nitrogen compounds from the nitrous vitriol is effected in Glover towers
(the invention of John Glover of Newcastle), which also serve to
concentrate to some extent the weak acid produced in the lead chambers,
and to cool the hot gases from the sulphur burners or pyrites kilns. The
weak chamber acid is mixed with the nitrous vitriol from the Gay-Lussac
tower, and the mixture is pumped to the top of the Glover tower, which
is of similar construction to the Gay-Lussac tower, but is generally
packed with flints. This Glover tower is placed between the sulphur
burners or pyrites kilns and the first lead chamber. The nitrous vitriol
passing down the tower meets the hot gases from the kilns, and a
threefold object is effected: (1) The nitrous fumes are expelled from
the nitrous vitriol, and are carried into the chambers, to again play
the part of oxygen-carriers; (2) the weak chamber acid which was mixed
with the nitrous vitriol is concentrated by the hot kiln gases; and (3)
the hot gases themselves are cooled. The acid from the Glover tower is
purified by special treatment--for example, the arsenic may be removed,
after precipitation with sulphuretted hydrogen, in the form of insoluble
arsenic sulphide,--and the purified acid is concentrated by heating in
glass or platinum vessels.

A considerable amount of sulphuric acid is now made by the so-called
"contact process," in which sulphur dioxide and oxygen unite to form
sulphuric acid in presence of a heated "contact" substance, usually some
form of finely-divided platinum.

_Nitric Acid._--This acid is usually prepared by distilling a mixture of
sodium nitrate and vitriol in cast-iron retorts or pots, the nitric acid
being collected in stoneware vessels connected one with another, or, as
is more generally the case at the present time, in condensing apparatus
consisting of stoneware pipes or coils cooled by water. The effluent