830 



SILVER ASSAYING 



1849 



agitator and tho ground ; but this is seldom made use of, because it is convenient to 

 be able to transport the agitator from ono place to another. When the agitation is 



finished, the catches are released, and 

 the bottles are placed in order upon 

 a table furnished with round cells 

 destined to receive them ,andto screen 

 them. When we place the bottles 

 upon this table, we must give them a 

 brisk circular motion to collect the 

 chloride of silver scattered round 

 their sides ; we must lift out their 

 stoppers, and suspend them in wire 

 rings, or pincers. We next pour 

 a thousandth of the decimal solu- 

 tion into each bottle ; and before 

 this operation is terminated, there 

 is formed in the first bottle, when 

 there should be a precipitate, a ne- 

 bulous stratum, very well marked, 

 of about a centimeter in thickness. 

 At the back of the table there is a 

 black board, divided into compart- 

 ments, numbered from 1 to 10, 

 upon each of which we mark, with 

 chalk, the thousandths of tho 

 decimal liquor put into the corre- 

 sponding bottle. The thousandths 

 of salt, which indicate an augmen- 

 tation of standard, are preceded by 

 the sign + , and the thousandths of 

 nitrate of silver by the sign . 

 When the assays are finished, the 

 liquor of each bottle is to be poured 

 into a large vessel in which a slight 

 excess of salt is kept ; and when 

 it is full, the supernatant clear 

 liquid must bo run off with a syphon. 

 The chloride of silver may be reduced without any perceptible loss. After having 

 washed it well, we immerse pieces of zinc in it, and add sulphuric acid in sufficient 

 quantity to keep up a feeble disengagement of hydrogen gas. Tho mass must not 

 be touched. In a few days the silver is completely reduced. This is easily recognised 

 by the colour and nature of the product ; or by treating a small quantity of it with 

 water of ammonia, we shall see whether there be any chloride unreduced, for it will 

 bo dissolved by the ammonia, and will again appear upon saturating the ammonia 

 with an acid. The chlorine remains associated with the zinc in a state of solution. 

 The first washings of the reduced silver must be made with an acidulous water, to 

 dissolve tho oxides which may have been formed, and tho other washings with com- 

 mon water. After decanting the water of the last washing, we dry the mass, and add 

 to it a little powdered borax. It must now be fused. The silver being in a bulky 

 powder is to be put in successive portions into a crucible as it sinks down. Tho heat 

 should be at first moderate ; but towards the end of the operation, it must be pretty 

 strong, to bring into complete fusion the silver and the scoriae, and to effect their 

 complete separation. In case it should be supposed that the whole of the silver had 

 not been reduced by the zinc, a little carbonate of potash should be added to tho borax. 

 The silver may also bo reduced by exposing tho chloride to a strong heat, in coatact 

 with chalk and charcoal. 



The following remarks by M. Gay-Lussac, tho author of the abovo method, upon 

 the effect of a little mercury in tho humid assay, are important: 



It is well known that chloride of si Ivor blackens the more readily when it is exposed 

 to an intense light, and that even in the diffused light of a room it becomes soon 

 sensibly coloured. If it contains 4 to 5 thousandths of mercury, it does not 

 blacken ; it remains of a dead white ; with 3 thousandths of mercury, there is no 

 marked discolouring in diffused light ; with 2 thousandths it is slight ; with 1 it 

 is much more marked, but still it is much less intense than with pure chloride. With 

 half a thousandth of mercury the difference of colour is not remarkable, and is per- 

 ceived only in a very moderate light. But when the quantity of mercury is so small 

 that it cannot be detected by tho difference of colour in tho chloride of silver, it may 



