OF ARTS AND SCIENCES. 75 



The arrangement here given makes it probable that at least one 

 series i-emains to be discovered. Fleitmann and Henneberg have 

 described a phosphate having the formula 6 Na20 . 5 P2O5. Such a 

 compound would belong to a class of phosphates represented by the 

 general formula — wanting among the vanadates — 



n R2O .271 — 2. P2O.. 



Roscoe long since established the correspondence of the vanadates 

 and phosphates up to a certain limit. Further investigation is required 

 to show how complete the correspondence is. From an examination 

 of the terms in the seven series of vanadates given above, it will appear 

 that various isomeric modifications are probable among vanadates, 

 and that these correspond, in form at least, to the various meta- 

 phosphates, — mono, di, tri, tetra, and hexa-metaphosphates of Fleit- 

 mann and Henneberg. These chemists have also described a class of 

 phosphates corresponding to the first term of the fourth series of 

 vanadates given above. The sodium salt has the formula 



3 Na20.2 PA+ 18 aq. 



In the imperfect state of our knowledge, all such coincidences are 

 worthy of attention. Another conclusion may perhaps be safely 

 drawn from the systematic arrangement of the vanadates which I have 

 given. This is that meta, pyro, and ortho-vanadates, and the similar 

 compounds of phosphorus, arsenic, antimony, etc., are not isolated, but 

 are terms of more complete series. Thus the fifth series embraces 

 the pyro-vanadates and their homologues, while the ortho-vanadates 

 form the first term of the seventh series. In all the series given 

 except the third, the common difference or homologizing term is 

 R2O . 2 VA ; in the third series, it is 2 R„0 . 2 VoO^. In some 

 cases only particular terms may exist to represent an entire series, 

 because the special conditions necessary for the stability of other 

 terms may be absent as regards degree. Taking now into account 



