na obtained
from the ammonio-muriate by heat. Its surface is most extensive and pure,
yet very accessible to the gases brought in contact with it: if placed in
impurity, the interior, as Thenard and Dulong have observed, is preserved
clean by the exterior; and as regards temperature, it is so bad a conductor
of heat, because of its divided condition, that almost all which is evolved
by the combination of the first portions of gas is retained within the
mass, exalting the tendency of the succeeding portions to combine.

* * * * *

638. I have now to notice some very extraordinary interferences with this
phenomenon, dependent, not upon the nature or condition of the metal or
other acting solid, but upon the presence of certain substances mingled
with the gases acted upon; and as I shall have occasion to speak frequently
of a mixture of oxygen and hydrogen, I wish it always to be understood that
I mean a mixture composed of one volume of oxygen to two volumes of
hydrogen, being the proportions that form water. Unless otherwise
expressed, the hydrogen was always that obtained by the action of dilute
sulphuric acid on pure zinc, and the oxygen that obtained by the action of
heat from the chlorate of potassa.

639. Mixtures of oxygen and hydrogen with _air_, containing one-fourth,
one-half, and even two-thirds of the latter, being introduced with prepared
platina plates (570. 605.) into tubes, were acted upon almost as well as if
no air were present: the retardation was far less than might have been
expected from the mere dilution and consequent obstruction to the contact
of the gases with the plates. In two hours and a half nearly all the oxygen
and hydrogen introduced as mixture was gone.

640. But when similar experiments were made with _olefiant gas_ (the
platina plates having been made the positive poles of a voltaic pile (570.)
in acid), very different results occurred. A mixture was made of 29.2
volumes hydrogen and 14.6 volumes oxygen, being the proportions for water;
and to this was added another mixture of 3 volumes oxygen and one volume
olefiant gas, so that the olefiant gas formed but 1/40th part of the whole;
yet in this mixture the platina plate would not act in forty-five hours.
The failure was not for want of any power in the plate, for when after that
time it was taken out of this mixture and put into one of oxygen and
hydrogen, it immediately acted, and in seven minutes cause