64 A SYSTEM OF 



more opaque, becaufe it is eafier to be 

 imitated by art f. 

 a . . • ■ 



S'E CT LVI. 





The Cat's Eye. Pfeudopalus. 



This ftone is opaque, and reflects green 

 and yellowifh rays from its furface, and is 

 found in Siberia *. 



Sometimes this Hone is furrounded with a white cruft, like 

 common flints in the Hrata of chalk ; which cruft has likevvife 

 the fame effect as that of the flint, when this laft-mentioned 

 has been previously freed from the adherent chalk; viz. i. It 

 does hot ferment nor difiblve in the acid of nitre ; ' 2. is not 

 fufible by itfelf in the Are; 3. but melts pretty eafily with 

 borax, though without any efFervefcence, 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, efpccially towards the end of 

 the operation ; but the giafs becomes quite clear and colour- 

 lefs, inilead of growing white and opaque, as with calcareous 

 fubftances. E. 



f Not only this, but alfo fome of the other kinds of opals, 

 have been well imitated by art, there being found compo- 

 sitions of 'giafs, which fhew very different colours by refrac- 

 tion from what appear by reflection. 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 outride, and (hews a fine 

 ruby colour when viewed againfl the light. And lately an 

 ingenious gentleman at London made fome paftes, which are 

 of a yellowifh dark brown by reflection; but fome of which, 

 when held againfl: the light, appear of a fine blue colour, and 

 others either purple, or like hyacints, garnets and rubies. 



* The earlier writers on flones mention other varieties of 

 this kind ; for inflance, the Oculus Mundi §, which, after hav_ 



§ There are in the Brittfh. Mufaeum at London, three of thefe ftones 

 called Oculus Mundi. The largeft of them is about the bignefs of a 

 cherry-ftone, though in an oval form. Ir is opaque, and its colour like 

 that of a common yellow pea ; it may be fcratched, though not with- 

 out difficulty, by a knife ; it feems however to leave a mark on com- 

 mon giafs, and does not ferment with the acid of nitre. 



When it has lain in water fome hours, it becomes tranfparent, and of 

 a yellow amber colour. This change begins foon after the immerfion, 

 and at one end, in form of a little fpot (but in a fmall one of the fame 



kind 



