829 
GEM 
GEM 
GELD, or Gild, in law, a fine or com- 
pensation for an offence. Hence wergeld, in 
our antient laws, was used for the value of a 
man slain ; and orsgeld, of a beast. 
GEM, innatuial history, a common name 
for all precious stones, of which there are two 
classes, the pellucid and semipellucid. See 
Gemma. 
GEMINI, the Twins, in astronomy, one of 
the twelve signs of the zodiac, the third in 
order, beginning with Aries. T his constella- 
tion, according to different authors, contains 
from 2 4 to 89 stars, it is represented by the 
figure of two twin-children, looking each 
other affectionately in the face, and supposed 
to be Castor and rollux. 
GEMMA, in natural history, a common 
name for all precious stones ; of which there 
are two classes, the pellucid and Semipellucid. 
The bodies composing the class of pellucid 
gems are bright, elegant, and beautiful fossils, 
naturally and essentially compound, ever 
found in small detached masses, extremely 
hard, and of great lustre. 
The bodies composing the class of semi 
pellucid gems are stones naturally and essen- 
fiallv compound, not intlmnmable nor solubl 
in watqr, found in detached masses, and com- 
posed of crystalline matter debased bv earth 
however, they are but slightly debased, and 
are of great beauty and brightness, of a mo- 
derate degree of transparency, and are usu- 
ally found in small masses. 
The knowledge of gems depends princi 
pally on observing their hardness and colour. 
For hardness, they are commonly allowed to 
■stand in the following order: the diamond 
the hardest of all ; then the ruby, sapphire, 
hyacinth, emerald, amethyst, garnet, earned, 
chalcedony, onyx, jasper, agate, porphyry, 
and marble. T his difference, however, is 
not regular and constant, but frequently va- 
ries. 'Good crystals may be allowed to suc- 
ceed the onyx; but the whole family of me- 
tallic glassy lluors seem to be still softer. Jn 
point of colour, the diamond is valued for its 
transparency, the ruby for its purple, the 
Sapphire for its blue, the emerald tor its green, 
the hyacinth tor its orange, the amethyst ear- 
ned for its carnation, the onyx for its tawny, 
the jasper, agate, and porphyry, for their 
vermilion, green, and variegated colours, and 
the garnet for its transparent blood-red. 
All these gems are sometimes found co- 
loured and spotted, and sometimes quite lim- 
pid and colourless. In this case the diamond- 
cutter or polisher knows how to distinguish 
their different species by their different de- 
grees of hardness upon the mill. For (he 
cutting or polishing of gems, the line powder 
of the fragments of those that are next in de- 
gree of hardness is always required to grind 
away the softer; but as none of them are 
harder than the diamond, this can only be 
polished by its own powder. 
Cronstedt observes of gems in general, that 
the colour of the ruby and emerald is said to 
remain in the tire, while that of the topaz flies 
off. Hence it is usual to burn the topaz, and 
thence substitute it for the diamond. 
With regard to the texture of gems, M. 
Magellan observes, that all of them are foli- 
ated or laminated, and of various degrees o! 
hardness. Whenever the edges of these la- 
mina: are sensible to the eye, they have a 
fibrous appearance, and relied various shades 
cl colour, which change successively accord- 
G E M 
ing to their angular position to the eye. 
These are called by the French chatorantes ; 
and what is a blemish in their transparency 
often enhances their value on account of their 
scarcity. But when the substance of a gem 
is composed of a broken texture, consisting 
of various sets of lamina; differently inclined 
to each other, it emits at the same time va- 
rious irradiations of different colours, which 
succeed one another according to their angle 
of position. This kind of gems has obtained 
the name of opals, and are valued in propor- 
tion to the brilliancy, beauty, and variety of 
their colours. Their crystallization, no doubt, 
depends on the same cause which produces 
that of salt, earths, and metals. The follow- 
ing table shows in general the component 
parts of gems according to the analysis of 
Bergman and M. Ac hard; the letter B pre- 
fixed to each denoting Bergman’s analysis, 
and A that of Achard. We must observe, 
however, that under the description of the 
different gems our readers will rind much 
more accurate statements. 
P C/} _ ^ £;* 
33 
39 
24 
23 
C 
8 
8 
10 
25 20 
22 20 
37 15 
34 11 
40 12 
48 11 
Tied oriental ruby, - B 40 39 9 
Ditto, - A 37.5 42.5 9 
Blue oriental sapphire," B 58 35 5 
Ditto, - - - A 53 
Yellow topaz from Saxony, B 46 
Green oriental emerald, B 60 
Ditto, - - A 60 
Y el low- brown oriental hy- 
acinth - - B 40 
Ditto, - - A 42 
Tourmalin from Ceylon, B 39 
Ditto from Brasil, - B 50 
Ditto from Tyrol, - B 42 
Garnet from Bohemia, - A 30 
GEMMA, a bud. See Botany ; and 
Plants, physiology of. 
GEMONliE scaloe, in Roman antiquity, 
a place for executing criminals. It was situ- 
ated on the Aventine mount, or tenth region 
of the city, and was, according to some, a 
place raised on several steps, whence they 
precipitated the criminals. But others will 
have it to have been a kind of dungeon, to 
which they descended by steps. 
GEMS, factitious , or counterfeit . See 
Glass. 
Gems , imitation of antique. There has 
been at different times a method practised by 
particular persons, of taking the impressions 
and figures of antique gems, with their en- 
gravings, in glass of the colour of the original 
cm. This has always been esteemed a very 
valuable method, and greatly preferable to 
the more ordinary ones of doing it on sealing- 
wax or brimstone: but, to the misfortune of 
the world, this art, being a secret only in the 
hands of some particular persons who got 
their bread by it, died with them, and every 
new artist was obliged to re-invent the me- 
thod; till at 1 ngth Mr. Ilomberg, having 
found it in great perfection, gave the whole 
process to the world to be aio more forgotten 
or lost ; and since that time it has been very 
commonly practised in France, and some- 
times in other places. 
Mr. Ilomberg was favoured in his attempts 
with all the engraved gems of the king’s ca 
binet, and took such elegant impressions, and 
made such exact resemblances of the origi- 
nals, and that in glasses so artfully tinged to 
the colour of the gems themselves, that the 
nicest judges were deceived in them, and 
often mistook them for the true antique stones. 
These counterfeit gems also serve, as well as 
the original ones, to make more copies from 
afterwards; so that there is no end of the 
numbers that may be made Item one; and 
there is this farther advantage, that the copy 
may be easily made perfect, though the ori- 
ginal should not be so, but should have sus- 
tained some damage from a blow or otherwise. 
r Fhe great cave in the operation is, to take 
the impression of the geni in a very fine 
earth, and to press down upon this a piece of 
proper glass, softened or half-melted at the 
lire, so that the figures of the impression 
made in the earth may be nicely and per- 
fectly expressed upon the glass.- In general, 
the whole process much resembles that of 
the common founders. But when it is brought 
to the trial, thqre are found a number of diffi- 
culties which were not to be foreseen, and 
which would not at all affect the common 
works of the founder. For his purpose every 
earth will serve that is fine enough to receive 
the impressions, and tough enough not to 
crack in the drying; these all serve tor their 
use, because the metals which they cast are 
of a nature incapable of mixing with earth, or 
receiving it into them, even it both are melt- 
ed together, so that the metal always easily 
and perfectly separates itself from the mould; 
but it is very difficult in these casts of glass. 
They are composed of a matter which differs 
in nothing from that of the mould, but that it 
has been run into this form by the force of 
fire, and the other has not yet been so run, 
and will mix itself inseparably with the glass 
in a large lire: consequently, if there is not 
great care used, as w r ell in the choice of the 
gias3 as tiie manner of using it, when the 
whole is finished, there will be found great 
difficulty in separating the glass from the 
mould; and often this cannot be done with- 
out wholly destroying the impression. 
All earths run more or less easily in the 
fire as they are more or lc*-,s mixed with sa- 
line particles in their natural formation. As 
all salts make earths run into glass, and as it 
is necessary to use an earth on this occasion 
for the making a mould, it being also neces- 
sary, to the perfection of the experiment, 
that this earth should not melt or run, it is 
our business to search out for this purpose 
some earth which naturally contains very 
little salt. Of all the species of earth which . 
Mr. Ilomberg examined on this occasion, 
none proved so much divested of salts, or so 
fit for the purpose, as the common tripda, or 
tripoli, used to polish glass and stones. Of; 
this earth there are two common kinds ; tlnx- 
one reddish, and composed of several Hakes- 
or strata ; the other yellowish, and of a simple 
structure. These are both to be had in the 
shops. The latter kind is from the Levant; 
the former is found in England, France, and 
many other places. This tripela must be 
chosen soft and smooth to the touch, and noti 
mixed w ith sandy or other extraneous matter. 
The yellowish kind is the best of the tw'o, 
and is commonly called Venetian tripoli. 
This receives the impression very beautifully, 
and never mixes with the glass in the opera- 
tion, which the red kind sometimes does. 
Mr. Homberg usually employed both kinds 
at once in the following manner: 1st, powder 
a quantity of the red tripela in an iron inor- 
