64 A S Y S T E M O P 
more opaque, becaufe it is caficr to be 
imitated by art f. 
SECT LVL 
The Cat’s Eye. Pfeudopalus, 
This ftone is opaque, and refledls green 
and yellowiih rays from its furface, and is 
found in Siberia 
Sometimes this ftone is furrounded with a white cruft, like 
common flints in the ftrata of chalk ; which cruft has likewife 
the fame efte6t as that of the flint, when this laft- mentioned 
has been previoufly freed from the adherent chalk; viz, i. It 
does not ferment nor diflblve in the acid of nitre ; 2. is not 
fuftble by itfelf in the fire ; 3. but melts pretty eafily with, 
borax, though without any elFervefcence, contrary to what we 
obferve with Calcareous fubftances ; and thus borax will diflblve 
a quantity equal to about three quarters of its own bulk, 
thg h not without difficulty, efpecially towards the end of 
the Operation ; but the glafs becomes quite clear and colour- 
lefs, inftead of growing white and opaque, as with calcareous 
fubftances. E. 
t Not only this, but alfo fome of the other kinds of opals, 
have been well imitated by art, there being found compo- 
iitions of glafs, which fliew very different colours by refrac- 
tion from what appear by reflexion. A curious antient one 
of this kind is to be feen in the Royal Abbey of St. Dennis, 
near Paris, which is green on the outfide, and {fiews a fine 
ruby colour when viewed againft the light. And Iat;dy an 
ingenious gentleman at London made fome paftes, vvhich are 
of ayellowifh dark brown by refiedlion; but fome of which, 
when held againft the light, appear of a ftne blue colour, and 
others either purple, or like hyacints, garnets and rubies. 
^ The earlier writers on flones mention other varieties of 
this kind ; for inftance, the Oculus Mundi §, which, after hav- 
§ There are in the Britiffi Mufaeum at London, three of thefe ftonea 
called Octtlv.s Mundi, The largeft of them is about the bignefs of a 
cherry-ftone, though in an oval form. It is opaque, and its colour like 
that of a common yellow pea ; it may be fciatched, though not with- 
out difficulty, by a knife ; it feems however to leave a mark on com- 
mon glafs, and does not ferment with the acid of nitre. 
When it has Iain in water fome hours, it becomes tranfparent, and of 
a yellow amber colour. This change begins foon after the immerfion, 
and at ons end, in form of a little fpot (but is a fmall one of the fame 
kind 
