THE ATOMIC WEIGHT OF TUNGSTEN. 5 



cipitate. This fact is important, otherwise the method must have been greatly 

 modified. It is further to be noted that the precipitation was gradual, hence 

 there was ample opportunity for the liquid to be well mixed, and the precipi- 

 tation was therefore not of a local character. The mode of procedure was the 

 following : — 



The liquid was boiled in a porcelain basin, and to it a measured quantity of 

 pure hydrochloric acid was added. The boiling was then continued till the 

 precipitate was formed in sufficient quantity, when the contents of the basin 

 were removed to a large beaker and allowed to settle. The supernatant liquid 

 was after some time decanted, and the precipitate washed once or twice by 

 decantation. The precipitate was set aside for future use, the separate 

 decantations were united and evaporated to the bulk of the original solution, 

 and acid added as before. This process was repeated till eleven fractionations 

 were obtained. 



The first of these had a dark green colour, probably because the current of 

 air had not thoroughly oxidised the liquid. The subsequent precipitates, as 

 far as the seventh, were pale yellow ; while the remaining fractionations were 

 dirty green, and not so finely divided as those which preceded. These 

 differences were, I think, caused by the fact that for the final precipitations the 

 liquid required to be boiled clown to small bulk, in order to obtain a reasonable 

 quantity of tungstic acid. The tartaric acid under these circumstances 

 probably exercises a reducing action, and in the small quantity of liquid the 

 precipitate was in all likelihood aggregated by the continued boiling. 



The third precipitate was purified, and used for estimation of the atomic 

 weight. The precipitate was washed several times by decantation, and then 

 repeatedly on a filter. It was then dissolved in pure ammonia, and after 

 filtration reprecipitated by addition of pure hydrochloric acid. The solution in 

 ammonia had a triple purpose. It insured the oxidation of the precipitate 

 in case any lower oxide of tungsten were present. It separated any slight 

 trace of impurity not soluble in ammonia. It aided the washing from sodium 

 salts, for the solution and reprecipitation presented fresh surfaces to the action 

 of the wash water. The washing (which was with water containing a little 

 hydrochloric acid in order to prevent the precipitate running through the filter) 

 was continued till the filtrate yielded no residue on evaporation, and a test 

 portion of the precipitate gave only a slight indication of sodium by the spectro- 

 scope. The precipitate was afterwards dried and ignited in a current of air. The 

 tungstic acid thus obtained had a beautiful pale canary- yellow colour, and was 

 quite uniform in appearance. It was reduced in a porcelain tube by a current 

 of pure hydrogen, the temperature being gradually raised from below dull 

 redness to the highest obtainable by a strong blast in a Fletcher's furnace. 



Assuming that tungstic acid has the composition expressed by the formula 



