COMBINATION OP OXYGEN AND HYDROGEN BY CLEAN PLATINA. 61 



cleansing of the surface of the platina is sufficient to enable it to exert its combining 

 power over oxygen and hydrogen at common temperatures. 



595. I now tried the effect of heat in conferring this property upon platina (584.). 

 Plates which had no action on the mixture of oxygen and hydrogen were heated by 

 the flame of a freshly trimmed spirit-lamp, urged by a mouth blowpipe, and when 

 cold were put into tubes of the mixed gases : they acted slowly at first, but after two 

 or three hours condensed nearly all the gases. 



596. A plate of platina, which was about one inch wide and two and three quarters 

 in length, and which had not been used in any of the preceding experiments, was 

 curved a little so as to enter a tube, and left in a mixture of oxygen and hydrogen for 

 thirteen hours : not the slightest action or combination of the gases occurred. It was 

 withdrawn at the pneumatic trough from the gas through the water, heated red hot 

 by the spirit-lamp and blowpipe, and then returned when cold into the same portion 

 of gas. In the course of a few minutes diminution of the gases could be observed, 

 and in forty-five minutes about one cubical inch and a quarter had disappeared. In 

 many other experiments platina plates when heated were found to acquire the power 

 of combining oxygen and hydrogen. 



597. But it happened not unfrequently that plates, after being heated, showed no 

 power of combining oxygen and hydrogen gases, though left undisturbed in them for 

 two hours. Sometimes also it would happen that a plate which, having been heated 

 to dull redness, acted feebly, upon being heated to whiteness ceased to act ; and at 

 other times a plate which, having been slightly heated, did not act, was rendered 

 active by a more powerful ignition. 



598. Though thus uncertain in its action, and though often diminishing the power 

 given to the plates at the positive pole of the pile (584.), still it is evident that heat 

 can render platina active, which before was inert (595.). The cause of its occasional 

 failure appears to be due to the surface of the metal becoming soiled, either from some- 

 thing previously adhering to it, which is made to adhere more closely by the action of 

 the heat, or from matter communicated from the flame of the lamp, or from the air 

 itself. It often happens that a polished plate of platina, when heated by the spirit- 

 lamp and a blowpipe, becomes dulled and clouded on its surface by something either 

 formed or deposited there ; and this, and much less than this, is sufficient to prevent it 

 from exhibiting the curious power now under consideration (634. 636.). Platina also 

 has been said to combine with carbon ; and it is not at all unlikely that in processes 

 of heating, where carbon or its compounds are present, a film of such a compound 

 may be thus formed, and thus prevent the exhibition of the properties belonging to 

 pure platina. 



599. The action of alkalies and acids in giving platina this property was now ex- 

 perimentally examined. Platina plates (569.) having no action on mixed oxygen and 

 hydrogen, being boiled in a solution of caustic potassa, washed, and then put into 

 the gases, were found occasionally to act pretty well, but at other times to fail. In 



