PURIFICATION OF SILVER. 21 



of traces of argentic phosphide from the metal prepared in this way. 

 His own observation, indeed, confirms this suspicion, for he states 

 that silver thus fused in an oxidizing flame became covered with a 

 yellow crystalline film, which instantly disappeared in a reducing 

 flame. The great instability of the oxide of silver affords reason for 

 doubting his assumption that this film was an oxide ; it seems more 

 probable that it was a phosphate, produced by oxidation of silver 

 phosphide, and decomposed by reduction to this compound again. 

 Stas, relying entirely upon the superficial appearance of the silver 

 and its flame test as a guide to its purity, would not have discovered 

 the presence of phosphide. 



The possibility of this danger is enough to incline the careful 

 chemist to avoid the piesence of phosphorus. As a matter of fact, 

 Stas never used silver prepared in this way for his atomic weight 

 researches,* and no one else seems to have used this procedure except 

 Scott, f who fused 5 grams of silver in this way for a single analysis. 

 Of course, however, a single analysis is too slender a basis upon 

 which to form any adequate judgment concerning its purity. Actu- 

 ated by the preceding considerations, we did not take the trouble to 

 test the method, preferring first to test others which gave more promise 

 of satisfactory results. 



Another possible material, which has been partially tested in 

 previous work, is carbon. Sugar-charcoal may be prepared in a state 

 very free from metallic impurities, and would serve excellently if it 

 were not that carbon is probably soluble to a slight extent in the 

 molten metal. We gave this possibility a fair test, and found that 

 when the silver is fused on a cupel of sugar-charcoal with a clean 

 blast lamp and moderately pure illuminating gas, it is as a matter of 

 fact very pure. (Experiments 67, 68, 71, on page 61.) Probably the 

 oxidizing flame used to melt the metal prevents the introduction of 

 much carbon at first, and during the brief reducing period in which 

 the metal must be cooled to eliminate an excess of oxygen there is not 

 time to dissolve much carbon. Evidently, however, a process which 

 depends upon the mutual elimination of opposing errors is an uncer- 

 tain one, and hence not suitable for the most accurate work. That 

 carbon is really dissolved was well shown by another experiment, in 

 which a carbon boat in a porcelain tube filled with pure hydrogen 

 was used to contain the fusing silver. The, experiment yielded silver 

 slightly less pure than the previous average ; it must have contained 



*Stas, Ouvres Com pi. , 3, 75. 



tA. Scott, J. Chem. Soc. Trans., 79, 147 (1901). 



