OF ARTS AND SCIENCES. 83 



and phospho-molybdates as well as of the corresponding arsenic com- 

 pounds. 



The general results to which the study of the phospho-molybdates 

 and arsenio-molybdates has led are as follows : — 



1 . The phospho-molybdates form a series of which the lowest term 

 contains five atoms of molybdic to one of phosphoric oxide, and the 

 highest twenty-four atoms of the former to one of the latter. 



2. As in the case of the phospho-tungstates, the greater number of 

 the molybdenum compounds contain an even number of atoms of tung- 

 stic oxide. The homologizing term is therefore 2 MoOg for these 

 cases. 



3. By far the greater number of phospho-molybdates contain three 

 atoms of fixed base (old style), or, in more modern language, may be 

 considered as derived from acids containing six atoms of hydroxyl. 

 Anhydrous compounds of this type occur, and are not always simply 

 residues obtained by heating salts which may be considered as acid, as 

 containing, for example, 3 E^O . 3 H^O. It seems therefore necessary 

 to admit the existence of acids of the general type 



n M0O3 • P2O5 • 3 H,0, 



which may, however, stand in the relation of pyro-acids to other acids 

 of the type 



n M0O3 . P2O5 . 6 HP . 



4. On the other hand, while no single phospho-molybdate containing 

 more than three atoms of fixed base for one of phosphoric oxide has 

 been obtained in a state of indubitable purity, it is probable that there 

 is at least one salt with six or more atoms of fixed base. I refer to 

 the silver salt which I have expressed by the formula 



22 M0O3 . Pp, . 7 Ag.O + 14 aq. 



5. Setting aside the evidence derived from the analogy of the phos- 

 pho-molybdates and phospho-tungstates, there is at present no sufficient 

 proof of the existence of a series of phospho-molybdates or arsenio- 

 molybdates containing more than three atoms of fixed base. Such 

 purely negative evidence must not be too highly regarded. 



6. As in the case of the phospho-tungstates, there exists a class of 

 phospho-molybdates in which the ratio of the number of atoms of base 

 to that of the number of atoms of phosphoric oxide is as 5 : 2, the num- 

 ber of atoms of molybdic oxide being even. 



Since the publication of my work on the phospho-tungstates and 



