70 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 



633. Clean platina, by which I mean such as has been made the positive pole of a 

 pile (570.), or has been treated with acid (605.), and has then been put into distilled 

 water for twelve or fifteen minutes, has a peculiar friction when one piece is rubbed 

 against another. It wets freely with pure water, even after it has been shaken and 

 dried by the heat of a spirit-lamp ; and if made the pole of a voltaic pile in a dilute 

 acid, it evolves minute bubbles from every part of its surface. But platina in its 

 common state wants that peculiar friction : it will not wet freely with water as the 

 clean platina does ; and when made the positive pole of a pile, it for a time gives off 

 large bubbles, which seem to cling or adhere to the metal, and are evolved at distinct 

 and separate points of the surface. These appearances and effects, as well as its 

 want of power on oxygen and hydrogen, are the consequences, and the indications, 

 of a soiled surface. 



634. I found also that platina plates which had been cleaned perfectly soon became 

 soiled by mere exposure to the air ; for after twenty-four hours they no longer moist- 

 ened freely with water, but the fluid ran up into portions, leaving part of the surface 

 bare, whilst other plates which had been retained in water for the same time, when 

 they were dried (580.) did moisten, and gave the other indications of a clean surface. 



635. Nor was this the case with platina or metals only, but also with earthy bodies. 

 Rock crystal and obsidian would not wet freely upon the surface, but being moistened 

 with strong oil of vitriol, then washed, and left in distilled water to remove all the 

 acid, they did freely become moistened, whether they were previously dry or whether 

 they were left wet ; but being dried and left exposed to the air for twenty-four hours, 

 their surface became so soiled that water would not then adhere freely to it, but ran 

 up into partial portions. Wiping with a cloth (even the cleanest) was still worse than 

 exposure to air ; the surface either of the minerals or metals immediately became as 

 if it were slightly greasy. The floating of small particles of metals under ordinary 

 circumstances is due to the effect of this kind of soiled surface. The extreme difficulty 

 of cleaning the surface of mercury when it has once been soiled or greased, is due to 

 the same cauSe. 



636. The same reasons explain why the power of the platina plates in some cir- 

 cumstances soon disappear, and especially upon use, and MM. Dulong and Thenard 

 have observed the same effect with the spongy metal*, as indeed have all those who 

 have used Dobereiner's instantaneous light machines. If left in the air, if put into 

 ordinary distilled water, if made to act upon ordinary oxygen and hydrogen, they can 

 still find in all these cases that minute portion of impurity which, when once in con- 

 tact with its surface, is retained there, and is sufficient to prevent its full action upon 

 oxygen and hydrogen at common temperatures : a slight elevation of temperature is 

 again sufficient to compensate for their effect, and cause combination. 



637. No state of things can be conceived more favourable for the production of this 

 effect than that which is possessed by platina obtained from the ammonio-muriate by 



* Annales de Chimie, torn. xxiv. p. 386. 



