Platina Cleansed by Acids and Alkalies   93
formed or deposited there; and this, and much less than this,
is sufficient to prevent it from exhibiting the curious power
now under consideration (370,, 372). Platina also has been
said to combine with carbon; and it is not at all unlikely that
in processes of heating, where carbon or its compounds are
present, a film of such a compound may be thus formed, and
thus prevent the exhibition of the properties belonging to pure
platina.1
 335.	The action of alkalies and acids in giving platina this
property was now experimentally examined.     Platina plates
(305) having no action on mixed oxygen and hydrogen, being
boiled in a solution of caustic potassa, washed, and then put
into the gases, were found occasionally to act pretty well, but
at other times to fail.   In the latter case I concluded that the
impurity upon the surface of the platina was of a nature not to
be removed by the mere solvent action of the alkali, for when
the plates were rubbed with a little emery, and the same solution
of alkali (328), they betarne active.
 336.	The action of acids was far more constant and satis-
factory.    A platina plate was boiled in dilute nitric acid: being
washed and put into mixed oxygen and hydrogen gases, it
acted well.    Other plates were boiled in strong nitric acid for
periods extending from half a minute to four minutes, and then
being washed in distilled water, were found to act very well,
condensing one cubic inch and a half of gas in the space of
eight or nine minutes, and rendering the tube warm (306).
 337.	Strong sulphuric acid was very effectual in rendering
the platina active.   A plate (305) was heated in it for a minute,
then washed and put into the mixed oxygen and hydrogen,
upon which it acted as well as if it had been made the positive
pole of a voltaic pile (306).
 338.	Plates which, after being heated or electrised in alkali,
or after other treatment, were found inert, immediately received
power by being dipped for a minute or two, or even only for
j      an instant, into hot oil of vitriol, and then into water.
|	339. When the plate was dipped into the oil of vitriol, taken
out, and then heated so as to drive off the acid, it did not act,
in consequence of the impurity left by the acid upon its surface.
i	340. Vegetable   acids,   as   acetic  and   tartaric,   sometimes
|      rendered inert platina active, at other times not.   This, I believe,
1 When heat does confer the property it is only by the destruction or
dissipation of organic or other matter which had previously soiled the
plate (368, 369, 370).—December 1838.