OF ARTS AND SCIENCES. 85 



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



molybdates, and corresponding arsenic compounds, have :i 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 MoO a . y.J), . 8 (NH 4 ) 2 + 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 I 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. a- in 

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

 tassic dichromate, 



Cr0 2 - O - K 



O 



Cr0 2 - O - K 

 which may be equally well applied to hexatomic tungsten, 



O 



II 

 O = W - O - K 







O = 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 



6 W0 8 . P g 6 . 6 H 2 or 6 W0 2 . P 2 6 . (HO) 12 



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

 bility, express the structure as follows: — 



