OF ARTS AND SCIENCES. 63 



I did not succeed in obtaining well-defined pyro-phospho-molybdates or 

 pyro-phospho-tungstates, though of course the residues of the ignition 

 of the acid salts may be regarded as such. When a phospho-molyb- 

 date is dissolved in ammonia-water and a current of sulphydric acid 

 gas is passed into the hot solution, sulpho-molybdates ai'e formed in 

 large quantity. This reaction distinguishes the phospho-molybdates 

 from the phospho-tungstates which are not decomposed U'uder the 

 same circumstances. 



Analytical Methods. — Tlie determination of the sum of the per- 

 centages of molybdic and phosphoric oxides was usually effected, as in 

 the case of the phospho-tungstates, by precipitating the two oxides 

 together by mercurous nitrate with addition of mercuric oxide to neu- 

 tralize the free nitric acid. It is best to precipitate from a boiling 

 solution, and to boil for a short time after adding mercuric oxide. 

 This last must always be in small excess. On account of the volatility 

 of molybdic teroxide, it is not possible to determine directly the sum 

 of the weights of the two oxides by simple ignition, but the difficulty 

 may be readily overcome by the following process. The filter with 

 the mercurous salts is to be cautiously heated in a platinum crucible 

 properly inclined to the vertical axis of the flame until the filter is 

 completely carbonized. On then regulating the heat and the supply 

 of air, the carbon may be readily burned off, leaving a mass of mer- 

 curous salts mixed with more or less mercuric oxide, no weighable 

 amount of molybdic teroxide being lost. An accurately weighed 

 quantity of anhydrous normal sodic tungstate in fine powder is then 

 to be added, and the contents of the crucible carefully mixed together 

 with a stout platinum wire previously weighed with the crucible 

 itself. The whole is to be heated at first by radiation from a small 

 iron dish, and afterward directly, until a clear white fused mass is 

 obtained. A second ignition and second weighing will determine 

 whether every trace of mercury has been expelled. It is almost 

 needless to remark, that all these operations must be conducted under 

 a flue with a good draught. This process gives excellent results, and 

 is much less tedious than would perhaps be supposed. 



After the estimation of the phosphoric oxide the molybdic teroxide 

 is best determined by difference from the sum of the weights of the 

 two oxides found as above. No really good general method for the 

 quantitative separation and estimation of molybdic oxide has yet been 

 given, at least no one which is sufficiently accurate to serve as a check 

 upon the method above described. The ammonium salts of this series 

 are most simply analyzed by igniting them directly with sodic tung- 



