often are thofe which are of a foft confidence, or a 

 particular figure, and which preferve the fame figure 

 in however minute particles they are broke -, for 

 inftance, the Calcareous Spar, the Sparry Gypfum, 

 Sparry Fluor, White Sparry Lead-ore, the Potters 

 Ore, (Galena tejfellata) the TefTellated Mock -lead 

 or Blende, &c. even all the common fluors which 

 have no determinate figure, and moil of the Mi- 

 ners metallorum calciforues crjfialhfatce or fpatofa : 

 All thefe are not fo compact as common hard 

 ftones •, and therefore, when the flame is imme- 

 diately pufhed at them, the heat forces itielf quick- 

 ly through and into their clefts or pores, and 

 caufes this violent expanfion and difperfion. Many 

 of the clays are like wife apt to crack in the fire, 

 which may be for the moil part afcribed to the 

 humidity, of which they always retain a portion, 

 Befides thefe enumerated, there may be found now 

 and then other mineral bodies of the fame nature ; 

 but it is, however, not fo common. 



The only way of preventing this inconveniency, 

 is to heat the body as (lowly as porTible. It is 

 be ft, prft of ail, to heat that place of the char^ 

 coal, where the piece is intended to be put on, 

 and afterwards lay it thereon ; a little crackling 

 wilj then enfue, but commonly of no great confe- 

 quence. After that, the fiame is to be blown 

 very (lowly towards it, in the beginning not di- 

 rectly upon, but fomewhat above it, and fo ap- 

 proaching nearer and nearer with the name until it 

 becomes red hot. This will do for the mod part; 

 but there are neverthelefs feme, which, notvvith- 

 flanding all thefe precautions, it is almGfc impofli- 

 bfc to keep on the charcoal. Thus the Fluors are 

 generally the moll difficult ; and as one of their 

 principal characters is difcovered by their effects 

 in the fire per fe^ (Sect, xviii. 6. ) they ought ne- 



ceiTarily- 



