All silicates insoluble in ivater and acids, no matter whether they
contain alkali metals or not, may he decomposed by fusion with alkaline
carbonates (fusion mixture). They are first ground up very finely
in an agate mortar, then intimately mixed with three or four times
their weight of fusion mixture and heated in a platinum crucible
as long as any CO2 is given ofP. This may be done over a good
Bunsen gas lamp or by means of a Herapath gas blowpipe. (A
platinum crucible can be employed with safety only when the
absence of easily reducible and fusible metals has been established
by a preliminary examination of the silicious substance in the dry
way.) The silica is thus obtained in combination with the alkalies
and dissolves in hot water, and the bases (CaO, MgO, Al20s, FejOs,
etc., etc.) are left as insoluble oxides or carbonates, and may be
separated by filtration ; or the whole mass is at once treated with
dilute hydrochloric acid and evaporated to dryness. Silicic anhy-
dride is left, and the bases are removed as chlorides by filtration.
The alkalies must be examined for in a separate portion by treat-
ment with hydrofluoric acid.
Pure amorphous silica dissolves completely, when boiled in a

• Prepared by precipitating a dilute solution of OaCl2 at 70* C. with excess
of OOAmo2, and washing the precipitate thoroughly with hot water by decanta-
tion.
140 Bouc Acm.
pbtmiim wemei with an aqueous aoludon of fixed caustic or cmr-
booated alkalies.
fliO} if tpmnUd horn TiOj (dtanie anhrdride) br fasHm vidi SOjHoKo
IB ft pbtimzm cmcible, and fobseqnait tmtmmt with wtcr ; the aHOj miiftiiiii
■ndiMolTed. The TiOj is praip^atad from, the addukCiid aqueoos wiation bj
Von^'tjooiijaaktd. boiling

QUESnOXS AJfTD EXEBCISES.
1. Gtre niffauioef of enrftalline umI mmorpboos iSica.
