ssac tower X. In order to prevent the escape of the oxides of
nitrogen regenerated in the reaction, the tower is filled with lumps of
coke, over which trickles concentrated sulphuric acid admitted from Y.
The nitrogen peroxide dissolves in the acid and the resulting solution
collects in H. This is pumped into E, where it is mixed with dilute
acid and allowed to trickle down through the chamber D (Glover tower),
which is filled with some acid-resisting rock. Here the nitrogen
peroxide is expelled from the solution by the action of the hot gases
entering from A, and together with them enters the first chamber
again. The acid from which the nitrogen peroxide is expelled collects in
F. Theoretically, a small amount of nitrogen peroxide would suffice to
prepare an unlimited amount of sulphuric acid; practically, some of it
escapes, and this is replaced by small amounts admitted at B.

The sulphuric acid so formed, together with the excess of condensed
steam, collect upon the floor of the chambers in the form of a liquid
containing from 62% to 70% of sulphuric acid. The product is called
_chamber acid_ and is quite impure; but for many purposes, such as the
manufacture of fertilizers, it needs no further treatment. It can be
concentrated by boiling it in vessels made of iron or platinum, which
resist the action of the acid, nearly all the water boiling off. Pure
concentrated acid can be made best by the contact process, while the
chamber process is cheaper for the dilute impure acid.

~Physical properties.~ Sulphuric acid is a colorless, oily liquid, nearly
twice as heavy as water. The ordinary concentrated acid contains about
2% of water, has a density of 1.84, and boils at 338 deg.. It is sometimes
called _oil of vitriol_, since it was formerly made by distilling a
substance called _green vitriol_.

~Chemical properties.~ Sulphuric acid possesses chemical properties which
make it one of the most important of chemical substances.

1. _Action as an acid._ In dilute solution sulphuric acid acts as any
other acid, forming salts with oxides and hydroxides.

2. _Action as an oxidizing agent._ Sulphuric acid contains a large
percentage of oxygen and is, like nitric acid, a very good oxidizing
agent. When the concentrated acid is heated with sulphur, carbon, and
many other substances, oxidation takes place, the sulphuric acid
decomposing according to the equation

H_{2}SO_{4} = H_{2}SO_{3} + O.

3. _Action on metals._ In