666 SEC. 1 3. CHEMISTRY. 



transferred to a similar boiler with acid of specific gravity 1-31. The silver is 

 removed by the action of the acid, and the gold remains in a spongy state (H). 



5th Process. The gold sponge (which retains the original form of the 

 cornet) is rendered coherent by annealing at a dull red heat. This is accom- 

 plished by introducing the platinum tray into the muffle. The cornets then 

 assume the appearance of (I). 



6th Process. The final operation consists in weighing the cornets. This 

 is done in a specially constructed " assay " balance, 6, which is capable of 

 indicating- about . a6 ^ 66 -th of a grain when loaded with 7*5 grains in each pan. 

 The weights employed bear a decimal relation to the original weight of the 

 piece of metal operated upon. The percentage of gold, therefore, present in 

 the alloy, is at once indicated without calculation. The weighing in the first 

 process was of course conducted on the same or a similar balance. 



2771. Appliances used in the Assay of Silver. 



1. Pipette for " standard " salt solution used in silver assaying. 



2. Pipette for " decimal " salt solution. 



3. Riemsdijk's apparatus for adding drops of decimal solution. 



The Master of the Mint. 



The assay of silver can be conducted by cupellation, an operation similar 

 to that already described in the 2nd process of the assay of gold, the only 

 differences being that no pure silver is added to the assay piece, and the opera- 

 tion terminates when the button (D) has been obtained and weighed. The 

 wet method of Gay Lussac is, however, now usually employed for the assay 

 of silver when the " standard " is approximately known, and the alloy contains 

 not less than 50 per cent, of silver. It consists in precipitating the precious 

 metal from the solution of a known weight of the alloy to be assayed, the 

 weight being so adjusted that sufficient silver is present to neutralize a given 

 volume of the solution employed as a precipitant. This solution is usually 

 one of common salt, but hydrochloric acid or hydrobromic acid may some- 

 times be used with advantage. The assay pieces, having been carefully 

 weighed, are placed in numbered bottles, and a definite amount of moderately 

 dilute nitric acid is added to each, the bottles being then moderately heated to 

 assist solution. The standard solution is then carefully introduced by means 

 of a pipette, and the bottles are vigorously shaken until the precipitate coheres 

 and the solution becomes clear. A cubic centimetre of " decimal " solution, 

 which is ^th as strong as the " standard " solution, is then added to each 

 bottle, and they are again shaken. This is repeated until the decimal solution 

 produces either no cloud or a very slight one. This indicates the conclusion 

 of the operation, as the amount of silver present can be calculated when the 

 weight of salt which is required to saturate it is known. 



From the above description it will be seen that the only special apparatus 

 required in assaying silver by the method of Gay Lussac is a pipette for 

 measuring out the " standard " solution, and one for adding " decimal " solu- 

 tion. 1 and 2 are the forms of these used in the Royal Mint. The pipette 

 for standard solution, 1, is fixed in a vertical position and filled by an india- 

 rubber tube from below. The opening at the upper end of the pipette is 

 closed by the finger, the india-rubber tube is removed, and the solution thus 

 accurately measured is added to the contents of a bottle. ' No. 2 is divided 

 into cubic centimetres, and the additions made by means of it as already 

 described. When great accuracy is required the decimal solution may be 

 added drop by drop by means of the apparatus No. 3, designed by Chevalier 

 Van Riemsdijk, of the Utrecht Mint. 



