80 PROCEEDINGS OF THE AMERICAN ACADEMY 



7 MoOs As.O^ . 3 H^O + 11 aq, 



and the three corresponding salts : — 



7 M0O3 . AsA • (NHJP . 2 Hp + 4 H^O 

 7 M0O3 . AS2O5 . 3 BaO 

 7 M0O3 . As^O. . 3 Ag.O. 



Debray obtained the acids and one or two salts of two different 

 series, which may be represented respectively by the formulas: — 



20 M0O3 . As,0- . 3 H,0 + 24 aq 



20 M0O3 . AS2O5 . 3 K^O 



20 M0O3 . AsA • 3 (NH,),0 

 6 M0O3 • A^Ps . 3 up -j- 13 aq 

 6 M0O3 . AsPs . 4 (NHJP 4- aq 

 6 M0O3 . AS2O5 . (NH,),0"+ 4 aq 

 6 M0O3 • AsA . Na,0 + 1 2 aq. 



Debray considers the formula of the 20-atom ammonium salt as 

 probable only, and regards the water determination in the corre- 

 sponding acid as not quite certain. Neither Seyberth nor Debray has 

 described the analytical methods employed, or given the details of 

 the analyses. 



I have found it most advantageous to separate arsenic from mo- 

 lybdic oxide by precipitating with magnesia mixture, redissolving the 

 amraonio-magnesic arsenate, and precipitating a second time with 

 ammonia after adding a little magnesia mixture. The ammonio-mag- 

 nesic arsenate may be digested with ammonic sulphide without de- 

 composition ; but after the second precipitation it does not retain 

 molybdic oxide, and the subsequent treatment is therefore unneces- 

 sary. To determine the sum of the molybdic and arsenic oxides 

 I precipitate the two together with mercurous nitrate and mercuric 

 oxide, in the manner already described for the estimation of molybdic 

 and phosphoric oxides, filter upon paper, and after drying roll U]) the 

 filter and its contents and ignite cautiously in a porcelain crucible. 

 By slow and careful heating the filter may be completely burned with- 

 out loss of molybdic or arsenic oxides, this result being attained by 

 the oxygen of the mercurous and mercuric oxides present. A 

 weighed quantity of sodic tungstate is then to be added in fine powder, 

 the mass well mixed in the crucible, and then cautiously heated until 

 mercury is completely expelled, and after cooling a white fused mass 

 remains. A second or even a third heating is necessary to insure a 



