Chapter XI: Part 11

The art of cameo-engraving, which, as we have seen, attained its greatest splendour at the beginning of the empire, followed on the whole a similar course. It waned in the early part of the 3rd century after the death of the emperor Severus, but under the first Christian emperor Constantine it enjoyed a brief period of revival. Fine cameo portraits of Constantine are extant; and it was during or shortly after his reign that Christian Scripture subjects began to appear on cameos. That class of subjects constituted the staple of such work--generally rude and artistically debased--as continued to be cultivated under the Byzantine empire down to nearly the epoch of the Renaissance. From the Byzantine period downward one peculiarity of gem-engraving becomes noticeable. Cameo-work as compared with intaglios in classical times was rare and infrequent, but now and onwards the opposite is the case, intaglio-sinking having almost died out, and cameos being chiefly produced. Commercial intercourse with the East still secured for the engravers a supply of magnificent sardonyxes, although blood-stone and other non-banded stones were very commonly used for works in relief. Cameos during the long dark ages were used chiefly for the decoration of reliquaries and other altar furniture, and as such their designs were purely ecclesiastical or scriptural. To this period also belongs the class of complimentary or motto cameos, which, containing only inscriptions and an ornamental border, executed in nicolo stones, were used as personal gifts and adornments.

In medieval times antique cameos were held in peculiar veneration on account of the belief, then universal, in their potency as medicinal charms. This power was supposed to be derived from their origin, of which two theories, equally satisfactory, were current. By the one they were held to be the work of the children of Israel during their sojourn in the wilderness (hence the name _Pierres d'Israël_), while the other theory held them to be direct products of nature, the engraved figures pointing to the peculiar virtue lodged in them. Interpreters less mystically inclined found Biblical interpretations for the subjects. Thus the cameo of the Sainte Chapelle was supposed to represent the triumph of Joseph in Egypt. A cameo with Poseidon, Athena and her serpent was Adam and Eve.

The revival of the glyptic arts in western Europe dates from the pontificate of the Venetian Paul II. (1464-1471), himself an ardent lover and collector of gems, to which passion, indeed, it is gravely affirmed he was a martyr, having died of a cold caught by the multiplicity of gems exposed on his fingers. The cameos of the early part of the 16th century rival in beauty of execution the finest classical works, and, indeed, many of them pass in the cabinets of collectors for genuine antiques, which they closely imitated. The Oriental sardonyx was not available for the purposes of the Renaissance artists, who were consequently obliged to content themselves with the colder German agate onyx. The scarcity of worthy materials led them to use the backs of ancient cameos, or to improve on classical works of inferior value executed on good material, and probably to this cause must also be assigned the development of shell cameos, which are rarely found, of an older period.

Among the means of distinguishing antique cameos from cinquecento work, the kind of stone is one of the best tests, the classical artists having used only rich and warm-tinted Oriental stones, which further are frequently drilled through their diameter with a minute hole, from having been used by their original Oriental possessors in the form of beads. The cinquecento artists also, as a rule, worked their subjects in high relief, and resorted to undercutting, no case of which is found in the flat low work of classical times. The projecting portions of antique work exhibit a dull chalky appearance, which, however, fabricators learned to imitate in various ways, one of which was by cramming the gizzards of turkey fowls with the gems. Another index of antiquity is found in the different methods of working adopted in classical and Renaissance times. The tools employed by the Renaissance engraver were the drill and the wheel, while the ancient artist also employed the diamond point.

The gem-engraver's art again during the 18th century revived under an even greater amount of encouragement from men of wealth and rank. In this last period the names of engravers who succeeded best in imitating classical designs were Natter, Pichler (fig. 14), and the Englishmen Marchant (fig. 15) and Burch. Compared with Greek gems, it will be seen that what at first sight is attractive as refined and delicate is after all an exaggerated minuteness of execution, entirely devoid of the ancient spirit. The success with which modern engravers imposed on collectors is recorded in many instances, of which one may be taken as an instructive type. In the Bibliothèque Nationale is a gem (Chabouillet's catalogue, No. 2337), familiarly known as the signet of Michelangelo, the subject being a Bacchanalian scene. So much did he admire it, the story says, that he copied from it one of the groups in his paintings in the Sistine chapel. The gem, however, is evidently in this part of it a mere copy from Michelangelo's group, and therefore a subsequent production, probably by da Pescia.

In our own day the engraving of cameos has practically ceased to be pursued as an art. Roman manufacturers cut stones in large quantities to be used as shirt-studs and for setting in finger-rings; and in Rome and Paris an extensive trade is carried on in the cutting of shell cameos, which are largely imported into England and mounted as brooches by Birmingham jewelry manufacturers. The principal shell used is the large bull's-mouth shell (_Cassis rufa_), found in East Indian seas, which has a sard-like underlayer. The black helmet (_Cassis tuberosa_) of the West Indian seas, the horned helmet (_C. cornuta_) of Madagascar, and the pinky queen's conch (_Strombus gigas_) of the West Indies are also employed. The famous potter Josiah Wedgwood introduced a method of making imitations of cameos in pottery by producing white figures on a coloured ground, this constituting the peculiarity of what is now known as Wedgwood ware.

_Gem Collectors._--The habit of gem-collecting is recorded first in the instance of Ismenias, a musician of Cyprus, who appears to have lived in the 4th century B.C. But though individual collectors are not again mentioned till the time of Mithradates, whose cabinet was carried off to Rome by Pompey, still it is to be inferred that they existed, if not pretty generally, yet in such places as Cyrene, where the passion for gems was so great that the thriftiest person owned one worth 10 minas, and where, according to Aelian (_Var. hist._ xii. 30), the skill in engraving was astonishing. The first cabinet (_dactyliotheca_) in Rome was that of Scaurus, a stepson of Sulla. Caesar is said to have formed six cabinets for public exhibition, and from the time of Augustus all men of refinement were supposed to be judges both of the art and of the quality of the stones.

In the middle ages the chief collections were incorporated in works of art in the church treasuries. The first collector of modern times was, as already mentioned, Pope Paul II., who was followed by a long succession of princely and noble collectors such as Lorenzo de' Medici and the great earl of Arundel. The collection of the latter passed into the hands of the dukes of Marlborough and thence into the possession of Mr David Bromilow. The collection was finally dispersed by auction in June 1899.

In modern times the principal collections are contained in state museums. The cabinets of Vienna and of the Bibliothèque Nationale are incomparably rich in the historic cameos. Those of the British Museum and of Berlin are the strongest in their range over the whole field of the gem-engraver's art.

BIBLIOGRAPHY.--For the fullest general account of the subject (with
especial attention to the gems of classical antiquity) see A.
Furtwängler, _Die antiken Gemmen, Geschichte der Steinschneiderkunst
im klassischen Altertum_, in 3 vols (1900). See also E. Babelon, _La
Gravure en pierres fines, camées et intailles_ (1894); A.H. Smith,
"Gemma" and "Sculptura," in the 3rd edition of Smith's _Dict. of
Antiquities_; J.H. Middleton, _The Engraved Gems of Classical Times_
(1891). Much curious information is in the works of C.W. King:
_Handbook of Engraved Gems_ (1866); _Antique Gems_ (1866); _The
Natural History, Ancient and Modern, of Precious Stones and Gems, and
of the Precious Metals_ (1865); _Antique Gems and Rings_ (2 vols.,
1872).

Special Periods:--_Babylonia, &c._--Menant, "Les Pierres gravées de la
haute Asie," _Recherches sur la glyptique orientale_ (1883-1886).

_Egypt._--For the early cylinder sealings, &c. see Petrie, "Royal
Tombs of the First Dynasty" (_Egypt Explor. Fund, XVIIIth Memoir_), p.
24; pls. 12, figs. 3 to 7, and pls. 18-29; Amélineau, "Nouvelles
Fouilles d'Abydos, 1897-1898," _Compte rendu_, pp. 78, 423; pl. 25,
figs. 1-3.

_The Bible._--Petrie, "Stones (Precious)," in Hastings' _Dict. of the
Bible_.

_Phoenician._--See M.A. Levy, _Siegel und Gemmen_, with three plates
of gems having Phoenician, Aramaic, old Hebrew and other inscriptions
(Breslau, 1869); and, on the same subject, De Voguë, in the _Revue
archéologique_, 2nd series (1868), xvii. p. 432, pls. 14-16.

_Crete._--Articles by A.J. Evans in _Journal of Hellenic Studies_,
xiv., xvii., xxi., and in _Annual of British School at Athens_, vi.
and onwards.

_Classical Gems._--See Furtwängler, op. cit.

_Gnostic Gems._--Cabrol, _Dict. d'archéologie chrétienne_, s.v.
"Abrasax."

For the controversy as to gems with artists' signatures, see Koehler,
_Abhandlung über die geschnittenen Steine, mit den Namen der
Künstler_; Koehler's collected works, ed. Stephani, vol. iii. (1851);
Stephani, Notes to Koehler as above; also _Über einige angebliche
Steinschneider des Alterthums_ (St Petersburg, 1851); Brunn,
_Geschichte der griechischen Künstler_, ii. (1859), pp. 442-637;
Furtwängler, _Jahrbuch d. k. deutsch. arch. Inst._ iii. (1888), pp.
105, 193, 297; iv. (1889), p. 46, and _Geschichte_, passim.

For the history of the Poniatowski gems, see Reinach, _Pierres
gravées_, p. 151.

_Catalogues._--The chief catalogues dealing with modern public
collections are: Berlin, A. Furtwängler, _Beschreibung der
geschnittenen Steine im Antiquarium_ (1896); British Museum, A.H.
Smith, _A Catalogue of Engraved Gems in the British Museum_ (_Dept. of
Greek and Roman Antiquities_) (1888); Paris, Bibliothèque Nationale,
Chabouillet, _Catalogue ... des camées et pierres gravées de la
Bibliothèque Impériale_ (1858); E. Babelon, _Catalogue des camées ...
de la Bibliothèque Nationale_ (1897).

_Modern Engraving._--Vasari vii. p. 113 (ed. Siena, 1792); continued
by Mariette, _Traité des pierres gravées_ (1750), i. p. 105. The older
books on gems are very numerous, but those of present-day importance
are not many. Faber, _Illustrium imagines ... apud Fulvium Ursinum_
(Antwerp, 1606); Stosch, _Gemmae antiquae caelatae, scalptorum
nominibus insignitae_ (Amsterdam, 1724); Winckelmann, _Description des
pierres gravées du feu Baron de Stosch_ (1760); Krause, _Pyrgoteles,
oder die edlen Steine der Alten_ (1856); a convenient reissue of
Stosch, and seven others of the older works, by S. Reinach, _Pierres
gravées, &c. ... réunies et rééditées, avec un texte nouveau_ (1895).

_Pastes._--The principal collection of glass and sulphur pastes from
gems was that issued by James Tassie of Glasgow, with _A Descriptive
Catalogue of a General Collection of ... Engraved Gems ... arranged
and described by R.E. Raspe_ (the author of _Baron Munchausen_)
(1791). (A. S. M.; A. H. Sm.)

FOOTNOTE:

[1] For Nos. 1-4 see Furtwängler, pl. 14; for Nos. 2-4 see Evans,
_Rev. archéologique_, xxxii. (1898) pl. 8.

GEM, ARTIFICIAL. The term "Artificial Gems" does not mean _imitations_ of real gems, but the actual formation by artificial means of the real precious stone, so that the product is identical, chemically, physically and optically, with the one found in nature. For instance, in chemical composition the lustrous diamond is nothing but crystallized carbon. Could we take black amorphous carbon in the form of charcoal or lampblack and dissolve it in a liquid, and by the slow evaporation of that liquid allow the dissolved carbon to separate out, it would probably crystallize in the transparent form of diamond. This would be a true synthesis of diamond, and the product would be just as much entitled to the name as the choicest products of Kimberley or Golconda. But this is a very different thing from the imitation diamond so common in shop windows. Here the chemist has only succeeded in making a paste or glass having limpidity and a somewhat high refractivity, but wanting the hardness and "fire" of the real stone.

_The Diamond._--Within recent years chemists have actually succeeded in making the real diamond by artificial means, and although the largest yet made is not more than one-fiftieth of an inch across, the process itself and the train of reasoning leading up to such an achievement are sufficiently interesting to warrant a somewhat full description. Attempts to make diamonds artificially have been numerous, but, with the sole exception of those of Henri Moissan, all have resulted in failure. The nearest approach to success was attained by J.B. Hannay in 1880 and R.S. Marsden in 1881; but their results have not been verified by others who have tried to repeat them, and the probability is that what was then thought to be diamond was in reality carborundum or carbide of silicon.

Attempts have been made by two methods to make carbon crystallize in the transparent form. One is to crystallize it slowly from a solution in which it has been dissolved. The difficulty is to find a solvent. Many organic and some inorganic bodies hold carbon so loosely combined that it can be separated out under the influence of chemical action, heat or electricity, but invariably the carbon assumes the black amorphous form. The other method is to try to fuse the carbon by fierce heat, when from analogy it is argued that on cooling it will solidify to a clear limpid crystal. The progress of science in other directions has now made it pretty certain that the true mode of making diamond artificially is by a combination of these two methods. Until recently it was assumed that carbon was non-volatile at any attainable temperature, but it is now known that at a temperature of about 3600° C. it volatilizes readily, passing without liquefying directly from the solid to the gaseous state. Very few bodies act in this manner, the great majority when heated at atmospheric pressure to a sufficient temperature passing through the intermediate condition of liquidity. Some few, however, which when heated at atmospheric pressure do not liquefy, when heated at higher pressures in closed vessels obey the common rule and first become liquid and then volatilize. Sir James Dewar found the critical pressure of carbon to be about 15 tons on the sq. in.; that is to say, if heated to its critical temperature (3600° C.), and at the same time subjected to a pressure of 15 tons to the sq. in., it will assume the liquid form. Enormous as such pressures and temperatures may appear to be, they have been exceeded in some of Sir Andrew Noble's and Sir F. Abel's researches; in their investigations on the gases from gunpowder and cordite fired in closed steel chambers, these chemists obtained pressures as great as 95 tons to the sq. in., and temperatures as high as 4000° C. Here then, if the observations are correct, we have sufficient temperature and enough pressure to liquefy carbon; and, were there only sufficient time for these to act on the carbon, there is little doubt that the artificial formation of diamonds would soon pass from the microscopic stage to a scale more likely to satisfy the requirements of science, if not those of personal adornment.

It has long been known that the metal iron in a molten state dissolves carbon and deposits it on cooling as black opaque graphite. Moissan carried out a laborious and systematic series of experiments on the solubility of carbon in iron and other metals, and came to the conclusion that whereas at ordinary pressures the carbon separates from the solidifying iron in the form of graphite, if the pressure be greatly increased the carbon on separation will form liquid drops, which on solidifying will assume the crystalline shape and become true diamond. Many other metals dissolve carbon, but molten iron has been found to be the best solvent. The quantity entering into solution increases with the temperature of the metal. But temperature alone is not enough; pressure must be superadded. Here Moissan ingeniously made use of a property which molten iron possesses in common with some few other liquids--water, for instance--of increasing in volume in the act of passing from the liquid to the solid state. Pure iron is mixed with carbon obtained from the calcination of sugar, and the whole is rapidly heated in a carbon crucible in an electric furnace, using a current of 700 amperes and 40 volts. The iron melts like wax and saturates itself with carbon. After a few minutes' heating to a temperature above 4000° C.--a temperature at which the lime furnace begins to melt and the iron volatilizes in clouds--the dazzling, fiery crucible is lifted out and plunged beneath the surface of cold water, where it is held till it sinks below a red heat. The sudden cooling solidifies the outer skin of molten metal and holds the inner liquid mass in an iron grip. The expansion of the inner liquid on solidifying produces enormous pressure, and under this stress the dissolved carbon separates out in a hard, transparent, dense form--in fact, as diamond. The succeeding operations are long and tedious. The metallic ingot is attacked with hot _aqua regia_ till no iron is left undissolved. The bulky residue consists chiefly of graphite, together with translucent flakes of chestnut-coloured carbon, hard black opaque carbon of a density of from 3.0 to 3.5, black diamonds--carbonado, in fact--and a small quantity of transparent colourless diamonds showing crystalline structure. Besides these there may be corundum and carbide of silicon, arising from impurities in the materials employed. Heating with strong sulphuric acid, with hydrofluoric acid, with nitric acid and potassium chlorate, and fusing with potassium fluoride--operations repeated over and over again--at last eliminate the graphite and impurities and leave the true diamond untouched. The precious residue on microscopic examination shows many pieces of black diamond, and other colourless transparent pieces, some amorphous, others crystalline. Although many fragments of crystals are seen, the writer has scarcely ever met with a complete crystal. All appear broken up, as if, on being liberated from the intense pressure under which they were formed, they burst asunder. Direct evidence of this phenomenon has been seen. A very fine piece of diamond, prepared in the way just described and carefully mounted on a microscopic slide, exploded during the night and covered the slide with fragments. This bursting paroxysm is not unknown at the Kimberley mines.

Sir William Crookes in 1906 communicated to the Royal Society a paper on a new formation of diamond. Sir Andrew Noble has shown that in the explosion of cordite in closed steel cylinders pressures of over 50 tons to the sq. in. and a temperature probably reaching 5400° were obtained. Here then we have conditions favourable for the liquefaction of carbon, and if the time of explosion were sufficient to allow the reactions to take place we should expect to get liquid carbon solidified in the crystalline state. Experiment proved the truth of these anticipations. Working with specially prepared explosive containing a little excess of carbon Sir Andrew Noble collected the residue left in the steel cylinder. This residue was submitted by Sir William Crookes to the lengthy operations already described in the account of H. Moissan's fused iron experiment. Finally, minute crystals were obtained which showed octahedral planes with dark boundaries due to high refracting index. The position and angles of their faces, and cleavages, the absence of bi-refringence, and their high refractive index all showed that the crystals were true diamond.

The artificial diamonds, so far, have not been larger than microscopic specimens, and none has measured more than about half a millimetre across. That, however, is quite enough to show the correctness of the train of reasoning leading up to the achievement, and there is no reason to doubt that, working on a larger scale, larger diamonds will result. Diamonds so made burn in the air when heated to a high temperature, with formation of carbonic acid; and in lustre, crystalline form, optical properties, density and hardness, they are identical with the natural stone.

It having been shown that diamond is formed by the separation of carbon from molten iron under pressure, it became of interest to see if in some large metallurgical operations similar conditions might not prevail. A special form of steel is made at some large establishments by cooling the molten metal under intense hydraulic pressure. In some samples of the steel so made Professor Rosel, of the university of Bern, has found microscopic diamonds. The higher the temperature at which the steel has been melted the more diamonds it contains, and it has even been suggested that the hardness of steel in some measure may be due to the carbon distributed throughout its mass being in this adamantine form. The largest artificial diamond yet formed was found in a block of steel and slag from a furnace in Luxembourg; it is clear and crystalline, and measures about one-fiftieth of an inch across.

A striking confirmation of the theory that natural diamonds have been produced from their solution in masses of molten iron, the metal from which has gradually oxidized and been washed away under cycles of atmospheric influences, is afforded by the occurrence of diamonds in a meteorite. On a broad open plain in Arizona, over an area of about 5 m. in diameter, lie scattered thousands of masses of metallic iron, the fragments varying in weight from half a ton to a fraction of an ounce. There is little doubt that these fragments formed part of a meteoric shower, although no record exists as to when the fall took place. Near the centre, where most of the fragments have been found, is a crater with raised edges, three-quarters of a mile in diameter and 600 ft. deep, bearing just the appearance which would be produced had a mighty mass of iron--a falling star--struck the ground, scattered it in all directions, and buried itself deeply under the surface, fragments eroded from the surface forming the pieces now met with. Altogether ten tons of this iron have been collected, and specimens of the Canyon Diablo meteorite are in most collectors' cabinets. Dr A.E. Foote, a mineralogist, when cutting a section of this meteorite, found the tools injured by something vastly harder than metallic iron, and an emery wheel used for grinding it was ruined. He attacked the specimen chemically, and soon afterwards announced to the scientific world that the Canyon Diablo meteorite contained diamonds, both black and transparent. This startling discovery was subsequently verified by Professors C. Friedel and H. Moissan, and also by Sir W. Crookes.

_The Ruby._--It is evident that of the other precious stones only the most prized are worth producing artificially. Apart from their inferior hardness and colour, the demand for what are known as "semi-precious stones" would not pay for the necessarily great expenses of the factory. Moreover, were it to be known that they were being produced artificially the demand--never very great--would almost cease. The only other gems, therefore, which need be mentioned in connexion with their artificial formation are those of the corundum or sapphire class, which include all the most highly prized gems, rivalling, and sometimes exceeding, the diamond in value. Here a remarkable and little-known fact deserves notice. Excepting the diamond and sapphire, each of the precious stones--the emerald, the topaz and amethyst--possesses a more noble, a harder, and more highly-prized counterpart of itself, alike in colour, but superior in brilliancy and hardness; still more strange, the precious stone to which its special name is usually attached is the variety the least prized. The ruby itself might almost be included in the same category. The true ruby consists of the earth alumina, in a clear, crystalline form, having a minute quantity of the element chromium as the colouring matter. It is often called the "Oriental Ruby," or red sapphire, and when of a paler colour, the "Pink Sapphire." But the ruby as met with in jewellers' shops of inferior standing is usually no true ruby, but a "spinel ruby" or "balas ruby," sometimes very beautiful in colour, but softer than the Oriental ruby, and different in chemical composition, consisting essentially of alumina and magnesia and a little silica, with the colouring matter chromium. The colourless basis of the true Oriental precious stones being taken as crystallized alumina or white sapphire, when the colouring matter is red the stone is called ruby, when blue sapphire, when green Oriental emerald, when orange-yellow Oriental topaz, and when violet Oriental amethyst. Clear, colourless crystals are known as white sapphire, and are very valuable. It is evident, therefore, that whosoever succeeds in making artificially clear crystals of white sapphire has the power, by introducing appropriate colouring matter, to make the Oriental ruby, sapphire, emerald, topaz and amethyst. All of these stones, even when of small size, are costly and readily saleable, while when they are of fine quality and large size they are highly prized, a ruby of fine colour, and free from flaws, a few carats in weight, being of more value than a diamond of the same weight.

This being the case, it is not surprising that repeated attempts have been made to effect the crystallization of alumina. This is not a matter of difficulty, but unfortunately the crystals generally form thin plates, of good colour, but too thin to be useful as gems. In 1837 M.A.A. Gaudin made true rubies, of microscopic size, by fusing alum in a carbon crucible at a very high temperature, and adding a little chromium as colouring matter. In 1847 J.J. Ebelmen produced the white sapphire and rose-coloured spinel by fusing the constituents at a high temperature in boracic acid. Shortly afterwards he produced the ruby by employing borax as the solvent. The boracic acid was found to be too volatile to allow the alumina to crystallize, but the use of borax made the necessary difference. But it was not till about the year 1877 that E. Frémy and C. Feil first published a method whereby it was possible to produce a crystallized alumina from which small stones could be cut. They first formed lead aluminate by the fusion together of lead oxide and alumina. This was kept in a state of fusion in a fireclay crucible (in the composition of which silica enters largely). Under the influence of the high temperature the silica of the crucible gradually decomposes the lead aluminate, forming lead silicate, which remains in the liquid state, and alumina, which crystallizes as white sapphire. By the admixture of 2 or 3% of a chromium compound with original materials the resulting white sapphire became ruby. More recently Edmond Frémy and A. Verneuil obtained artificial rubies by reacting at a red heat with barium fluoride on amorphous alumina containing a small quantity of chromium. The rubies obtained in this manner are thus described by Frémy and Verneuil: "Their crystalline form is regular; their lustre is adamantine; they present the beautiful colour of the ruby; they are perfectly transparent, have the hardness of the ruby, and easily scratch topaz. They resemble the natural ruby in becoming dark when heated, resuming their rose-colour on cooling." Des Cloizeaux says of them that "under the microscope some of the crystals show bubbles. In converging polarized light the coloured rings and the negative black cross are of a remarkable regularity."

Other experimentalists have attacked the problem in other directions. Besides those already mentioned, L. Eisner, H.H. De Senarmont, Sainte-Claire Deville, and H. Caron and H. Debray have succeeded with more or less success in producing rubies. The general plan adopted has been to form a mixture of salts fusible at a red heat, forming a liquid in which alumina will dissolve. Alumina is now added till the fused mass will take up no more, and the crucible is left in the furnace for a long time, sometimes extending over weeks. The solvent slowly volatilizes, and the alumina is deposited in crystals, coloured by whatever colouring oxide has been added.

Mention has been made above of a stone frequently substituted for the true ruby, called the "spinel" or "balas" ruby. The spinel and ruby occur together in nature, stones from Burma being as often spinel as true Oriental ruby. In the artificial production of the ruby it sometimes happens that spinel crystallizes out when true Oriental ruby is expected. The fusion bath is so arranged that only red-coloured alumina shall crystallize out, but it is difficult to have all the materials of such purity as to ensure the complete absence of silica and magnesia. In this case, when these impurities have accumulated to a certain point they unite with the alumina, and spinel then separates, as it crystallizes more easily than ruby. When all the magnesia and silica have been eliminated in this way the bath resumes its deposition of crystalline ruby. Rubies of fine colour and of considerable size have been shown in London, made on the Continent by a secret process. The writer has seen several cut stones so made weighing over a carat each, the uncut crystals measuring half an inch along a crystal edge, and weighing over 70 grains, and a clear plate of ruby cut from a single crystal weighing over 10 grains. Ruby has been made by Sir W. Roberts-Austen as a by-product in the production of metallic chromium. Oxide of chromium and aluminium powder are intimately mixed together in a refractory crucible, and the mixture is ignited at the upper part. The aluminium and chromium oxide react with evolution of so much heat that the reduced chromium is melted. Such is the intensity of the reaction that the resulting alumina is also completely fused, floating as a liquid on the molten chromium. Sometimes the alumina takes tip the right amount of chromium to enable it to assume the ruby colour. On cooling the melted alumina crystallizes in large flakes, which on examination by transmitted light are seen to be true ruby. The development of the red colour is said by C. Greville-Williams only to take place at a white heat. It is not due to the presence of chromic acid, but to a reaction between alumina and chromic oxide, which requires an elevated temperature.

Artificially made but real rubies have been put on the market, prepared by a process of fusion by A. Verneuil. He finds that certain conditions have to be fulfilled in order to get the alumina in a transparent form. The temperature must not be higher than is absolutely necessary for fusion. The melted product must always be in the same part of the oxyhydrogen flame, and the point of contact between the melted product and the support should be reduced to as small an area as possible. M. Verneuil uses a vertical blowpipe flame directed on a support capable of movement up and down by means of a screw, so that the fused product may be removed from the zone of fusion as it gets higher by addition of fresh material. The material employed is either composed of small, valueless rubies, or alumina coloured with the right amount of chromium. It is very finely powdered and fed in through the blowpipe orifice, whence it is blown in a highly heated condition into the zone of fusion. The support is a small cylinder of alumina placed in the axis of the blowpipe. As the operation proceeds the fine grains of powder driven on to the support in the zone of fusion form a cone which gradually rises and broadens out until it becomes of sufficient size to be used for cutting. Rubies prepared in this way have the same specific gravity and hardness as the natural ruby, and they are also dichroic, and in the vacuum tube under the influence of the cathode stream they phosphoresce with a discontinuous spectrum showing the strong alumina line in the red. When properly cut and mounted it is almost impossible to distinguish them from natural stones.

_The Sapphire._--Auguste Daubrée has shown that when a full quantity of chromium is added to the bath from which white sapphire crystallizes the colour is that of ruby, but when much less chromium is added the colour is blue, forming the true Oriental sapphire. The real colouring matter of the Oriental sapphire is not definitely known, some chemists considering it to be chromium and others cobalt. Artificial sapphires have been made of a fair size and perfectly transparent by the addition of cobalt to the igneous bath of alumina, but the writer does not consider them equal in colour to true Oriental sapphire.

_The Oriental Emerald._--The stone known as emerald consists chemically of silica, alumina and glucina. Like the ruby, it owes its colour to chromium, but in a different state of oxidation. As already mentioned, there is another stone which consists of crystallized alumina coloured with chromium, but holding the chromium in a different state of oxidation. This is called the Oriental emerald, and, owing to its beauty of colour, its hardness and rarity, it is more highly prized than the emerald itself and commands higher prices. The Oriental emerald has been produced artificially in the same way as the ruby, by adding a larger amount of chromium to the alumina bath and regulating the temperature.

_The Oriental Amethyst._--The amethyst is rock crystal (quartz) of a bluish-violet colour. It is one of the least valuable of the precious stones. The sapphire, however, is found occasionally of a beautiful violet colour; it is then called the Oriental amethyst, and, on account of its beauty and rarity, is of great value. It is evident that if to the igneous bath of alumina some colouring matter, such as manganese, is added capable of communicating a violet colour to the crystals of alumina, the Oriental amethyst will be the result. Oriental amethyst has been so formed artificially, but the stone being known only as a curiosity to mineralogists and experts in precious stones, and the public not being able to discriminate between the violet sapphire and amethystine quartz, there is no demand for the artificial stone.

_The Oriental Topaz._--The topaz is what is called a semi-precious stone. It occurs of many colours, from clear white to pink, orange, yellow and pale green. The usual colour is from straw-yellow to sherry colour. The exact composition of the colouring matter is not known; it is not entirely of mineral origin, as it changes colour and sometimes fades altogether on exposure to light. Chemically the topaz consists of alumina, silica and fluorine. It is not so hard as the sapphire. There is also a yellow variety of quartz, which is sometimes called "false topaz." The Oriental topaz, on the other hand, is a precious stone of great value. It consists of clear crystalline sapphire coloured with a small quantity of ferric oxide. It has been produced artificially by adding iron instead of chromium to the matrix from which the white sapphire crystallizes.

_The Zircon._--The zircon is a very beautiful stone, varying in colour, like the topaz, from red and yellow to green and blue. It is sometimes met with colourless, and such are its refractive powers and brilliancy that it has been mistaken for diamond. It is a compound of silica and zirconia. H. Sainte-Claire Deville formed the zircon artificially by passing silicon fluoride at a red heat over the oxide zirconia in a porcelain tube. Octahedral crystals of zircon are then produced, which have the same crystalline form, appearance and optical qualities as the natural zircon.

BIBLIOGRAPHY.--Sir William Crookes, "A New Formation of Diamond,"
_Proc. Roy. Soc._ vol. lxxvi. p. 458; "Diamonds," a lecture delivered
before the British Association at Kimberley, South Africa, 5th
September, 1905, _Chemical News_, vol. xcii. pp. 135, 147, 159; J.J.
Ebelmen, "Sur la production artificielle des pierres dures," _Comptes
rendus_, vol. xxv. p. 279; "Sur une nouvelle méthode pour obtenir, par
la voie sèche, des combinations crystallisées, et sur ses applications
à la réproduction de plusieurs espèces minérales," _Comptes rendus_,
vol. xxv. p. 661; Edmond Frémy and C. Feil, "Sur la production
artificielle du corindon, du rubis, et de différents silicates
crystallisées," _Comptes rendus_, vol. lxxxv. p. 1029; C. Friedel,
"Sur l'existence du diamant dans le fer météorique de Cañon Diablo,"
_Comptes rendus_, vol. cxv. p. 1037, vol. cxvi. p. 290; H. Moissan,
"Étude de la météorite de Cañon Diablo," _Comptes rendus_, vol. cxvi.
p. 288; "Expériences sur la réproduction du diamant," _Comptes
rendus_, vol. cxviii. p. 320; "Sur quelques expériences relatives à la
préparation du diamant," _Comptes rendus_, vol. cxxiii. p. 206; _Le
Four électrique_ (Paris, 1897); H. Sainte-Claire Deville and H. Caron,
"Sur un nouveau mode de production à l'état cristallisé d'un certain
nombre d'espèces chimiques et minéralogiques," _Comptes rendus_, vol.
xlvi. p. 764; A. Verneuil, "Production artificielle des rubis par
fusion," ibid. vol. cxxxv. p. 791; J. Boyer, _La Synthèse des pierres
précieuses_ (Paris, 1909). (W. C.)

GEMBLOUX, a town in the province of Namur and on the borders of Brabant, Belgium, 25 m. S.E. of Brussels on the main line to Namur and Luxemburg. Pop. (1904) 4643. It is a busy place with large railway and engine works, and the junction for several branch lines. On the 31st of January 1578 Don John of Austria gained here a signal victory over the army of the provinces led by Antony de Goignies.

GEMINI ("The Twins," i.e. Castor and Pollux), in astronomy, the third sign in the zodiac, denoted by the symbol II. It is also a constellation, mentioned by Eudoxus (4th century B.C.) and Aratus (3rd century B.C.), and catalogued by Ptolemy, 25 stars, Tycho Brahe 25, and Hevelius 38. By the Egyptians this constellation was symbolized as a couple of young kids; the Greeks altered this symbol to two children, variously said to be Castor and Pollux, Hercules and Apollo, or Triptolemus and Iasion; the Arabians used the symbol of a pair of peacocks. Interesting objects in this constellation are: [alpha] Geminorum or Castor, a very fine double star of magnitudes 2.0 and 2.8, the fainter component is a spectroscopic binary; [eta] Geminorum, a long period (231 days) variable, the extreme range in magnitude being 3.2 to 4; [zeta] Geminorum, a short period variable, 10.15 days, the extreme range in magnitude being 3.7 to 4.5; _Nova_ Geminorum, a "new" star discovered in 1903 by H.H. Turner of Oxford; and the star cluster M.35 Geminorum, a fine and bright, but loose, cluster, with very little central condensation.

GEMINIANI, FRANCESCO (c. 1680-1762), Italian violinist, was born at Lucca about 1680. He received lessons in music from Alessandro Scarlatti, and studied the violin under Lunati (Gobbo) and afterwards under Corelli. In 1714 he arrived in London, where he was taken under the special protection of the earl of Essex, and made a living by teaching and writing music. In 1715 he played his violin concertos with Handel at the English court. After visiting Paris and residing there for some time, he returned to England in 1755. In 1761 he went to Dublin, where a servant robbed him of a musical manuscript on which he had bestowed much time and labour. His vexation at this loss is said to have hastened his death on the 17th of September 1762. He appears to have been a first-rate violinist, but most of his compositions are dry and deficient in melody. His _Art of Playing the Violin_ is a good work of its kind, but his _Guida armonica_ is an inferior production. He published a number of solos for the violin, three sets of violin concertos, twelve violin trios, _The Art of Accompaniment on the Harpsichord, Organ_, &c., _Lessons for the Harpsichord_ and some other works.

GEMISTUS PLETHO [or PLETHON], GEORGIUS (c. 1355-1450), Greek Platonic philosopher and scholar, one of the chief pioneers of the revival of learning in Western Europe, was a Byzantine by birth who settled at Mistra in the Peloponnese, the site of ancient Sparta. He changed his name from Gemistus to the equivalent Pletho ("the full"), perhaps owing to the similarity of sound between that name and that of his master Plato. He invented a religious system founded on the speculative mysticism of the Neoplatonists, and founded a sect, the members of which believed that the new creed would supersede all existing forms of belief. But he is chiefly memorable for having introduced Plato to the Western world. This took place upon his visit to Florence in 1439, as one of the deputies from Constantinople on occasion of the general council. Cardinal Bessarion became his disciple; he produced a great impression upon Cosimo de' Medici; and though not himself making any very important contribution to the study of Plato, he effectually shook the exclusive domination which Aristotle had exercised over European thought for eight centuries. He promoted the union of the Greek and Latin Churches as far as possible, but his efforts in this direction bore no permanent fruit. He probably died before the capture of Constantinople. The most important of his published works are treatises on the distinction between Plato and Aristotle as philosophers (published at Venice in 1540); on the religion of Zoroaster (Paris, 1538); on the condition of the Peloponnese (ed. A. Ellissen in _Analekten der mittel- und neugriechischen Literatur_, iv.); and the [Greek: Nomoi] (ed. C. Alexandre, Paris, 1858). In addition to these he compiled several volumes of excerpts from ancient authors, and wrote a number of works on geography, music and other subjects, many of which still exist in MS. in various European libraries.

See especially F. Schultze, _Geschichte der Philosophie der
Renaissance_, i. (1874); also J.A. Symonds, _The Renaissance in Italy_
(1877), ii. p. 198; H.F. Tozer, "A Byzantine Reformer," in _Journal of
Hellenic Studies_, vii. (1886), chiefly on Pletho's scheme of
political and social reform for the Peloponnese, as set forth in the
pamphlets addressed to Manuel II. Palaeologus and his son Theodore,
despot of the Morea; W. Gass, _Gennadius und Pletho_ (1844). Most of
Pletho's works will be found in J.P. Migne, _Patrologia Graeca_, clx.;
for a complete list see Fabricius, _Bibliotheca Graeca_ (ed. Harles),
xii.

GEMMI PASS, a pass (7641 ft.) leading from Frutigen in the Swiss canton of Bern to Leukerbad in the Swiss canton of the Valais. It is much frequented by travellers in summer. From Kandersteg (7½ m. by road above Frutigen, which is 12 m. by rail from Spiez on the Berne-Interlaken line) a mule path leads to the summit of the pass, passing over the Spitalmatte plain, where in 1782 and again in 1895 a great avalanche fell from the Altels (11,930 ft.) to the S.E., causing on both occasions great loss of life and property. The mule path descends on the south side of the pass by an extraordinary series of zigzags, made accessible for mules (though no rider is now allowed to descend on mule-back) by a band of Tirolese workmen in 1740-1741. They are cut in a very steep wall of rock, about 1800 ft. in height, and lead down to the village of Leukerbad, which is 9½ m. by carriage road past Leuk above the Susten station in the Rhône valley and on the Simplon line. (W. A. B. C.)

GENDARMERIE, originally a body of troops in France composed of _gendarmes_ or men-at-arms. In the days of chivalry they were mounted and armed cap-à-pie, exactly as were the lords and knights, with whom they constituted the most important part of an army. They were attended each by five soldiers of inferior rank and more lightly armed. In the later middle ages the men-at-arms were furnished by owners of fiefs. But after the Hundred Years' War this feudal gendarmerie was replaced by the _compagnies d'ordonnance_ which Charles VII. formed when the English were driven out of France, and which were distributed throughout the whole extent of the kingdom for preserving order and maintaining the king's authority. These companies, fifteen in number, were composed of 100 lances or gendarmes fully equipped, each of whom was attended by at least three archers, one _coutillier_ (soldier armed with a cutlass) and one _varlet_ (soldier's servant). The states-general of Orleans (1439) had voted a yearly subsidy of 1,200,000 livres in perpetuity to keep up this national soldiery, which replaced, and in fact was recruited chiefly amongst, the bands of mercenaries who for about a century had made France their prey. The number and composition of the _compagnies d'ordonnance_ were changed more than once before the reign of Louis XIV. This sovereign on his accession to the throne found only eight companies of gendarmes surviving out of an original total of more than one hundred, but after the victory of Fleurus (1690), which had been decided by their courage, he increased their number to sixteen. The four first companies (which were practically guard troops) were designated by the names of _Gendarmes écossais_, _Gendarmes anglais_, _Gendarmes bourguignons_ and _Gendarmes flamands_, from the nationality of the soldiers who had originally composed them; but at that time they consisted entirely of French soldiers and officers. These four companies had a captain-general, who was the king. The fifth company was that of the queen; and the others bore the name of the princes who respectively commanded them. This organization was dissolved in 1788. The Revolution swept away all these institutions of the monarchy, and, with the exception of a short revival of the _Gendarmes de la garde_ at the Restoration, henceforward the word "gendarmerie" possesses an altogether different significance--viz. military police.

GENEALOGY (from the Gr. [Greek: genos], family, and [Greek: logos], theory), a pedigree or list of ancestors, or the study of family history.

1. _Biblical Genealogies._--The aims and methods of ancient genealogists require to be carefully considered before the value of the numerous ancestral lists in the Bible can be properly estimated. Many of the old "genealogies," like those of Greece, have arisen from the desire to explain the origin of the various groups which they include. Information relating to the subdivision of tribes, their relation to each other, the intermingling of populations and the like are thus frequently represented in the form of genealogies. The "sons" of a "father" often stand merely for the branches of a family as they existed at some one period, and since in course of time tribal relations would vary, lists which have originated at different periods will present discrepancies. It is obvious that many of the Biblical names are nothing more than personifications of nations, tribes, towns, &c., which are grouped together to convey some idea of the bond by which they were believed to be connected.

For the personification of a people or tribe, cp. Gen. xxxiv. 30
("Jacob said ... I am a few men"), Josh. xvii. 14 ("the children of
Joseph said ... I am a numerous people"), Ex. xiv. 25 ("Egypt said,
let me flee"), Jos. ix. 7, 1 Sam. v. 10, &c.; see G.B. Gray on
Numbers, xx. 14 (_Internat. Crit. Comm._). Thus we find among the
"sons" of Japhet: (the nations) Gomer, Javan, Tubal; Canaan "begat"
Sidon and Heth; the "sons" of Ishmael include the well-known tribes
Kedar and Jetur; Jacob, or the synonym Israel, personifies the
"children of Israel" (cf. use of "I," "thou" of the Israelites in
Deut., and in poetical passages). The recognition of this
characteristic usage often furnishes an ethnological interpretation to
those genealogical stories which obviously do not relate to persons,
but to tribes or peoples personified. The Edomites and Israelites are
regarded as "brothers" (cf. Num. xx. 14, Deut. ii. 4, Am. i. 11), and
since Esau (Edom) was born before Jacob (Israel) it would appear that
the Edomites were held to be the older nation. The union of two clans
is expressed as a marriage, or the wife is the territory which is
dominated by the husband (tribe); see CALEB. If the woman is not of
noble blood, but is a handmaiden or concubine, her children are
naturally not upon the same footing as those of the wife; consequently
the descendants of Ishmael, the son of Hagar (Sarah's maid), are
inferior to Isaac and his descendants, whilst the children of Keturah
("incense"), Abraham's concubine, are still lower--from the Israelite
point of view. This application of the terms of relationship is
characteristic of the Semites. The "father" of the Rechabites is their
head or founder (cf. 1 Sam. x. 12: "who is their father?"), and a
common bond, which is not necessarily physical, unites all "sons,"
whether they are "sons of the prophets" (members of prophetic guilds)
or "sons of Belial" (worthless men).

The interpretation of ethnological or statistical genealogies may easily be pushed too far. Every case has to be judged upon its own merits, and due allowance must be made both for the ambition of the weaker to claim or to strengthen an alliance with the stronger, and for the not unnatural desire of clans or individuals to magnify the greatness of their ancestry. The first step must always be the careful comparison of related lists in order to test the consistency of the tradition. Next, these must be critically studied in the light of all available historical material, though indeed such evidence is not necessarily conclusive. Finally, (a) literary criticism must be employed to determine if possible the dates of such lists, since obviously a contemporary register is more trustworthy than one which is centuries later; (b) a critical estimate of the character of the names and of their use in various periods of Old Testament history is of importance in estimating the antiquity of the list[1]--for example, many of the names in Chronicles attributed to the time of David are indubitably exilic or post-exilic; and (c) principles of ordinary historical probability are as necessary here as in dealing with the genealogies of other ancient peoples, and attention must be paid to such features as fluctuation in the number of links, representation of theories inconsistent with the growth of national life, schemes of relationship not in accordance with sociological conditions, &c.

The Biblical genealogies commence with "the generations of the heaven and earth," and by a process of elimination pass from Adam and Eve by successive steps to Jacob and to his sons (the tribes), and finally to the subdivisions of each tribe (cp. 1 Chron. i.-ix. 1). According to this theory every Israelite could trace back his descent to Jacob, the common father of the whole nation (Josh. vii. 17 seq., 1 Sam. x. 21). Such a scheme, however, is full of manifest improbabilities. It demands that every tribe and every clan should have been a homogeneous group which had preserved its unity from the earliest times, that family records extending back for several centuries were in existence, and that such a tribe as Simeon was able to maintain its independence in spite of the tradition that it lost its autonomy in very early times (Gen. xlix. 7). The whole conception of the unity of the tribes cannot be referred to a date previous to the time of David, and in the older writings a David or a Jeroboam was sufficiently described as the son of Jesse or of Nebat. The genealogical zeal as represented in the Old Testament is chiefly of later growth, and the exceptions are due to interpolation (Josh. vii. 1 18, contrast v. 24), or to the desire to modify or qualify an older notice. This, in the case of Saul (1 Sam. ix. 1), has led to textual corruption; a list of such a length as his should have reached back to one of the "sons" of Benjamin (cf. e.g. Gen. xlvi. 21), else it were purposeless. The genealogies, too, are often inconsistent amongst themselves and in contradiction to their object. They show, for example, that the population of southern Judah, so far from being "Israelite" was half-Edomite (see Judah), and several of the clans in this district bear names which indicate their original affinity with Midian or Edom. Moreover, there was a free intermixture of races, and many cities had a Canaanite (i.e. pre-Israelite) population which must have been gradually absorbed by the Israelites (cf. Judg. 1.). That spirit of religious exclusiveness which marked later Judaism did not become prominent before the Deuteronomic reformation (see DEUTERONOMY), and it is under its influence that the writings begin to emphasize the importance of maintaining the purity of Israelite blood, although by this time the fusion was complete (see Judg. iii. 6) and for practical purposes a distinction between Canaanites and Israelites within the borders of Palestine could scarcely be discerned.