Determining Columhium and Tantalum. 395 



to 0*22 per cent. This result includes several determinations 

 in which the amount of material used was under one ^ram, 

 which increased the percentage error considerably. "When 

 two grams or more of material were used, duplicates seldom 

 disagreed by more than 0"15 per cent. To illustrate : the den- 

 sities found for a mixture containing 90 percent tantalic oxide 

 were 8-103 and 8"078 when 1-5 grams were used. Upon repeat- 

 ing the experiment with 3'3 grams of material, the densities 

 found were 8-090 and 8-092. 



We next determined that the mixed oxides became constant 

 in density after heating for an hour over the blast lamp. For 

 this purpose a sample of the mixed oxides from a Branchville 

 columbite was ignited over the blast lamp for an hoiir in a 

 platinum crucible. The density was determined and the mate- 

 rial then ignited for live-minute periods, a density determi- 

 nation being made after each ignition. The results were; 

 4-908, 4-908, 4-919, 4-921, 4-923, 4-912. The material was then 

 heated an hour logger and the density redetermined, the results 

 being 4-924 and 4-923. In these and the following determi- 

 nations the ignition was accomplished in 30 gm. platinum cruci- 

 bles over a fairly powerful blast lamp. The question whether 

 different preparations of the oxides in the same proportions 

 had the same specific gravity could only be answered by mak- 

 ing a series of such determinations. Complete duplicate series 

 of mixtures were not made on account of lack of material, but 

 the results given in the. table, which appears below, show what 

 agreement was obtained in each case. The average difference 

 in density between different preparations of oxides having the 

 same composition was 1-1 per cent. This appears to be the 

 greatest source of error in the determinations, and shows that 

 tlie method of preparing the oxides must be fixed as definitely 

 as possible. 



The detailed method of preparing the mixed oxides in con- 

 dition for specific gravity determination was as follows : About 

 three grams of the oxides were fused in a platinum dish with 

 six times their weight of acid potassium fluoride till the mass 

 was jnst liquid. The fusion was dissolved in 200'^'= of hot water 

 in a platinum dish, adding a little hydrofluoric acid to obtain 

 a clear solution. The solution was made alkaline with ammo- 

 nia, and after allowing the pi-ecipitate to settle, filtered on a 

 rubber funnel and washed well with water containing ammo- 

 nia. The precipitate runs through the paper if pure water is 

 used. The precipitate was redissolved in the platinum dish, 

 using as little dilute hydrofluoric acid and water as possible 

 and evaporated to dryness. Ten cubic centimeters of concen- 

 trated sulphuric acid were added in such a way that the resi- 

 due was completely moistened. The liquid was evaporated 



