s even to forty minutes, and yet ignition and explosion would
result. This effect is due to the removal of a portion of acid which
otherwise adheres firmly to the plate [A].

[A] In proof that this is the case, refer to 1038.--_Dec. 1838._

583. Occasionally the platina plates (569.), after being made the positive
pole of the battery, were washed, wiped with filtering-paper or a cloth,
and washed and wiped again. Being then introduced into mixed oxygen and
hydrogen, they acted apparently as if they had been unaffected by the
treatment. Sometimes the tubes containing the gas were opened in the air
for an instant, and the plates put in dry; but no sensible difference in
action was perceived, except that it commenced sooner.

584. The power of heat in altering the action of the prepared platina
plates was also tried (595.). Plates which had been rendered positive in
dilute sulphuric acid for four minutes were well-washed in water, and
heated to redness in the flame of a spirit-lamp: after this they acted very
well on mixed oxygen and hydrogen. Others, which had been heated more
powerfully by the blowpipe, acted afterwards on the gases, though not so
powerfully as the former. Hence it appears that heat does not take away the
power acquired by the platina at the positive pole of the pile: the
occasional diminution of force seemed always referable to other causes than
the mere heat. If, for instance, the plate had not been well-washed from
the acid, or if the flame used was carbonaceous, or was that of an alcohol
lamp trimmed with spirit containing a little acid, or having a wick on
which salt, or other extraneous matter, had been placed, then the power of
the plate was quickly and greatly diminished (634. 636.).

585. This remarkable property was conferred upon platina when it was made
the positive pole in sulphuric acid of specific gravity 1.336, or when it
was considerably weaker, or when stronger, even up to the strength of oil
of vitriol. Strong and dilute nitric acid, dilute acetic acid, solutions of
tartaric, citric, and oxalic acids, were used with equal success. When
muriatic acid was used, the plates acquired the power of condensing the
oxygen and hydrogen, but in a much inferior degree.

586. Plates which were made positive in solution of caustic potassa did not
show any sensible action upon the mixed oxygen and hydrogen. Other plates
made positive in solutions of carbonates of potassa and soda exhibited the
a