the Attractive Powers of Mineral Acids. yy 
though with difficulty ; for after 24 hours the galls ftill ft ruck 
a black. I Ihould alio add, that iron does not precipitate zinc 
from the vitriolic acid. 
Moft metallic fubftances, precipitated by iron from the ni- 
trous acid, are in fome meafure re-diftolved lhortly after, as 
the nitrous acid foon dephlogifticates the iron too much, then 
lets it fall, and re-abts on the other metals and re-diftolvcs 
them. 
The precipitation of the argillaceous earth from allum bv 
iron is owing to the excefs of acid in the allum which fir ft 
dephlogifticates the iron ; and when this is dephlogifticatcd, it 
attra&s the acid more ftrongly. Earth of allum, on the other 
hand, precipitates iron when the folution of iron is dephlo- 
gifticated by heat. It may alfo produce this eftecl by depriving 
iron of its excefs of acid which keeps it in folution. 
Of precipitations of and by copper. 
When fiver is dillolved in the nitrous acid, and a piece of 
copper is put into the folution, it fometimes happens, that the 
fiver is not precipitated, as Dr. lewis has obferved *. This 
happens either when the nitrous acid is fuperfaturated with 
fiver having taken up fome in its metallic form, as already 
obferved ; or when the fiver is not much dephlogifticated, for 
then its affinity to phlogifton, which is the principal caule of 
its precipitation, is lefs than 491 ; therefore, the remedy is to 
heat it and add more acid, by which it is dephlogifticated fur- 
ther. However, the nitrous acid always retains a little fiver. 
schlutter. 362. Hift. Mem. Par. 1728. 
It is commonly faid, that if filings of copper be put into 
boiling folution of allum, vitriol of copper will be found, and- 
* Commercium Philof. p. 157. 
the 
