[ a86 3 
SECT. XVI. 
Of thofe things that are to be aflayed, only a 
fmall piece muft be broke off for that purpofe, not 
bigger than that the flame of the candle (Sed. vii. 
xlv.) may be able to ad upon it at once, if re- 
quired ; which is fometinies neceffary ; for in- 
ftance, when the matter requires to be made red 
hot throughout. A piece of about an eighth part 
of an inch fquare is reckoned of a moderate fize, 
and fittefl; for experiments *, feldom more, but 
rather lefs. This proportion is only mentioned as 
a diredion in regard to the quantity, the figure 
being of no confequence at all, a piece broke off 
from a flone feldom or never happening to be 
fquare. But here it is to be obferved, that the 
piece ought to be broke as thin as polTible, at 
leafl: the edges : The advantage thereof is eafily 
feen, the fire having then more influence upon 
the fubjed, and the experiment being quicker 
made. This is particularly necefiary to be ob- 
ferved v/hen fuch ftones are to be aflayed, which 
although in fome refpeds fufibie by themfelves, 
yet refift confiderably the adion of the fire ; be- 
caufe they may by thefe means be brought into 
fufion, at leafl: at their edges, which elfe would 
have been very difEcult if the piece had been 
thick, 
* SEC T. XVIL 
Some of the mineral bodies are very difficult to 
keep fleady upon the charcoal during the experi- 
ment, before they are made red hot ; becaufe, as 
foon as the flame begins to ad upon them, they 
fplit afunder with violence, and diiperfe. Such 
