Permanent Colours of Opake Bodies. 1 1 3 



"ivhite, when pure, and free from heterogpneous 

 matters. 



The colours, which they afiume, are adventi- 

 tious, and arife from phlogiftic, or metallic, mix- 

 tures Such compounds form ochreous, cupre- 

 ous, and other metallic earths, and coloured clays, 

 and boles ; which may be artificially obtained, 

 by precipitating metals from their folutions, 

 and uniting them with calcareous, or aluminous 

 bafes. 



Calcareous earth indurated conftitutes marble, 

 which may be tinged, by means of metallic fo- 

 lutions, with dyes which, both in their colours, 

 and materials, are fimilar to thofe produced by 

 nature. 



From filiceous earths, flints are formed, which 

 owe their colour to phlogifton, and when fuffici- 

 ently heated, are rendered white, by the lofs of 

 their inflammable contents. 



Siliceous earths impregnated withmetals, form 

 agates, cornelians, jafper, and coloured cryflals. 



The emerald, fapphire, topaz, hyacinth, and 

 ruby are compofed of a mixture of filiceous, cal- 

 careous, and argillaceous earths, and receive 

 their colours from iron. 



All thefe coloured fl:ones may be imitated by 

 glafl^es tinged with fuch phlngiflic, or metallic 

 matters, as enter the compofition of the original 

 fubn:ances. 



P 3 Liquors, 



