. ( ? 7 < 5 , ) 
it vitrifies, and helps to difTolve them. The Plates of 
Fire-ftone bear the Heat of the Focus much longer than 
any other Mitter ^ but great care is to be taken in heat' 
ing them without breaking, /till they become red-hot, 
and when they are hot the leaft cold Air makes them melt. 
Neverthelefs this is the only fubfrance that 1 have ufed 
with moft fuccels, to hold Metals a long time in fufion, 
tho’ with thegreateft caution that waspoilible, to avoid 
the Inconveniences aforememioa’d. 
An other thing that has hinder’d me from carrying on 
thefe Inquiries upon Metals fo far as I could have wi(h- 
ed, has been the few clear Days we have had for thefe 
two Years pah ^ for the greateft part of ‘thefe Experi- 
ments require a bright, ftrong, and conftant Sun to keep 
the Matter a long time in perfeft fufion : And I have 
fcarce had, for this laft Year, above three or four fuch days 
as I could wifii for^ the Sky having been almofi: every 
day cover’d with Clouds about Noon, which is the only, 
time of the day fit for thefe Experiments. 
I come row to the Particulars of what Experiments I 
have made 5 and (ball begin with thofe on Iron. 
Of Iron, 
I placed in the Focus of the Burning- glafs a piece of 
forged Iron of about a Drahm weight; It turn’d red* hoc, 
and its furface was cover’d with a black Matter like Pitch 
or Tar. If one withdraws the Iron out of the Focus in 
this ftate, this Matter fixes itfelf on the furface of the Me- 
tal, and there forms a fraall Skin or very fine blackifh 
Scale, which is commonly very eafily feparared byftriking 
upon it 5 and that part of the Iron that was cover’d with 
this Scale appears blacker than ordinary. This Scale is 
feme of the fulphurons part of the Iron that rifes to 
and Surface of the Metal when it is ready to melt, 
the there remains for fome time before it exhales. 
It 
