232 COLOURED GLASSES OF THt ANCIENTS. 



mined for cobalt *. It contained only a flight trace of copper 

 and lime. The firft of thefe was made to appear, by com- 

 bining the fluid with priiffiate of potajh, and the brown-red 

 precipitates thus obtained amounted to a little more than two 

 grains, which are to be coniidered equivalent to about one 

 grain of oxided copper. 



(e) At lafl:, carbonated foda threw down about half a grain 

 of calcareous earth. 

 Component parts In confequence of this decompofition, thofe txvo hundred 

 °s ename"/ ^^ ^grains of the antique blue glafs-pafte, mufl: have contained the 

 following earthy and metallic conftituent parts : 



J 6.3. grains. 



19. 



3. 



1. 



0.5 ' 



Silex - 



(a) 



Oxide of Iron 



(c) 



Alumine - 



(b) 



Oxide of Copper 



(d) 



Lime 



(O 



186.5 



Other experl- As I have fubjefled the above blue glafs to feveral other 

 ""^U'^vrTfli experiments, merely with an intention of difcovering the co- 

 ed no cobalt, baltic portion it might poflibly contain, yet without finding 

 the leaft trace of it, there appears to be no doubt, that its 

 Iron can aff rd blue colour entirely depends on the ingredient iron. That 

 a blue enamel; j^^j^^ under fome circumftances, is capable of producing a 

 fmelting works, blue enamel, is clearly fliewn by the beautifully blue coloured 

 fcoriae of iron, which frequently are met with in the high 

 furnaces on fmelting filiceous iron-ftones. But we are not 

 fufiicienlly acquainted with the circumflances and conditions 

 under which this colour is produced ; for the aflertion of 

 Henckel, and fome other earlier authors, that by means of 

 iron, cemented with arfenic, the fame blue tingfe can be given 

 glafs which it acquires from cobalt, has not yet been fufficiently 



* It is well known that nature tinges the fapphire, lapis'lazuli, 

 blue clays, &c. by means of iron without cobalt; but man is not 

 j^,,.. pofTeffed of her means. A chemical friend, with whom the tianf- 



* Jator had a converfation on this fubje6> and on the difficulty of 



proving the accuracy of the above analyfis by a fynthetical procefsy 

 luggefted the idea, that fuch a blue pafte could, perhaps, be made 

 without cobalt by the intermedium of lapis-lazuli ; an idea which: 

 fnay afford a lubjeCl fox experiment. — Tranjl, 



confirmed. 



