824 SILVER ASSAYING 



combine. As the heat rises, the oxide of lead flows round about over the surface, 

 till it is absorbed by the cupel. When the lead is wasted to a certain degree, a 

 very thin film of it only remains on the silver, which causes the iridescent appearance 

 like the colours of soap-bubbles : a phenomenon called, by the old chemists, 

 fulguration. 



When the cupel cools in the progress of the assay, the oxidation of the lead 

 ceases ; and, instead of a very liquid vitreous oxide, an imperfectly-melted oxide is 

 formed, which the cupel cannot absorb. To correct a cold assay, the temperature of 

 the furnace ought to be raised, and pieces of paper put into the cupel, till the oxide of 

 lead which adheres to it be reduced. On keeping up the heat, the assay will resume 

 its ordinary train. Pure silver is more liable to vegetate. Some traces of copper 

 destroy this property, which is obviously duo to the oxygen which the silver can 

 absorb while it is in fusion, and which is disengaged the moment it solidifies. An 

 excess of lead, by removing all the copper at an early stage, tends to cause the vege- 

 tation. The brightening is caused by the heat evolved when the button passes from 

 the liquid to the solid state. Many other substances present the same phenomenon. 

 In the above operation it is necessary to employ lead which is very pure, or at least 

 free from silver. This is called poor lead. The lead reduced from Pattinson's 

 1 oxychloride ' is very free from silver ; the lead reduced from the litharge of com- 

 merce usually contains 10 dwts. or more of silver per ton. 



2. Wet Methods, (a.) By means of a Standard Solution of Salt or Chloride of 

 Sodium. The process by the humid way, recommended at the Royal Mint in 1829, and 

 exhibited as to its principles before the Right Honourable John Herries, then Master, 

 in 1830, has all the precision and certainty we could wish. It is founded on the well- 

 known property which silver has, when dissolved in nitric acid, to be precipitated as an 

 insoluble chloride of silver by a solution of salt or by muriatic acid ; but, instead of 

 determining the weight of the chloride of silver, we take the quantity of the solution 

 of salt which has been necessary for the precipitation of the silver. To put the process 

 in execution, a liquor is prepared composed of water and salt in such proportions that 

 1,000 measures of this liquor may precipitate completely 10 grains of silver, perfectly 

 pure or of the standard 1,000, previously dissolved in nitric acid. The liquor thus 

 prepared gives, immediately, the true standard of any alloy whatever, of silver and 

 copper, by the quantity of which it may be necessary to precipitate the silver in a 

 known weight of this alloy. 



The process by the humid way is, so to speak, independent of the operator, the mani- 

 pulations being very easy ; and the term of the operation is very distinctly announced 

 by the absence of any sensible turbidity on the addition of salt to the silver solution, 

 while there remains in it one quarter of a thousandth of metal. The process is not 

 tedious, and in experienced hands it may rival the cupel in rapidity; it has the 

 advantage over the cupel of being more within the reach of ordinary operators, and 

 of not requiring a long apprenticeship. It is particularly useful to such assayers 

 as have only a few assays to make daily, as it will cost them very little time and 

 expense. 



By agitating briskly, during two minutes, the liquid rendered milky by the pre- 

 cipitation of the chloride of silver, it may be sufficiently clarified to enable us to 

 appreciate, after a few moments of repose, the disturbance that can be produced in 

 it by the addition of 1000th of a grain of silver. The presence of lead and copper, 

 or any other metal, except mercury, has no perceptible influence on the quantity of 

 salt necessary to precipitate the silver ; that is to say, the same quantity of silver, 

 pure or alloyed, requires for its precipitation a constant quantity of the solution of 

 salt. 



Supposing that we operate upon a gramme of pure silver, the solution of salt ought 

 to be such that 100 cubic centimeters may precipitate exactly the whole of the silver. 

 The standard of an alloy is given by the number of thousandths of solution of salt 

 necessary to precipitate the silver contained in a gramme of alloy. 



When any mercury is accidentally present, which is however, a rare occurrence, 

 it is made obvious by the precipitated chloride remaining white when exposed to 

 daylight ; whereas, when there is no mercury present, it becomes speedily first grey 

 and then purple. Silver so contaminated must be strongly ignited before being 

 assayed, and the loss of weight noted ; or a cupel assay may be had recourse to. 



The following is a description of the process and apparatus as first introduced by 

 Gay-Lussac : 



Preparation of the Normal Solution of Salt wlitn it is measured by Weight. Sup- 

 posing the salt pure as well as the water, we have only to take these two bodies in 

 the proportion of 0-5437 k. of salt to 99'4573 k. of water, to have 100 k. of solution, 

 of which 100 grammes will precipitate exactly one gramme of silver. But instead of 

 pure salt, which is to be procured with difficulty, and which besides may be altered 



