USE OF THE BLOWPIPE. 105 



The preparatory operation, termed roasting, or calcining, 

 consists in submitting the metallic arseniurets, sulphurets, 

 and other substances, to a low, red heat; the 

 supports, that is the substances on which it is 

 placed, being either charcoal, mica, the open glass 

 tube, or the tube with a bulb at the end, called the 

 glass matrass (fig. 56). 



Unless this operation be carefully performed, so 

 as to expel all the sulphur or other volatile matter, 

 it will be found impossible to reduce the ore with 



, . ,, , FIG. 56. 



any satisfactory result. 



Fluxes are substances added to metallic ores, or other 

 minerals, in order to promote their fusion. The most useful 

 of these are borate of soda, or borax ; carbonate of soda, and 

 microcosmic salt, which should be procured pure, and kepi; 

 in phials. 



The mode of proceeding with the fragment of a mineral to 

 be examined by the blowpipe is this : First, heat it gently in 

 the glass matrass, observing whether it give off water, change 

 colour, decrepitate, give off volatile matter, and whether such 

 matter be sulphur, known by its peculiar odour ; arsenic, 

 which in smell resembles garlic ; selenium, which gives an 

 odour resembling decayed horseradish ; or mercury, which is 

 known by its peculiar appearance. 



The next step, will be to heat another fragment of the 

 same mineral, without fluxes, on charcoal ; observing whe- 

 ther, on being gently heated in the outer flame, it gives off 

 volatile matter, and whether such matter be sulphur, arsenic, 

 or selenium, &c. ; whether it decrepitate, become magnetic, 

 fuse, and whether such fusion be complete, partial, quick, or 

 slow ; with a pasty aspect, or with the appearance of a 

 liquid ; whether it intumesce, and that slightly or violently ; 

 whether it volatilise ; whether it give off fumes, and whether 

 they be copious or slight, and of what colour ; whether 

 they leave a remainder or not, and whether they condense 

 or not on the support. It is also to be observed, whether 

 the assay colour the jet; burn either with or without 

 a flame; change colour once, or more than once; whether 

 it become absorbed by the charcoal ; whether it fuse with a 

 result, and whether such result be a bead of metal, or ashes, 

 or powder ; a glassy globule, or a white enamel ; whether the 



