The Soda of commerce is a carbonate of soda, and it is made from
sea salt. It is used in large quantities for the general purposes
of washing and cleansing, and very extensively in bleaching and
soap making. Soda occurs in two forms--soda ash and in crystals;
the first is the crude soda before crystallization. To make soda,
oil of vitriol (sulphuric acid) is poured, by degrees, on a layer
of sea salt, in a sort of funnel connected with a tall chimney; on
the addition of the vitriol, copious fumes of the hydrochloric acid
are given off; this is the acid formerly called spirit of salt. When
the furnace is heated and all the acid driven off, the dried residue
is taken out, it is sulphate of soda. It was formerly the custom to
allow the hydrochloric acid to pass up a very tall chimney so that
it may be dispersed in the air; but such an injurious effect was
produced on the surrounding vegetation that this could no longer be
allowed; the plan adopted was, to cause the acid to be condensed by
filling the chimney with coke, and causing water to trickle through
it; the acid vapours, coming into contact with this porous wet
surface, is condensed into a liquid, which runs down into a cistern
placed to receive it. The sulphate of soda, when taken from the
furnace and cooled, is next ground in a mill with rather more than
its weight of chalk, and about half its weight of coal. This mixture
is placed in a furnace and raised to a sufficient heat to partly
fuse it, during which time it is stirred about; the black mass which
results is called by the workmen “black ball.” It is taken out and
put into a cistern; water is then poured over it, and after stirring,
it is drained off and evaporated to a dry mass; this is impure soda.
It is mixed with coal-dust, again burnt, again washed and evaporated,
by this second process the soda ash is produced, which, being
dissolved, filtered and evaporated, produces large crystals of soda.
The large quantity of hydrochloric acid produced in the first part
of the process, is used in the of making chloric of lime. A few years
back, soda was got from the ashes of the plant called “salsola soda,”
and sold in the form of an impure carbonate called “barilla.”