[ 22 5 ] 
black covering, like foot, which would not conduct 
in the lead. But thefe fame pieces of glafs, thus 
covered with the black coating, being put into 
a crucible full of fand, and urged with a ftrong 
heat, came out white, and conducted exactly as 
before. 
With a lefs degree of heat the black covering wo$ 
changed to white, but it did not adhere fo firmly to 
the glafs, as when the heat had been greater ; though 
it adhered more clofely than the black covering, 
which might be wiped off with a feather. But this 
white coating, produced by a moderate heat, would 
not conduct at all. 
In fome cafes I have found this whitifh matter to 
be difperfed by feveral explofions, as Mr. Franklin 
found gilding with leaf gold to be. 
In whatever manner the pieces of glafs were 
covered, the coating vanifhed when it was made red 
hot in an open fire ; and the glafs that remained 
would not condudt, any more than it did before. 
This circumfcance exadlly refembled the efcape of 
phlogifton from charcoal and metal, burnt in the 
open air. 
In a microfcope, this whitifh matter looked exa#!y 
like metal, or rather fome of the femi-metals, hav- 
ing a bright polifli, tho’ it foon became, as it were, 
tarn iflied. 
To try whether it was metal, I dipped the pieces 
of glafs that were covered with it in the acids, but 
found that they had little or no effeft upon it, though 
it is by no means fixed in the pores of the glafs, but 
covers it quite fuperficia'dv. 
Vol. LX. Gg It 
