INTERFERENCES WITH THE USUAL ACTION OF PLATINA. 71 



heat : its surface is most extensive and pure, yet very accessible to the gases brought 

 in contact vrith it. If placed in impurity, the interior, as Thenard and Dulong have 

 observed, is preserved clean by the exterior ; and as regards heat, it is so bad a con- 

 ductor, because of its divided condition, that almost all which is evolved by the com- 

 bination 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 pheno- 

 menon, 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 

 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 access of gas. 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 re- 

 sults 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 1 volume olefiant gas, so that the olefiant gas formed but 

 ^Vth 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 



