of the combination (630.).  But notwithstanding the effect of these causes, diminution, 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.

582.  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 [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.).