of Phosphoric Acid. 329 



metaphosphate of silver appeared to have been formed, in ad- 

 mixture with a Uttle undecomposed f-phosphoric acid. 



Some uncertainty still continues regarding the composition 

 of fused phosphoric acid. A very long time since I made 

 several experiments upon this point*, and found that the acid, 

 fused for a considerable time over a spirit-lamp, in three ex- 

 periments, contained a slightly less amount of water than is 

 required by the compound P^O^+HO. In another experi- 

 ment, probably with some acid which had been heated more 

 strongly, and for a longer time, the amount of water was still 

 less, and nearly corresponded to the compound 3P^O^ + 5 

 ^p2Q5_j_ piO) : hence it is thus rendered probable that phos- 

 phoric acid vk^ould be obtained in a perfectly anhydrous statf 

 by a very long and continuous application of heat. 

 ' My experiments have been recently confirmed by M. 

 Weber, who examined an acid which had been exposed for a 

 considerable time to a temperature at which it began to be 

 slightly volatilized; 3*127 grms. of this acid, when treated 

 with 16*891 grms. of oxide of lead, left, after having been 

 heated to redness, a residue of 19*700 grms. The per-centage 

 composition of the acid was therefore — 



Phosphoric acid .... 89'84< 



Water 10*16 



100*00 

 In this case also the quantity of water is slightly less than 

 the composition P'^0^ + HO requires. The quantities of oxygen 

 are in the proportion of 50*34; : 9*03. 



In addition to these three submodifications of metaphos- 

 phoric acid, there are undoubtedly others. That acid which 

 is formed on burning phosphorus in dry atmospheric air or 

 oxygen gas, may be considered as a fourth submodification, for, 

 as has been stated above, the reactions of its solution are dif- 

 ferent from those of the other modifications. The salts which 

 it forms with bases have not been prepared and examined. I 

 shall merely remark here, that anhydrous phosphoric acid does 

 not exhibit any affinity towards dry ammoniacal gas, nor does 

 it absorb it; hence it differs in this respect from anhydrous sul- 

 phuric acid. Probably the various submodifications of me- 

 taphosphoric acid should be considered as conjugate acids, 

 as the difference in their reactions would then be more satis- 

 factorily explained. Anhydrous phosphoric acid may consti- 

 tute the conjunct, which is capable of combining in different 

 proportions with pyrophosphoric acid or with c-phosphoric 

 * PoggendorfF's Annden, vol. viii. p. 203. 



