COLOURED GLASSES OF THE ANCIENTS. <231* 



0«"igin. It is true that Gmelin could, in his examination, 

 emfploy no more than the fmall quantity of a few grains ; 

 but the refults were fufficient to Ihew, that the colouring 

 principle in his fpeciraen originated not from cobalt but from 

 iron. 



A like refult is afforded by the following decompofltion of and by our au, 

 the blue glafs from the ruins at Capri. ''^°''* 



Its colour is a fapphire-blue verging towards that of fmalt. Antique blue 



It is tranfparent on (he edges only. Its frafture, as well as s'^'f '_ f ^"™***, 

 , ' " ■' . perfectly opakc. 



that ot the preceding, comes nearer to the Icoriaceous and 



conchoidal than to the fplintery. Some of thefe blue glafs- Some plates are 

 plates are particularly diftinguiftied by this circumftance, that ^^"^.^"'j^^^j^* 

 they are not coloured blue throughout the whole of their mafs, 

 but only to about two-thirds of their thicknefs. Each of the 

 lirata is fo nicely diftincl from the other, as to give the appear- 

 ance of two plates adhering at their broad furfaces; the one 

 blue, the other colourlefs. 



(a) Two hundred grains of the above blue pafte yvere re- Pufion with 

 duced to a fubtile powder, and fufed with 400 grains of caz(/?2cfoda, &c, gave 

 foda. The obtained mafs, foftened with water, was faturated 



to excefs, and evaporated to a moderate drynefs. When re- 

 diflblved in boiling water, it depofited Jiliceoia earth, which, 

 after wafhing and ignition, amounted to 163 grains. 



(b) The fluid was then fuperfalurated with caufiic ammonia. Alumint by the 

 A brown precipitate thence enfued, which, upon edulcoration, procefs with am- 

 I digefted with a folution oi caiiftic pntujli. The flight portion * 

 taken up of it by this alkali was again, after faturating the lix- 

 ivium with an excefs of muriatic acid, precipitated by means 



of carbonated Joda, and proved, upon edulcoration and red 

 heat, to be aluminous earth, amounting to three grains. 



(c) What remained undiflblved by the cauftic potafli, was Qxided Iron, 

 merely oxided iron, weighing nineteen grains when ignited 



and waftied. 



(d) The liquor that had been fuperfaturated with cauftic The liquid con- 

 ammonia and pofleflTed a blueifli tinge, was by flow evapora- tained a little 

 tion fo far reduced to a fma'ller volume, that the greateft part ^JJPP^J TobllT^'* 

 of the muriate of foda, which had been generated and con- 

 tained in it, could feparate in cryflals. The fluid feparated from 



thefe, in which the acid predominated, and which now hardly 

 exhibited any perceivable greeniih colour, was in vain exa- 



(nined 



