4 DR JOHN WADDELL ON 



till no more precipitate was produced. The liquid was then evaporated to 

 dryness, and thus a mixture of sodium tungstate and carbonate was obtained. 

 For my purpose the presence of carbonate was no disadvantage, and I did not 

 crystallise out the tungstate. 



As I have already remarked, the method of purification hitherto almost 

 universally adopted was recrystallisation. I employed the method of fractional 

 precipitation as more likely to give a decided test. In the case of all the 

 precipitations being alike, the probability of purity would nearly amount to a 

 certainty, for it is extremely unlikely that the proportions of the impurities 

 would be the same in each of the precipitates. In case of a difference 

 appearing in the precipitations, it is natural to suppose that those most widely 

 separated would exhibit the greatest dissimilarity, and that the middle frac- 

 tionations would be similar and practically pure. 



It was known that the great bulk of my salt was tungstate. Any impurities 

 more readily precipitated than tungsten ought to be concentrated in the first 

 fractionation, while anything less readily precipitated would be chiefly found 

 in the last portion. 



The question presented itself in what form it was best to precipitate the 

 tungsten. From the soluble tungstate it was possible to throw down either an 

 insoluble tungstate or tungstic acid. As I wished to determine the atomic 

 weight of the metal by reduction of tungstic acid, it is evident that if the 

 former method were employed it would be well to produce a tungstate which 

 could be easily decomposed and changed to tungstic acid. Such a compound 

 is mercurous tungstate, which loses its mercury on ignition. From some 

 experiments tried with mercurous nitrate as precipitant, I decided that this 

 method of fractionation was not so feasible as the precipitation of tungstic 

 acid direct by means of hydrochloric acid. 



Before proceeding with the fractionation, however, I freed the sodium 

 tungstate so far as possible from molybdenum. The method employed was 

 that recommended by Rose. Sufficient tartaric acid was added to a solution 

 of the alkaline salt to prevent the precipitation of tungstic acid on acidification 

 with hydrochloric acid. A stream of hydrogen sulphide was then passed 

 through the solution, and an appreciable though small precipitate of molyb- 

 denum sulphide was thus obtained. 



The filtered solution containing about 300 grammes of solid tungstate had 

 a blue colour, owing to the presence of a small quantity of one of the lower 

 oxides of tungsten. The liquid was decolorised by the passage of a current 

 of air, and was then ready for fractional precipitation. It is to be noted that 

 though sufficient tartaric acid had been added to the solution to prevent 

 precipitation of tungstic acid by a small quantity of dilute hydrochloric acid, 

 yet a considerable excess of the latter was capable of producing quite a pre- 



