prismatic borax,
Na_2B4O7.10H2O, and "jewellers'" or octahedral borax, Na_2B4O7.5H2O. It
is to be noted that the term "borax" was used by the alchemists in a
very vague manner, and is therefore not to be taken as meaning the
substance now specifically known by the name. Prismatic borax is found
widely distributed as a natural product (see below, _Mineralogy_) in
Tibet, and in Canada, Peru and Transylvania, while the bed of Borax
Lake, near Clear Lake in California, is occupied by a large mass of
crystallized borax, which is fit for use by the assayer without
undergoing any preliminary purification. The supply of borax is,
however, mainly derived from the boric acid of Tuscany, which is fused
in a reverberatory furnace with half its weight of sodium carbonate, and
the mass after cooling is extracted with warm water. An alternative
method is to dissolve sodium carbonate in lead-lined steam-heated pans,
and add the boric acid gradually; the solution then being concentrated
until the borax crystallizes. Borax is also prepared from the naturally
occurring calcium borate, which is mixed in a finely divided condition
with the requisite quantity of soda ash; the mixture is fused, extracted
with water and concentrated until the solution commences to crystallize.

From a supersaturated aqueous solution of borax, the pentahydrate,
Na2B4O7.5H2O, is deposited when evaporation takes place at somewhat
high temperatures. The same hydrate can be prepared by dissolving
borax in water until the solution has a specific gravity of 1.246 and
then allowing the solution to cool. The pentahydrate is deposited
between 79 deg. C. and 56 deg. C.; below this temperature the
decahydrate or ordinary borax, Na2B4O7.10H2O, is deposited. Crystals
of ordinary borax swell up to a very great extent on heating, losing
their water of crystallization and melting to a clear white glass. The
crystals of octahedral borax fuse more easily than those of the
prismatic form and are less liable to split when heated, so that they
are preferable for soldering or fluxing. Fused borax dissolves many
metallic oxides, forming complex borates which in many cases show
characteristic colours. Its use in soldering depends on the fact that
solder only adheres to the surface of an untarnished metal, and
consequently a little borax is placed on the surface of the metal and
heated by the soldering iron in order to remove any superficial film
of