342 PHILOSOPHICAL TRANSACTIONS. [ANNO 1783. 



gisticated by heat. It may also produce this effect by depriving iron of its excess 

 of acid which keeps it in solution. 



Of precipitations of and by cupper. — When silver is dissolved in the nitrous 

 acid, and a piece of copper is put into the solution, it sometimes happens that 

 the silver is not precipitated, as Dr. Lewis has observed. This happens either 

 when the nitrous acid is supersaturated with silver having taken up some in its 

 metallic form, as already observed ; or when the silver is not much dephlogisti- 

 cated, for then its affinity to phlogiston, which is the principal cause of its pre- 

 cipitation, is less than 49 1 ; therefore, the remedy is to heat it and add more 

 acid, by which it is dephlogisticated further. However, the nitrous acid always 

 retains a little silver. 



Of precipitations of and by tin. — Tin is not precipitated, in its metallic form, 

 by any metallic substance ; and the reason is, because its precipitation is not the 

 effect of a double affinity, but of the single greater affinity of its menstruum to 

 every other metallic earth. Metals that are precipitated from the nitrous acid 

 by tin, are afterwards re-dissolved, because the acid soon quits the tin, it 

 becoming too much dephlogisticated. 



Of precipitations of and by lead. — Metals dissolved in the vitriolic and marine 

 acids, and precipitable by lead, according to the indication of the balance of 

 affinities, are yet slowly precipitated, because the first portions of lead that are 

 dissolved form salts of difficult solution, which cover its surface, and protect it 

 from the further action of the acid ; and yet it contains so little phlogiston, that 

 a great deal of it must be dissolved before it gives out enough to precipitate the 

 dissolved metals. 



Of precipitations oj and by mercury. — Though the difference between the 

 quiescent and divellent powers be very small, yet mercury is quickly precipitated 

 from the vitriolic acid by copper , because the attraction of the calx of mercury 

 to phlogiston is very strong, and a very small proportion of that contained in 

 copper is sufficient to revive it. Silver does not precipitate mercury from the 

 vitriolic acid, unless it contains copper; yet if silver and turpeth mineral be dis- 

 tilled, the mercury will pass in its metallic form, Wenzel 42 ; which shows that 

 the affinity of calx of mercury to phlogiston is increased by heat. Mercury pre- 

 cipitates silver from the nitrous acid, not by virtue of the superiority of the 

 usual divellent powers, but by reason of the attraction of mercury and silver to 

 each other, for they form partly an amalgama and partly vegetate, and scarce any 

 of either remains in the solution. The same thing happens, that is, they vege- 

 tate, if solutions of both metals in the same acid be mixed together. Silver does 

 not precipitate mercury from the solution of sublimate corrosive; but, on the 

 contrary, mercury precipitates silver from the marine acid: and it a solution of 

 horn silver in volatile alkali be triturated with mercury, the silver will be freed 



