gases are passed through a scrubber in order to free them from the last
traces of acid before discharging them into the atmosphere.

_Hydrochloric Acid._--The greater part of the hydrochloric acid
manufactured in Great Britain is obtained as an intermediate product in
the Leblanc alkali process, which will presently be described, being
produced by heating common salt with vitriol. A large quantity is,
however, also produced by the so-called direct process of Hargreaves &
Robinson, which is, in principle, the same method as that employed in
the Leblanc process, except that the intermediate product, vitriol, is
not separated. It consists essentially in passing the hot gases from
pyrites kilns, as used in the manufacture of vitriol, through large
cast-iron vessels containing common salt heated to a high temperature.
Various physical conditions must be complied with in order to make the
process a success. For example, the salt is used in the form of moulded
hard porous cakes made from a damp mixture of common salt and rock salt.
The cast-iron vessels must be heated uniformly, and the hot pyrites kiln
gases must be passed downwards through the salt in order to ensure
uniform distribution. The hydrochloric acid is condensed in stoneware
pipes connected with towers packed with coke or stoneware.

_Alkali: Leblanc Process._--The manufacture of vitriol, as I have
described it to you, is the first step in the Leblanc process. The next
stage consists in the manufacture of sodium sulphate (salt-cake) and
hydrochloric acid from the sulphuric acid and common salt; this is
called the salt-cake process. The production of salt-cake or crude
sodium sulphate is carried out in two stages. A large covered iron pan,
called the decomposing pan or salt-cake pot, is mounted in one part of
the salt-cake furnace, and alongside it is the hearth or bed on which
the second stage of the process, the drying or roasting, is effected.