Prof. H. Rose on the Composition of Samarskite. 143 



posed in the ordinary mode by fusion with bisulphate of potash, 

 the acids of niobium and tantalum are, indeed, very well sepa- 

 rated by treatment of the fused mass with water; but if the 

 separated metallic acids are not examined with the greatest care, 

 errors may be fallen into ; for these acids may be contaminated 

 with many substances, the presence of which in them may often 

 be unsuspected. It is well known that they always contain no 

 inconsiderable quantities of peroxide of iron, which cannot be 

 separated from them by acids, but only by converting it into 

 sulphide of iron by sulphide of ammonium, and dissolving the 

 latter in very dilute hydrochloric acid, during which process 

 there is always danger of dissolving at the same time a small 

 quantity of the metallic acids, especially tantalic acid. As per- 

 oxide of iron, after fusion with bisulphate of potash, dissolves 

 completely, although slowly, in water, it is the tantalic acid and 

 the acids of niobium which, after they have lost their sulphuric 

 acid by ignition, expel the sulphuric acid from the sulphate of 

 iron, and combine to form salts, from which the peroxide of 

 iron cannot be extracted by dilute acids, but only by heating 

 with concentrated sulphuric acid. It is only strong bases, from 

 the compounds of which with sulphuric acid, tantalic acid and 

 the acids of niobium are incapable of expelling the sulphuric 

 acid, that can be perfectly separated from the above-mentioned 

 acids by fusion with bisulphate of potash. 



Other oxides, as well as peroxide of iron, may remain un- 

 dissolved during the treatment of the mineral fused with bisul- 

 phate of potash with water, and not only weakly basic oxides, 

 but also such as form compounds with sulphuric acid or with 

 sulphate of potash, which are insoluble or difficult of solution, 

 especially in the solution of sulphate of potash, or, if they are 

 soluble in sulphuric acid at ordinary temperatures, separate 

 from the solution when heated, or on the addition of a large 

 quantity of water. Of this kind are silicic acid, stannic acid, 

 zirconia, thorina, tungstic acid, titanic acid, and also the oxide 

 of cerium (and those of lanthanium and didymium). From 

 many of these oxides it is difficult to separate the acids of 

 niobium and tantalum by decomposition with bisulphate of 

 potash ; and if they are not particularly sought for, or their 

 presence is not suspected, they may readily escape detection. 

 Zirconia and thorina, especially, may either be entirely over- 

 looked, or their quantity incorrectly determined in the analysis 

 of minerals containing tantalum and niobium, if these are de- 

 composed by fusion with bisulphate of potash. Baryta, strontia, 

 and oxide of lead (the latter at least not in appreciable quan- 

 tities) have not hitherto been found in those minerals, but their 

 separation also would be attended with no small difficulty. 



