ployed
until recent years, and still in very extensive use, is much more
complicated. The reactions are quite involved, but the conversion of
water, sulphur dioxide, and oxygen into sulphuric acid is accomplished
by the catalytic action of oxides of nitrogen. The reactions are brought
about in large lead-lined chambers, into which oxides of nitrogen,
sulphur dioxide, steam, and air are introduced in suitable proportions.

~Reactions of the chamber process.~ In a very general way, the
various reactions which take place in the lead chambers may be
expressed in two equations. In the first reaction sulphur
dioxide, nitrogen peroxide, steam, and oxygen unite, as shown
in the equation

(1) 2SO_{2} + 2NO_{2} + H_{2}O + O = 2SO_{2} (OH) (NO_{2}).

The product formed in this reaction is called nitrosulphuric
acid or "chamber crystals." It actually separates on the walls
of the chambers when the process is not working properly. Under
normal conditions, it is decomposed as fast as it is formed by
the action of excess of steam, as shown in the equation

(2) 2SO_{2} (OH) (NO_{2}) + H_{2}O + O = 2H_{2}SO_{4} + 2NO_{2}.

The nitrogen dioxide formed in this reaction can now enter into
combination with a new quantity of sulphur dioxide, steam, and
oxygen, and the series of reactions go on indefinitely. Many
other reactions occur, but these two illustrate the principle
of the process.

The relation between sulphuric acid and nitrosulphuric acid can be seen
by comparing their structural formulas:

O= -OH O= -OH
S S
O= -OH O= -NO_{2}

The latter may be regarded as derived from the former by the
substitution of the nitro group (NO_{2}) for the hydroxyl group (OH).

[Illustration: Fig. 44]

~The sulphuric acid plant.~ Fig. 44 illustrates the simpler parts of a
plant used in the manufacture of sulphuric acid by the chamber process.
Sulphur or some sulphide, as FeS_{2}, is burned in furnace A. The
resulting sulphur dioxide, together with air and some nitrogen peroxide,
are conducted into the large chambers, the capacity of each chamber
being about 75,000 cu. ft. Steam is also admitted into these chambers at
different points. These compounds react to form sulphuric acid,
according to the equations given above. The nitrogen left after the
withdrawal of the oxygen from the admitted air escapes through the
Gay-Lu