88o Dr. watson’s Experiments and 
upon the furface of the lead was red-hot alfo, but it was 
ftill without colour. The fame parcel of lead was boiled 
in a crucible for a conliderable time ; during the boiling 
a copious fleam was difcharged, and the furface of the 
lead, as is ufual, became covered with a half vitrified 
fcoria. The lead which remained unvitrified was then 
examined, and it had acquired the property of forming a 
fucceflion of coloured pellicles during the whole time of 
continuing in a Hate of fufion. 
Another portion of the fame kind of lead was expofed 
to a flrong calcining heat for a long time ; the part which 
remained uncalcined did, at length, acquire the property 
of exhibiting colours fufiiciently vivid. 
Thefe experiments induced me to conclude, that the 
Chinefe lead was mixed with fome fubflance from which 
it was neceffary to free it, either by fublimation or calci- 
nation, before it would exhibit its colours. It would be 
ufelefs to mention all the experiments which I made be- 
fore I difcovered the heterogeneous fubflance with 
which I fuppofed the Chinefe lead was mixed. At laft 
I hit upon one which feems fully fufficient to explain 
the phenomenon. Into a ladle full of melted Derby fir ire 
lead, which manifefled a fucceflion of the mofl vivid 
colours, I put a fmall portion of tin, and obferved, that as 
foon as the tin was melted, and mixed with the lead, no 
more 
