SILVER. 265 



powder, in dissolving such coins in nitric acid. Its 

 color and lustre may be brought out by rubbing. 



663. THE SILVER ASSAY. Assaying is 



What is as- , n ,, . 



laying, and tne process by which the proportion of met- 

 why necessa- a i s j n an a }} oy j s ascertained. In all estab- 

 lishments where money is coined, assaying 

 is an important part of the work of the establishment. 

 The precious metals, as received at the mint, commonly 

 contain a certain proportion of other metals. But it 

 may be too much or too little. It is the business of the 

 assayer to ascertain its precise composition, that the 

 metal may be rendered purer, if necessary, or be fur- 

 ther alloyed, if found purer than the standard. 



664. As a preparation for the silver as- 



Descmbc the 



process of as- say, a sample, containing an ounce, or other 

 definite weight of the impure metal, is dis- 

 solved in nitric acid. The dissolved silver has the pro- 

 perty of becoming solid again, and sinking 

 to the bottom of the clear solution as a white 

 curd, just in proportion as common salt is fur- 

 nished to it. But the other metals which 

 may be present, as impurities, have no such 

 effect. It follows, that the amount of silver 

 present, is just in proportion to the amount of 

 salt it is necessary to supply, before the pre- 

 cipitation, or formation of the curd, ceases. Now, the 

 assayer knows beforehand, how much salt he must 

 supply to the solution of an ounce of metal, if it be all 

 silver. If he finds that an ounce of the sample, re- 

 quires to be supplied with the same quantity, before the 

 precipitation ceases, he knows that the metal is all silver ; 



12 



