mineralogy. 65 
SECT. LVIL 
The Onyx. O^yx Camehuja. Memphites. 
This ftoile is the hardeft of the flinty 
tribe, and confifts of differently coloured 
veins, which run parallel to one another, 
fometimes in ftraight, fometimes in curved 
lines. It is found of two forts. 
a. Nail- coloured onyx, having pale fiefh- 
coloured, and v/hite lines. From the 
river Tomm in Siberia. 
h* .With black and white lines. The Oriental 
onyx. 
The old Romans were accii domed to 
cut figures on the ftraight-lined onyxes in 
relief, which they called Camehuja , thefe 
are dill counterfeited, and called Carna- 
yeu. Thofe which confid of concentric 
circles v/ere called Memphites *, and we 
have now of this kind cut to be fet in 
ang been laid in water, (bines like apiece of red-hot charcoal ; 
the Alleria, which is faid to (hew luminous ftars on its furface, 
&c. But thefe are no longer to be met with, fmce falhioii 
has given preference to the more tranfparent hard (tones j 
and it is alfo very difficult rightly to underdand the defcrip- 
tions of antient authors, in regard to colours, and their dif- 
ferent mixtures with one another J. 
kind the beginning is round the edges) which Increafes by flow degrees 
until the whole ftone is become uniformly dear througliout : when 
taken out of the water, it lofes its tranfparency, firft at one end, 
then gradually over the remainder, until the whole lias recovered 
3,ts former opacity ; and this change happens in lefs time than that of its 
becoming tranfparent. No other experiments have yet been . made 
upon this ftone, becaufeit is fo very feldom to be met with ; and thefe 
are not fufficient to determine exadly of what kind it is. E. 
p Amongfl: thefe cannot be reckoned the Tourmaline, fo much re- 
nowned of l.ate for its eledrical qualities. It is but a cryftallifed cockle, 
of a green or brown colour, more or lefs deep, fo as to turn to a 
black, and fometimes to a bluifli colour, when looked through j others 
appear quite black. [See Se<S. LXXV.) E. 
The Brafilian emerald feems to have the fame properties of becoming 
eledrified pofitively in one fide, and negatively in the other : it belongs 
^Ifo to the Shid kind. [See Sea. XLVIU. Notef,] 
F 
nngs^ 
