Mr. D. Forbes on Evansite, a new Mineral Species. 345 



little insoluble residue, and then precipitated by the addition of a 

 previously mixed solution of sulphate of magnesia, chloride of 

 ammonium, and ammonia in excess, allowed to stand twelve hours, 

 and the precipitated phosphate of ammonia and magnesia then 

 filtered off and determined as in the last case ; the pyrophosphate 

 of magnesia amounted to 4*605 grs., equivalent to 2*944 grs. 

 phosphoric acid, or 19*01 per cent. 



A third determination of the phosphoric acid was now made 

 upon 13*365 grs. dissolved in hydrochloric acid, some 50 

 grs. crystallized tartaric acid added, and then ammonia in 

 excess ; the solution remained clear, and was then precipitated by 

 a mixed solution of sulphate of magnesia, chloride of ammonium, 

 and liquid ammonia, and allowed to stand twelve hours. The 

 supernatant solution was now carefully decanted, and the pre- 

 cipitate redissolved in hydrochloric acid, a little tartaric acid 

 added, and then ammonia in excess : after standing twelve hours 

 the precipitated phosphate of ammonia and magnesia was col- 

 lected and determined as usual ; the pyrophosphate of magnesia 

 weighed 4*14 grs., equivalent to 2*63 grs. phosphoric acid, or 

 19*73 per cent, in the mineral. The mean of these three de- 

 terminations of phosphoric acid will consequently amount to 

 1905 per cent. 



Determination of the Alumina. 



22*22 grs. (the same as employed as before mentioned in deter- 

 mining the phosphoric acid by the molybdate-of-ammonia me- 

 thod) were here made use of, and the solution, after separating 

 the precipitate of phosphomolybdate of ammonia, was now sub- 

 jected to the action of a stream of sulphuretted hydrogen gas 

 until no more precipitate of sulphide of molybdenum fell ; it was 

 then filtered from this precipitate, and the solution, after boiling 

 to remove any excess of the gas, precipitated by ammonia, by 

 which the alumina present was thrown down, which, being 

 washed, dried, and incinerated, weighed 8*90 grains, or conse- 

 quently 40*05 per cent, in the mineral. 



Another determination of the alumina was made on the 

 quantity of mineral (15*38 grs.) used in determining the 

 phosphoric acid according to the tin method. The matter 

 insoluble in sulphide of ammonium was, as far as possible, 

 dissolved in nitrohydrochloric acid, this solution was added to 

 the nitric-acid solution obtained in the first instance after filter- 

 ing off the oxide and phosphate of tin, and the whole then pre- 

 cipitated by ammonia and the alumina collected. From its ap- 

 pearance, however, it was suspected that it might contain tin ; it 

 was redissolved in sulphuric acid and a stream of sulphuretted 

 hydrogen passed through the solution, when a considerable 



Phil. Mag. S. 4. Vol. 28. No. 190. Nov. 1864. 2 A 



