, is used instead of sand in the making of a solid
and impermeable mortar or cement, for the parts of buildings that are
covered with water.
121. _BLACK CHALK, or DRAWING SLATE, is an earthy substance, of slaty
texture; generally of a greyish, sometimes a bluish black colour._
_It is soft and smooth to the touch, and, in handling, stains the
fingers._
To crayon painters, and other artists, black chalk is a very useful
article. Considerable quantities of it are imported from France, Spain,
and Italy. The best is brought from Italy. This is more free from gritty
particles, more firm and compact in its texture, and in its touch much
smoother than the chalk of any other country. It contains somewhat more
than one─tenth part of its weight of charcoal. When prepared for use, it
is cut into square pieces, which are sometimes enclosed in wooden cases,
like black lead pencils. These pencils are said to become dry, hard, and
unfit for use, by long keeping. To preserve them in greatest perfection,
they should be kept in a moist place. Some artists prefer pencils that
are made of the chalk finely ground, mixed with a certain proportion of
gum water, and cast in moulds. Care should be taken not to put too much
gum, as the pencils will not, in such case, leave any mark on the paper.
Drawing slate is sometimes used as a black colour for painting. For this
purpose it is pounded or ground, and then mixed with oil or size,
according to the kind of work for which it is required. When black chalk
is strongly heated, it loses its colour, and assumes that of a reddish
grey.
122. _HONE, or WHET SLATE, is a well─known kind of stone, of somewhat
slaty texture, and generally of dull white, or greenish grey colour. Its
surface is smooth, and feels unctuous to the touch._
These stones, when properly cut and smoothed, are of indispensable
utility to carpenters, cutlers, and others, for sharpening their cutting
instruments upon. Those of the finest grain are used for lancets,
penknives, and razors. For this purpose their surface, when used, is
covered with a small quantity of oil; by which, after a while, they are
rendered considerably harder than they were at first. They ought to be
kept in damp and cool places; for, if much exposed to the sun, they
become too hard and dry for many purposes to which they are applied.
There is a vulgar and erroneous notion that hones are holly wood, which
by lying in petrifying water, have been thereby converted into stone.
The greater number of them have a fine and a coarse side. From the
circumstance of their having been originally brought into this country
from Turkey and the Levant, they are sometimes called _Turkey stones_.
They are now found in Saxony and Bohemia, in North Wales, and near
Drogheda, in Ireland.
The powder of whet slate is sometimes used, instead of emery, for the
cutting and polishing of metals.
MICA FAMILY.
123. _COMMON MICA, GLIMMER, or MUSCOVY GLASS, is a mineral substance of
foliated texture, which is capable of being divided into extremely thin
leaves that have a sensible elasticity, and are transparent._
_The colour of mica is greenish, sometimes nearly black, reddish, brown,
yellow, or silvery white, with, occasionally, a metallic lustre on the
surface. Mica is so soft as easily to be scratched; and, when divided
across the plates, seems rather to tear than break._
This is one of the most abundant mineral substances that is known. It
not only occurs in a massive and crystallized state, but it enters into
the composition of many rocks; is found filling up their fissures, or
crystallized in the cavities of the veins which traverse them. In some
countries, as in Siberia, it is an article of commerce, and is obtained
from mines like other minerals. From these it is extracted by hammers
and chisels. It is then washed, to free it from the impurities which
adhere to it; split into thin leaves or pieces; and assorted into
different kinds, according to their goodness, purity, and size. We are
informed by the Abbé Haüy, that plates of mica a yard or more in width
have been obtained from the mines in some parts of Russia.
Thin plates of mica are adopted, in many parts of Siberia and Muscovy,
to supply the place of glass for windows. In the shipping of Russia it
is considered preferable to glass, as the concussion produced by the
firing of the guns does not shatter it. It is employed instead of window
glass in Peru and New Spain; and also in Pennsylvania. Mica may be
advantageously substituted for horn in lanterns, as it is not only more
transparent, but is not susceptible of injury from the flame of the
candle. It has, however, the inconvenience of soon becoming dirty; and
of having its transparency destroyed by long exposure to the air. Mica
is used for enclosing objects that are intended to be viewed by
microscopes.
So plentiful is this substance in Bengal, that, for the value of five
shillings, as much of it may be purchased as will yield a dozen panes,
each measuring about twelve inches in length and nine in breadth, and so
clear as to allow of ordinary objects being seen through them at the
distance of twenty or thirty yards.
Mica, when powdered, is sold by stationers on the Continent, in place of
sand, for absorbing ink in writing, but it does not dry sufficiently
quick to be of much use in this respect. In Russia it is employed in
different kinds of inlaid work. It is sometimes powdered, and intermixed
with the glaze in particular kinds of earthen ware. The heat which melts
the glaze has no effect on the mica: hence it appears, dispersed
throughout the glaze, like plates or scales of silver or gold. Some
artists use it in making artificial avanturines (85).
It must be observed that the best mica is of a pure pearl colour; and,
when split into leaves, presents a smooth surface.
SOAPSTONE FAMILY.
124. _STEATITE, or SOAPSTONE, is a soft and unctuous substance, which
has much the appearance of soap; and is generally of a white or grey
colour, intermixed with greenish or yellowish shades._
_It is somewhat more than twice as heavy as water; and is distinguished
from indurated talc (135) by not splitting, like that substance, into
slaty fragments._
In the counties of Devon and Cornwall, and the islands in the vicinity
of the Lizard Point, this mineral is found in considerable abundance. It
possesses many of the same properties as fullers’ earth, and is, like
that substance, employed in the scouring of woollen cloths. When mixed
with water it may be formed into a paste; and, in this state, it is
easily worked, like clay, for the manufacture of earthen ware. In the
porcelain manufactory at Worcester considerable quantities of steatite
are employed. According to Dr. Shaw, the Arabs use it in their baths,
instead of soap, to soften the skin.
As it becomes hard in the fire, and does not alter its shape, this
substance has been successfully adopted for imitations of engraved gems.
The subjects are engraved upon it with great ease in its natural state;
it is then exposed to a strong heat; afterwards polished, and then
coloured by means of certain metallic solutions.
We are informed by travellers, that some of the savage tribes eat
steatite, either alone, or mixed with their food, to deceive hunger. The
inhabitants of New Caledonia eat considerable quantities of it.
Humboldt, the South American traveller, assures us that the Otomacks, a
savage race of people, who live on the banks of the Orinoco, are almost
wholly supported, during three months of the year, by eating species of
steatite, or potter’s clay, which they first slightly bake, and then
moisten with water. M. Golberry says that the negroes near the mouth of
the Senegal mix their rice with a white kind of steatite, and eat it
without inconvenience.
In some parts of Spain a variety of steatite is found, which is used by
artists under the name of Spanish chalk. When slightly burned, this
mineral is sometimes used as the basis of rouge.
125. FIGURE STONE _is a kind of steatite, which has, internally, a
glimmering and resinous lustre, and a slaty or splintery fracture_.
From its softness, and yet solidity of texture, this mineral can easily
be fashioned into various shapes, even with a knife. Hence in China,
where it frequently occurs, it is cut into grotesque figures of various
kinds, which the French call _magots de la Chine_, into cups, vases,
pagodas, snuff─boxes, and other articles.
126. _MEERSCHAUM, or SEA─FROTH, is a singular kind of mineral, of
yellowish or greyish white colour, sometimes so light as to float in
water: when fresh dug it has nearly the consistence of wax._
_If exposed to a strong heat, it becomes so hard as to yield sparks with
steel._
The principal use to which meerschaum is applied is in the formation of
the bowls or heads of tobacco─pipes used by the Turks, and the quantity
consumed for this purpose is very great. It is found in a fissure of
grey, calcareous earth, about six feet wide, near Konie, in Natolia,
where upwards of six hundred men are employed in the digging and
preparation of it; and the sale of it supports a monastery of dervises
established at that place. The workmen assert that it grows again in the
fissure, and puffs itself up like froth. It is prepared for use by being
first agitated with water in great reservoirs, then allowed to remain at
rest for some time. The mixture soon passes into a kind of fermentation,
and a disagreeable odour, resembling that of rotten eggs, is exhaled. As
soon as this smell ceases, the mass is further diluted with water,
which, after a while, is poured off. Fresh water is repeatedly added,
until the mass is sufficiently washed and purified. The meerschaum, in
this state, is dried to a certain degree. It is then pressed into a
brass mould, and, some days afterwards, is hollowed out so as to form
the head of the pipe. It is subsequently dried in the shade, and lastly
is baked. In this state the pipe heads are brought to Constantinople,
where they are subjected to further processes. They are first bailed in
milk, and next in linseed oil and wax; and, when perfectly cool, are
polished with rushes and leather. The boiling in oil and wax renders
them capable of receiving a higher polish than could otherwise be given.
When thus impregnated, they also acquire, by use, various shades of red
and brown, which are thought to add considerably to their beauty. In
Turkey, and even in Germany, meerschaum pipes that have been much used
are more valued than those newly made, and this solely on account of the
colouring they possess. Indeed there are people in those countries whose
only employment consists in smoking tobacco pipes, until they acquire
the favourite tints of colour. By long use, the heads become black: but
if boiled in milk and soap, they are soon rendered white again.
It is asserted that the Turks spread meerschaum on bread, and eat it as
a medicine; and that they cover with it the heads and eyes of dead
bodies, previously to interment. As it lathers with water like soap, it
is used by the Turkish women for washing their hair; and, as it absorbs
oily matters, it is occasionally used, as fuller’s earth is with us, for
the cleansing and scouring of cloth.
We are informed by Pliny, that a kind of bricks were made by the
ancients, so light that, when dried, they would float in water. He
describes them to have been formed of a spongy kind of earth, and to
have had some resemblance to pumice stone, which he says might perhaps
be applied to the same purposes as these bricks, if it could be obtained
and wrought in sufficient quantity. Bricks of similar description have
lately been made of a mineral substance found near Sienna, in Italy, and
which is supposed to be meerschaum.
A kind of meerschaum has lately been discovered, in veins, in the
serpentine (132) of Cornwall.
127. _BOLE is an earthy mineral, of yellowish or reddish brown colour,
soft, and somewhat unctuous to the touch, and generally found in a
massive state._
_It exhibits internally a glimmering lustre; and, when put into water,
immediately absorbs it, and breaks down into small pieces with a
crackling noise. This mineral is farther distinguished by its fracture
being conchoidal, or appearing somewhat like the impression of a shell;
and by its adhering strongly to the tongue._
Although bole is at present little used except as a basis of tooth
powder, and a coarse kind of paint, it was formerly considered an
important article in medicine, and used as an astringent. We are
informed that tobacco pipes are sometimes made of this mineral; and that
it is employed as an ingredient in the glaze of some kinds of earthen
ware.
It is chiefly imported from the Levant; though it has also been found in
considerable beds in Silesia and Saxony.
128. _LEMNIAN EARTH is a kind of bole of yellowish grey, or yellowish
white colour, sometimes marbled with rust─like spots._
_It is distinguished from bole by being dry and not unctuous to the
touch, dull internally, adhering slightly to the tongue, and its
fracture being earthy._
With the ancients this mineral was considered an almost invaluable
medicine. They procured it chiefly from Armenia, and the island of
Lemnos, in the Grecian Archipelago. The Lemnian bole was held so sacred
that it was dug in the presence of the priests of Venus, and, after
having been mixed by them with goat’s blood, was moulded into cakes,
which were impressed with the figure of a goat, to authenticate them.
This done, it was administered as a consecrated remedy; and, even so
lately as the sixteenth century, the vein of bole in Lemnos was annually
opened on the sixth of August, and, after certain prayers by the
priests, so much of the earth was taken out as was thought sufficient
for the consumption of the ensuing year. The entrance was then closed,
and the severest punishments were denounced against any one who should
open it without permission. A portion of the earth was sent to
Constantinople, where it was made into small cakes, and sealed by the
ministers of the Emperor; the remainder was prepared in the island, and
was impressed with the seal of the Governor. Not many years ago, it was
customary with certain empirics on the Continent, to sell this substance
in sealed packets, as a nostrum of great value, and particularly as
possessing astringent properties of very extraordinary nature.
129. _FULLER’s EARTH is a well─known mineral, generally of greenish
colour, more or less mixed with brown, grey, or yellow: of soft and
almost friable texture, and somewhat unctuous to the touch._
_When put into water it immediately absorbs it, and breaks down into a
fine pulp._
This earth is valuable for its property of taking grease out of woollen
and other cloths, which, on a large scale, is effected by the operation
called _fulling_, whence its name has been derived. This operation,
which is performed by a kind of water mill, called a _fulling mill_, is
particularly necessary with respect to new cloths, to extract from them
the grease and oil that have been used in their preparation.
Fuller’s earth was formerly considered an article of such importance in
England that its exportation was prohibited under severe penalties. It
was then employed for most of those purposes for which soap has since
been so extensively applied. In the dressing of cloth it is now so
indispensable, that foreigners, although they can procure the wool, are
never able, without fuller’s earth, to reach the perfection of the
English cloths: and, in this country, incalculable quantities of it are
consumed. As an article of domestic utility, it might be much more
frequently used than it is, as a substitute for soap, in the cleaning
and scouring of wooden floors and wainscots.
There are extensive beds of fuller’s earth in several of the counties of
England. London is principally supplied from those of Kent, Sussex, and
Surrey. At Wavedon, near Woburn, in Bedfordshire, a peculiarly fine kind
is dug up from pits at the depth of ten or twelve feet below the surface
of the ground; and no country in the world is known to produce fuller’s
earth of quality so excellent as that obtained in England.
TALC FAMILY.
130. _JADE, or NEPHRITE, is a very hard and tough species of stone, of
greenish or olive colour, somewhat unctuous to the touch, and looking as
if it had imbibed oil._
_It is found massive, in blunt─edged or rounded pieces._
Nothing has so much tended to make this stone known, as a superstitious
notion that a piece of it suspended to the neck will dissolve stones in
the kidneys. Hence has been attained its appellation of nephrite, or
divine stone; and hence have originated all those numerous amulets in
the form of oval plates, hearts, fishes, birds, &c. pierced with holes
for ribbons to pass through, which are seen in collections of the
curious. Some of the Indian nations make talismans of jade.