vantage, as far as poffible, fuch a fyfterri miift be 

 more generally received, and at the fame time 

 the eafier underftood, as it includes the mineral 

 bodies in a lefs number of clafTes, orders, &c. by 

 which the memory is not fo much clogged, as if 

 only their furfaces had been defcribed. 



SECT. IV. 



This granted, let us confider what difficulties 

 there are to be met with in examining mineral bodies. 

 Thefe are often like one another as to their external 

 appearances, although their conftituent parts are 

 quite different, and confequently make them ufe- 

 ful in different ways : Mod part of them ought 

 alfo to be changed from their natural form, and 

 even often difTolved, before they can be made any 

 ufe of. Their figure and colour, or, in fhort, their 

 furfaces, are therefore not folely to be depended 

 upon -, we muft penetrate into them ; and they 

 muft be decompounded according to the princi- 

 ples of chemiftry. 



SECT. V. 



By examining the mineral kingdom in this man- 

 ner, we may now and then find the fubjects of 

 our experiments (if even nearly the fame) to differ 

 in fome of their effects, which is particularly owing 

 to the difficulty of juftly determining the degrees 

 of the fire employed •, a difficulty not yet re- 

 moved, but which, however, ought not to hinder 

 us from going as far as pdffibly we can, frrice 

 we find by practice, that fuch cbltacies often are 

 remedied by repeated experiments ; and of thefe 

 we never can make too many, if judicioully per- 

 formed. 



T 3 SECT. 



