386 On the Method of Assaying Silver. [No. 



cup, so as to cause to sink any minute particles that may happen to 

 remain floating. He then, after an interval of ten minutes, drains 

 off about three-fourths of the supernatant water, which he lets run 

 down the rod into a vessel near him, and with a tap or two with 

 the rod to the outside of the cup to still further loosen the deposit, 

 this part of the manipulation is concluded. 



The crucibles are next taken to the drying furnace, where a 

 steam bath is ready to receive them, on the perforated upper plate 

 of this they are ranged, and allowed to remain for about an 

 hour. This gradually and without spurting, frees the chlorides 

 from moisture, which may be known by their caking, •*. e., leaving 

 the sides of the cups round the edges and forming at the 

 bottom of each a loose cake, resembling somewhat a gun-wad. The 

 crucibles are then arranged on a hot air plate and there exposed 

 to a temperature of between 300° and 350° (Fahr.) for about 2 

 hours, till thoroughly dried, when they are ready for weighing.* 

 When the above manipulations have been carefully and satisfac- 

 torily gone through, each little cup contains an unbroken, tole- 

 rably firm, cake of chloride of silver, lying unattached, which 

 admits of being easily grasped with a pair of forceps, and cleanly 

 lifted out of the cup and conveyed to the skiff of the assay balance 

 in which it is weighed. The cups are generally brought from the 

 laboratory to the assayer at the balance, in batches of 8 or 10. A 

 " Standard," synthetically prepared of pure silver and copper, and 

 an assay pound of pure silver are introduced with each day's set of 

 assays and their chlorides dried with the others, and the analysis 

 of them verified before weighing the rest. Occasionally these 

 " checks" are also fused and weighed in a porcelain capsule, but 

 the weight found never differs from that of the chloride merely dried 

 as above. 



Once, or twice a month, the silver is recovered from the accumula- 

 ted chlorides, which are well pounded in a mortar and brought to 

 a powder and then mixed with a proper proportion of chalk and 

 charcoal, and put into a wrought iron crucible and reduced with 



* The chlorides are weighed warm, to obviate the risk of their absorbing 

 moisture ; a precaution especially necessary in the heavy monsoon weather 

 in this country. 



