ogether, though they are elements having such powerful affinity as to
unite naturally under a thousand different circumstances, they do not
combine by mere mixture. Still it is evident that, from their perfect
association, the particles are in the most favourable state possible for
combination upon the supervention of any determining cause, such either as
the negative action of the platina in suppressing or annihilating, as it
were, their elasticity on its side; or the positive action of the metal in
condensing them against its surface by an attractive force; or the
influence of both together.

629. Although there are not many distinct cases of combination under the
influence of forces external to the combining particles, yet there are
sufficient to remove any difficulty which might arise on that ground. Sir
James Hull found carbonic acid and lime to remain combined under pressure
at temperatures at which they would not have remained combined if the
pressure had been removed; and I have had occasion to observe a case of
direct combination in chlorine[A], which being compressed at common
temperatures will combine with water, and form a definite crystalline
hydrate, incapable either of being formed or of existing if that pressure
be removed.

[A] Philosophical Transactions, 1823, p. 161.

630. The course of events when platina acts upon, and combines oxygen and
hydrogen, may be stated, according to these principles, as follows. From
the influence of the circumstances mentioned (619. &c.), i.e. the
deficiency of elastic power and the attraction of the metal for the gases,
the latter, when they are in association with the former, are so far
condensed as to be brought within the action of their mutual affinities at
the existing temperature; the deficiency of elastic power, not merely
subjecting them more closely to the attractive influence of the metal, but
also bringing them into a more favourable state for union, by abstracting a
part of that power (upon which depends their elasticity,) which elsewhere
in the mass of gases is opposing their combination. The consequence of
their combination is the production of the vapour of water and an elevation
of temperature. But as the attraction of the platina for the water formed
is not greater than for the gases, if so great, (for the metal is scarcely
hygrometric,) the vapour is quickly diffused through the remaining gases;
fresh portions of this latter, therefore, come into jux