OP ARTS AND SCIENCES. 85 



we are not justified in holding that the phospho-tungstates, phospho- 

 molybdates, and corresponding arsenic compounds, have a basicity of 

 which the higher limit is six. I may here mention that I shall here- 

 after describe a vanadio-molybdate of ammonium the analyses of which 

 agree well with the formula 



18 M0O3 . VA . 8 (NHJP + 15 aq. 



The risk of drawing hasty conclusions from purely negative evi- 

 dence is particularly great in discussing the degree of basicity of this 

 whole class of compounds, but 1 shall endeavor to show that it is possi- 

 ble to devise structural formulas which will embrace and explain all 

 degrees of basicity which appear to be possible under the general con- 

 ditions of the problem. 



We may, as in the case of the alkaline tungstates already discussed, 

 assume that both tungsten and molybdenum are hexatomic, and, as in 

 that case, we may start from the commonly received formula for po- 

 tassic dichromate, 



CrO^ - - K 



I 





 I 



CrOa - - K 



which may be equally well applied to hexatomic tungsten, 





 II 

 0=W-0-K 



I 



o 



I 

 = AY - O - K 

 II 

 



If we further suppose that the separate terms of the structural 

 formulas are symmetrically arranged, and take a 6 : 1 phospho-tung- 

 state 



6W03.PA-6H,0 or 6WO,.PA.(HO)j2 



as an illustration, we may, with at least a certain degree of proba- 

 bility, express the structure as follows : — 



