MINERALOGY. 85 
manufacture of silver, and of the baser metals, is effected at the mining 
establishment, and depends principally on the separation of those substances 
which give to the ore its distinguishing feature. Thus from the arsenids we 
obtain the combined metal, silver for instance, by roasting the ore: this drives 
off the arsenic in the form of vapor of arsenious acid. Sulphur is dissipated 
in the same way; and the processes by which a separation from injurious 
combinations is effected, are sometimes so complicated as to form a consider- 
able portion of the occupation of the workers in metals. 
Of the oxydes some are important as being ores from which their metals 
may be economically obtained. Thus iron is obtained from its oxydes, or 
from minerals rich in this latter substance. Not less important is silica, in 
its different varieties. All our different forms of glass are combinations of 
silica with powerful bases, especially with potassa and soda. By the combi- 
nation of impure carbonate of potassa, impure silica, and a certain quantity 
of lime, we obtain the well known green bottle and window glass. White 
window glass is obtained in the same way, by the addition of some 
decolorizing agent which shall remove the green color produced by the 
presence of protoxyde of iron. Peroxyde of manganese is used for this 
purpose, as by it the protoxyde of iron may be changed into peroxyde: this, 
when in large quantity, gives the glass a yellow color, but in less amount 
scarcely tinges it at all. For the better sorts of glass, only the purest materials 
are used, the silica, for instance, being furnished by the quartz rock, which 
contains it in great purity. By the addition of oxyde of lead the glass 
becomes more fluid, and such easily melted glass is used for preparing the 
colors for glass and porcelain painting, by melting with it various metallic 
oxydes. Thus, by the addition of oxyde of cobalt we get a dark blue glass, by 
that of oxyde of copper one of a beautiful reddish brown, which is pounded and 
rubbed to a fine powder. This powder, mixed with a drying oil, is applied 
to the glass or porcelain surface to be painted. After the paint has some- 
what dried, the objects to which it is applied are exposed to a red heat, 
which melts the glass into an enamel. 
Besides the application of silica in the manufacture of glass, no less 
important is the use made of several of its natural combinations. Porc elain 
is obtained by strongly heating together alumina, an alkali, and silica; for 
this purpose a substance of native occurrence is employed, namely, the 
kaolin or porcelain earth, which is found in large quantities at various 
localities. Perfectly clear and colorless rock crystal is used for purposes 
of ornament. Some colored varieties of silica, as the amethyst, are also 
applied to the same purpose. Chalcedony and agates also consist essentially 
of silica, handsomely colored by various oxydes; they furnish an excellent 
material for many instruments on account of their great hardness. Thus, 
plates of agate are used to suspend the knife-edges of delicate balances ; the 
pivot holes of magnetic needles are also made of this same material, to avoid 
undue friction. Valuable dishes and capsules, mortars for chemical 
purposes, and many other useful implements of art, are constructed of these 
substances. 
Some of the silicates constitute minerals of great value. The most 
ICONOGRAPHIC ENCYCLOPADIA.—VOL. I. 33 513 
