THE A.TOMIC WEIGHT OF TUNGSTEN. 3 



This is the most expeditious method of oxidation, for 200 grammes of metal 

 can readily be ignited in one operation, and two-thirds of the total quantity are 

 oxidised in a few seconds. I found it most convenient to mould the materials 

 into a cone, sufficient water having been added to make this operation possible. 

 The cone having been placed on a large iron tray was lighted at the apex, so 

 that the propagation of the combustion was downward. By this precaution, 

 loss of material caused by spurting was in large measure avoided. The 

 objection to this mode of oxidationis the great difficulty experienced in re- 

 covering tungstic acid from the great bulk of salt. If, however, appliances are 

 at hand for evaporation, with strong hydrochloric acid on an oil bath and long 

 continued drying at 120°, the process might be made fairly successful, otherwise 

 the difficulties encountered in recovering the tungstic acid far outweigh the 

 advantage gained by rapid oxidation. 



The method which I found most convenient and satisfactory was the igni- 

 tion of the metal in a porcelain tube, a current of air being passed over the 

 red hot mass. The metal must form only a thin layer, else the oxidised 

 material will obstruct the passage of air, for tungstic acid occupies nearly 

 four times the volume of metallic tungsten. In my experiments, sulphurous 

 fumes were produced in considerable quantity, evidently showing the presence 

 of sulphide in the commercial tungsten. The vapours were absorbed by 

 caustic soda solution, and in this solution traces of molybdenum were found. 

 After about six hours' heating, the metal was, as a rule, to a great extent 

 oxidised, and had acquired a green colour. If the operation be long continued, 

 say for thirty hours, the compound produced possesses a canary-yellow colour, 

 and is practically tungstic acid. 



The green colour usually obtained was doubtless due to the presence of 

 partially oxidised and totally unoxidised material. In ordinary cases complete 

 oxidisation was not considered necessary, for in subsequent operations the 

 small amount which had escaped the action of the hot air was easily elimin- 

 ated. The greater part of the tungstate obtained was prepared from the im- 

 pure tungstic acid just mentioned. The mode of treatment was as follows : — 



The partially oxidised mass was fused in a platinum basin with one half of 

 its weight of sodium carbonate. The fusion was complete in a few minutes. 

 The fused mass after cooling was disintegrated with water, and the tungstate 

 dissolved out, leaving the unoxidised metal as residue. It was found that the 

 platinum basin was somewhat attacked, and subsequent examination proved 

 the presence of lead in the tungsten residue. The solution of tungstate was 

 boiled in a large silver basin with addition of ammonia carbonate, the latter 

 being employed to separate out any silica or iron and aluminium hydrates 

 possibly present. A very slight precipitate formed and was filtered off. The 

 filtrate was again treated with ammonium carbonate, the process being repeated 



