£ **85 1 
^he lips grow weak or tired, after having continued 
to blow for a while in one ftrain ; but they fooii 
recover their former ftrength, by ceafing to blow 
for fome minutes, 
SECT, XI¥. 
The candle ufed for this purpofe (Sed. vii.) 
pught to be fnuffed often, but fo, that the top of 
the wick may retain fome fat in it, becaufe the 
flame is not hot enough when the wick is almoft 
burnt to afhes *, but only the top muft be fnuffed off, 
becaufe a low wick gives too fmall a flame. The 
blue flame is the hotteft ; this ought therefore to 
be forced out when a great heat is required, and 
only the point of the flame muft be direded upon 
the fubjed which is to be affayed. 
SECT. XV. 
The pieqe of charcoal made ufe of in thefe ex- 
periments (Sed. vii,), muft not be of a difpofition 
to crack. If this Ihould happen, it muft gradually 
be heated until it docs not crack any more, before 
any affay is rnade upon it. If this is not obferved, 
but the affay made immediately with a ftrong 
|iame, fmall pieces of it will Iplit off in the face 
and eyes of the affayer, and often throw along 
with them the matter that was to be affayed. 
Charcoal which is too mqch burnt confumes toq 
quick during the experiment, leaving fmall holesf 
in it, wherein the matter tq be tried may be loft t 
And charcoal that is burnt too little catches flame 
from the candle, burning by itfelf like a piece of 
'^Qod, which like wife hinders the procefs. 
§ E C Tr 
