38 



MINERALS AND GEOLOGY 



Metallic compounds referable to the third group, yield no metal 

 charcoal, or by other treatment in open contact with the atmosphere. 

 The presence of arsenic, however, is easily made known by the garlic - 

 like odour evolved during fusion with reducing agents (or alone) on 

 charcoal. Cadmium and zinc may also be recognized by the oxidized 

 sublimates which they deposit on the charcoal. The Cadmium subli- 

 mate is reddish-brown ; the zinc snblimate, lemon-yellow and phos- 

 phorescent whilst hot, and white when cold. Mercury forms no 

 incrustation on charcoal ; but its presence in any compound may be 

 determined by reduction with carbonate of soda or iron-fillings in a 

 glass tube of narrow diameter. A small test-tube or piece of glass 

 tubing closed at one end before the blow-pipe, may be used for the 

 experiment. The test-substance, in powder, mixed with 3 or 4 vols. 

 of perfectly dry carb. soda, is inserted into the tube by means of a 

 narrow strip of glazed writing-paper bent into the form of a trough, 

 so as to prevent the sides of the glass from being soiled, and the mix- 

 ture is strongly ignited by the spirit-lamp or by the blowpipe-flame. 

 If mercury be present, a grey metallic sublimate will be formed near 

 the upper part of the tube. By friction with an iron wire, or the 

 narrow end of a quill-pen, &c., the sublimate may be brought into the 

 form of fluid globules, which can be poured out of the tube, and are 

 thus easily recognized as metallic mercury. 



7. Cupellation: Gold and silver are separated by this process 

 from other metals. The test-metal is fused with several times its 

 weight of pure lead. The button, thus obtained, is exposed to an 

 oxidating fusion on a porous support of bone-ash, known as a cupel. 

 The lead and other so-called base metals become oxidized by this treat- 

 ment, and are partly volatilized, and partly absorbed by the bone-ash, 

 a globule of gold or silver (or the two combined) being finally left on 

 the surface of the cupel. For blowpipe operations, cupels are gen- 

 erally made by pressing a small quantity of dry boneash into a cir- 

 cular iron mould, the latter being fixed, when presented to the flame, 

 in a special support, consisting essentially of a wooden foot and pillar 

 with three or four short cross-wires (between which the cupel-mould 

 rests) at the top of the latter. Instruments of this kind cannot be 

 obtained in remote places, but the process may be performed equally 

 well by the use of a small iron spoon, of about half-an-inch in dia- 

 meter. Enough bone-ash to fill this, is taken up in it, and warmed 



