Power of the Plates Affected 89

went on more and more slowly, and at last ceased altogether.
The first effect of an increase in the rapidity of combination
depended in part upon the water flowing off from the platina
plate, and allowing a better contact with the gas, and in part
upon the heat evolved during the progress of the combination:
(366). But notwithstanding the effect of these causes, diminu-
tion., and at last cessation of the power, always occurred. It
must not, however, be unnoticed, that the purer the gases
subjected to the action of the plate, the longer was its combining
power retained. With the mixture evolved at the poles of the
voltaic pile, in pure dilute sulphuric acid, it continued longest;
and with oxygen and hydrogen, of perfect purity, it probably
would not be diminished at all.

318. Different modes of treatment applied to the platina plate,
after it had ceased to be the positive pole of the pile, affected
its power very curiously. A plate which had been a positive
pole in diluted sulphuric acid of specific gravity 1.336 for four
or five minutes, if rinsed in water and put into mixed oxygen
and hydrogen, would act very well, and condense perhaps one
cubic inch and a half of gas in six or seven minutes; but if that
same plate, instead of being merely rinsed, had been left in
distilled water for twelve or fifteen minutes, or more, it would
rarely fail, when put into the oxygen and hydrogen, of becoming,
in the course of a minute or two, ignited, and would generally
explode the gases. Occasionally the time occupied in bringing
on the action extended to eight or nine minutes, and sometimes
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.1

319. Occasionally the platina plates (305), after being made
the positive pole of the battery, were washed, wiped with filter-
ing-paper or a cloth, and washed and wiped again. Being then
introduced into mixed oxygen and hydrogen, they acted ap-
parently 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.

320. The power of heat in altering the action of the prepared
platina plates was also tried (331). 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

1 In proof that this is the case, refer to 774.—December 1838.!xtent of decomposable matter in the direction of the current*ectly to