128 SMITH AND EXNER — ATOMIC WEIGHT OF TUNGSTEN-. [April? 



Ignition of the ammonium salt, and resolution in ammonium 

 hydroxide will not eliminate it. Nor will ammonium sulphide 

 remove it. In fact it seems likely that it has never been wholly 

 extracted from any previous material. 



''The purification by Pennington and Smith^ did not remove it, 

 for though closely following the method outlined by Schneider, 

 and adding to it the complete elimination of molybdenum, they 

 omitted the final repeated precipitations with acid. 



''To understand the effect of possible impurity, the following 

 table is given. A molecular mixture of tungsten and the impurity 

 is treated as though it were all tungsten, and the resulting atomic 

 weight calculated. 



Reduction Series Oxidation Series 

 Molecular Mixture. Atomic Weight. Atomic Weight. 



W -f W 184 184 



W + Mo 140 140 



W 4- 2Fe 148 148 



W + 3MnO 298 298 



Loss of Material Low High 



"A consideration of these numbers shows that : molybdenum and 

 iron would produce a low value ; manganese a high value ; volatility 

 a low value on reduction and a high value on oxidation. The error 

 introduced by manganese is more than three times as costly as that 

 introduced by iron, and more than two and a half times that intro- 

 duced by molybdenum. These ratios would apply regardless of the 

 proportion of the mixture. 



" From these considerations it is believed, that the presence 

 of manganese and iron will account for the high oxidation 

 values, for their presence would affect the result in a twofold 

 manner: Manganese through its inherent molecular changes 



[Mn.,Oi:e- ^ sMnO], and iron through its secondary action on 



the volatility. Further, that the presence of iron, molybdenum, 

 manganese, and volatility will explain the numerous discrepancies 

 noted in the published work on this subject. Again, since iron and 

 molybdenum decrease the value, and manganese and volatility 

 increase the value, and iron and molybdenum influence the vola- 

 tility, it is quite possible that such a mixture of these factors might 

 occur that the errors would be compensated. 



"Viewed in this way there still remains the necessity for deter- 

 minations with material from which every trace of molybdenum, 

 iron, and manganese, together with other possible impurities, has 

 been removed. 



" The method of determining atomic weights from the loss of 

 carbon dioxide has been applied to a number of the elements. Its 

 application to tungsten, and the special modification of the method 

 necessary for accurate determinations has not been before recorded. 



1 Proc. Am. Philos. Soc.,33, 332 (1894). 



