1836.] 



On Assaying Silver. 



82 



to this quantity of salt to make up the weight of 1,000 grains, 

 it would become a properly prepared solution for the purpose 

 required. This is the theory, but practical difficulties occur 

 which require the solution thus formed to be verified. 



17, These difficulties are, first ; that as these two sub- 

 stances are most delicate mutual tests, the salt, which imbibes 

 moisture, must be weighed warm, and cannot be weighed 

 with sufficient precision so as to completely decompose the 

 silver and neither exceed nor fall short of it. And secondly, 

 that if it could be weighed with the required accuracy, the 

 foregoing equivalent numbers may not be perfectly correct ; 

 some former tables give them at 110 and 60 instead of 108 

 and 58.75, and the 110 and 60 are extremely near the propor- 

 tional numbers laid down by the French chemists. But in 

 the quantity of salt required for 1 0 grains of silver the differ- 

 ence between the two sets of equivalents is but 0.02 of a 

 grain, and an error to this extent might easily be made in 

 weighing at different temperatures. The solution must there- 

 fore be verified ; and it may be done thus. 



18. Prepare a solution of pure dry common salt so that 

 5.44 grains shall be contained in 994.56 grains of distilled 

 water; i. e. 1.000 grains of the solution will contain 544 

 grains of salt, and 994.56 of water. This quantity ought to 

 precipitate exactly 10 grains of pure silver. Dissolve therefore 

 10 grains of pure silver in 3 or 4 drachms of dilute nitric 

 acid ; and when dissolved, add thereto half an ounce of dis- 

 tilled water, and shake it up and mix it well together : then 

 pour in the 1,000 grains of the salt solution, shake it well, let 

 it subside and when clear test a portion with a single drop of 

 nitrate of silver, and another portion with a single drop of 

 the solution of salt. If the prepared solution of salt be of 

 the exact strength, neither one nor the other of the tests 

 will have any effect. But if it be too strong, the test by 

 nitrate of silver will cause a discolouration ; if too weak, the 

 other test will do the same. Suppose it too weak and that 

 5 grains in weight more of the solution are required to pro- 

 duce complete precipitation of the silver ; it may then be 

 made of the proper strength either by evaporating it in the 

 proportion of 5 grains in the 1 ,000 ; or by adding more 



