de, gives up its oxygen at a much lower temperature than
when heated alone. Hydrogen dioxide decomposes very rapidly when
powdered manganese dioxide is sifted into its concentrated solution.

On the other hand, the catalytic agent sometimes retards chemical
action. For example, a solution of hydrogen dioxide decomposes more
slowly when it contains a little phosphoric acid than when perfectly
pure. For this reason commercial hydrogen dioxide always contains
phosphoric acid.

Many reactions are brought about by the catalytic action of traces of
water. For example, phosphorus will not burn in oxygen in the absence of
all moisture. Hydrochloric acid will not unite with ammonia if the
reagents are perfectly dry. It is probable that many of the chemical
transformations in physiological processes, such as digestion, are
assisted by certain substances acting as catalytic agents. The principle
of catalysis is therefore very important.

~Sulphuric acid~ (_oil of vitriol_) (H_{2}SO_{4}). Sulphuric acid is one
of the most important of all manufactured chemicals. Not only is it one
of the most common reagents in the laboratory, but enormous quantities
of it are used in many of the industries, especially in the refining of
petroleum, the manufacture of nitroglycerin, sodium carbonate, and
fertilizers.

~Manufacture of sulphuric acid.~ 1. _Contact process_. The reactions
taking place in this process are represented by the following equations:

SO_{2} + O = SO_{3},

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

To bring about the first of these reactions rapidly, a catalyzer is
employed, and the process is carried out in the following way: Large
iron tubes are packed with some porous material, such as calcium and
magnesium sulphates, which contains a suitable catalytic substance
scattered through it. The catalyzers most used are platinum powder,
vanadium oxide, and iron oxide. Purified sulphur dioxide and air are
passed through the tubes, which are kept at a temperature of about 350 deg..
Sulphur trioxide is formed, and as it issues from the tube it is
absorbed in water or dilute sulphuric acid. The process is continued
until all the water in the absorbing vessel has been changed into
sulphuric acid, so that a very concentrated acid is made in this way. An
excess of the trioxide may dissolve in the strong sulphuric acid,
forming what is known as _fuming sulphuric acid_.

2. _Chamber process._ The method of manufacture exclusively em