266 METALS. 



if but half as much is required, he knows that it is 

 but half silver. Having ascertained the true proportion, 

 the assay is completed. The salt required in the pro- 

 cess is employed in the form of a solution, and the 

 quantity used is known by pouring it from a graduated 

 vessel. 



665. EXPLANATION. The curd, which 



Explain the 



chemical ac- forms in the above process, is insoluble 

 abovTprocess. chloride of silver, formed from the silver of 

 * the solution, and the chlorine of the salt. 

 The nitric acid and oxygen, which were combined 

 with the silver, at the same time unite with the sodium, 

 forming nitrate of soda, which remains in solution. 



666. SILVER SEPARATED FROM COPPER. 

 Describe the ,/. 



method of ex- Copper, obtained from certain ores, con- 

 tams so mucn silver as to make their separa- 

 tion an object of importance. The method 

 pursued is, to fuse the copper with lead. As the lead 

 flows out again by subsequent fusion, it brings with it 

 all the silver, and the copper remains behind as a spongy 

 mass. This process is called liquation. The silver is 

 then freed from lead by the process of cupellation al- 

 ready described. 



Mention some ^67. IfsES OF SILVER. Most US6S of 



uses of silver, silver are so familiar that they need not be 

 here mentioned. Its employment for daguerreotype 

 plates depends on the fact that the color of many of 

 its compounds is readily changed by light. This sub- 

 ject is more fully considered in the section on chlo- 

 rides. The nitrate of silver, or lunar caustic, is used 

 in surgical operations, to burn or cauterize the flesh. 



