C 2S 9 ] 



quick heat is felt on the Bdii. When the calca- 

 reous fi1bft3p.ce is mixed with the vitriolic acidj 

 as in the gypfum ; or with a clay, as in the marie ; 

 it 'commonly, melts by itfeif ; yet more or iefs diffi- 

 cult in proportion to the differences of the mix- 

 tures : The gypfum produces generally a white, 

 and the marie a grey glais or flag. When there i.s 

 any iron in it, as in white iron ore, it becomes 

 tiark, and fometimes quite black, &c. 



2. The Siliceae never melt, alone, but become 

 generally more brittle after being burnt : Such of 

 them as are coloured become colourlcfs, and the 

 fooner when it does not arife from any contained 

 metal; for inftance, the Topazes, Amethifts* &c. 

 iome of the precious ftones, however, excepted. 

 And iuch as are mixed with a quantity of iron, 

 grow dark in the fire, as fome of the Jafpers, &c. 



3. The Garnet-kind melt always into a black 

 Hag, and that fometimes fo eafy, that it may be 

 brought into a round globule upon the charcoal. 



4. The Argiliaceae, when pure, never melt, 

 but become white and hard : The fame effects 

 follow when they are mixed with phlogifton ; for 



ance, the Soap-rock is eafily cut with the 

 knife-, but, being burnt, it cuts glafs, and would 

 ftrike fire with the fteel, if as large a piece, as is 

 neceflary for that purpofe, could be tried in this 

 wiy. The Soap-recks are fometimes found of a 

 dark brown and nearly black colour, but become 

 for all that quite white in the fire, as a piece of 



ina ware : However, care mud be taken not to 



Hi the flame from the top of the wick, there 

 - for the moil part a fboty fmoke, which com- 

 monly will darken ail that it touches*, and if this 

 is not obferved, a miftake in the experiment might 



!y happen : But if it is mixed with iron, as it 

 b linnet: mes found, it does not fo eafily part with 



U its 



