Mr. kirwan’s Experiments and Obfervations on 
the earth of allum precipitated. If this were true it would be 
very furpriling, as copper is foluble, even with the afliftance of 
heat, only in the concentrated vitriolic acid. Hence I made 
the experiment, and found, that after 20 hours boiling not 
the fmalleft particle of copper was diflolved, for the colour of 
rhe folution was not altered by volatile alkalies : and though the 
allum was precipitated, it {fill retained its faline form, fo that 
it loft only its excefs of acid in this experiment. 
Of precipitations of and by tin. 
t ■ 
Tin is not precipitated, in its metallic form, by any metallic 
fubftance ; and the reafon is, becaufe its precipitation is not the 
effedt of a double affinity, but of the lingle greater affinity of 
its menflruum to every other metallic earth. Metals that are 
precipitated from the nitrous acid by tin, are afterwards re- • 
diflolved, becaule the acid foon quits the tin, it becoming too 
much dephlogifticated. 
Of precipitations of and by lead. 
Metals diflolved in the vitriolic and marine acids, and preci- . 
pitable by lead, according to the indication of the balance of 
affinities, are yet flowly precipitated, becaufe the firft portions 
of lead that are diflolved form falts of difficult folution, which 
cover its furface, and protect it from the further adtion of the 
acid; and yet it contains lo little phlogifton, that a great deal 
of it muft be diflolved before it gives out enough to precipitate 
the diflolved metals. 
Mr. bergman obferved, that a very faturate folution of lead 
is difficultly, if at all, precipitable by iron. Does not this 
alfo arife from fome lead being taken up in its. metallic form ? 
Iron does not precipitate lead from the marine acid, though a 
j • pre- 
