

the lips grow weak or tired, after having continue4 

 to blow for a while in one ftrain ; but they foon 

 recover their former ftrength, by ceafing to blow 

 for fome minutes, 



SECT. XIV. 



The candle ufed for this purpofe (Seel, vii.) 

 pught to be fnuffed often, but fo, that the top of 

 the wick may retain fome fat in it, becayfe 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 qught therefore tp 

 be forced out when a great heat is 1 required, an4 

 only the point of the flame muft be directed upon; 

 the fubject which is to be aflayed. 



SECT. XV. 



The piece of charcoal made ufe of in thele ex- 

 periments (Se£t. vii.), mu{t not be of a difpofition 

 to crack. If this fhould happen, it muft gradually 

 be heated until it does not crack any more, before 

 any aflay is made upon it. If this is not obferved^ 

 but the aflay made immediately with a ftrongj 

 flame, fmall pieces of it will fplit off in the face 

 and eyes of the aflayer, and often throw along 

 with them the matter that was to be aflayed. 

 Charcoal which is too much burnt confumes too 

 quick during the experiment, leaving fmall holes 

 in it, wherein the matter to be tried may be loft : 

 And charcoal that is burnt too little catches flame 

 from the candle, burning by itfelf like a piece of 

 wood, which likewife hinders the procefs. 



SECT. 



