704 PHILOSOPHICAL TRANSACTIONS. [aNNO 1 7Q0. 



covering the circumstances, and of investigating the cause, if I should be able, 

 of this irregularity and exception to the generally received laws of affinity. 



2. I digested a piece of fine silver in pure and pale nitrous acid, and while the 

 dissolution was going on, and before the saturation was completed, I poured a 

 portion of the solution on pieces of clean and newly-scraped iron wire into a wine 

 glass, and observed a sudden and copious precipitation of silver. The precipi- 

 tate was at first black, then it assumed the appearance of silver, and was 5 or 6 

 times larger in diameter than the piece of iron wire which it enveloped. The 

 action of the acid on the iron continued some little time, and then it ceased; 

 the silver re-dissolved, the hquor became clear, and the iron remained bright and 

 undisturbed in the solution at the bottom of the wine glass, where it continued 

 during several weeks, without suffering any change, or affecting any precipitation 

 of the silver. 



3. When the solution of silver was completely saturated, it was no longer 

 affected by iron, according to Bergman's observation. 



4. Having found that the solution acted on the iron, and was thus precipitated, 

 before it had been saturated, and not afterwards, I was desirous of knowing, 

 whether the saturation was the circumstance which prevented the action and pre- 

 cipitation. For this purpose I added to a portion of the saturated solution some 

 of the same nitrous acid, of which a part had been employed to dissolve the 

 silver; and into this mixture, abounding with a superfluous acid, I threw a piece 

 of iron, but no precipitation occurred. It was thence evident, that the satura- 

 tion of the acid was not the only circumstance which prevented the precipitation. 



5. To another portion of the saturated solution of silver I added some red 

 smoking nitrous acid ; and I found, on trial, that iron precipitated the silver from 

 this mixture, and that the same appearances were exhibited as had been observed 

 with the solution before its saturation. 



quiescent affinities being 625, and that of the divellent 746. Yet Mr. Bergman observed, that a 

 very saturated solution of silver was very difficultly precipitated, and only by some sorts of iron, even 

 though the solution was diluted, and an access of acid added to it. The reason of this curious phe- 

 nomenon appears to me deducible from a circumstance first observed by Scheele, in dissolving mer- 

 cury, namely, that the nitrous acid when saturated with it will take up more of it in its metallic form. 

 The same thing happens in dissolving silver in the nitrous acid in a strong heat ; for, as I before re- 

 marked, the last portions of silver thrown in afford no air, and consequently are not dephlogisticated. 

 Now this compound of calx of silver, and silver in its metallic form, may well be unprecipitable by 

 iron the silver in its metallic form preventing the calx from coming into contact with the iron, and 

 extracting phlogiston from it." In this paper I shall not enter into the explanation of these appear- 

 ances • but I thought it necessar)- to premise what so eminent a chemist as Mr. Kirwan has suggested 

 on the subject, that the reader may see at once the present state of the question. I shall only remark, 

 that the above explanation, not being founded on any peculiarity in the nature of iron, seems to sup- 

 pose that the silver is also incapable of being precipitated from such solutions as iron cannot act on by 

 any other metal. But this is not the case : copper and zinc readily precipitate silver frQ;u thete so- 

 lutions. — Orig. 



