268 CHEMISTRY. 



with antimony and arsenic. There is always some silver 

 in the common ore of lead, galena, and sometimes there is 

 so much of it that it is profitable to submit the lead ob- 

 tained from this ore to certain chemical processes for ex- 

 tracting the silver alloyed with it. 



379. Extraction of Silver from Galena. We have already 

 told you, in 369, how the galena is freed from the sul- 

 phur. This gives you an alloy of lead and silver. This 

 is melted in a large basin and allowed to cool slowly. As 

 it cools a crust continually forms over the surface, which 

 is composed of crystallized lead without any of the silver, 

 this settling down in the liquid below simply because it 

 does not crystallize as readily as lead does. This crust is 

 taken off with an iron colander as fast as it forms, until 

 there is left only a small amount of the melted metal. 

 You see what the result is. You have an alloy containing 

 much more silver in proportion than the mass which you 

 melted. This alloy, after cooling, is submitted to a proc- 

 ess called cupellation. The cupel is a shallow dish made 

 of bone ashes, and is very porous. In this is placed the 

 alloy, and it is submitted to a strong heat. When it is at 

 a full red-heat a powerful current of air is thrown across 

 it by bellows in order to blow away the litharge or oxide 

 of lead which forms on the surface. What is not thus 

 blown away is absorbed by the pores of the cupel. When 

 the lead is all disposed of, and the silver is left alone, the 

 surface suddenly becomes brilliant, and the workman, see- 

 ing this flashing or lightening, as it is technically termed, 

 knows that the process is completed, and withdraws the 

 cupel from the fire. 



380. Salts of Silver. Silver forms many useful salts. 

 Silver nitrate is often called by physicians lunar caustic, 

 being used as a caustic by the surgeon. As it grows 

 black rapidly when exposed to the light in contact with 



