T Hough the following Dif- 

 courfe was at firft written 

 by way of Appendix to the Trea- 

 tife of the ImperfeSion of the 

 Chymical Do&rine of Qualities; 

 yet the bulk of it, fwelling be- 

 yond what was forefeen, made it 

 feem expedient to publifh it as a 

 Trad by it lelf. 



