given, as in the common preparation of coloured glasses,, 
by the designed admixture of the oxyd of the metal usedo 
Thus the scoriae of reduced cobalt ore are of a deep 
blue ; of fine copper, green or brown according to cir- 
cumstances ; of iron grass- green, and the like* The loss 
of metal is still greater when the ore retains a portion of 
sulphur, which last uniting with the alkali, acts with still 
more power on the metallic oxyds. 
As naked alkaline fluxes are peculiarly liable to this in- 
convenience, some eminent docimastic chemists employ 
with considerable success fluxes of equal power, but with 
less disposition to dissolve the metallic oxyds. Of this 
kind are very fusible glasses made of much alkali and si- 
lex without litharge or other metallic admixture, fluor 
spar, lime, and above all, borax. This last produces a 
thinner fusion of vitrifying mixtures than an equal quan- 
tity of any other substance whatever, and hence a smaller 
dose of this than of the naked alkalies, will answer all the 
purposes of a flux, and of course the loss by solution of 
the metallic oxyd is much less. 
A reduction of the sulphuretted ores is sometimes con- 
veniently effected by a single operation which gives a but- 
ton of metal of considerable purity. The roasting, which 
is always a tedious operation, is saved by mixing the ore 
with two or three times its weight of nitre, and projecting 
it into a hot crucible. The mixture dephlagrates on reach- 
ing the crucible, the sulphur is burnt and converted to 
sulphuric acid which unites with the potash of the de- 
composed nitre, and the metal, now freed from the sul- 
phur, becomes fully oxygenated by the nitric acid 
and ready for reduction. This is effected by throw- 
ing in a reducing flux of tartar and pitch or any sim- 
ilar matter, and applying a strong heat for the requisite 
time. 
It appears, however, that though nitre is often a good 
and expeditious method of freeing ores from their sul 
