The following table of M. Peclet is perhaps more instructive:--_A Dictionary of Arts, Manufactures and Mines containing a clear exposition of their principles and practice

+------------+------+--------+-------------------+----------+--------+

+------------+------+--------+---------+---------+----------+--------+

|lamp |100 | 42 | 1·40 | 5·8 | 42 | 5·8 |

|mechan. do.| 12·05| 11 | 1·40 | 1·5 | 88 | 12·3 |

|lamp | 31·0 | 26·714 | 1·40 | 3·7 | 86·16 | 12·0 |

|lamp | 85 | 43 | 1·40 | 6·0 | 50·58 | 7·0 |

|vase | 41 | 18 | 1·40 | 2·5 | 43·90 | 6·1 |

|above | 90 | 43 | 1·40 | 6·0 | 47·77 | 6·6 |

|lamp | 63·66| 34·71 | 1·40 | 4·8 | 54·52 | 7·6 |

|do. |107·66| 51·143 | 1·40 | 7·1 | 47·5 | 6·6 |

|in lb. | 10·66| 8·51 | 1·40 | 1·2 | 70·35 | 9·8 |

|Do. 8 in do.| 8·74| 7·51 | 1·40 | 1·0 | 85·92 | 12·0 |

|smaller wick| 7·50| 7·42 | 2·40 | 1·7 | 98·93 | 23·7 |

|5 in lb. | 13·61| 8·71 | 7·60 | 5·7 | 64·04 | 48·6 |

|candle, do. | 14·40| 8·92 | 7·60 | 5·8 | 61·94 | 47·8 |

|candle, do. | 14·30| 9·35 | 6·00 | 5·5 | 65·24 | 37·1 |

|Coal gas |127 |136 | | 5·0 | 107 | 3·9 |

|Oil gas |127 |136 do. | | 5·0 | 30 | 3·9 |

+------------+------+--------+-------------------+----------+--------+

The light of the mechanical lamp is greatly over-rated relatively to

that of gas. The cost of the former is at least 5 times greater than of

the latter, in London.

LAMP OF DAVY consists of a common oil lamp, surmounted with a covered

cylinder of wire gauze, for transmitting light to the miner without

endangering the kindling of the atmosphere of fire-damp which may

surround him; because carburetted hydrogen, in passing through the

meshes of the cylindric cover, gets cooled by the conducting power of

the metallic gauze, below the point of its accension.

The apertures in the gauze should not be more than 1-20th of an inch

square. Since the fire-damp is not inflamed by ignited wire, the

thickness of the wire is not of importance, but wire from 1-40th to

1-60th of an inch in diameter is the most convenient.

[Illustration: 614]

The cage or cylinder should be made by double joinings, the gauze being

folded over in such a manner as to leave no apertures. When it is

cylindrical, it should not be more than two inches in diameter; because

in larger cylinders, the combustion of the fire-damp renders the top

inconveniently hot; a double top is always a proper precaution, fixed

1/2 or 3/4 of an inch above the first top. See _fig._ 614.

[Illustration: 615]

The gauze cylinder should be fastened to the lamp by a screw _b_, _fig._

615., of four or five turns, and fitted to the screw by a tight ring.

All joinings in the lamp should be made with hard solder; as the

security depends upon the circumstance, that no aperture exists in the

apparatus, larger than in the wire-gauze.

The parts of the lamp are,

1. The brass cistern _a_, _d_, _fig._ 615., which contains the oil. It

is pierced at one side of the centre with a vertical narrow tube, nearly

filled with a wire which is recurved above, at the level of the burner,

to trim the wick, by acting on the lower end of the wire _e_ with the

fingers. It is called the safety-trimmer.

2. The rim _b_ is the screw neck for fixing on the gauze cylinder, in

which the wire-gauze cover is fixed, and which is fastened to the

cistern by a screw fitted to _b_.

3. An aperture _c_ for supplying oil. It is fitted with a screw or a

cork, and communicates with the bottom of the cistern by a tube at _f_.

A central aperture for the wick.

4. The wire-gauze cylinder, _fig._ 614., which should not have less than

625 apertures to the square inch.

5. The second top, 3/4 of an inch above the first, surmounted by a brass

or copper plate, to which the ring of suspension may be fixed. It is

covered with a wire cap in the figure.

6. Four or six thick vertical wires, _g´ g´ g´ g´_, joining the cistern

below with the top plate, and serving as protecting pillars round the

cage. _g_ is a screw-pin to fix the cover, so that it shall not become

loosened by accident or carelessness. The oil-cistern _fig._ 615. is

drawn upon a larger scale than _fig._ 614., to show its minuter parts.

When the wire-gauze safe-lamp is lighted and introduced into an

atmosphere gradually mixed with fire-damp, the first effect of the

fire-damp is to increase the length and size of the flame. When the

inflammable gas forms so much as 1-12th of the volume of the air, the

cylinder becomes filled with a feeble blue flame, while the flame of the

wick appears burning brightly within the blue flame. The light of the

wick augments till the fire-damp increases to 1-6th or 1-5th, when it is

lost in the flame of the fire-damp, which in this case fills the

cylinder with a pretty strong light. As long as any _explosive_ mixture

of gas exists in contact with the lamp, so long it will give light; and

when it is extinguished, which happens whenever the foul air constitutes

so much as 1-3d of the volume of the atmosphere, the air is no longer

proper for respiration; for though animal life will continue where flame

is extinguished, yet it is always with suffering. By fixing a coil of

platinum wire above the wick, ignition may be maintained in the metal

when the lamp itself is extinguished; and from this ignited wire the

wick may be again rekindled, on carrying it into a less inflammable

atmosphere.

“We have frequently used the lamps where the explosive mixture was so

high as to heat the wire-gauze red-hot; but on examining a lamp which

has been in constant use for three months, and occasionally subjected to

this degree of heat, I cannot perceive that the gauze cylinder of iron

wire is at all impaired. I have not, however, thought it prudent, in our

present state of experience, to persist in using the lamps under such

circumstances, because I have observed, that in such situations the

particles of coal dust floating in the air, fire at the gas burning

within the cylinder, and fly off in small luminous sparks. This

appearance, I must confess, alarmed me in the first instance, but

experience soon proved that it was not dangerous.

“Besides the facilities afforded by this invention to the working of

coal-mines abounding in fire-damp, it has enabled the directors and

superintendents to ascertain, with the utmost precision and expedition,

both the presence, the quantity, and correct situation of the gas.

Instead of creeping inch by inch with a candle, as is usual, along the

galleries of a mine suspected to contain fire-damp, in order to

ascertain its presence, we walk firmly on with the safe-lamps, and, with

the utmost confidence, prove the actual state of the mine. By observing

attentively the several appearances upon the flame of the lamp, in an

examination of this kind, the cause of accidents which happened to the

most experienced and cautious miners is completely developed; and this

has hitherto been in a great measure matter of mere conjecture.

“It is not necessary that I should enlarge upon the national advantages

which must necessarily result from an invention calculated to prolong

our supply of mineral coal, because I think them obvious to every

reflecting mind; but I cannot conclude without expressing my highest

sentiments of admiration for those talents which have developed the

properties, and controlled the power, of one of the most dangerous

elements which human enterprise has hitherto had to encounter.”--See

Letter to Sir H. Davy, in Journal of Science, vol. i. p. 302., by John

Buddle, Esq., generally and justly esteemed one of the most scientific

coal-miners in the kingdom.

Mr. Buddle, in a letter dated 21st August, 1835, which is published in

Dr. Davy’s life of his brother Sir Humphrey, says;--

“In the evidence given in my last examination before a committee of the

House of Commons, I stated that after nearly twenty years’ experience of

‘the Davy’ with from 1000 to 1500 lamps in daily use, in all the variety

of circumstances incidental to coal mining, without a single accident

having happened which could be attributed to a defect in its principle,

or even in the rules for its practical application, as laid down by Sir

Humphrey--I maintained that ‘the Davy’ approximated perfection, as

nearly as any instrument of human invention could be expected to do. We

have ascertained distinctly that the late explosion did not happen in

that part of the mine where the Davys were used. They were all found in

a perfect state after the accident--many of them in the hands of the

dead bodies of the sufferers.”

LAMP-BLACK. See BLACK.

LAMPATES and LAMPIC ACID. When a spirit of wine lamp has its cotton wick

surmounted with a spiral coil of platinum wire, after lighting it for a

little, it may be blown out, without ceasing to burn the alcohol; for

the coil continues ignited, and a current of hot vapour continues to

rise, as long as the spirit lasts. This vapour was first condensed and

examined by Professor Daniell, who called it lampic acid. It has a

peculiar, strongly acid, burning taste, and a spec. grav. of 1·015. It

possesses in an eminent degree the property of reducing certain metallic

solutions; such as those of platinum, gold, and silver. The _lampates_

may be prepared by saturating the above acid with the alkaline and

earthy carbonates.

LAPIDARY, _Art of_. The art of the lapidary, or that of cutting,

polishing, and engraving gems, was known to the ancients, many of whom

have left admirable specimens of their skill. The Greeks were passionate

lovers of rings and engraved stones; and the most parsimonious among the

higher classes of the Cyrenians are said to have worn rings of the value

of ten minæ (about 30_l._ of our money.) By far the greater part of the

antique gems that have reached modern times, may be considered as so

many models for forming the taste of the student of the fine arts, and

for inspiring his mind with correct ideas of what is truly beautiful.

With the cutting of the diamond, however, the ancients were

unacquainted, and hence they wore it in its natural state. Even in the

middle ages, this art was still unknown; for the four large diamonds

which enrich the clasp of the imperial mantle of Charlemagne, as now

preserved in Paris, are uncut, octahedral crystals. But the art of

working diamonds was probably known in Hindostan and China, in very

remote periods. After Louis de Berghen’s discovery, in 1476, of

polishing two diamonds by their mutual attrition, all the finest

diamonds were sent to Holland to be cut and polished by the Dutch

artists, who long retained a superiority, now no longer admitted by the

lapidaries of London and Paris.

The operation of gem cutting is abridged by two methods; 1. by cleavage;

2. by cutting off slices with a fine wire, coated with diamond powder,

and fixed in the stock of a hand-saw. Diamond is the only precious stone

which is cut and polished with diamond powder, soaked with olive oil,

upon a mill plate of very soft steel.

Oriental rubies, sapphires, and topazes, are cut with diamond powder

soaked with olive oil, on a copper wheel. The facets thus formed are

afterwards polished on another copper wheel, with tripoli, tempered with

water.

Emeralds, hyacinths, amethysts, garnets, agates, and other softer

stones, are cut at a lead wheel, with emery and water; and are polished

on a tin wheel with tripoli and water, or, still better, on a zinc

wheel, with putty of tin and water.

The more tender precious stones, and even the pastes, are cut on a

mill-wheel of hard wood, with emery and water; and are polished with

tripoli and water, on another wheel of hard wood.

Since the lapidary employs always the same tools, whatever be the stone

which he cuts or polishes, and since the wheel discs alone vary, as also

the substance he uses with them, we shall describe, first of all, his

apparatus, and then the manipulations for diamond-cutting, which are

applicable to every species of stone.

[Illustration: 616]

The lapidary’s mill, or wheel, is shewn in perspective in _fig._ 616. It

consists of a strong frame made of oak carpentry, with tenon and

mortised joints, bound together with strong bolts and screw nuts. Its

form is a parallelopiped of from 8 to 9 feet long, by from 6 to 7 high;

and about 2 feet broad. These dimensions are large enough to contain two

cutting wheels alongside of each other, as represented in the figure.

Besides the two sole bars B B, we perceive in the breadth, 5 cross bars,

C, D, E, F, G. The two extreme bars C and G, are a part of the

frame-work, and serve to bind it. The two cross-bars D and F, carry each

in the middle of their length, a piece of wood as thick as themselves,

but only 4-1/2 inches long (see _fig._ 617.), joined solidly by mortises

and tenons with that cross bar, as well as with the one placed opposite

on the other parallel face. These two pieces are called _summers_

(lintels); the one placed at D is the upper; the one at F, the lower.

[Illustration: 617]

In _fig._ 617. this face is shewn inside, in order to explain how the

mill wheel is placed and supported. The same letters point out the same

objects, both in the preceding and the following figures.

In each of these _summers_ a square hole is cut out, exactly opposite to

the other; in which are adjusted by friction, a square piece of oak _a

a_, _fig._ 617., whose extremities are perforated with a conical hole,

which receives the two ends of the arbor H of the wheel I, and forms its

socket. This square bar is adjusted at a convenient height, by a double

wooden wedge _b b_.

The cross bar in the middle E supports the table _c c_, a strong plank

of oak. It is pierced with two large holes whose centres coincide with

the centre of the conical holes hollowed out at the end of the square

pins. These holes, of about 6 inches diameter each, are intended to let

the arbor pass freely through, bearing its respective wheel. (See one of

these holes at I, in _fig._ 621. below.)

[Illustration: 618 619]

Each wheel is composed of an iron arbor H, _fig._ 618., of a

grinding-wheel I, which differs in substance according to circumstances,

as already stated, and of the pulley J, furnished with several grooves

(see _fig._ 619.), which has a square fit upon the arbor. The arbor

carries a collet _d_, on which are 4 iron pegs or pins that enter into

the wheel to fasten it.

The wheel plate, of which the ground plan is shown at K, is hollowed out

towards its centre to half its thickness; when it is in its position on

the arbor, as indicated in _fig._ 619., a washer or ferrule of wrought

iron is put over it, and secured in its place by a double wedge. In

_fig._ 619. the wheel-plate is represented in section, that the

connection of the whole parts may be seen.

A board _g_ (see _fig._ 616. and _fig._ 624.), about 7-1/2 inches high,

is fixed to the part of the frame opposite to the side at which the

lapidary works, and it prevents the substances made use of in the

cutting and polishing, from being thrown to a distance by the

centrifugal force of the wheel-plate.

Behind this apparatus is mounted for each grinding-plate, a large wheel

L (see _fig._ 616.), similar to a cutler’s, but placed horizontally.

This wheel is grooved round its circumference to receive an endless cord

or band, which passes round one of the grooves of the pulley J, fixed

below the wheel-plate. Hence, on turning the fly-wheel L, the plate

revolves with a velocity relative to the velocity communicated to the

wheel L, and to the difference of diameter of the wheel L and the pulley

J. Each wheel L, is mounted on an iron arbor, with a crank (see M,

_fig._ 620.)

[Illustration: 620]

The lower pivot of that arbor _h_ is conical, and turns in a socket

fixed in the floor. The great wheel L rests on the collet _i_, furnished

with its 4 iron pins, for securing the connection. Above the wheel an

iron washer is laid, and the whole is fixed by a double wedge, which

enters into the mortise _l_, _fig._ 620.

[Illustration: 621]

[Illustration: 622]

_Fig._ 621. exhibits a ground-plan view of all this assemblage of parts,

to explain the structure of the machine. Every thing that stands above

the upper _summer-bar_ has been suppressed in this representation. Here

we see the table _c c_; the upper _summer_ _m_; the one wheel-plate _l_,

the other having been removed to shew that the endless cord does not

cross; the two large wheels L L, present in each machine, the crank bar

N, seen separate in _fig._ 622, which serves for turning the wheel L.

This bar is formed of 3 iron plates _n_, _o_; _p_, _q_; and _q_, _r_;

(_fig._ 622.) The first is bent round at the point _n_, to embrace the

stud _s_; the second _p q_, is of the same breadth and thickness as the

first; and the third, is adjusted to the latter with a hinge joint, at

the point _q_, where they are both turned into a circular form, to

embrace the crank M. When all these pieces are connected, they are fixed

at the proper lengths by the buckles or square rings _t t t_, which

embrace these pieces, as is shown in _fig._ 622.

[Illustration: 623]

The stud _s_, seen in _fig._ 622., is fixed to the point _v_ by a

wedge-key upon the arm P, represented separately, and in perspective, in

_fig._ 623. The labourer seizing the two upright pegs or handles _x x_;

by the alternate forward and backward motion of his arm, he communicates

the same motion to the crank rod, which transmits it to the crank of the

arbor M, and impresses on that arbor, and the wheel which it bears, a

rotatory movement.

[Illustration: 624]

_Fig._ 624. shows piece-meal and in perspective, a part of the

lapidary’s wheel-mill. There we see the table _c c_, the grind-plate I,

whose axis is kept in a vertical position by the two square plugs _a a_,

fixed into the two _summers_ by the wedges _b b_. On the two sides of

the wheel-plate we perceive an important instrument called a _dial_,

which serves to hold the stone during the cutting and polishing. This

instrument has received lately important ameliorations, to be described

in _fig._ 625. The lapidary holds this instrument in his hand, he rests

it upon the iron pins _u u_ fixed in the table, lest he should be

affected by the velocity of the revolving wheel-plate. He loads it

sometimes with weights _e_, _e_, to make it take better hold of the

grinding plate.

One of the most expert lapidaries of Geneva works by means of the

following improved mechanism, of his own invention, whereby he cuts and

polishes the facets with extreme regularity, converting it into a true

dial.

[Illustration: 625]

_Fig._ 625. shows this improvement. Each of the two jaws bears a large

conchoidal cavity, into which is fitted a brass ball, which carries on

its upper part a tube _e_, to whose extremity is fixed a dial-plate _f

f_, engraved with several concentric circles, divided into equal parts,

like the toothed-wheel cutting engine-plate, according to the number of

facets to be placed in each cutting range. The tube receives with

moderate friction the handle of the cement rod, which is fixed at the

proper point by a thumb-screw, not shown in the figure, being concealed

by the vertical limb _d_, about to be described.

A needle or index _g_, placed with a square fit on the tail of the

cement rod, marks by its point the divisions on the dial plate _f f_. On

the side _m n_ of the jaw A, there is fixed by two screws, a limb _d_,

forming a quadrant whose centre is supposed to be at the centre of the

ball. This quadrant is divided as usual into 90 degrees, whose highest

point is marked 0, and the lowest would mark about 70; for the remainder

of the arc down to 90 is concealed by the jaw. The two graduated plates

are used as follows:--

When the cement rod conceals zero or 0 of the limb, it is then vertical,

and serves to cut the table of the brilliant; or the point opposite to

it, and parallel to the table. On making it slope a little, 5 degrees

for example, all the facets will now lie in the same zone, provided that

the inclination be not allowed to vary. On turning round the cement rod

the index _g_ marks the divisions, so that by operating on the circle

with 16 divisions, stopping for some time at each, 16 facets will have

been formed, of perfect equality, and at equal distances, as soon as the

revolution is completed.

Diamonds are cut at the present day in only two modes; into a rose

diamond, and a brilliant. We shall therefore confine our attention to

these two forms.

The rose diamond is flat beneath, like all weak stones, while the upper

face rises into a dome, and is cut into facets. Most usually six facets

are put on the central region, which are in the form of triangles, and

unite at their summits; their bases abut upon another range of

triangles, which being set in an inverse position to the preceding,

present their bases to them, while their summits terminate at the sharp

margin of the stone. The latter triangles leave spaces between them

which are likewise cut each into two facets. By this distribution the

rose diamond is cut into 24 facets; the surface of the diamond being

divided into two portions, of which the upper is called the crown, and

that forming the contour, beneath the former, is called _dentelle_

(lace) by the French artists.

According to Mr. Jefferies, in his Treatise on Diamonds, the regular

rose diamond is formed by inscribing a regular octagon in the centre of

the table side of the stone, and bordering it by eight right-angled

triangles, the bases of which correspond with the sides of the octagon;

beyond these is a chain of 8 trapeziums, and another of 16 triangles.

The collet side also consists of a minute central octagon, from every

angle of which proceeds a ray to the edge of the girdle, forming the

whole surface into 8 trapeziums, each of which is again subdivided by a

salient angle (whose apex touches the girdle) into one irregular

pentagon and two triangles.

To fashion a rough diamond into a brilliant, the first step is to modify

the faces of the original octahedron, so that the plane formed by the

junction of the two pyramids shall be an exact square, and the axis of

the crystal precisely twice the length of one of the sides of the

square. The octahedron being thus rectified, a section is to be made

parallel to the common base or _girdle_, so as to cut off 5 eighteenths

of the whole height from the upper pyramid, and 1 eighteenth from the

lower one. The superior and larger plane thus produced is called the

_table_, and the inferior and smaller one is called the _collet_; in

this state it is termed a _complete square table diamond_. To convert it

into a brilliant, two triangular facets are placed on each side of the

table, thus changing it from a square to an octagon; a lozenge-shaped

facet is also placed at each of the four corners of the table, and

another lozenge extending lengthwise along the whole of each side of the

original square of the table, which with two triangular facets set on

the base of each lozenge, completes the whole number of facets on the

table side of the diamond; viz. 8 lozenges, and 24 triangles. On the

collet side are formed 4 irregular pentagons, alternating with as many

irregular lozenges radiating from the collet as a centre, and bordered

by 16 triangular facets adjoining the girdle. The brilliant being thus

completed, is set with the table side uppermost, and the collet side

implanted in the cavity made to receive the diamond. The brilliant is

always three times as thick as the rose diamond. In France, the

thickness of the brilliant is set off into two unequal portions; one

third is reserved for the upper part or table of the diamond, and the

remaining two thirds for the lower part or collet (_culasse_). The table

has eight planes, and its circumference is cut into facets, of which

some are triangles, and others lozenges. The collet is also cut into

facets called _pavillons_. It is of consequence that the pavillons lie

in the same order as the upper facets, and that they correspond to each

other, so that the symmetry be perfect, for otherwise the play of the

light would be false.

Although the rose-diamond projects bright beams of light in more

extensive proportion often than the brilliant, yet the latter shows an

incomparably greater play, from the difference of its cutting. In

executing this, there are formed 32 faces of different figures, and

inclined at different angles all round the table, on the upper side of

the stone. On the _collet_ (culasse) 24 other faces are made round a

small table, which converts the culasse into a truncated pyramid. These

24 facets, like the 32 above, are differently inclined and present

different figures. It is essential that the faces of the top and the

bottom correspond together in sufficiently exact proportions to multiply

the reflections and refractions, so as to produce the colours of the

prismatic spectrum.

The other precious stones, as well as their artificial imitations,

called _pastes_, are cut in the same fashion as the brilliant; the only

difference consists in the matter constituting the wheel plates, and the

grinding and polishing powders, as already stated.

[Illustration: 626]

In cutting the stones, they are mounted on the cement-rod B, _fig._

626., whose stem is set upright in a socket placed in the middle of a

sole piece at A, which receives the stem of the cement-rod. The head of

the rod fills the cup of A. A melted alloy of tin and lead is poured

into the head of the cement-rod, into the middle of which the stone is

immediately plunged; and whenever the solder has become solid, a portion

of it is pared off from the top of the diamond, to give the pyramidal

form shown in the figure at B.

[Illustration: 627]

There is an instrument employed by the steel polishers for pieces of

clock work, and by the manufacturers of watch-glasses for polishing

their edges. It consists of a solid oaken table, _fig._ 627. The top is

perforated with two holes, one for passing through the pulley and the

arbor of the wheel-plate B, made either of lead or of hard wood,

according to circumstances; and the other C for receiving the upper part

of the arbor of the large pulley D. The upper pulley of the wheel-plate

is supported by an iron prop E, fixed to the table by two wooden screws.

The inferior pivots of the two pieces are supported by screw-sockets,

working in an iron screw-nut sunk into the summer-bar F. The legs of the

table are made longer or shorter, according as the workman chooses to

stand or sit at his employment. Emery with oil is used for grinding

down, and tin-putty or colcothar for polishing. The workman lays the

piece on the flat of the wheel-plate with one hand, and presses it down

with a lump of cork, while he turns round the handle with the other

hand.

The _Sapphire_, _Ruby_, _Oriental Amethyst_, _Oriental Emerald_, and

_Oriental Topaz_, are gems next in value and hardness to diamond; and

they all consist of nearly pure alumina or clay, with a minute portion

of iron as the colouring matter. The following analyses show the

affinity in composition of the most precious bodies with others in

little relative estimation.

+---------------+---------+---------------+------+

| +---------+---------------+------+

|Alumina or clay| 98·5 | 89·50 | 86·0 |

|Silica | 0·0 | 5·50 | 3·0 |

|Oxide of iron | 1·0 | 1·25 | 4·0 |

|Lime | 0·5 | 0·00 | 0·0 |

| +---------+---------------+------+

| | 100·0 | 96·25 | 93·0 |

+---------------+---------+---------------+------+

_Salamstone_ is a variety which consists of small transparent crystals,

generally six-sided prisms, of pale reddish and bluish colours. The

corundum of Battagammana is frequently found in large six-sided prisms:

it is commonly of a brown colour, whence it is called by the natives

_curundu gallé_, cinnamon stone. The hair-brown and reddish-brown

crystals are called adamantine spar. Sapphire and salamstone are chiefly

met with in secondary repositories, as in the sand of rivers &c.,

accompanied by crystals and grains of octahedral iron-ore and of several

species of gems. Corundum is found in imbedded crystals in a rock,

consisting of indianite. Adamantine spar occurs in a sort of granite.

The finest varieties of sapphire come from Pegu, where they occur in the

Capelan mountains near Syrian. Some have been found also at Hohenstein

in Saxony, Bilin in Bohemia, Puy in France, and in several other

countries. The red variety, the ruby, is most highly valued. Its colour

is between a bright scarlet and crimson. A perfect ruby above 3-1/2

carats is more valuable than a diamond of the same weight. If it weigh

one carat, it is worth 10 guineas; 2 carats, 40 guineas; 3 carats, 150

guineas; 6 carats, above 1000 guineas. A deep coloured ruby, exceeding

20 carats in weight, is generally called a carbuncle; of which 108 were

said to be in the throne of the Great Mogul, weighing from 100 to 200

carats each; but this statement is probably incorrect. The largest

oriental ruby known to be in the world, was brought from China to Prince

Gargarin, governor of Siberia. It came afterwards into the possession of

Prince Menzikoff, and constitutes now a jewel in the imperial crown of

Russia.

A good blue sapphire of 10 carats is valued at 50 guineas. If it weighs

20 carats, its value is 200 guineas; but under 10 carats, the price may

be estimated by multiplying the square of its weight in carats into half

a guinea; thus, one of 4 carats would be worth 4² × 1/2 G. = 8 guineas.

It has been said that the blue sapphire is superior in hardness to the

red, but this is probably a mistake arising from confounding the

corundum ruby with the spinelle ruby. A sapphire of a barbel blue

colour, weighing 6 carats, was disposed of in Paris by public sale for

70_l._ sterling; and another of an indigo blue, weighing 6 carats and 3

grains, brought 60_l._; both of which sums much exceed what the

preceding rule assigns, from which we may perceive how far fancy may go

in such matters. The sapphire of Brasil is merely a blue tourmaline, as

its specific gravity and inferior hardness show. White sapphires are

sometimes so pure, that when properly cut and polished they have been

passed for diamonds.

The yellow and green sapphires are much prized under the names of

Oriental topaz and emerald. The specimens which exhibit all these

colours associated in one stone are highly valued, as they prove the

mineralogical identity of these varieties.

Besides these shades of colour, sapphires often emit a beautiful play of

colours, or _chatoiement_, when held in different positions relative to

the eye or incident light; and some likewise present star-like

radiations, whence they are called star-stones or _asterias_; sending

forth 6 or even 12 rays, that change their place with the position of

the stone. This property so remarkable in certain blue sapphires, is not

however peculiar to these gems. It seems to belong to transparent

minerals which have a rhomboid for their nucleus, and arises from the

combination of certain circumstances in their cutting and structure.

Lapidaries often expose the light-blue variety of sapphire to the action

of fire, in order to render it white and more brilliant; but with regard

to those found at Expailly in France, fire deepens their colour.

3. _Chrysoberyl_, called by Haüy Cymophane, and by others Prismatic

corundum, ranks next in hardness to sapphire, being 8·5 on the same

scale of estimation. Its specific gravity is 3·754. It usually occurs in

rounded pieces about the size of a pea, but it is also found

crystallised in many forms, of which 8-sided prisms with 8-sided summits

are perhaps the most frequent. Lustre vitreous; colour asparagus green,

passing into greenish-white and olive-green. It shows a bluish

opalescence, a light undulating as it were in the stone, when viewed in

certain directions; which property constitutes its chief attraction to

the jeweller. When polished, it has been sometimes mistaken for a yellow

diamond; and from its hardness and lustre is considerably valued. Good

specimens of it are very rare. It has been found only in the alluvial

deposits of rivers, along with other species of gems. Thus it occurs in

Brasil, along with diamonds and prismatic topaz; also in Ceylon. Its

constituents are, alumina 68·66; glucina 16·00; silica 6·00; protoxide

of iron 4·7; oxide of titanium 2·66; moisture 0·66, according to

Seybert’s analysis of a specimen from Brasil. It is difficultly but

perfectly fusible before the blowpipe, with borax and salt of

phosphorus. In composition it differs entirely from sapphire, or the

rhombohedral corundum.

4. _Spinelle Ruby_, called Dodecahedral corundum by some mineralogists,

and Balas ruby by lapidaries. Its hardness is 8. Specific gravity 3·523.

Its fundamental form is the hexahedron, but it occurs crystallized in

many secondary forms: octahedrons, tetrahedrons and rhombohedrons.

Fracture conchoidal; lustre vitreous; colour red, passing into blue and

green, yellow, brown and black; and sometimes it is nearly white. Red

spinelle consists of, alumina 74·5; silica 15·5; magnesia 8·25; oxide of

iron 1·5; lime 0·75. Vauquelin discovered 6·18 _per cent._ of chromic

acid in the red spinelle. The red varieties exposed to heat, become

black and opaque; on cooling they appear first green, then almost

colourless, but at last resume their red colour. _Pleonaste_ is a

variety which yields a deep green globule with borax.

Crystals of spinelle from Ceylon have been observed imbedded in

limestone, mixed with mica, or in rocks containing adularia, which seem

to have belonged to a primitive district. Other varieties like the

pleonaste occur in the drusy cavities of rocks ejected by Vesuvius.

Crystals of it are often found in diluvial and alluvial sand and gravel,

along with true sapphires, pyramidal zircon, and other gems, as also

with octahedral iron ore, in Ceylon. Blue and pearl-gray varieties occur

in Südermannland in Sweden, imbedded in granular limestone. Pleonaste is

met with also in the diluvial sands of Ceylon. Clear and finely coloured

specimens of spinelle are highly prized as ornamental stones. When the

weight of a good spinelle exceeds 4 carats, it is said to be valued at

half the price of a diamond of the same weight. M. Brard has seen one at

Paris, which weighed 215 grains.

5. _Zircon_ or _Hyacinth_. Its fundamental form is an isosceles 4-sided

pyramid; and the secondary forms have all a pyramidal character.

Fracture conchoidal, uneven; lustre more or less perfectly adamantine;

colours, red, brown, yellow, gray, green, white; which with the

exception of some red tints, are not bright. Hardness 7·5. Specific

gravity 4·5. Zircon and hyacinth consist, according to Klaproth, of

almost exactly the same constituents; namely, zirconia 70; silica 25;

oxide of iron 5. In the white zirconia there is less iron and more

silica. Before the blowpipe the hyacinth loses its colour, but does not

melt. The brighter zircons are often worked up into a _brilliant_ form,

for ornamenting watch cases. As a gem, hyacinth has no high value. It

has been often confounded with other stones, but its very great specific

gravity makes it to be readily recognized.

6. _Topaz._ The fundamental form is a scalene 4-sided pyramid; but the

secondary forms have a prismatic character; and are frequently observed

in oblique 4-sided prisms, acuminated by 4 planes. The lateral planes of

the prism are longitudinally striated. Fracture conchoidal, uneven;

lustre vitreous; colours, white, yellow, green, blue, generally of pale

shades. Hardness 8; specific gravity 3·5. Prismatic topaz consists,

according to Berzelius, of alumina 57·45; silica 34·24; fluoric acid

7·75. In a strong heat the faces of crystallization, but not those of

cleavage, are covered with small blisters, which however immediately

crack. With borax, it melts slowly into a transparent glass. Its powder

colours the tincture of violets green. Those crystals which possess

different faces of crystallization on opposite ends, acquire the

opposite electricities on being heated. By friction, it acquires

positive electricity.

Most perfect crystals of topaz have been found in Siberia, of green,

blue, and white colours, along with beryl, in the Uralian and Altai

mountains, as also in Kamschatka; in Brazil, where they generally occur

in loose crystals, and pebble forms of bright yellow colours; and in

Mucla in Asia Minor, in pale straw-yellow regular crystals. They are

also met with in the granitic detritus of Cairngorm in Aberdeenshire.

The blue varieties are absurdly called _oriental aquamarine_ by

lapidaries. If exposed to heat, the Saxon topaz loses its colour and

becomes white; the deep yellow Brazilian varieties assume a pale pink

hue; and are then sometimes mistaken for spinelle, to which, however,

they are somewhat inferior in hardness. Topaz is also distinguishable by

its double refractive property. Tavernier mentions a topaz, in the

possession of the Great Mogul, which weighed 157 carats, and cost

20,000_l._ sterling. There is a specimen in the museum of natural

history at Paris which weighs 4 ounces 2 gros.

Topazes are not scarce enough to be much valued by the lapidary.

7. _Emerald_ and _Beryl_, are described in their alphabetical places.

Emerald loses its lustre by candle-light; but as it appears to most

advantage when in the company of diamonds, it is frequently surrounded

with brilliants, and occasionally with pearls. Beryl is the aqua-marine

of the jewellers, and has very little estimation among lapidaries.

A Dictionary of Arts, Manufactures and Mines containing a clear exposition of their principles and practice · 第 242 / 421 题