396 Foote and Langley — Indirect Method for 



over asbestos till all hydrofluoric acid was expelled, stirring if 

 there seemed to be danger of spattering. The residue was 

 cooled, 200"' of water added, and the solution made alkaline 

 with annnonia, filtered, and the preci])itate washed with dilute 

 ammonia. The precipitate was transferred while moist to a 30 

 gram platinum crucible and ignited over a blast lamp for an hour. 

 The residue was ground to a tine powder with water and was 

 then ready for the densitj' determination previously described. 

 This metl)od of preparing the oxides may appear longer than 

 necessary, but we adopted it only after preliminary work had 

 made it appear essential. Acid potassium fluoride was used in 

 decomposing the oxides instead of potassium bisulphate because 

 the latter does not render the oxides completely soluble iu 

 hydrofluoric acid if they have previously been ignited. This 

 fusion with acid fluoride does not involve any loss of either 

 columbium or tantalum by volatilization. To prove this, 1*3576 

 gms. of mixed columbic and tantalic oxides were subjected to 

 the process outlined above and 1-3603 gms. were obtained. If 

 the mixed oxides have not been ignited and are completely 

 soluble in hydrofluoric acid, this may be used to dissolve them 

 instead of acid potassium fluoride. The first precipitate with 

 ammonia from hydrofluoric acid solution can not be ignited 

 and used for density determination, as it contains fluorine after 

 being ignited. Washing the precipitate with dilute ammonia 

 will not remove all the fluorides. 



JResults. 



In preparing the mixtures for density determinations, we 

 were fortunate in obtaining very pure tantalum and columbium 

 oxides. Dr. W. E. Ford supplied a sample of columbic oxide 

 which had been given him by Prof. E. F. Smith. It had 

 been prepared from the chloride and was exceedingly pure. 

 Dr. Ford also supplied some tantalum oxide from Dr. W. R. 

 Whitney of Schenectady, who stated that the material was 

 considerably over 99 per cent pure. Dr. T. B. Osborne fur- 

 nished us with considerable amounts of potassium tantalum 

 and potassium columbium double fluorides which we recrys- 

 tallized and converted into the oxides. The densities were as 

 follows : 



Columbic oxide (Smith) 4*552 



" " (Osborne) 4-564 



Tantalic " (Whitney) 8-716 



" " (Osborne) 8-683 



In making the mixtures, it was assumed that Smith's and 

 Whitney's oxides were pure and that Osborne's columbic oxide 

 contained 0-61 per cent tantalic oxide and his tantalic oxide 



