taposition with the
metal, combine, and the fresh vapour formed is also diffused, allowing new
portions of gas to be acted upon. In this way the process advances, but is
accelerated by the evolution of heat, which is known by experiment to
facilitate the combination in proportion to its intensity, and the
temperature is thus gradually exalted until ignition results.

631. The dissipation of the vapour produced at the surface of the platina,
and the contact of fresh oxygen and hydrogen with the metal, form no
difficulty in this explication. The platina is not considered as causing
the combination of any particles with itself, but only associating them
closely around it; and the compressed particles are as free to move from
the platina, being replaced by other particles, as a portion of dense air
upon the surface of the globe, or at the bottom of a deep mine, is free to
move by the slightest impulse, into the upper and rarer parts of the
atmosphere.

632. It can hardly be necessary to give any reasons why platina does not
show this effect under ordinary circumstances. It is then not sufficiently
clean (617.), and the gases are prevented from touching it, and suffering
that degree of effect which is needful to commence their combination at
common temperatures, and which they can only experience at its surface. In
fact, the very power which causes the combination of oxygen and hydrogen,
is competent, under the usual casual exposure of platina, to condense
extraneous matters upon its surface, which soiling it, take away for the
time its power of combining oxygen and hydrogen, by preventing their
contact with it (598.).

633. Clean platina, by which I mean such as has been made the positive pole
of a pile (570.), or has been treated with acid (605.), and has then been
put into distilled water for twelve or fifteen minutes, has a _peculiar
friction_ when one piece is rubbed against another. It wets freely with
pure water, even after it has been shaken and dried by the heat of a
spirit-lamp; and if made the pole of a voltaic pile in a dilute acid, it
evolves minute bubbles from every part of its surface. But platina in its
common state wants that peculiar friction: it will not wet freely with
water as the clean platina does; and when made the positive pole of a pile,
it for a time gives off large bubbles, which seem to cling or adhere to the
metal, and are evolved at distinct and separate points of the surface.
These appearan