GEM
a lump of dough between the fingers : this pafte muft be put in- to a fmall crucible of a flat fhape, and about half an inch or a little more in depth, and of fuch a breadth at the furface a* is a little more than that of the ftone, whofe impreflion is to be taken. This crucible is to be nicely filled with this pafte lightly preffed down into it, and the furface of the pafte muft be ftrewed over with the fine powder of the yellow tri- pela not wetted. When this is done, the ftone, whofe im- preflion is to be taken, muft be laid upon the furface, and preffed evenly down into the pafte with a finger and thumb, fo as to make it give a ftrong and perfect impreflion ; the tripela is then to be preffed nicely even to its fides with the fingers, or with an ivory knife : the ftone muft be thus left a few moments for the humidity of the pafte to moiften the dry powder of the yellow tripela, which is ftrewed over it ; then the ftone is to be carefully raifed by the point of a needle fixed in a handle of wood, and the crucible being then turned bottom upwards, it will fall out, and the im- preflion will remain very beautifully on the tripela. Mem. Acad. Par. ibid.
If the fides of the cavity have been injured in the falling out of the ftone, they may be repaired, and the crucible muft then be fet, for the pafte to dry, in a place where it will not be incommoded by the duft.
The red tripoli being the more common and the cheaper kind, is here made to fill the crucible only to fave the other, which alone is the fubftance fit for taking the impreflion. When the ftone is taken out, it muft be examined to fee whether any thing be lodged in any part of the engraving, hecaufe if there be any of the tripela left there, there will certainly be fo much wanting in the impreflion. When the crucible and pafte are dry, a piece of glafs muft be chofen of a proper colour, and cut to a fize proper for the figure ; this muft be laid over the mould, but in fuch a manner, that it does rfot touch the figures, other wife it would fpoil them. The crucible is then to be brought near the furnace by degrees, and gradually heated till it cannot be touched without burn- ing the fingers ; then it is to be placed in the furnace un- der a muffle, furroUnded with charcoal. Several of thefe fmall crucibles may be placed under one muffle, and when they are properly difpofed, the aperture of the muffle fliould have a large piece of burning charcoal put to it, and then the operator is to watch the procefs, and fee when the glafs begins to look bright: this is the fignal of its being fit to re- ceive the impreflion. The crucible is then to be taken out of the fire, and the hot glafs muft be preffed down upon the mould with an iron inftrument, to make it receive the regular impreflion : as foon as this is done, the crucible is to be fet at the fide of the furnace out of the way of the wind, that it may cool gradually without breaking. When it is cold, the glafs is to be taken out, and its edges fliould be grated round with pincers, which will prevent its flying af- terwards, which Is an accident that fometimes happens when this caution has been omitted, efpecially when the glafs is naturally tender. The different coloured glafles are of dif- ferent degrees of hardnefs, according to their compofition, but the hard eft to melt are always the beft for this purpofe, and this is known by a few trials.
If it be defired to copy a ftone in relief, which is naturally in creux ; or to take one in creux, which is naturally in relief, there needs no more than to take an impreflion firft in wax or fulphur, and to mould that upon the pafte ©f tripela in- ftead of the ftone itfelf : then proceeding in the manner be- fore directed the procefs will have the defired fuccefs. Mem. Acad. Par. 1712.
Semlpe'hicid Gems. Thefe are defined to be ftones, compofed of a but flightly debafed cryftalline matter, of great beauty and brightnefs, of a moderate degree of tranfparence, and ufually found in fmall maffes. The bodies of this clafs are divided into two general orders, and under thefe into eight diftinct genera. Of the firft order are thofe of but few variegations, and frequently of one plain fimple colour, though fometimes veined. Of the fecond are thofe remarkable for their veins, ■zones, and variegations. The genera of the firft order, are the cornelians, chalcedonies, hydrophanx, and pramnion. Thofe of the fecond, are the agates, onyxes, fardonyxes, and earner. See each of thefe under its proper head. Hill's Hift. Foff.
Indian Gem, Indica Gemma,, a name given by Pliny, and fome other of the antients, to a gem of a reddifti colour, which according to their accounts, afforded a purple fweat on be- ing rubbed : another kind of it is alfo mentioned, which was white and looked dufty.
Gemma Veneris, in natural hiftory, a name given by fome of the Latin poets, and other authors in that language, to a gem, which was much efteemed by the Romans, and fet in rings. It was called by the Greeks aphrodites. Many have fuppofed this to be the opal j but it is certain from the oldeft accounts, that the moft beautiful kind of the amethyft -was the gem they called by this name. They diftinguifhed the amethyft as all the other gems into feveral kinds, according to the degrees of the colour, and to the moft eminent of thefe they gave peculiar names, as is evident in the feveral kinds of the emerald and beryl. The worft amethyfts were thofe 5
GEN
which had very little colour boyond that of cryftal, and the very beft were thofe which had the fine red of the carbuncle, mixed with the purple of the violet; and" the whole fo pale, that a rofe colour feemed blended with them and predominating over the reft. Thefe Pliny tells us were called pjederotes, and anterotes, and were the gems called from their beauty aphrodites lapis, and gemma veneris ; but all thefe names have been Jince given by others to the opal.
GEMMEUM, an epithet ufed by the Latins to exprefs fuch veflels, as were called dialitha and lithocolla by the Greeks. The more correct Roman authors however call this gemma- turn poadtim, and not gemmeum ; the word gemmeum, fignifying properly a vafe or cup, cut out of a fingle ftone, as an agate or the like ; but gctnmatum exprefling thofe five vafes of gold, which were ornamented with emeralds, and other precious ftones.
GEMONIDES, a name by which fome of the antients have called the peantides ; probably the fame with our ftalactites, a ftone famous with them for its fuppofed virtue, in pro- moting delivery. Plin. Hift. Nat.
GEMURSA, a word ufed by the antient writers on medicine, for a diftemper which was an excrefcence of flefh between the toes ; a difeafe not known with us. Cajl. Lex. Med. in voc.
GENECANTHE, in botany, a name ufed by fome authors, for the black bryony. Ger. Emac. Ind. 2.
GENERATION (Cycl.)<— The aft of generation is very diffe- rently performed in different animals, and is not left won- derful in the fmaller than in the larger creatures.
Generation of Butterflies. The butterfly, whofe whole life in that ftate is employed only in the' propagation of its fpecies, givee the curious obferver much caufe of wonder in the manner of it. In all the fpecies, the male, in the time of coupling, throws out from the laft ring of his body a part which enters into the hinder part of the female. This organ may be feen on prefling lightly with the fingers the body of a male butterfly. The end of the body where this is fituated is extremely flexile, and the creature turns it with great facility upward or downward, or to either fide ; and to this it is owing, that there are many different attitudes in the generation of different butterflies. In many fpecies the male lays his body in a parallel befide the body of the female, and their wings being fpread horizontally, make a fort of clofe covering for both their bodies, and the hinder part of the body of the male being capable of turning fideways, is now able to perform all its functions. In other fpecies the male gets up- on the body of the female, and the work is then performed by means of bis power of bending the end of his body down- wards. In this ftate, if any thing difturbs them, the female arifes from her feat, and flies away with the male upon her back j and this feems almoft the only ufe the female has for wings, for file is too heavy to fly with any pleafure. The power the male has of bending this part of his body upwards, is of ufe when he meets the female on the wing ; but this is feldom the cafe, and that only in a few fpecies. But befide* all thefe forms of generation, they are not unfrequently feeu joined tail to tail, with their bodies in a ftrait line, and their heads ftanding oppofite ways. The act of generation in moft fpecies of butterflies, pailes without any emotion, and feem- ingly in a ftate of reft and perfect tranquillity j but this is not die cafe in all, for in the butterfly bred of the filkworm ca- terpillar, the male raifes and falls his wings all the time with great celerity. Mr. Malpighi has had the patience to count the number of thefe vibrations of the butterfly of this fpecies wings, and found them 130 in number ; after this the crea- ture falls into a languor and faintnefs, which lafts about a quarter of an hour, and fometimes feparates himfelf from the female ; if not, at the end of this time he recovers himfelf and renews the agitations of his wings, but for a lefs number of times. The fame author ufually counted at this latter time thirty fix, and after this he becomes languid again, and afterwards revives feveral other times, but the intervals are longer and the clappings of the wings lefs numerous. Memoirs Acad. Scient. Par. 1736.
Unnatural Gener at ion. One of the moft fingular accounts ever publifhed as to the unnatural genei-ation of animals, is that re- corded in the Philofophical Tranfactions, under the names of Mr. Roberts and Mr. Halley of Chefter, of a male greyhound, which voided by the anus a whelp, refembling a grey- hound puppy. They fay that the creature came into the world alive, and after a few hours died ; but that it was pre- ferved in fpirit of wine. A white mucous matter was void- ed with it, and it had a fhot on one fide exactly in the fame part where the greyhound had, which voided it. The dog was killed and diffected, but nothing particular was found in the internal parts. Many people attefted the truth of this ftrange ftory, but if true, by whatever accident' it happened, it is odd that hiftory fhould have produced no one inftance of fuch an accident before, nor the world have met with any thing like it fince. Philof. Tranf. N 1 . 222. We read alfo in the fame book of generation performed, not- withftanding that a ftone fo filled the urethra, that for fe- veral years the urine did not pafs, but by the introduction of a knitting needle. The author of the relation docs not pretend to account for the difficulty of the cafe ; he only
increafes
