102 Faraday's Researches

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.

365. Although there are not many distinct cases of combina-
tion 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 Hall found carbonic
acid and lime to remain combined under pressure at tempera-
tures 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,1 which being com-
pressed 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.

366. The course of events when platina acts upon, and com-
bines oxygen and hydrogen, may be stated, according to these
principles, as follows. From the influence of the circumstances
mentioned (355, etc.), 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 exist-
ing 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 com-
bination is the production of the vapour of water and an eleva-
tion 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 the
latter, therefore, come into juxtaposition with the metal, com-
bine, and the fresh vapour formed is also diffused, allowing
new portions of gas to be acted upon. In this way the process

1 Philosophical Transactions, 1823, p. 161.mining circumstance ;