Page:Cyclopaedia, Chambers - Supplement, Volume 2.djvu/495

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time, no increafe in the number of their branches, yet a probable guefs may be made from their being all thicker and more robuft, and the tops more open.

SLOUGH (Cycl.) — SiouGH-fifoer, in our old writers, a rent paid in fome places in lieu of certain days work in harveft, heretofore referved to the lord from his tenants. Blount.

SLOW {Cycl.) — SLOW-worm, the Engl ifli name of the cceciiia, called alio the blind-worm, and by fome the deaf-adder. See the article Coecili a.

SLUDS, a term ufed by the miners in Cornwall for half-roafted ores. See the article Ore.

SMALL (Cycl.) — $MALL-craft, in the fea language. See the article Craft.

SMALL-pox. See the article Pox.

SMALL-Jlones, among jewellers, denote diamonds under the weight of a carrat. Jeffreys on Diamonds, p. 21.

SwiALL-wort, among jewellers* is ufed to denote the ftar and fhell-facets of diamonds. Jeffreys on Diamonds. See the article Facets.

SMALLAGE, in botany, &c. See Apium.

SMALT (Cyc/.)— There is not a more precarious ■ procefs in all metallurgy, than the preparation of this fine blue glafs from cobalt, nor any manufacture, in which fo expert work- men are neceflary. The carrying the heat to too great a degree, in the roafting the cobalt, is a very mifchievous er- ror, and the fuffering as much of the ar fen it, as may be, to remain in the earth, is greatly to the advantage of the /malt; and its colour may be greatly heightened, by adding com- mon arfenic to it while it is in fufion.

It is always necefTary to make careful and repeated experi- ments on a new ore, or a kind of cobalt, which has not been worked before ; by trying it with various degrees of heat in the roafting, and with different proportions of the flints and pot-afh in the fufion. On comparing the feveral products of thefe experiments, it will eafily be feen which yields the fineft colour, and this muft be always after followed in the great operations.

It is well known that the beautiful blue glafs, called fmalt, is prepared from the fixed earth of cobalt ; but experiments have proved, that this earth alone will not acquire that fine colour. Dr. Link tried fome of it, by keeping it eight hours in the moft in enfe heat, yet on taking it out, it was only a grey glafs, excepting that there were a few blue fpecks in it near the edges of the crucible. Thefe {pecks make no objection to this doctrine, when properly consider- ed ; for they were owing, doubtlefs, to the fame fubftance which is found neceflary to be ufed, in order to procure the blue colour.

Flints are the neceflary addition to cobalt for the making the blue glafs, or /malt, and the crucible being made of earth, might eafily contain fome particles of fand. Every piece of fand is a fmall ftone, of the nature of flint; that is, a piece of cryftal debafed by earth; and thefe fmall flints mixing with the glafs of the cobalt-earth, while in fufion, rendered the fmall fpecks, where they were mixed, true fmalty while the reft remained grey and unaltered. This earth of cobalt evidently contains fome portion of copper, though fo fmall in quantity, that it can never be difcerned by the common figns, and fo intimately blended with the earth, that it can never be procured feparate. The lightnefs of this earth alfo, in which the copper refides after the fublimation of the arfenic, is a proof that its quantity muft be very fmall ; yet that it exifts there, is evident from the colours it affords.

Copper is well known to yield a green colour with acids, and a fine blue with alkalies. Thus a folution of this earth in aqua fortis is of a deep green, and its glafs, when mixed with the alkali of flints and of pot-afh, is the fine deep blue fmalt : nor is it wonderful, that the vitrified earth of cobalt will never become blue by calcination alone, fince, if it owes this colour to copper, it cannot be fuppofed to fhew itfelf till called forth by an alkali. Philof. Tranf. N° 396. p. 201.

Some cobalt yields fmalt before roafting, even better than it would afterwards ; this is a peculiar kind of cobalt, but is fo like the reft, that it cannot be diftinguifhed by the eye, but experiment alone fhews this property in it. Though cobalt in general requires roafting, in order to fit it for yield- ing the fmalt, yet its different kinds require fome more, and fome lefs roafting ; and the degree can never be judged of by the inflection of the mineral, but is only known by the experienced artift in the procefs. Hence it is, that ex- pert and intelligent perfons are neceflary in the fmalt- works more than in almoft any other branch of this fort of bufinefs.

The addition of a fmall quantity of arfenic, or of the arfe- nical flowers, during the time that the fmalt is in fufion, adds greatly to the beauty of the colour : this is a practice kept fecret by the workmen of fome places ; and by this means their fmalt is always rendered better than that of their neighbours. It is eafy to fee from this, that the roaftinc of the cobalt is the neceflary beginning of the fmalt-woik, not the divefting it of its arfenic, which only happens acci- dentally in that procefs ; and it would be much better if it Supfl. Vol. II.

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did not happen, fince we find the arfenic added afterwards exalts the colour. From hence it is evident, that thofe co- balts which will make fmalt without previous roafting, muft, as they are found to do, make the very fineft fmalt, be- caufe their arfenic is yet left in them in great pan : and from this alfo appears the neceflity of having expert work- men for the fmalt-mzking ; fince the knowing the degree of fire necefTary to the ore is a moft cflen ial article, and after the roafting has been carried to a fumcient degree, every moment's heat diverting it of more of its arfenic than was neceflary, makes the colour of the fmalt to be made after- wards fo much the worfe. From this alfo appears the rea- fon why the blue, made by precipitation from a folution of cobalt, is fo much fuperior to the common fmalt made by fire; becaufe in this the arfenic is ail preferved, whereas, in the common wav of preparing it by fire, it is driven off. Philof. Tranf. N° 396. SMARAGDUS, the emerald, (Cycl.) a very beautiful gerri of a fine green colour.

The gem known by the antients under the name of the fma- ragd, or emerald, was the fame with that we call at this time by the fame name, which has not been the cafe in the hyacinth, fapphire, and many others.

In the days of Pliny we find, however, that a great deal of error and confufion had crept into the world In regard to this gem, that author giving a long lift of the different kinds of emeralds, as he calls them ; among which there are evi- dently jafpers, coloured cryftals, and fpars, reckoned among the fpecies of this gem.

Our jewellers know only what they call two fpecies of the emerald, the Oriental and the Occidental, and fuch there are without queftion ; that is,- the emeralds of the Eaft-Indies are, in their pureft ftate, much finer than thofe of any other part of the world, as is the cafe in the other gems : but our jewellers feldom meeting with thefe fine ftones, call the American emeralds Oriental, and ufually fell cryftal, acci- dentally tinged green, under the name of Occidental emeraldsi 1 hefe Iaft being alfo greatly more common than the others^ there has grown an opinion among our lapidaries, that the emerald is no harder than cryftal ; and this has no other foundation, than their ufually meeting with cryftal under the name of the emerald.

The genuine emerald, when in its moft perfect ftate, is per- haps the fineft and moft beautiful of all the gems. It is found of various fizes, but ufually fmall. Multitudes are met with of about the fixteenth of an inch in diameter,' and they are found from this up to the fize of a wallnut. We have accounts of fome of immenfe fize anions the antients ; but they were evidently not emeralds, but jafpers, or other green ftones. The large fpecimens at prefent are very fcarce, and whether they are Oriental or Occidental, are of very great value. Hill's Hift. of Fofl". p. 596 The emerald is naturally of different figures, like the dia- mond, and many of the other gems. It is fometimes found in a roundifh, or pebble-like form, but much more fre- quently in a columnar, or angular one, refembling common cryftal. The pebble emeralds are always the bardeft and brighteft, but are feldom found exceeding the fize of a pea, very rarely coming up to that of a hoffe-bean. The cryftalli- form ufually grow feveral together, and are often confider- ably large. The pebble emeralds are found loofe in the earth of mountains, and in the fands of rivers; the colum- nar are ufually found adhering to, or bedded in a white opake cryftalline matter, and fometimes to the jafper, or to the prufius; which laft has been therefore called by fome the mother of the emerald.

The rough emerald, of whatever form, is ufually of a very bright, and naturally polifhed furfacc, and is ever of a pure and beautiful green, without the leaft admixture of any other' colour. It is of all the various fhades of green, from the deepeft to the paleft, and doubtlefs is fometimes wholly co- lourlefs; but it is then efteemed a fapphire, all the harder gems, when colourlefs, being called by our jewellers white Jappbires.

The Oriental emerald is of the hardnefs of the fapphire and! ruby, and is fecond only to the diamond in luftfe and bright- ness ; the American is of the hardnefs of the garnet ; and the European is ftill fofter than that, but considerably harder than cryftal ; yet the coloured cryftals, ufually fold under the name of emeralds, have very much debated the credit of this gem. It lofes its colour in the fire, and becomes undiftinguifhable from the white fapphire. The Oriental emeralds are very fcarce, and are at prefent found only in the kingdom of Cambay : very few of them have of late been imported into Europe, infomuch, that fome have fuppofed that there were in nature no Oriental emeralds ; but within thefe few years fome have been brought from Cambay into Italy, which greatly exceed all the American ones in beauty, as well as hardnefs.

The American emeralds, which, as they are the fineft that come to us, are called by our jewellers Oriental, are found principally about Peru, in the earth of fome mountains. The European emeralds come principally from Silefia, though there arc of them in other places ; and the coloured cryftals, 2 Q.4 1 whick