CHEMICAL ANALYSIS. 333 



solved in acid, the former may be precipitated by am- 

 monia. The latter remains in solution and may be 

 afterward removed by some other agent. 



847. INDIRECT METHODS. Indirect me- 



II lust rate the , , - 



indirect mcth- thods of analysis are much more frequently 

 od - employed than either of the above. The 



detection of silver in a copper alloy may serve as an 

 example. The alloy being first dissolved, hydrochloric 

 acid is added to the solution, as a test. The appearance 

 of a white insoluble curd, is taken as conclusive evi- 

 dence of the presence of silver. No other metal of an 

 alloy ever combines with the chlorine of hydrochloric 

 acid to form such a precipitate. The evidence is quite 

 as satisfactory to the chemist as that which would be 

 obtained by the separation of the silver in the metallic 

 form. 



848. Neither is separation necessary, in 



How is the . 



weight calcu- order to ascertain the exact weight of the 

 lated? metal which has been precipitated. Ad- 



vantage is here taken of the well-established law of 

 combination by definite proportions. The chloride of 

 silver produced in the experiment, is invariably of the 

 same proportional composition. It is made up of an 

 atom of silver, to every atom of chlorine. Its weight 

 being ascertained by the balance, the amount of silver 

 which it contains may be calculated with absolute pre- 

 cision, by help of the table of atomic weights. This 

 weight being compared with that of the original alloy, 

 gives, by a simple calculation, the per centage propor- 

 tion of silver which the alloy contains. The nature and 

 quantity of other constituents, whether of compounds 



