hree volumes of oxygen and hydrogen, the carbonic oxide
being then only 1/18th part of the whole, the plate acted, slowly at first,
and at the end of forty-two minutes the gases exploded.

648. These experiments were extended to various gases and vapours, the
general results of which may be given as follow. Oxygen, hydrogen,
nitrogen, and nitrous oxide, when used to dilute the mixture of oxygen and
hydrogen, did not prevent the action of the plates even when they made
four-fifths of the whole volume of gas acted upon. Nor was the retardation
so great in any case as might have been expected from the mere dilution of
the oxygen and hydrogen, and the consequent mechanical obstruction to its
contact with the platina. The order in which carbonic acid and these
substances seemed to stand was as follows, the first interfering least with
the action; _nitrous oxide, hydrogen, carbonic acid, nitrogen, oxygen_: but
it is possible the plates were not equally well prepared in all the cases,
and that other circumstances also were unequal; consequently more numerous
experiments would be required to establish the order accurately.

649. As to cases of _retardation_, the powers of olefiant gas and carbonic
oxide have been already described. Mixtures of oxygen and hydrogen,
containing from 1/16th to 1/20th of sulphuretted hydrogen or phosphuretted
hydrogen, seemed to show a little action at first, but were not further
affected by the prepared plates, though in contact with them for seventy
hours. When the plates were removed they had lost all power over pure
oxygen and hydrogen, and the interference of these gases was therefore of a
different nature from that of the two former, having permanently affected
the plate.

650. A small piece of cork was dipped in sulphuret of carbon and passed up
through water into a tube containing oxygen and hydrogen (638.), so as to
diffuse a portion of its vapour through the gases. A plate being introduced
appeared at first to act a little, but after sixty-one hours the diminution
was very small. Upon putting the same plate into a pure mixture of oxygen
and hydrogen, it acted at once and powerfully, having apparently suffered
no diminution of its force.

651. A little vapour of ether being mixed with the oxygen and hydrogen
retarded the action of the plate, but did not prevent it altogether. A
little of the vapour of the condensed oil-gas liquor[A] retarded the action
still more, but not nearly so much a