PURIFICATION OF SILVER. 27, 



All the silver used in the three final series of experiments which 

 follow was thus ignited in pure, well-seasoned lime boats, capable of 

 holding from 30 to 50 grams at each fusion. The unglazed porcelain 

 boats in which the lime was molded were made on purpose for this 

 object by the Royal Berlin Porcelain Manufactory. 



The environing atmosphere around the silver during fusion next 

 claims attention, since that, too, may vitiate molten metal. Obviously 

 air is impossible. A vacuum seemed a priori to be the best means of 

 preventing the access of impurity, but, to our surprise, we found that 

 silver fused in a vacuum was sometimes less pure than that fused in 

 the open flame of the blast lamp. (Exps. 72 to 76, p. 61.) This 

 puzzling circumstance was ascribed finally to the supersaturation of 

 silver with oxygen , derived either from included argentic nitrate or 

 from the calcic nitrate in the boat.* Silver precipitated by formiate 

 ignited on a pure boat does not show this impurity, because free from 

 oxidizing substances; and a few minutes' ignition in pure hydrogen 

 quickly removes in chemical fashion this supersaturation when it 

 exists, thus purifying the silver. The amount of hydrogen which 

 remains must be on the limit of the weighable ; at least, we were 

 unable to detect it quantitatively, because silver fused in hydrogen 

 yielded exactly the same amount of argentic chloride as that contain- 

 ing no oxygen which had been fused in a vacuum. This is shown by 

 the comparison of Exps. 77, 78, 80, 81, 83, and R. 2, with 84, 85, 

 and R. 3 (p. 65). Hence Stas's conclusion that silver does not dissolve 

 over 0.0004 per cent of hydrogen is supported, although indeed his 

 method of preparing the silver was somewhat different from ours, and 

 his method of distinguishing the hydrogen was somewhat doubtful. 



The discovery of the residual trace of oxygen in the fused elec- 

 trolytic metal, which had contained occluded nitrate, was not made 

 until the work on sodium was finished, but fortunately three pieces of 

 the silver used in that work still remained, so that it could be com- 

 pared with the purest silver which we were able to make. The 

 comparison was made by weighing the chloride obtainable from known 

 weights of each sample, and the details will be recorded in full in the 

 chapter concerning the synthesis of this salt. For the present it is 

 enough to say that the silver used in the work on sodium yielded 

 0.004 per cent less chloride than the purest silver, f and therefore was 



*A trace of chlorine, which, however, could hardly have heen present, would 

 have the same effect. 



tThe average of analyses 65, 66, 70 was compared with the average of the 

 final series (see pp. 61 and 65). 



