OF ARTS AND SCIENCES. 107 



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The solution of this salt gives with manganous sulphate after a 

 time a fiue cauary-yellow crystalline precipitate. The ammonium 

 salt appears to be decomposed by evaporation to dryness, or perhaps 

 by the act of solution. The dry residue, on addition of water, leaves 

 a white insoluble crystalline substance. 



The existence of classes of vanadio-phospho-tungstates and vanadio- 

 phospho-raolybdates has already been pioved. These may be embraced 

 under the general ibrmulas 



m V„0, . n PoO, . p WO3, m' V.O^ . n' P.^O^ . p' M0O3. 



I believe that I am fully justified, though from qualitative experi- 

 ments only, in maintaining that the formula 



m R.O, . n R',,0- . p ^".P^ 



is general, and that RoOj ^^^^ ^'f^o "^^^7 I'epresent any two pentoxides, 

 at least of the series PoO^ . VjOg . As^O. and SbaO^. 



PYROPHOSPHO-TUNGSTATES. 



The phospho-tungstates and pliospho-molybdates which have hitherto 

 been described must be regarded as containing pliosphoric oxide in 

 the modification in which it exists in the class of tribasic or ortho 

 phosphates. This appears to be clearly shown, first, by the synthesis 

 of these compounds from orthophosphoric acid and ortliophosphates 

 and the acid tungstates and molybdates ; and, secondly, from the fact 

 that in all cases, after the addition of an excess of ammonia, mao^nesia- 

 mixture at once throws down the characteristic ammonio-magnesic 

 orthophosphate. I shall now show that the other modifications of 

 phosphoric oxide or acid enter as such into similar compounds, so 

 that we have perfectly well defined groups of pyrophospho-tungstates, 

 metaphospho-tungstates, etc., and corresponding compounds containing 

 molybdenum; and that, in the compounds so formed, the various modi- 

 fications of phosphoric oxide preserve as it were their individuality. 

 I shall further render it at least probable that there are groups — of 

 phospho-tungstates for instance — in whicli two different modifications 

 of phosphoric oxide may exist, each preserving its own characteristic 

 properties. I do not now assert tliat in each of the complex groups 

 which I have described orthophosphoric acid may be replaced by 

 some other modification. That remains for experimental investigation. 

 It is at least clear that the field already opened for study is one of 



