78 PROCEEDINGS OF THE AMERICAN ACADEMY 



will be found at least substantially correct, and that the whole subject 

 will soon receive the attention of other chemists, who may have a 

 more abundant supply of vanadium salts at their disposal. 



ARSENOSO-TUNGSTATES. 



The alkaline salts belonging to this series are easily prepared by 

 boiling arsenous oxide with strong solutions of acid tungstates, as, for 

 example, with 12:5 or 10:4 sodic salt. The arsenous oxide should 

 be in excess to secure complete saturation. The filtered solution on 

 standing usually deposits crystals of arsenous oxide in greater or less 

 quantity. The salts of this series are either colorless or pale yellow. 

 They are not very easily oxidized in acid solutions, but in presence of 

 an excess of alkali they absorb oxygen from the air and are con- 

 verted into arsenio-tungstates. Bromine and chlorine readily oxidize 

 such solutions. The arsenoso-tungstates are, as a rule, more soluble 

 than arsenio-tungstates. They are very stable and well defined, and 

 may as a class be embraced undei*the general formula 



m W0„ . n As,,03 . p R.O, 



in which n is usually greater than unity. 



Analytical Methods. — Hydric sulphide precipitates arsenous sul- 

 phide from these salts, and usually without sensible reduction of the 

 tungstic teroxide. It is best to precipitate from a hot solution, and to 

 pass the current of the gas for a long time to insure complete decom- 

 position. The arsenous sulphide may then be collected and weighed 

 upon a Gooch asbestos filter. The tungstic oxide may be determined 

 in the filtrate by mercurous nitrate in the usual manner. The simplest 

 and most satisfactory method of determining the arsenic is by titrition 

 with iodine in a solution containing an excess of sodic or potassic 

 bicarbonate. Finally, it is possible to determine the arsenic by oxid- 

 izing the solution with bromine after adding an excess of an alkaline 

 carbonate, and then precipitating the arsenic in the form of ammonio= 

 magnesic arsenate. The precipitate must be redissolved in dilute 

 chlorhydric acid and precipitated a second time. 



The alkaline arsenoso-tungstates are, as a rule, easily soluble ; the 

 crystalline salts are often soft and gummy, though well defined. They 

 appear in some cases to be decomposed more or less completely by 

 solution and evaporation, veith separation of arsenous oxide and forma- 

 tion of new salts of different series. In certain cases the addition of a 

 salt of one of the heavy metals determines the precipitation of crystals 



