VOL. LXXX.J PHILOSOPHICAL TRANSACTIONS. 707 



When a fresh piece of iron was then held in the liquor, so as not to touch the' 

 two pieces of altered iron, they were also soon acted on by the acid solution, and 

 suddenly covered with silver precipitate as before ; and these phenomena may be 

 repeated with the same solution of silver, till the superfluous acid of the solution 

 becomes saturated by the iron, and then the re-dissolution of the precipitated 

 silver must cease. 



17. I poured some dephlogisticated nitrous acid on a piece of altered iron, 

 without any action ensuing, though this acid readily acted on fresh iron ; and 

 when, to the dephlogisticated nitrous acid, with a piece of altered iron lying im- 

 mersed in it, I added a piece of fresh iron, this immediately began to dissolve, 

 and soon afterwards the altered iron was acted on also by the acid. 



18. On a piece of altered iron I poured a solution of copper in nitrous acid ; 

 but the copper was not precipitated by the iron ; neither did this iron precipitate 

 copper from a solution of blue vitriol. 



19. Altered iron was acted on by a dilute phlogisticated nitrous acid ; but not 

 by a red concentrated acid, which is known to be highly phlogisticated. 



20. I put some pieces of clean fresh iron wire into a concentrated and red 

 fuming nitrous acid. No apparent action ensued ; but the iron was found to be 

 altered in the same manner as it is by a solution of silver ; that is, it was rendered 

 incapable of being attacked either by a phlogisticated solution of silver, or by 

 dephlogisticated nitrous acid. 



21. Iron was also altered by being immersed some little time in a saturated so- 

 lution of silver, which did not show any visible action on it. 



22. The alteration thus produced on the iron is very superficial. The least 

 rubbing exposes some of the fresh iron beneath the surface^ and thus subjects it 

 to the action of the acid. It is therefore with difficulty that these pieces of altered 

 iron can he dried, without losing their peculiar property. For this reason, I 

 generally transferred them out of the solution of silver, or concentrated nitrous 

 acid, into any other liquor, the effects of which I wanted to examine. Or they 

 may be transferred first into a glass of water, and thence into the liquor to be 

 examined. But it is to be observed, that if they are allowed to remain long in 

 the water, they lose their peculiar property or alteration. They may be preserved 

 in their altered state by being kept in spirit of sal ammoniac. 



23. To a saturated solution of copper in nitrous acid, which was capable of be- 

 ing readily precipitated by fresh iron, I added some saturated solution of silver. 

 From this mixture a piece of fresh iron neither precipitated silver nor copper : 

 nor did the addition of some dephlogisticated nitrous acid effect this precipitation. 



24. A solution of copper, formed by precipitating silver from nitrous acid by 

 means of copper, was very reluctantly and slowly precipitated by a piece of fresh 

 iron ; and the iron thus acted on by the acid was changed to an ochre. 



4x2 



