591.  Accordingly, a platina plate (569.) was cleaned by being rubbed with a cork, a little water, and some coal-fire ashes upon a glass plate:  being washed, it was put into mixed oxygen and hydrogen, and was found to act at first slowly, and then more rapidly.  In an hour, a cubical inch and a half had disappeared.

592.  Other plates were cleaned with ordinary sand-paper and water; others with chalk and water; others with emery and water; others, again, with black oxide of manganese and water; and others with a piece of charcoal and water.  All of these acted in tubes of oxygen and hydrogen, causing combination of the gases.  The action was by no means so powerful as that produced by plates having been in communication with the battery; but from one to two cubical inches of the gases disappeared, in periods extending from twenty-five to eighty or ninety minutes.

593.  Upon cleaning the plates with a cork, ground emery, and dilute sulphuric acid, they were found to act still better.  In order to simplify the conditions, the cork was dismissed, and a piece of platina foil used instead; still the effect took place.  Then the acid was dismissed, and a solution of potassa used, but the effect occurred as before.

594.  These results are abundantly sufficient to show that the mere mechanical 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  It was withdrawn at the pneumatic trough from the gas through the water, heated red-hot by the spirit-lamp and blowpipe, and then returned when cold into the same portion of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 infrequently 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.