stokes.J HIGHER METAPHOSPIILMIC ACIDS. 139 
by the mean angle of least tension, 135°, and this configuration in the 
case of the molecule of amido-triimidotetraphosphorip acid will be either 
a zigzag line or the form represented by 
POOH NH 
NH 
/ 
\ 
p0 \i ! 
iOH: 
y 
POOH 
\ 
/ 
Nil JH 1 
NH POOH 
a configuration which admits of the formation of the anhydride or lac- 
tam, which is nothing else than tetrametaphosphimicacid. A theoret- 
ical yield of this can not be expected, for when a chain, P 5 N 5 , for 
example, is further broken up, the disruption may occur at any one of 
nine points, but only when one of the resulting products contains P 4 N 4 
can tetrametaphosphimic acid be formed. A simple calculation shows 
that at most only 27 per cent of that required by the equation 
Pe^OioHio + 2H 2 = P 4 N 4 8 H 8 + NH 4 H 2 PO^ 
can be expected. A chain P G N G , however, offers more chances for the 
formation of fragments containing P 4 N 4 , and hence, as actually found, 
the yield of tetrametaphosphimic acid is greater, instead of less, as would 
be the case if the nucleus of this acid existed as such in hexameta- 
phosphimic acid. 
The first two chloronitrides, PNC1 2 and P 2 N 2 C1 4 , have not been 
obtained, and of the corresponding mono- and dimetaphosphimic acids 
the former is unknown and the existence of the latter questionable. 
Mente 1 has described salts of an acid to which he gives the formula of 
dimetaphosphimic acid, P 2 N 2 4 H 4 , but no proof is given that it is not 
some other polymer of PN0 2 H 2 , and the method by which he obtained 
it is one which is not likely to give a body of this molecular weight. 
The dimetaphosphimic ring would have a mean angle of 90°, which 
differs from that of tetrametaphosphimic acid by 45°. Such a ring 
should be much less stable than even that of heptametaphosphimic 
acid, and it is therefore quite possible that it is incapable of existence 
as such, and can exist only as the hydrate P 2 N 2 5 H G . This is perhaps 
the acid described by Gladstone 2 as pyrophosphodiamic acid. 
The chloronitride series presents a maximum of stability in triphos- 
phonitrilic chloride, but this is by no means as marked as that existing 
in the acid series. That such a maximum should occur in one case in 
a ring of G sides, and in the other in an 8-sided ring, and that the 
series should be limited in one case and unlimited in the other, involves 
no contradiction. Triphosphonitrilic chloride is stable only in the 
sense that it is formed in the largest amount and that its chlorine 
> Ann. Chem. (Liebig), Vol. CCXLVIII, p. 244. 
2 Quart. Jour. Chem. Soc. London, Vol. Ill, pp. 135, 354; Ann. (.'hem. (Liebig), Vol. LXXVI, 
p. 79; Vol. LXXVII, p. 315; Jour. Chem. Soc London [2J, Vol. II, p. 231. 
