40 



MINERALS AND GEOLOGY 



soon as fusion takes place, the cupel must be moved somewhat farther 

 from the flame, so as to allow merely the outer envelope of the latter, 

 or the warm air which surrounds this, to play over the surface of the 

 globule. By this treatment, the lead will become gradually con- 

 verted into a fusible and crystalline slag. When this collects in large 

 quantity, the position of the cupel must be slightly altered, so as to 

 cause the globule to flow towards its edge, the surface of the lead 

 being thus kept free for continued oxidation. When the globule 

 becomes reduced to about a fourth or fifth of its original bulk, the 

 process is discontinued, and the cupel set on the anvil to cool.* This 

 is the first or concentration stage of the process. Another cupel is 

 then prepared and dried ; and the concentrated globule (after being 

 carefully separated from the slag in which it is imbedded) is placed 

 on this, and again subjected to the oxidizing influence of the flame. 

 During this second part of the process, the flame is made to play 

 more on the surface of the cupel around the lead button, than on the 

 button itself, by which a complete absorption of the oxidized lead is 

 effected. The flame should be sharp and finely pointed, and urged 

 down on the cupel at an angle of forty or forty-five degrees. Finally, 

 if the test-metal contain gold or silver, a minute globule of one (or 

 both) of these metals will be left on the surface of the bone-ash. By 

 concentrating several portions of a test-substance, melting the concen- 

 trated globules together, again concentrating, and finally completing 

 the cupellation, as small an amount as half-an-ounce of gold or silver 

 in a ton of ore or in round numbers, about one part in sixty- 

 thousand --may be readily detected by the blowpipe. 



During cupellation, the process sometimes becomes suddenly 

 arrested. This may arise from the temperature being too low, in 

 which case the point of the blue flame must be brought for an instant 

 on the surface of the globule, until complete fusion again ensue. Or 

 the hindrance may arise from the boneash becoming saturated, when 

 a fresh cupel must be taken. Or. it may be occasioned, especially if 

 much copper or nickel be present, by an insufficient quantity of lead. 

 In this latter case, a piece of pure lead must be placed in contact 

 with the globule, and the two. fused together ; the cupel being then 



* This is not always necessary, as in many cases the entire cupellation may be effected 

 without interruption on the same cupel. 



