OP ARTS AND SCIENCES. 135 



by mercurous nitrate, mercuric oxide being employed to secure per- 

 fect neutrality. The mercurous salt was then separated upon an 

 asbestos filter, and, after drying, ignited — finally with the blast-lamp — 

 until a constant weight was obtained. In this manner very nearly the 

 whole of the arsenic oxide was volatilized. There is no danger of a 

 reduction to metallic arsenic if the crucible containing the asbestos and 

 precipitate is placed within another, covered, and then cautiously heated. 

 The arsenic oxide is best determined as ammonio-magnesian arsenate, 

 using a large excess of magnesia-mixture in the first precipitation. 

 Two precipitations are necessary to secure a perfect separation ; the 

 salt is to be collected on an asbestos filter, and dried in the usual man- 

 ner. In determining water, or water and ammonia, in these salts, it is 

 best to ignite with a weighed quantity of fused sodic tungstate, as sug- 

 gested to me by Dr. Gooch ; only it must be observed that the fused 

 tungstate is rather deliquescent. With all these precautions fairly 

 good results may be obtained. I endeavored to separate arsenic from 

 tungstic oxide by boiling the salt with dilute phosphoric acid, reducing 

 the arsenic to arsenous acid by sulphurous acid, and then precipi- 

 tating by sulphydric acid as As 2 S 8 . This method appears to give a 

 complete separation, but is very tedious and circumstantial. 



Acid 3-atom Potassic Arsenio-tungstate. — When 12:5 acid potassic 

 tungstate is dissolved, and a solution of potassic arsenate, As0 4 KII 2 , 

 is added, a white very fine-grained precipitate is formed. When an 

 excess of the arsenate is employed, and the mixed solutions are evap- 

 orated upon a water-bath, a perfectly white insoluble salt is separated, 

 which is the acid arsenio-tungstate 



6 W0 3 . As,0 5 . 3 K,0 . 3 H 2 0. 



The formation of this salt may be represented by the equation 

 12 W0 3 . 5 K 2 + As 2 0. . K 2 . 2 H 2 -f 4 H 2 = 

 2 {6 W0 3 . As 2 0. . 3 K 2 . 3 H 2 0} . 



For analysis the salt was washed upon a filter with hot water, then 

 dried upon paper, and afterward — as the mass remained pasty — 

 upon a water-bath, where it finally dried to a hard white mass. Of 

 this salt, — 



0.8276 gr. gave 0.5902 gr. W0 3 = 71.31% 



0.7197 gr. " 0.5130 gr. " =71.28% 



1.9815 gr. " 0.3841 gr. As 2 K Mg 2 (MII 4 ) 2 + H 2 0== 11.73% As 2 6 

 1.3507 gr. lost on ignition 0.0357 gr. water = 2.04% 



