stokes.] TBIMETAPHOSPHIMIC ACID. 95 
conditions, and should be found present to the extent of at least 00 x>er 
cent instead of only 15 per cent. On this ground, it seems to me, we 
must reject the amidopyrophosphoric formula, and regard the body as 
imidodiphosphoric acid, 
NH< 
PO(OH) 
PO(OH)/ 
This formula does not account for the pyrophosphoric acid. We may, 
perhaps, conceive that owing to the greater affinity of phosphorus for 
oxygen than for nitrogen, a group, PO.(OH) 2 , is able to change places 
with a hydroxylic hydrogen atom: 
HO.PO.O— H HO.PO.O— PO(OH) 2 
HN— PO(OH) 2 HN— H 
the latter, in common with other amido phosphoric acids, being unstable, 
and passing easily into pyrophosphoric acid. It may be noted that this 
transformation seems to be promoted by heat, apparently more pyro- 
phosphoric acid being formed by decomposing trimetaphosphimic acid 
in hot than in cold solution. 
The second acid of the series, diimidotriphosphoric acid, seems really 
to have the constitution, 
PO(OH) 2 .NH.PO(OH).NH.PO(OH) 2 , 
implied by the name. It forms an acid salt with 3, and a neutral 
suit with 5 atoms of silver, both of which are colorless. If it were a 
diamide of triphosphoric acid, 
^>PO.O.PO(OH).O.PO<^§ 2 , 
the penta-silver salt would have 2 atoms united to nitrogen, and 
such a salt, to judge from all phosphorus-nitrogen salts, where this is 
known to be the case, would be distinctly colored. 
Still another intermediate acid is possible, preceding imidodiphos- 
phoric acid, viz, amidoimidodiphosphoric acid, 
^>PO.NH.PO(OH) 2 . 
I have been unable to detect this acid thus far, but the too high per- 
centage of nitrogen invariably found in imperfectly purified imidodi 
phosphoric acid, may be due to the presence of small amounts of this 
body. 
In giving formulas to these bodies, I wish to be understood as using 
them with full appreciation of their uncertainty. A much more 
exhaustive study is necessary before they can be regarded as estab- 
lished as firmly as those of many organic compounds. There are several 
directions in which such a study might be of value. Besides the ques- 
tion of tautomerism, the stereochemistry of nitrogen might be attacked 
with profit from this side. The analogy of nitrogen and phosphorus 
suggests the possibility of steric phenomena being shown by the latter 
also. The formula (b) suggested for trimetaphosphimic acid, for 
example, may imply a cis- and cis-trans-form depending on the posi- 
