88i 
Obfervations on Lead Ore. 
more colours were to be feen. I do not know precifely 
the fmalleft poffible quantity of tin, which will be fufii- 
cient to deprive a given quantity of lead of its property 
of forming coloured pellicles, but I have reafon to be- 
lieve, that it does not exceed one five thoufandth part of 
weight of the lead. 
Derbylhire lead, which has loft its property of exhi- 
biting colours by being mixed with tin, acquires it again, 
as is mentioned of the Chinefe lead, by being expofed to 
a calcining heat for a fufficient time ; the tin it is fup- 
pofed being feparated from the lead by calcination be- 
fore all the lead is reduced to a calx. 
Some calcined Chinefe lead was reduced to its metallic 
form by burning fome tallow over it. The reduced lead 
gave, when melted, coloured pellicles; the calx of tin, 
which we fuppofe to have been mixed with the calcined 
lead, not being fo eafily reducible as that of lead. 
I find that zinc is another metallic fubftance which 
has the fame property as tin with refpecft to the depriv- 
ing lead of its power of forming coloured pellicles ; but 
it does hot, I think, poflefs this power in fo eminent a 
degree as tin. I put fmall portions of bifmuth alfo into 
melted lead, but the lead ftiil retained its quality of form- 
ing colours. I melted together fome filver and lead, but 
the lead did not thereby lofe its power of forming colours. 
Vol. LXVIII. 5 R A little 
