On Trihasic Phosphoric Acid. 1 85 



become very great. After long-continued boiling, the fluid must 

 be allowed to settle and become cold. It is then separated 

 from the residue by filtration and washing. After dissolving 

 the residue in hydrochloric acid, it, as well as the solution, is 

 freed from silica in the ordinary way. When silica is contained 

 in the fluid in large quantities, it may be preferable to separate 

 it by a current of IIS in an ammoniacal solution. The process 

 of evaporation, in cases like these, has always to be repeated 

 at least twice, in order to get rid of all traces of silica ; the iron 

 precipitate yielded on a second evaporation 1.9 p. c. SiO„ on a 

 third evaporation, 1.6 p. c. Si0 2 , and would.no doubt, have 

 yielded some Si0 2 a fourth time. The solution containing 

 phosphoric acid was then evaporated and precipitated with 

 magnesia mixture. The loss of phosphoric acid here was 

 owing to the formation of the fS-modificatioD, which was 

 disregarded, and therefore lost. The results are as follows : 

 (a) 1.2399 (b) 1.4159 



P0 5 32.19 32.32 



Fe 2 3 33.70 33.97 



HO 33.81 33.71 



The 1 p. c. which is in excess of the amount of water 

 actually contained in the substance, belongs to PO,, ami would 

 have been precipitated here under precautions. Taking tin.-, 

 case into account, we have the mean 



P0 5 =36.40 

 Fe 2 0,=33.83 

 IK) =29.77 



IV. Method with MI, S x . 



The Bubstance was dissolved in hydrochloric acid, then am- 

 monia and poly-sulphide of ammonium a«l<i«-<l. and the whole 

 digested for about a <lay until the supernatant liqnid was per- 

 fectly clear ami of a yellow color. 



After filtering and washing out with water containing some 



