250 Mr Silliman on the 



magnesia, together with a little persalt of iron, held up by the 

 alcohol. 



F. The mixture (E) being filtered, and the precipitate 

 washed quite clean with alcohol, the filtrate is evaporated 

 until all the alcohol is expelled, and then supersaturated with 

 ammonia ; a little trace of alumina and iron separates, which 

 may be added to that to be obtained from the other portion 

 (H). We may now either add an excess of pure chloride of 

 calcium to the filtrate (F), or a portion of perchloride of iron. 

 The object in either case is to separate the phosphoric acid 

 in combination with a base, from whose weight its quantity 

 may be directly estimated, which is an indispensable step, 

 since the fluorine, according to this plan of analysis, can be 

 estimated only by the amount required to saturate the excess 

 of bases. In case the chloride of calcium is employed, we 

 have all the phosphoric acid in the form of phosphate of lime, 

 mixed with a large quantity of sulphate of lime, derived from 

 the sulphuric acid and sulphate of potassa previously em- 

 ployed. This mixture of phosphate and sulphate of lime is 

 collected, washed, and redissolved in chlorohydric acid. The 

 sulphate of lime is separated by alcohol, and the phosphoric 

 acid remains in solution, which, after the excess of alcohol 

 has been expelled, may be thrown down by ammonia, ignited 

 and weighed, or, preferably, may be estimated by a magnesian 

 salt. If we employ the method by perchloride of iron, we 

 form in the acid solution containing the phosphoric acid, a 

 basic perphosphate of iron, on supersaturating the solution 

 with ammonia. This compound is mixed with a bulky mass 

 of peroxide of iron, which being thrown on a filter and 

 thoroughly washed, is subsequently decomposed completely 

 by hydrosulphuret of ammonia, into sulphuret of iron and 

 phosphate of ammonia. Care must be taken to use a suffi- 

 cient quantity of perchloride of iron, otherwise a white pre- 

 cipitate of neutral perphosphate of iron is formed, which is 

 soluble in an excess of ammonia. In either case (the em- 

 ployment of the chloride of calcium, or the perchloride of 

 iron) the phosphoric acid eliminated may be finally best esti- 

 mated by a magnesian salt and ammonia, as the ammonio- 

 phosphate of magnesia, from whose known constitution the 



