nitric acid: being washed and put into mixed
oxygen and hydrogen gases, it acted well. Other plates were boiled in
strong nitric acid for periods extending from half a minute to four
minutes, and then being washed in distilled water, were found to act very
well, condensing one cubic inch and a half of gas in the space of eight or
nine minutes, and rendering the tube warm (570.).

601. Strong sulphuric acid was very effectual in rendering the platina
active. A plate (569.) was heated in it for a minute, then washed and put
into the mixed oxygen and hydrogen, upon which it acted as well as if it
had been made the positive pole of a voltaic pile (570.).

602. Plates which, after being heated or electrized in alkali, or after
other treatment, were found inert, immediately received power by being
dipped for a minute or two, or even only for an instant, into hot oil of
vitriol, and then into water.

603. When the plate was dipped into the oil of vitriol, taken out, and then
heated so as to drive off the acid, it did not act, in consequence of the
impurity left by the acid upon its surface.

604. Vegetable acids, as acetic and tartaric, sometimes rendered inert
platina active, at other times not. This, I believe, depended upon the
character of the matter previously soiling the plates, and which may easily
be supposed to be sometimes of such a nature as to be removed by these
acids, and at other times not. Weak sulphuric acid showed the same
difference, but strong sulphuric acid (601.) never failed in its action.

605. The most favourable treatment, except that of making the plate a
positive pole in strong acid, was as follows. The plate was held over a
spirit-lamp flame, and when hot, rubbed with a piece of potassa fusa
(caustic potash), which melting, covered the metal with a coat of very
strong alkali, and this was retained fused upon the surface for a second or
two[A]: it was then put into water for four or five minutes to wash off the
alkali, shaken, and immersed for about a minute in hot strong oil of
vitriol; from this it was removed into distilled water, where it was
allowed to remain ten or fifteen minutes to remove the last traces of acid
(582.). Being then put into a mixture of oxygen and hydrogen, combination
immediately began, and proceeded rapidly; the tube became warm, the platina
became red-hot, and the residue of the gases was inflamed. This effect
could be repeated at pleasure, and thus the maximum phenomen