577.  Some plates (569.) were made positive for four minutes in dilute sulphuric acid of specific gravity 1.336:  they were rinsed in distilled water, after which two were put into a small bottle and closed up, whilst others were left exposed to the air.  The plates preserved in the limited portion of air were found to retain their power after eight days, but those exposed to the atmosphere had lost their force almost entirely in twelve hours, and in some situations, where currents existed, in a much shorter time.

578.  Plates were made positive for five minutes in sulphuric acid, specific gravity 1.336.  One of these was retained in similar acid for eight minutes after separation from the battery:  it then acted on mixed oxygen and hydrogen with apparently undiminished vigour.  Others were left in similar acid for forty hours, and some even for eight days, after the electrization, and then acted as well in combining oxygen and hydrogen gas as those which were used immediately after electrization.

579.  The effect of a solution of caustic potassa in preserving the platina plates was tried in a similar manner.  After being retained in such a solution for forty hours, they acted exceedingly well on oxygen and hydrogen, and one caused such rapid condensation of the gases, that the plate became much heated, and I expected the temperature would have risen to ignition.

580.  When similarly prepared plates (569.) had been put into distilled water for forty hours, and then introduced into mixed oxygen and hydrogen, they were found to act but very slowly and feebly as compared with those which had been preserved in acid or alkali.  When, however, the quantity of water was but small, the power was very little impaired after three or four days.  As the water had been retained in a wooden vessel, portions of it were redistilled in glass, and this was found to preserve prepared plates for a great length of time.  Prepared plates were put into tubes with this water and closed up; some of them, taken out at the end of twenty-four days, were found very active on mixed oxygen and hydrogen; others, which were left in the water for fifty-three days, were still found to cause the combination of the gases.  The tubes had been closed only by corks.

581.  The act of combination always seemed to diminish, or apparently exhaust, the power of the platina plate.  It is true, that in most, if not all instances, the combination of the gases, at first insensible, gradually increased in rapidity, and sometimes reached to explosion; but when the latter did not happen, the rapidity of combination diminished; and although fresh portions of gas were introduced into the tubes, the combination 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