708 PHILOSOPHICAL TRANSACTIONS. [aNNO I79O. 



25. A saturated solution of silver having been partly precipitated by copper, 

 acquired the property of acting on fresh iron, and of being precipitated by it. 



26. Fresh iron immersed some time in solutions of nitre of lead, or of nitre of 

 mercury in water, did not occasion any precipitation of the dissolved metals ; but 

 acquired an altered quality. These metals then in this respect resemble silver. 



27. It is well known, that a solution of martial vitriol, added to a solution of 

 gold in aqua regis, precipitates the gold in its metallic state. I do not recollect, 

 that the precipitation of a solution of silver by the same martial vitriol has been 

 observed. However, on pouring a solution of martial vitriol into a solution of 

 silver in the nitrous acid, a precipitate will be thrown down, which acquires in a 

 few minutes more and more of a metallic appearance, and is indeed perfect silver. 

 When the 2 solutions are pretty concentrated, a bright argentine film swims on 

 the surface of the mixture, or silvers the sides of the glass in which the experiment 

 is made. When a phlogisticated solution of silver is used, the mixture is black- 

 ened, as happens generally to a solution of martial vitriol, when a phlogisticated 

 nitrous acid is added to it. 



I added about equal parts of water to a mixture of a phlogisticated solution of 

 silver and a solution of martial vitriol, in which all the silver had been precipitated, 

 and digested the diluted mixture with heat, by which means most of the preci- 

 pitated silver was re-dissolved. Bergman has observed a similar re-dissolution of 

 gold precipitated by martial vitriol on boiling the mixture ; but he attributes the 

 re-dissolution to the concentration of the aqua regis by the evaporation. As this 

 explanation did not accord with my notions, I diluted the mixture with water, 

 and found that the same re-dissolution occurred both with the solution of silver 

 and with that of gold. But with neither of the metals did I find that the re- 

 dissolution ever took place, unless there had been a superabundant acid in the so- 

 lutions of gold and silver employed. 



28. Mercury is also precipitated in its metallic state from its solution in nitrous 

 acid by a solution of martial vitriol. When the liquor is poured off from the 

 precipitate, this may be changed into running mercury by being dried near 

 the fire. 



29. I found also, that silver may be precipitated in its metallic state, from its 

 solution in vitriolic acid, by addition of a solution of martial vitriol. A vitriol of 

 mercury may also be decompounded by a solution of martial vitriol, and the mer- 

 curial precipitate, which is a black powder, forms globules, when dried and warmed. 



30. Luna cornea is not decompounded by martial vitriol ; consequently there 

 is no operation of a double aflinity. Yet this luna cordea may be decompounded 

 by the elements of martial vitriol, while they are in the act of dissolution ; that 

 is, the silver may be precipitated in its metallic state, by digesting luna cornea 

 with a dilute vitriolic acid, to which some pieces of iron are added. And it is to 

 be observed, that this reduction of the silver and precipitation take place, while 



