tric oxide give red fumes of the peroxide. In organic chemistry many
catalytic actions are met with. In the class of reaction known as
"condensations," it may be found that the course of the reaction is
largely dependent upon the nature of some substance which acts
catalytically. One of the most important is the Friedel and Craft's
reaction, in which an aromatic compound combines with an alkyl haloid in
the presence of aluminium, zinc or ferric chloride. It seems in this, as
in other cases, that additional compounds are first formed which
subsequently react with the re-formation of the catalyst. The formation
of benzoin from benzaldehyde in the presence of potassium cyanide is
another example; this action has been investigated by G. Bredig and
Stern (_Zeit. Elektrochem._, 1904, 10, p. 582).

The second class of catalytic actions, viz. those occasioned by the
presence of a metal or some other substance which undergoes no change,
is of especial interest, and has received much attention. The
accelerating influence of a clean platinum plate on the rate of
combination of hydrogen and oxygen was studied by Faraday. He found that
with the pure gases the velocity of reaction increased until the mixture
exploded. The presence of minute quantities of carbon monoxide, carbon
disulphide, sulphuretted hydrogen and hydrochloric acid inhibited the
action; in the case of the first two gases, there is no alteration of
the platinum surface, since the plate brings about combination when
removed to an atmosphere of pure hydrogen and oxygen; with the last two
gases, however, the surface is altered, since the plate will not
occasion the combination when placed in the pure gases. M. Bodenstein
(_Zeit. phys. Chem._, 1904, 46, p. 725) showed that combination occurs
with measurable velocity at ordinary temperatures in the presence of
compact platinum. More energetic combination is observed if the metal be
finely divided, as, for instance, by immersing asbestos fibres in a
solution of platinum chloride and strongly heating. The "spongy"
platinum so formed brings about the combination of ammonia and oxygen to
form water and nitric acid, of nitric oxide and hydrogen to form ammonia
(see German Patent, 1905, 157,287), and of sulphur dioxide and oxygen to
form sulphur trioxide. The last reaction, which receives commercial
application in the contact process of sulphuric acid manufacture, was
studied by M. Bodenstein and W. Pohl (_Zeit. Elektrochem._, 1