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 action, but only in a feeble degree.

587.  When a neutral solution of sulphate of soda, or of nitre, or of chlorate of potassa, or of phosphate of potassa, or acetate of potassa, or sulphate of copper, was used, the plates, rendered positive in them for four minutes, and then washed in water, acted very readily and powerfully on the mixed oxygen and hydrogen.

588.  It became a very important point, in reference to the cause of this action of the platina, to determine whether the positive pole only could confer it (567.), or whether, notwithstanding the numerous contrary cases, the negative pole might not have the power when such circumstances as could interfere with or prevent the action were avoided.  Three plates were therefore rendered negative, for four minutes in diluted sulphuric acid of specific gravity 1.336, washed in distilled water, and put into mixed oxygen and hydrogen. All of them acted, though not so strongly as they would have done if they had been rendered positive.  Each combined about a cubical inch and a quarter of the gases in twenty-five minutes.  On every repetition of the experiment the same result was obtained; and when the plates were retained in distilled water for ten or twelve minutes, before being introduced into the gas (582.), the action was very much quickened.

589.  But when there was any metallic or other substance present in the acid, which could be precipitated on the negative plate, then that plate ceased to act upon the mixed oxygen and hydrogen.

590.  These experiments led to the expectation that the power of causing oxygen and hydrogen to combine, which could be conferred upon any piece of platina by making it the positive pole of a voltaic pile, was not essentially dependent upon the action of the pile, or upon any structure or arrangement of parts it might receive whilst in association with it, but belonged to the platina at all times, and was always effective when the surface was perfectly clean.  And though, when made the positive pole of the pile in acids, the circumstances might well be considered as those which would cleanse the surface of the platina in the most effectual manner, it did not seem impossible that ordinary operations should produce the same result, although in a less eminent degree.