[ 298 ] 



and might then roll from the charcoal, if its fur- 

 face was plain ; but when borax is put to it, this 

 inconveniency is not fo much to be feared. 



SECT. XXXII. 



"Whenever an ore is to be tried, a fmall bit i$ 

 broke off for that purpofe, of fuch a fize as is di- 

 rected in Sed:. xvi. this bit is laid upon the char- 

 coal, and the flame blown on it (lowly : Then the 

 fulphur or arfenic begins to part from it in form 

 of imoke •, thefe are eafily dillinguifhed from one 

 another by their fmell, that of iulphur being fuf- 

 ficiently known, and the arfenic fmelling like gar- 

 lick. The flame ought to be blown very foftly, 

 as long as any fmoke is feen to part from the ore ; 

 but, after that, the heat muft be augmented by 

 degrees, in order to make the calcination as per- 

 fect as poflible. If the heat is applied very ftrong 

 from the beginning upon an ore, that contains 

 much of the fulphur, or arfenic, this ore will pre* 

 fently melt, and yet lofe very little of its mine-, 

 ralifing bodies, and by that means render the cal- 

 cination very imperfect. It is however, impoflible 

 to calcine the ores in this manner to the utmoft 

 perfection, which is eafily ieen in the following 

 ;nflance, viz. in melting down a calcined Potter's 

 ore with borax, it will be found to bubble upon 

 the coal, which depends on the fulphur, which is 

 ftill left, the vitriolic acid of this uniting with the 

 borax, and caufing this motion. However, lead 

 in its metallic form, melted in this manner, bub- 

 bles alone upon the charcoal, if any fulphur re- 

 mains in it. But, as the lead, as well as fomc of 

 the other metals, may raife bubbles upon the 

 charcoal, although they are quite free from the 

 fulphur, only by the flames being forced too vio- 

 lently 



