OP ARTS AND SCIENCES. 73 



It will be noticed that in the experiments with phosphates and 

 phosphomolybdates a large excess of the magnetic chloride mixture 

 was not markedly prejudicial to the accuracy of results when pre- 

 cautions were taken to induce a slow formation of the precipitate, 

 while in the experiments with phosphotungstates the reverse was true. 

 The reason for this exceptionally unfavorable action in the case of 

 the phosphotungstates is probably Indicated in the facts that the 

 addition of free ammonia beyond a certain amount to the solution of 

 a phosphotungstate produces turbidity, and that the addition of mag- 

 nesia mixture to a clear solution of normal sodic tungstate, amnionic 

 chloride and ammonia renders the solution opalescent and occasions 

 the deposition, after some hours, of an almost invisible precipitate ; 

 but it is to be noted in this connection that the action of magnesia 

 mixture upon a clear ammoniacal solution of amnionic molybdate and 

 ammonic chloride is similar to its action upon the tungstate solution. 



Summary. 



It would appear from the preceding account that in determining the 

 phosphoric oxide of alkaline phosphates, free from sulphates or 

 other substances likely to contaminate a precipitate, accuracy is most 

 conveniently and surely attained by adding to the somewhat dilute 

 solution of the phosphate ammonia in slight but quite distinct excess, 

 then an excess of magnesic chloride mixture containing no free 

 ammonia (made by dissolving three parts of crystallized magnesic 

 chloride and eight parts of ammonic chloride in water, adding water 

 containing ammonia until the volume of the solution reaches forty- 

 eight parts, filtering and boiling off the free ammonia), and, after the 

 precipitate has settled, ammonia until the liquid evolves a strong odor 

 of it. In determining the p>hosphoric oxide of phosphotungstates the 

 best results are to be got by adding to the solution containing free 

 ammonia an excess of either of the magnesia mixtures, washing the 

 precipitate with the precipitant, dissolving in hydrochloric acid, di- 

 luting if necessary and reprecipitating with a little dilute ammonia, 

 adding strong ammonia after the precipitate has settled ; or, when 

 working with small amounts, by proceeding as in the case of the phos- 

 phates, taking special care, however, that the solution is distinctly 

 ammoniacal before precipitating, and that no great excess of the 

 precipitant is used. In the determination of the phosphoric oxide of 

 phosphomolybdates, the method recommended for use in the case of 

 the phosphates serves very well when the amount of phosphoric oxide 



