706 PHILOSOPHICAL TRANSACTIONS. [aNNO ]7Q0. 



in it, during several hours, gave a precipitate of silver in the form of a black 

 powder. 



^ 2. On the alterations which iron or its surface undergoes by the action of a 

 solution of silver in nitrous acid, or of a pure concentrated nitrous acid. — 13. It 

 has been said, that when iron is exposed to the action of a phlogisticated solu- 

 tion of silver, it instantly precipitates the silver, is itself acted on or dissolved by 

 the acid solution during a certain time, longer or shorter, according to the degree 

 of phlogistication, quantity of superabundant acid, and other circumstances, and 

 that at length the solution of the iron ceases; the silver precipitate is re-dissolved, 

 if there is superfluous acid ; the liquor becomes clear again, but only rendered a 

 little browner by its having dissolved some iron ; while the piece of iron remains 

 bright and undisturbed at the bottom of the liquor, where it is no longer able to 

 affect the solution of silver. 



14. I poured a part of the phlogisticated solution of silver which had passed 

 through these changes, and which had ceased to act on the piece of iron, into 

 another glass, and dropped another piece of fresh iron wire into the liquor ; on 

 which I observed a precipitation of silver, a solution of part of the iron, a re- 

 dissolution of the precipitated silver, and a cessation of all these phenomena, with 

 the iron remaining bright and quiet at the bottom of the liquor, as before. It 

 appeared then, that the liquor had not lost its power of acting on fresh iron, 

 though it ceased to act on that piece which had been exposed to it. 



\5. To one of the pieces of iron which had been employed in the precipitation 

 of a solution of silver, and from which the solution, no longer capable of acting 

 on it, had been poured ofi^, I added some phlogisticated solution of silver which 

 had never been exposed to the action of iron, but no precipitation happened. It 

 appeared then, that the iron itself, by having been once employed to precipitate 

 a solution of silver, was rendered incapable of any further action on any solution 

 of silver. And it is to be observed, that this alteration was produced without the 

 least diminution of its metallic splendour, or change of colour. The alteration 

 however was only superficial, as may be supposed ; for by scraping off its altered 

 coat, it was again rendered capable of acting on a solution of silver. To avoid 

 circumlocution, I shall call iron thus affected, altered iron ; and iron which is 

 clean, and has not been altered, fresh iron. 



l6. To a phlogisticated solution of silver, in which a piece of bright altered 

 iron lay, without action, I added a piece of fresh iron, which was instantly en- 

 veloped with a" mass of precipitated silver, and acted on as usual ; but, what is 

 very remarkable, in about a quarter of a minute, or less, the altered iron suddenly 

 was covered with another coat of precipitated silver, and was now acted on by 

 the acid solution like the fresh piece. In a .little time the silver precipitate was 

 re-dissolved, as usual, and the two pieces of iron were reduced to an altered state. 



