238 ANNUAL OF SCIENTIFIC DISCOVERY. 



Dr. Karsten, a German chemist, several years ago made some 

 experiments with lead and zinc, and found that when a mixture of 

 these metals was allowed to cool very gradually, lead with a minute 

 trace of zinc was found at the bottom of the crucible, and zinc with a 

 small amount of silver at the top. If the lead contained silver, it was 

 almost entirely transferred to the zinc. Hearing that in Carmarthen 



/ c? 



silver is withdrawn from lead by means of zinc, he resumed his exam- 

 ination of the subject. 



He found that silver may be entirely separated from lead by zinc, 

 and that the following method gives the best results : A tube of 

 cast-iron 1^- inch in diameter is fitted to the crucible, so that the desil- 

 verized lead may be let off from the bottom. One end of this tube, 

 dipping nearly to the bottom of the crucible, is furnished with a slide 

 moving in grooves at the edge of the crucible, so that it can be shut 

 when required by means of a rod. In this way the stream of melted 

 lead may be regulated, and the fall of level gradual and uniform. In 

 the crucible were put 25 cwt. of lead, containing seven-eighth of an 

 ounce of silver to the cwt., and 4 cwt. of zinc. The whole was tUen 

 fused, and stirred together for one hour at a bright red heat. This 

 large amount of zinc was used because it was intended to attempt a pro- 

 cess of concentration in which the same quantity of zinc should serve to 

 desilverize subsequent charges of lead. After the stirring apparatus was 

 withdrawn, and the melted mass kept for four hours at a red heat, the 

 lead, perfectly freed from silver, was drawn off until only about 6 cwt. of 

 metal remained in the crucible. To this residue a second 25 cwt. of 

 zinc were likewise added, for reasons given below. A fourth, fifth, 

 and sixth charge of lead were introduced and treated in like manner, 

 2 cwt, of zinc having again been added to the fourth charge. The 

 lead drawn off, in each case, was entirely freed from silver. But when 

 lead was introduced without an addition of zinc, the lead, when^drawn 

 off, still retained silver to the extent of three-eighths of an ounce to the 

 cwt. The desilverizing of 150 cwt. of lead in this manner requires 8 

 cwt. or 5^- per cent of zinc, a quantity differing widely from that indi- 

 cated by former experiments namely H per cent. 



An addition of H per cent, of zinc is quite sufficient for the per- 

 fect desilverization of lead when only one charge is worked. Thus 

 25 cwt. of lead may very well be freed from silver 42 Ibs. of zinc, but 

 the difficulty of separating the small quantity of argentiferous metal 

 from the desilverized zinc is so great that this plan is not practicable. 

 On the other hand, there is a certain limit to the size of the crucible, 

 which cannot be exceeded, and recourse must, therefore be had to a 

 process of concentration. The silver is separated from the lead very 

 imperfectly, if twice or thrice as much zinc as is required for one 

 charge of lead is added at once, with the view of making it serve for 

 several charges. It is likewise imperfect when, on introducing into 

 the crucible the several charges of lead, the 1^ per cent, needed for 

 desilvering the lead is added with each charge. If, therefore, with 

 reference to the above example, the first melting is made with 25 cwt. 

 of lead, and 42 Ibs. of zinc, the second, third, fourth, &c., charges 



