G E M. 
fire; and when the fal ammoniac is rublimed, weigh it, 
and add to the mixture an equal quantity. Tlicn dillil 
and fubliine as before, and repeat the operation fix times, 
being careful at each time to mix the fal ammoniac 
and the manganefe upon the porphyry with diluted oil 
of vitriol. 
Of the Heat necessary for Factitious 
Gems. —M. Fontanieu obferves, that there are three 
degrees of heat very different in their energy. The fire 
kept up in the wind furnaces in the laboi'atories of che- 
mifls, is lefs adtive than that whofe effedl is accelerated 
by the means of bellows ; and a fire fupported by wood, 
and kept up during fixty hours without interruption, 
produces fingular effedfs in vitrification, and renders the 
glafs finer and lefs alterable. When recourfe is had to 
the forge, in order to operate a vitrification, it is necef- 
lary to turn about the crucible from time to time, that 
4 lie mafs may melt equally. Some coal alfo fhould be 
replaced, in proportion as it confumes towards the nozel 
•of the bellows; for without this precaution, we fliould 
run the lifk of cooling the crucible oppofite to the 
flame, and probably of cracking it, when all the melted 
mafs running among the coals would be totally loft. 
Though this is tJie readieft way of melting, it fhould 
not be employed out of choice; for the crucible often 
breaks, or coals get into it, and reduce the calx of lead 
to a metallic ftate. 
Compositions for imitating the choicest 
Gems. —i. For the white diamond : Take the bafe of 
Mayencc. This cryftal is very pure, and has no co¬ 
lours.—2. For the yellow diamond : To an ounce of the 
fourth bafe, add for colour twenty-five grains of lima 
cornea, or ten grains of glafs of antimony.—3. For the 
emerald 1 To fifteen ounces of either of the bafes, add 
for colour one dram of mountain-blue, and fix grains 
of glafs of antimony. Or, to an ounce of the lecond 
bale, add for colour twenty gTains of glafs of antimony 
.and three grains of calx of cobalt.—4. For the fap- 
phire : To twenty-four ounces of the Mayence bafe, 
add for colour two drams forty-fix grains of the calx of 
cobait.—5. For the amethift : To twenty-four ounces 
of the Mayence bafe, add for colour four drams of pre¬ 
pared manganefe and four grains of precipitate of Caf- 
fuis.—6. Fortheberil: To twenty-four ounces of tlie 
third bafe, add for colour ninety-fix grains of glafs of 
.antimony and four grains of calx of cobalt.—7. For 
the black agate : To twenty-four ounces of either of 
the bafes, add two ounces of the mixture direfted above 
under Cobalt. —8, For the opal: To an ounce of the 
third bafe, add for colour ten grains of luna cornea, two 
grains of magnet, and twenty-fix grains of abforbent 
earth.—9. For the oriental topaz ; To twenty-four 
ounces of the firft or tliird bafe, add for colour five 
drams of glafs of antimony.—10. For the topaz of Sax¬ 
ony ; To twenty-four of the fame bafe, add for colour 
fix drams of the glafs of antimony.-—ii. For tlie topaz 
of Bralil ; To twenty-four ounces of the fecond or third 
bafe, add for colour one ounce twenty-four grains of the 
glafs of antimony and eight grains of precipitate of Caf- 
fius.—12. f'orthe hyacinth : To twenty-four ounces of 
the bafe made with rock-cryftal, add for colour two 
drams forty-eiglit grains of glafs of antimony.—13. For 
the oriental ruby : To fixteen ounces of the Mayence 
bale, add for colour a mixture of two drams forty-eight 
grains of the precipitate of CalTius, the fame quantity of 
crocus Martis prepared in aquafortis, the fame of golden 
fulphiir of antimony and of fufible manganefe, with tlie 
addition of two ounces of mineral cryftal. Or, to twenty 
ounces of llie bafe made with flint, add h.alf an ounce 
of fufible manganefe and two ounces of mineral cryftal. 
—14. For the balafs ruby: To fixteen ounces of the 
Mayence bale, add tlie above colouring powder, but di- 
minifhed a fourth part. Or, to twenty ounces of the 
bafe made with flints, add the fame colouring powder,' 
but with a fourth lels of the manganefe. 
Voi,. VIII, No. 503, 
297 
The faSlkious gems are diftinguifhed from the natural, 
by their loftnefs and fufibillty ; by their folubility in 
acids ; by their caufing only a fingle iefra6t:ion of the 
rays of light; and, in many cafes, by their fpecific gra¬ 
vity, which exceeds 2-76 in all precious gems of the 
firft order, as the diamond, -ruby, fappliire, &c. 
BeTides the imitation of the gems above deferibed, the 
jewellers have a method of making counterfeit brilliants, 
called doublets, which are formed of two pieces of white 
tranfparent glafs, or of cryftal, laid over each other, and 
cemented uniformly together, with the colouring matter 
inclofed in the centre. The manner of conflriuStingtheni 
is as follows : Let the cryftal or glafs be firft cut in the 
form of a brilliant, except that the figure nuift be com- 
pofed from two feparate pieces, divided in an horizontal 
direftion a little lower than the middle. After the two 
plates of the intended gem are thus cut, and fitted fo ex- 
aiStiy that no divilion can appear when they are laid to¬ 
gether, the upper part muft be polifhed ready for let¬ 
ting; and then tlie colour muft be put betwixt the two 
plates by the following method. “ Take of Venice or 
Cyprus turpentine two fcruples; and add to it one feru- 
ple of the grains of maftich perfectly pure, and finely 
powdered. Melt them together in a fmall lilver ladle, 
or other veffel, and put to them gradually any of the co¬ 
loured fubftances mentioned below, ftirring them toge¬ 
ther as the colour is put in, that they may be thorouglily 
commixed. Warm then the doublets to the fame de¬ 
gree of heat as the melted mixture ; and paint the upper 
furface of the lower plate, and put the upper one iii- 
ftantly upon it, preffing them to eacli otiier, but taking 
care that they may be conjoined in the moft equable man¬ 
ner. When the cement or paint is quite cold a'nd fet, 
the redundant part of it, which has been preffed out of 
the joint of the two pieces, fhould be carefully feraped 
off the fide, till there be no appearance of any colour On 
the outfide of tlie doublets: and they fliould tlien be^ 
fkilfully fet; obferving to carry the mounting over tlie 
joint, that the deception might not be difeovered, and 
that the upper piece may be well fecured from fepar.atiiag 
from the under one.—The colours are communicated by 
the following pigments, See. —The ruby may be imitated 
by mixing a fourth part of carmine with fome of the 
fineft crimfon lake that can be procured.—The fappliire 
may be counterfeited by very bright Prulfian blue, mixed 
with a little of the above-mentioned crimfon lake, to 
give it a caft of purple. The Prufiian blue ftiould not 
be very deep-coloured, or but little of it fliould be ufed; 
for otherwife, it will give a black ftiade that will be in¬ 
jurious to the luftreof the doublets.—The emerald may 
be counterfeited by diftilled verdigris, with a little 
powdered aloes. But the mixture fliould not be ftrongly 
heated, nor kept long over the fire after the verdigris is 
added : for the colour may be foon impaired by it. 
—The refejiiblance of the garnet may be made by dra¬ 
gon’s blood ; which, if it cannot be procured of f ufii- 
cient brightnefs, may be alTiftedby a very linall quantity 
of carmine.—The amethyftmay be imitated by the iiiix- 
ture of fome Pruliian blue with the. crimfon lake; but 
tlie proportions can only be regulated by experiment, as 
different parcels of the lake and Prufiian blue vary ex¬ 
tremely in the degree of ftreiigth of tlie. colour.—-The 
yellow topazes may be counterfeited by mixing pow¬ 
dered aloes with a little dragon’s blood, or by good 
Spanifli arnotto : but the colour muft be very fparin^ly 
ufed, or the tinge will be too firoiig for the natural ap¬ 
pearance of that ftone.—The chryfolite, luachuii, 
vinegar garnet, eagle marine, and other more dilute co¬ 
lours, may be formed in the fame manner, by leli'ening 
the proportions of the colours, or by compounding 
them together correfpondently to the iiue of the gem to, 
be imitated; to which end it is proper to have an ori¬ 
ginal ftone, or an exaft imitation of one, at hafli.l when 
tile mixture is made, in order to the more perfect adap¬ 
tation of the colours to the effect deiired ; and wiien 
4 G thefe' 
