1904.] SMITH AND EXNER — ATOMIC WEIGHT OF TUNGSTEN. 137 



Weight of residue 0.47 1 2 gram. 



" WO3 0.4160 '« 88.28% 



" " SiOg 0.0149 '< 3.16 '« 



" FcgOg 0.0337 •' 7-15" 



Manganese, platinum, etc Undet. 



98.59% 

 Even pure sodium carbonate will not remove all of the impuri- 

 ties, although it may serve to test the purity of the oxide as to the 

 iron, etc., which may be present. 



Ignition of Ammonium Paratungstate. 



The ignition of this salt in platinum vessels, as ordinarily con- 

 ducted, contaminates the trioxide with platinum. To minimize 

 this contamination a platinum crucible was fitted tightly two- 

 thirds of its length into an asbestos board. A platinum wire shaped 

 into a tripod was set upon the bottom of the crucible. A smaller 

 platinum crucible was supported by the tripod. Into the latter 

 were introduced from time to time not more than from two to three 

 grams of ammonium tungstate. A red heat was applied to the outer 

 crucible. The ammonia was expelled in the course of half an hour, 

 when the crucibles were covered with an inverted porcelain lid, it 

 being lifted from time to time to admit air. Constant weight was 

 obtained in two hours. This procedure gave the best results 

 which could be gotten by the use of platinum crucibles. While the 

 oxide is cooling it should be protected from all reducing atmos- 

 pheric dust, because the hot oxide is extremely sensitive to the 

 action of such substances. This is evident from the following : 

 a platinum rod previously heated in a flame and applied to the hot 

 oxide produces no change, but if the rod be touched quickly to the 

 skin and then laid on the hot oxide, a green spot will appear at the 

 point of contact. 



The efforts to substitute silver and gold crucibles for those of 

 platinum demonstrated that these metals, too, were appreciably 

 absorbed by the oxide. Porcelain crucibles were used, notwith- 

 standing the absorption of silica, which would of course become 

 greater as the time of ignition was prolonged and as the heat was 

 increased. Further, the oxide in immediate contact with the 

 porcelain invariably showed a green color. The glaze of the 

 crucible always indicated etching. With an unglazed crucible the 

 action was not so evident, hence the contamination was not so 

 great, and the most satisfactory results were gotten by setting the 



