SILVER ASSAYING 831 



bo rendered quite evident by a very simple process of concentration. Dissolve one 

 gramme of the silver supposed to contain a quarter of a thousandth of mercury, and 

 let only a quarter of it be precipitated, by adding a quarter of the common salt neces- 

 sary to precipitate it entirely. In thus operating, the quarter thousandth of mercury 

 is concentrated in a quantity of chloride of silver four times smaller : it is as if the 

 silver having been entirely precipitated, four times as much mercury, equal to 2 

 thousandths, have been precipitated -with it. On taking two grammes of silver, and 

 precipitating only a quarter by common salt, the precipitate would be, with respect to 

 the chloride of silver, as if it amounted to 4 thousandths. By this process, which 

 occupies only five minutes because exact weighing is not necessary, one tenth of a 

 thousandth of mercury may be detected in silver. It is not useless to observe that, in 

 making these experiments, the most exact manner of introducing small quantities of 

 mercury into a solution of silver, is to weigh a minute globule of mercury, and to 

 dissolve it in nitric acid, diluting the solution so that it may contain as many cubic 

 centimeters as the globule weighs of centigrammes. Each cubic centimeter, taken by 

 means of a pipette, will contain one milligramme of mercury. If the ingot of silver 

 to be assayed be found to contain a greater quantity of mercury 1 thousandth, for 

 example the humid process ought, in this case, either to be given up or to be com- 

 pared with cupollation. When the silver contains mercury, the solution from which 

 the mixed chlorides are precipitated does not really become clear. Silver containing 

 mercury, put into a small crucible, and mixed with lamp-black, to prevent the volatil- 

 isation of the silver, was heated for three-quarters of an hour in a muffle, but the 

 silver increased sensibly in weight. This process for separating the mercury, there- 

 fore, failed. It is to be observed, that mercury is the only metal which has thus the 

 power of disturbing analysis by the humid way. The error caused by the presence of 

 mercury may be avoided by the addition of a small quantity of acetate of soda to the 

 solution of the silver in nitric acid, previous to the addition of the chloride of sodium, 

 as this salt prevents the precipitation of the mercury. 



Since the above process was first introduced by Gay-Lussac, several modifications in 

 the form of apparatus and other details have been introduced ; but the principles 

 \ipon which the method is worked are essentially the same. The normal solution of 

 salt is preserved in a vessel of glass or stoneware, instead of metal. The \ise of metal 

 tubes is dispensed with. Various modes of filling the pipettes from below or other- 

 wise are in use. Instead of the thermometer placed within the tube tq indicate the 

 temperature of the salt-solution, the standard is verified once a day JOT oftener if 

 necessary by check assays. The assay of silver, or silver alloys by a standard solution 

 of salt may be conducted as follows : Ten grs. or more of the metal according to cir- 

 cumstances, is weighed out, transferred to the bottle, dilute nitrjc acid added and 

 solution effected by placing the bottle in a water-bath. The red fumes are expelled, 

 and the solution diluted with water. The bottle is now placed under the lower end of 

 the pipette, 1000 grs. of the normal solution of salt (equal to ten grs. of silver) run in, 

 and the contents briskly shaken until clear. Ten grs. of the decimal solution of salt 

 (1000 grs. of which are equal to 1 gr. of silver) is now added from a pipette and, as 

 precipitate forms, the solution is again shaken until clear. This process is repeated 

 until the last 10 grs. added, does not produce any precipitate. As the last 10 grs. of 

 decimal solution added does not give any precipitate, it proves that it is in excess ; it 

 is therefore, deducted from the total quantity used, and also the half, say of the 

 previous 10 grs. added, as it is obvious that the previous 10 grs. added were not 

 sufficient to precipitate the whole of the silver. For example, 1 1 grs. of the alloy 

 require 1000 grs. of the normal solution, and 50 grs. of the decimal solution of salt, for 

 the working of the assay. The amount found necessary is, therefore, 35 grs. of the 

 decimal solution, which is equal to 3'5 of the normal solution, which added on to the 

 1000 grs. makes the total quantity required for the precipitation of the silver 1003-5. 

 Therefore 



Salt-solntion Salt-solution Silver Silver 



1000 : 1003-5 : 10 : 10-035 

 Then as 



Alloy Alloy Silver Silver 



11 grs. : 1000 : 10-035 : 912-2 



The weight of alloy operated on should contain about 10 grs. of silver. The contents 

 of silver should, therefore, be approximately determined by cupellation or otherwise 

 before submitting it to assay by this method. It is also desirable to take a quantity 

 of the metal for the assay, so as to require tho decimal solution of salt to complete 

 it ; by this means tho error noticed by Mulder and other assayers is obviated. However, 

 if it is found in the working of an assay that the first 10 grs. of the decimal solution 

 of salt does not yield any precipitate, excess of the decimal solution of silver (1000 grs.. 



