OF ARTS AND SCIENCES. 19 



W0 2 = W0 2 



I I 



Na - - W0 2 — WO, - O - Na 



Na - O - W0 2 — W0 2 - O - Na 



O 



1 I 



Na - O - W0 2 — W0 2 - O - Na 



I " I 



o 



1 I 



Na - O - W0 2 — W0 2 - O - Na 



I ! 



o o 



Na - O - W0 2 — WO,, - O - Na 

 i i 



o 



1 I 



Na - O - W0 2 = W0 2 - O - Na 



It will be seen that, with this view of the subject, those terms in the 

 series in which the number of atoms of sodic oxide is even are repre- 

 sented by formulas in which the free atoms of W0 2 are united, in 

 part directly, and in part by oxygen, while the union is direct when 

 the number of atoms of sodic oxide is odd. I shall return to this 

 subject in speaking of the phosphotungstates and other complex 

 inorganic acids. No great value can, in the present state of our 

 knowledge, be attributed to formulas like the above. They afford, 

 however, some assistance in showing the possible mode of formation 

 of the different terms of the series, but various other constructions 

 may be devised which are perhaps equally probable. In adopting 

 provisionally the particular construction which I have used, I have 

 simply followed the clew given by the commonly received formula for 

 potassic dichromate, 



Cr0 2 - O - K 



O 



Cr0 2 - O - K, 



which of course gives a similar expression for the homologizing term 

 in the acid tungstate series. So far as I am aware, no attempt 



