[ 186 ] 
A quantity of the brown folution in vitriolic acid 
was diluted with water till it became very pale. I then 
poured to it fome infufion of galls, which turned it 
immediately black. 
A parcel of the blue fubflance, being placed at the 
diflance of a foot from the fire, was changed to a 
greenifh colour. 
Thefe experiments, compared with its natural hi- 
ffory, feem to throw fome light on the nature and 
compoiition of this curious produ&ion. It is the 
known property of all vegetable aftringents, to af- 
fedl the colour of iron, either when it is combined 
with vitriolic acid, in the form of green vitriol, or by 
itfeif ; and I believe they have no fuch effedt on any 
other metal. The colour they produce with it is 
various, inclining indeed to black, but almoft of every 
different (hade between black and blue; and it feems 
to me, that they occafion a more pure black with 
vitriol, and a purple blue with iron itfeif, as is feen 
for inftance on dropping a little infufion of tea on a 
knife. 
Now vve find, that when a vegetable aflringent is 
added to a folution of this fubflance in vitriolic 
acid, it flrikes a black colour with it, and redores the 
original blue to the white precipitate from that acid. 
We alfo find, that there actually is iron contained in 
it; becaufe, when fluxed with the black flux, its par- 
ticles are attracted by the loadftone j and we can draw 
no argument, from our not having difcovered iron in 
the experiment with the blow-pipe, againfl the pre- 
fence of that metal, as fo little can be examined in 
that way. I therefore think it probable, that the 
principal ingredients of it, and thofe on which its 
colour 
