llow and brown purple.

CATALYSIS (from the Gr. [Greek: kata], down, and [Greek: luein], to
loosen), in chemistry, the name given to chemical actions brought about
by a substance, termed the "catalyst," which is recovered unchanged
after the action. The term was introduced by Berzelius, who first
studied such reactions. It is convenient to divide catalytic actions
into two groups:--(1) when the catalyst first combines with one of the
reaction components to form a compound which immediately reacts with the
other components, the catalyst being simultaneously liberated, and free
to react with more of the undecomposed first component; and (2), when
the catalyst apparently reacts by mere contact. The theory of catalysis
is treated under CHEMICAL ACTION; in this article mention will be made
of some of the more interesting examples.

A familiar instance of a catalytic action is witnessed when a mixture
of potassium chlorate and manganese dioxide is heated to 350 deg.,
oxygen being steadily liberated, and the manganese dioxide being
unchanged at the end of the reaction. The action may be explained as
follows:--part of the chlorate reacts with the manganese dioxide to form
potassium permanganate, chlorine and oxygen, the chlorine subsequently
reacting with the permanganate to produce manganese dioxide, potassium
chloride and oxygen, thus

2KClO3 + 2MnO2 = 2KMnO4 + Cl2 + O2 = 2KCl + 2MnO2 + 3O2.

This explanation is supported by the facts that traces of chlorine are
present in the gas, and the pink permanganate can be recognized when
little dioxide is used. Other oxides bring about the same decomposition
at temperatures below that at which the chlorate yields oxygen when
heated alone; but since such substances as kaolin, platinum black and
some other finely powdered compounds exercise the same effect, it
follows that the explanation given above is not quite general. Another
example is Deacon's process for the manufacture of chlorine by passing
hydrochloric acid gas mixed with air over heated bricks which had been
previously impregnated with a copper sulphate solution. The nitrous
gases employed in the ordinary chamber process of manufacturing
sulphuric acid also act catalytically. Mention may be made of the part
played by water vapour in conditioning many chemical reactions. Thus
sodium will not react with dry chlorine or dry oxygen; carbon, sulphur
and phosphorus will not burn in perfectly dry oxygen, neither does
ni