ASSAY OP GOLD AND SILVER COINS. 



425 



thousandth of silver, and a white ch^ud of chloride will show itself. More doses 

 are added if the indications require it. 



The liquid is again shaken and. cleai'ed ; and the process is thus repeated 

 until the addition of the salt water shows only a faint trace of chloride below 

 the upper surface of the l»iquid. 



Let us suppose, for the sake of an e^rample, that three measures of the decimal 

 solution have been used with effect. This will show that the' 11 1-5 parts of the 

 coin contained 1003 of pure silver; and thus the proportion of pure silver in the 

 whole alloyed metal is ascertained. 



TEST ASSAY. 



For the foregoing process to be exact, it is necessary that the saline solution 

 be of the true standard strength, or be such that the quantity of it, measm'ed in 

 the large pipette, shall be just sufficient to precipitate 1000 parts of silver. This 

 cannot be assumed ^\•ithout proof, and a test assay is accordingly made as fol- 

 lows : 



A roll of silver, known to be of absolute purity, is kept from year to year, in 

 an enveloj)e, under the seal of the chairman of the assay commissioners. This 

 being opened in their presence, a portion of the silver is taken, and 1004 parts 

 carefully weighed off and submitted to the process of assay described above. If 

 the salt water used be of the exact standard, it is evident that as the solution in 

 the larger pipette will preci})it<ate 1000 parts of silver, four measures of the deci- 

 mal solution will be required to precipitate the remaining four parts. 



But as the nonual or standard solution is affected, from day to daj'', by changes 

 of temperatm'e or other influences, the finishing decimal doses may be more or 

 fewer ; and the other assays are to be corrected by the proof-piece accordingly. 



CALCULATION OF FIN E.NESS. 



By the assay thus corrected, the number of parts of silver contained in 1115 

 of the metal under trial is ascertained ; and the fineness, in thousandths, is then 

 found by the proportion: As 1115 is to the number of parts of fine silver, so is 

 1000 to the fineness of the alloyed silver, in thousandths. 



Thus, if the assay show the presence 1005i parts of fine silver, the fineness 

 of the alloyed silver will be 901.8 thousandths, as found by the proportion — 



1115 : 1005.5 : : 1000 : 901.79. 



It is on this principle that the following table is constructed. The numbers 

 at the top and the fractions at the side correspond to the measures of the decimal 

 solution used, corrected by the test assay. The numbers in the body of the table 

 show the corresponding fineness of the assay-piece, of which the weight was 1115 

 parts : 



