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 caused explosion of the gas.  This result was obtained several times, and when larger proportions of olefiant gas were used, the action seemed still more hopeless.

641.  A mixture of forty-nine volumes oxygen and hydrogen (638.) with one volume of olefiant gas had a well-prepared platina plate introduced.  The diminution of gas was scarcely sensible at the end of two hours, during which it was watched; but on examination twenty-four hours afterwards, the tube was found blown to pieces.  The action, therefore, though it had been very much retarded, had occurred at last, and risen to a maximum.

642.  With a mixture of ninety-nine volumes of oxygen and hydrogen (638.) with one of olefiant gas, a feeble action was evident at the end of fifty minutes; it went on accelerating (630.) until the eighty-fifth minute, and then became so intense that the gas exploded.  Here also the retarding effect of the olefiant gas was very beautifully illustrated.

643.  Plates prepared by alkali and acid (605.) produced effects corresponding to those just described.

644.  It is perfectly clear from these experiments, that olefiant gas, even in small quantities, has a very remarkable influence in preventing the combination of oxygen and hydrogen under these circumstances, and yet without at all injuring or affecting the power of the platina.

645.  Another striking illustration of similar interference may be shown in carbonic oxide; especially if contrasted with carbonic acid.  A mixture of one volume oxygen and hydrogen (638.) with four volumes of carbonic acid was affected at once by a platina plate prepared with acid, &c. (605.); and in one hour and a quarter nearly all the oxygen and hydrogen was gone.  Mixtures containing less carbonic acid were still more readily affected.