place in the presence of very fine
platinum dust, the reaction is rapid and nearly complete.

[Illustration: Fig. 43]

~ Experimental preparation of sulphur trioxide.~ The experiment
can be performed by the use of the apparatus shown in Fig. 43,
the fine platinum being secured by moistening asbestos fiber
with a solution of platinum chloride and igniting it in a
flame. The fiber, covered with fine platinum, is placed in a
tube of hard glass, which is then heated with a burner to about
350 deg., while sulphur dioxide and air are passed into the tube.
Union takes place at once, and the strongly fuming sulphur
trioxide escapes from the jet at the end of the tube, and may
be condensed by surrounding the receiving tube with a freezing
mixture.

~Properties of sulphur trioxide.~ Sulphur trioxide is a colorless liquid,
which solidifies at about 15 deg. and boils at 46 deg.. A trace of moisture
causes it to solidify into a mass of silky white crystals, somewhat
resembling asbestos fiber in appearance. In contact with the air it
fumes strongly, and when thrown upon water it dissolves with a hissing
sound and the liberation of a great deal of heat. The product of this
reaction is sulphuric acid, so that sulphur trioxide is the anhydride of
that acid:

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

~Catalysis.~ It has been found that many chemical reactions, such as the
union of sulphur dioxide with oxygen, are much influenced by the
presence of substances which do not themselves seem to take a part in
the reaction, and are left apparently unchanged after it has ceased.
These reactions go on very slowly under ordinary circumstances, but are
greatly hastened by the presence of the foreign substance. Substances
which hasten very slow reactions in this way are said to act as
catalytic agents or _catalyzers_, and the action is called _catalysis_.
Just how the action is brought about is not well understood.

DEFINITION: _A catalyzer is a substance which changes the velocity of a
reaction, but does not change its products._

~Examples of Catalysis.~ We have already had several instances of such
action. Oxygen and hydrogen combine with each other at ordinary
temperatures in the presence of platinum powder, while if no catalytic
agent is present they do not combine in appreciable quantities until a
rather high temperature is reached. Potassium chlorate, when heated with
manganese dioxi