found, in Portland Island, near Bath, and at Ketton, in the county of
Rutland._
Of Ketton stone several of the colleges in Cambridge are built. Its
grain has a singular resemblance to the petrified roe of a fish, whence
also it is sometimes called _roestone_. The bridges, St. Paul’s
Cathedral, the Monument, and nearly all the buildings of late date in
London, are constructed of Portland stone.
Some of these kinds of stone, when first dug out of the quarry, are so
soft that they are readily worked into any form which use or ornament
may require. This is owing to the moisture with which they are naturally
impregnated; but when they once become hardened, by exposure to the sun
and air, they are extremely firm and solid. On the contrary, other kinds
of limestone that are used for buildings imbibe and retain the moisture
of the atmosphere, in consequence of which they burst or are crumbled by
frost.
We are informed that Portland stone was first used in London in the
reign of James the First, that monarch, by the advice of his architects,
having employed it in the construction of the banquetting house at
Whitehall. After the great fire in London, it was brought into general
use by Sir Christopher Wren.
188. _MARL is a combination of clay, silex (76), and lime: and is
denominated calcareous, argillaceous, or siliceous, as the lime, clay,
or silex, is most abundant._
The calcareous part of marl is frequently composed of shells, whence it
frequently has the name of _shell marl_; and where these are
predominant, it affords an excellent manure for sandy, dry, gravelly, or
light lands. Marl likewise produces very beneficial effects on mossy and
clayey soils; and these effects, where it has been properly applied,
have been observable for twelve or fourteen years. Some kinds of marl
that contain but a small portion of lime have been successfully used in
the manufacture of earthenware.
This mineral is usually found at the depth of from five to nine feet
beneath the surface of the ground, and deposited between beds of clay
and sand. It is dug out with spades; and, in the digging of it, in
Ireland, the workmen not unfrequently meet with the horns of deer and
other curious fossils.
The usual mode by which persons, generally unacquainted with minerals,
distinguish this from other clayey substances, is, to break a small
piece of dry marl into a glass of vinegar. If it be marl it will
immediately dissolve with considerable effervescence; and the briskness
of the effervescence will be in proportion to the quantity of lime which
it contains.
189. _FLORENCE MARBLE is a kind of indurated or hardened marl, and is
remarkable for presenting, when polished, the appearance of ruined
edifices or rocks._
This kind of marble is never used in architecture. Little slabs of it
are cut for Mosaic work, and to be framed like pictures; and the latter,
when of considerable dimensions, are sometimes purchased at a high
price. If held at a distance from the eye, an inexperienced observer
might mistake a slab of Florence marble for a drawing in bistre. Here,
observes a French writer, we remark a shattered Gothic castle, there the
mouldering fragments of a cathedral; in one part ruined walls, and in
another shattered bastions and towers. But, when we approach the
picture, the illusion vanishes, and those imaginary figures which, at a
distance, appeared to be so correctly drawn, become changed into
irregular spots, lines, and shades, which present nothing distinct to
the view.
190. COTTAM MARBLE, which, when cut and polished, also exhibits the
appearance of a landscape, is a kind of compact marl. It has its name
from being found at Cottam, near Bristol.
191. _LIAS, or CALP is a kind of limestone of bluish black, or greyish
blue colour, and composed chiefly of lime, silex (76), clay, and oxide
of iron (21)._
This stone, when burned, forms a cement which has the property of
setting very strongly under water. It has also, of late years, been
employed in a manner which merits particular notice, for the multiplying
of copies of drawings and penmanship. A drawing is made on prepared
paper with a peculiar kind of ink. A slab of lias, about an inch thick,
is then heated; the drawing is placed upon it, and both are passed
through a rolling press. The paper is afterwards wetted, and washed from
the stone; but the ink, being of a gummy or glutinous quality, becomes
in part absorbed by the stone, and remains. The stone is then ready for
the printer. Previously to taking off each impression, the stone is
wetted with a sponge; fresh ink (which is said somewhat to resemble
printers’ ink, and is put on with a ball similar to that used by
letter─press printers) is then applied. This is prevented, by the water,
from adhering to any part except to the ink that had been absorbed, by
the stone, from the paper on which the drawing was originally made.
Paper is then placed on the stone, both are passed through a rolling
press as before, and a perfect impression of the drawing is made upon
the paper.
This art has been practised in Germany with great success; and with the
difference only of the original drawing being made upon the stone
instead of paper. Many beautiful specimens of drawings, taken from slabs
of lias, may be seen in this country. It is said that copies of military
drawings and orders were, to a very large amount, multiplied by this
means at the headquarters of the armies lately employed on the
Continent.
An artificial composition is sometimes used instead of lias.
Considerable quarries of this stone are wrought in Germany. It is also
found at Leixlip, near Dublin; in beds at Aberthaw, in Glamorganshire;
in Dorsetshire, and near Bath.
SULPHAT OF LIME.
192. _ALABASTER, or GYPSUM, is a kind of sulphat of lime, or of lime in
combination with sulphuric acid (24), which has a shivery and glittering
texture; and is of white colour tinged with grey or red, and sometimes
striped, veined, or spotted. When crystallized, the primitive form of
its crystals is a regular four─sided prism_ (Pl. II, Fig. 14.)
Being considerably softer than marble, this mineral is not capable of
receiving a good polish. From this circumstance it is, however, the more
easily worked. It is manufactured into chimney─pieces, columns, busts,
ornamental vases, and lamps; the latter of which transmit a soft and
pleasing light. Such is sometimes the transparency of alabaster, that it
has been employed for windows; and, at Florence, there is now a church
which receives its light through the medium of this substance.
The ancients, though acquainted with the art of making glass, had not
attained the knowledge of reducing it into thin transparent plates; and
frequently employed alabaster for windows. Of this stone the Temple of
Fortune, which was built by order of the Emperor Nero, was erected. It
had no windows whatever, and received only a soft kind of light through
its walls; appearing rather as if the light issued from the interior,
than that it was admitted from without.
The hot springs of St. Philip, which supply the baths of Tuscany, are so
strongly impregnated with alabaster, that artists take advantage of this
to obtain impressions of bas─reliefs, by merely exposing their moulds to
a current of the water until they become filled with the earthy deposit.
These impressions, when taken out, are found to be as hard as marble,
and are very beautiful. There are, in the British Museum, some casts of
medals formed from the water of these springs.
When alabaster is heated, it falls into a soft white powder, which, on
being mixed with water, absorbs it so rapidly, that if it be formed into
a paste, it dries and becomes hard in a few minutes. In this state it is
called _plaster of Paris_; and is employed for the making of statues,
casts, and other ornamental work, which, though of a beautiful white
colour, are very brittle. When mixed with coloured gummy or glutinous
substances, it yields plasters of different hues, and has the name of
_stucco_; and, in this state, is used for lining the walls and ceilings
of rooms. This plaster is much in request in the northern counties of
England, for the floors of dairies, store─rooms, granaries, and other
apartments; and, when properly formed, it constitutes a very smooth and
durable flooring.
The fine white varieties of gypsum are used as an ingredient in the
composition of earthenware and porcelain; and the glaze, or enamel, with
which porcelain is covered, has the purest gypsum for one of its
ingredients. Of late years this mineral has been advantageously employed
as a manure for fertilizing the soil.
Gypsum is found in Cheshire and Derbyshire, as well as in several parts
of the Continent. That which is imported into this country from Italy
and Spain is considered the best.
193. FIBROUS GYPSUM.—There is a variety of gypsum which has a somewhat
fibrous appearance, and which, when cut in a convex form, and polished,
reflects a light not much unlike that of the cats─eye (86). Hence it is
sometimes sold to ignorant persons for that stone. It has also been
imposed upon purchasers for the gem called moonstone (113). Fibrous
gypsum is cut into ear─pendants, crosses, beads for necklaces, and other
female ornaments; but its softness is such as to allow of its being
easily injured both by dirt and friction.
FLUAT OF LIME.
194. _FLUOR SPAR, or DERBYSHIRE SPAR, is a mineral formed by the
combination of lime with fluoric acid (27)._
_It sometimes occurs in a massive, and sometimes in a crystallized
state; the primitive form of its crystals being a regular octohedron_
(Pl. II, Fig. 5). _Its colour is usually bluish, green, yellow, whitish,
or a mixture of some of these._
When heated, this substance cracks, and shines brightly in the dark. But
if kept hot for some time, it ceases to be luminous, and this property
cannot be restored to it. If also two pieces be rubbed strongly
together, they become luminous in the dark.
From this spar are made several kinds of ornamental vases of
considerable size, columns, and toys, which, from being extremely varied
in their colours and appearance, and admitting of a high polish, are
very beautiful. When a piece of fluor spar is to be wrought into a vase,
or any similar article, it is first carved with a mallet and chisel into
a somewhat spherical form. It is then fixed to a turner’s lathe, and,
with great care, is formed into the shape that is required. When this is
complete, it has to be polished, which is done first with gritstone and
pumice (108), and lastly with emery (58) and putty. The lathes formerly
in use were worked by the foot; but those now adopted are worked by
machinery, the advantage of the more steady motion of which has been
that ornaments of much more delicate structure can now be formed than
before. The manufacture of articles from fluor spar gives employment to
a great number of industrious families in Derbyshire. This mineral
occurs in several parts of that county, where it has the name of _Blue
John_, and where it is obtained from caverns at a considerable depth
beneath the surface of the earth. It is also found in various countries
both of the European and American continents.
The acid produced from fluor spar is called _fluoric acid_ (27), and has
the peculiar property of corroding glass and flint, and consequently
cannot be kept in glass bottles. Artists, by means of fluoric acid, are
enabled to etch on glass, in the same manner as, with aqua fortis
(nitric acid), they do on copper. The process is sufficiently simple.
The glass is first a little heated, for the purpose of covering it
thinly over with wax; then, with a needle or other fine point the
drawing is to be made, by cutting through the wax to the surface of the
glass. The edges are next to have a little wall of wax raised upon them.
This done, the glass must be placed in an horizontal position, and
sifted over with fluor finely pounded; and lastly, a mixture of one part
of spirit of vitriol or sulphuric acid (24) with two or three parts of
water is to be poured gently upon it. The acid will be prevented from
running off by the wax; and, in the course of a little while, if these
be cleared away, the glass will be found corroded in all the lines along
which the needle passed.
The mode of obtaining fluoric acid for chemical purposes is, by pouring
sulphuric acid upon powdered spar in a leaden retort, and applying to it
a gentle heat. This acid should be used with great caution; for, when
applied to the skin, it instantly disorganizes it, and produces very
painful sores.
BARYTES FAMILY.
195. These minerals are sometimes called _ponderous earths_, and have
their name from a Greek word signifying _heavy_. They comprehend all the
combinations of barytes with acids.
When purified, they form a greyish white, porous substance, which is
easily reducible to powder; has no perceptible smell, but has a harsh
and more burning taste than lime, and changes the blues of vegetable
colours to green.
Although barytes is one of the most useful chemical tests that we are
acquainted with, it is not much employed in the arts, because, when
purified, it is found too expensive. It is capable of being made into a
very tenacious cement; and painters use a preparation that is made from
it as a white colour which will not change. This is sold in the shops
under the name of “Hume’s permanent white.” Barytes taken into the
stomach proves a virulent poison; yet a preparation of it is used in
medicine, and particularly for the removal of scrophulous complaints.
When finely pounded and mixed with oatmeal, _carbonat of barytes_ has
been found an efficacious poison for rats.
196. _SULPHAT of BARYTES is a mineral formed by the combination of
sulphuric acid (24) with barytes._
_It sometimes occurs in a state of powder, frequently in shapeless
masses, and often crystallized: the primitive form of its crystals being
a four─sided prism. It is not soluble in any other than sulphuric acid._
With us sulphat of barytes is of no use in the arts. The Chinese,
however, employ it as an ingredient in the composition of porcelain; and
it is said to form a good manure for clover fields.
_The_ BOLOGNA PHOSPHORUS, _or_ BONONIAN STONE, _a very remarkable kind
of barytes, has its name from being found near Bologna in Italy._ This
substance, when detached, is usually observed in roundish, flat,
kidney─shaped pieces, from about the size of a walnut to that of an
orange, which have a shining and somewhat fibrous texture within.
When the outer coat of this stone is washed away by heavy rains, it has
sometimes the appearance of burnished silver. An Italian shoemaker, in
the year 1630, deceived by this appearance, carried home several pieces,
hoping, by means of fire, to extract silver from them. But at the same
time that he was disappointed in this expectation, he was surprised by a
very unlooked─for phenomenon. All the pieces which he had thus attempted
to melt, when they were afterwards exposed to the light, became
themselves luminous. It is the singular property of the Bologna
phosphorus, after it has undergone calcination in a particular manner,
to become capable of imbibing so much light on exposure, for a little
while, to the light of the sun, or even to the flame of a candle, that
it will afterwards shine in the dark for an interval of from eight to
fifteen minutes, like a glowing coal, but without any sensible heat. The
light which it emits is sufficient to read by, provided the letters be
placed near it. If well prepared, the stone will retain this
extraordinary property for five or six years.
The preparation of it is thus conducted. Pieces of sulphat of barytes
are made red hot, for a few minutes, in a covered crucible placed in the
middle of a fire, and then left to cool. When cool, they are pounded in
a stone mortar, and sifted. The powder thus formed is made into a paste
with a little gum arabic, and divided into long cakes, or cylinders,
each about a quarter of an inch thick. These pieces are dried in a
moderate heat, and then, by degrees, are exposed to a more violent heat,
among charcoal, in a wind furnace. As soon as the coals of the furnace
are half consumed, it must be filled a second time, and the phosphorus
must be left undisturbed. When the coals are quite consumed the ashes
must be carefully blown off with a pair of bellows, and the phosphorus
will be found at the bottom of the grate.
────────────────────────────────────────────────────────────────────────
CLASS II.—SALTS.
ORDER I.—EARTHY SALTS.
───────
ALUMINE FAMILY.
197. _ALUM is a substance of yellowish or greyish white colour, usually
opaque, but sometimes transparent. When purified, it consists of
slender, irregular, hair─shaped fibres, and has a sweetish, astringent
taste._
The alum of commerce is an artificial production from the different
kinds of stones which contain it. That called _Roman alum_, from its
being procured from the neighbourhood of Rome, is usually considered
preferable to the other sorts; but good alum of our own manufacture is
equal to it in quality. The Levant, or _Roche alum_, is said to have had
its name from the village of Rocca, the present Edessa, in Syria.
There is a famous alum mine at Tolfa, near Civita Vecchia, in Italy. The
alum is obtained from this mine nearly in a pure state; and it is so
extremely hard, that it can only be wrought by means of pickaxes, and
gunpowder. At Solfatara, near Naples, and in other volcanic countries,
an abundance of alum is found, in a state of efflorescence, from the
lava.
The alum of our own country is manufactured from a kind of slaty stone
which is found near Whitby, in Yorkshire. This manufactory was first
established about the conclusion of the sixteenth century, by Sir Thomas
Chaloner, who is supposed to have obtained his knowledge of the process,
from the alum works which had then lately been introduced into Germany
and Spain. The rock of _alum slate_, near Whitby, is supposed to be
nearly twelve miles in extent: and affords an abundant supply of alum.
The workmen tear open the rock; after which the different fragments are
loosened, in the form of slaty leaves or plates, that are of a dark grey
colour. To obtain the alum, a bed of fagots is formed from ten to twelve
feet in depth. By the side of this a scaffold is erected, which enables
the workmen to form a pile of mineral about fifty feet long, and forty
feet high. While this pile is forming, the fagots are lighted. By the
gradual operation of the heat, a calcination takes place, in consequence
of which the alum is afterwards rendered capable of being more easily
separated than it otherwise would be from the stone in which it was
contained, and from other extraneous matters that are combined with it.
After this, the mineral is washed in shallow vessels, so arranged that
the water may be poured from one into the other. By this process the
alum becomes suspended in the water, while all the earthy particles
subside to the bottom. The next operation is to evaporate the water
saturated with alum. This is done by boiling it in large leaden
caldrons, fixed, on cast iron bars, over a furnace. As soon as the
contents of the caldrons are brought to a proper state, they are drawn
off into casks, where the alum concretes into a mass. The hoops are then
taken off, and the alum is broken and left to dry; after which it is
packed in casks for sale.
Alum is an article of indispensable importance to dyers, not only on
account of its cleansing and opening the pores of the substances to be
dyed, and thus rendering them fit to receive the colouring particles,
but also from its more essential property of fixing the colours in such
manner that they cannot afterwards be washed out. By tanners it is in
great request for giving firmness to the skins after they have been
rendered flaccid in the lime─pits. It is employed in the manufacture of
paper, and by engravers, and other artists. In the making of candles,
alum is added to the tallow, to render it glossy, and to give it greater
firmness and consistence; and, mixed with cream, it aids the separation
of butter. It has a tendency to retard ignition. Paper soaked in alum
water does not easily take fire, and is thereby better fitted for the
preservation of gunpowder. Such paper is likewise used in the whitening
of silver, and the silvering of brass. It has been recommended that
ladies’ muslin dresses should be dipped in a solution of this substance,
for the purpose of rendering them less liable to catch fire. A solution
of alum also retards the putrefaction of animal substances, and affords
useful, as well as economical, means of preserving natural productions
that are imported from foreign countries. Alum is frequently mixed with
paste, to prevent its losing its tenacity by the absorption of moisture.
It is asserted that bakers occasionally use it as an ingredient in
bread, and that its presence may be discovered by thrusting a heated
knife into a loaf before it is cold: if free from alum, scarcely any
alteration will be visible on the blade, but if the contrary, the
surface, when cool, will appear slightly covered with an incrustation of
alum. A very important purpose to which alum may be applied is in the
purifying and sweetening of water that has become fetid and unfit for
use; from five to ten grains of burned alum, and double or treble that
quantity of pounded charcoal, will correct the fetor of a gallon of
water. Printers’ cushions, and the blocks used for the printing of
calicos, are rubbed with burned alum to remove any greasiness, which
otherwise would prevent the ink or colour from sticking. This substance
is also occasionally employed by surgeons to stop the bleeding of small
vessels, to corrode fungous or proud flesh, and for other purposes in
medicine.
198. MAGNESIA FAMILY.
This is a family of minerals which comprehends all the combinations of
magnesia with acids.
When freed from extraneous matters, magnesia is a powdery substance of
limpid white colour.
199. _EPSOM SALTS, or SULPHAT OF MAGNESIA, consist of magnesia in
conjunction with sulphuric acid (24)._
It is said that Epsom salts have been found in the Alps, and in
Switzerland, under a powdery form, and sometimes even in masses, or a
state of incrustation on stones and rocks. They are, however, chiefly
found dissolved in mineral waters, and particularly in those at Epsom in
Surrey, and Sedlitz in Bohemia. Their taste is bitter and unpleasant. So
little are they affected by exposure to the air, that the Abbé Haüy kept
some by him for more than twelve years without any sensible alteration.
These salts are much used in medicine, and are sometimes manufactured
from the waters of Epsom (290) and Sedlitz (289), but more frequently,
and in much greater abundance, from sea─water.
The _magnesia_ of the shops is prepared by dissolving Epsom salts in
water, and adding to the solution half their weight of potash (205). The
substance that sinks to the bottom is magnesia; and this, washed with a
sufficient quantity of water and dried, has the appearance of a light,
soft, and white powder, of insipid taste.
Magnesia is used in medicine, both in a simple state and when calcined
or burned. It is also employed in some chemical processes; and is in
considerable request in the manufacture of enamel and porcelain. If
putrid water be agitated with a small quantity of magnesia, it will lose
a considerable portion of its bad taste and smell.
200. SODA FAMILY.
Soda, like potash (205), is an extremely caustic alkali (42). It has a
greyish white colour, and agrees exactly with potash (205) in taste,
smell, and corrosive quality, but it is not so heavy.
In a mineral state soda has hitherto been found only in combination with
some acid.
Common salt (202) is a compound of soda with muriatic acid (29).
The soda of commerce is obtained from sea─water; and from the ashes of
different kinds of plants that grow on the sea─shores, but particularly
from that called _salsola soda_, which is found in great abundance on
the coasts of the southern parts of Europe; and from which it has its
name. It is sometimes called _barilla_, from the salsola soda being so
denominated in Spain.