IroKEs.] TETRAMETAPHOSPHTMTC ACID. 117 
the acid potassium and ammonium salts arc anhydrous and conform to 
the above formula. 
Notwithstanding the proximity of these two acids in the scries, and 
the general similarity of the chloronitrid.es from which they are derived, 
they differ exceedingly, trimetaphosphimic acid being extremely solu- 
ble, uncrystallizable, undergoing rapid spontaneous decomposition, and 
yielding but two series of salts ; while tetrametaphosphimic acid is very 
difficultly soluble, highly crystalline, permanent when dry, offers 
marked resistance to the action of acids, and forms three series of salts, 
P.NAII.M^, P 4 N 4 8 H 4 M' 4 , and P 4 N 4 8 M' 8 . Those metals which form 
insoluble salts are' precipitated from solutions of their chlorides or 
nitrates even by the free acid, but a very slight excess of either being 
required to make the precipitation complete. As is the case with tri- 
metaphosphimic acid, the 8-atom silver salt exists in a white and a 
yellow modification, the former, however, being very unstable ; these 
may be regarded as corresponding to the tautomeric forms (PN(OH) 2 ), 
and (PO.NH.OH)^. Under the action of stronger acids it is ultimately 
converted into orthophosphoric acid and ammonia, but its great stabil- 
ity, and the instability of the intermediate products, have rendered the 
isolation of the latter impossible. 
Experimental data for a discussion of the constitution of the acid 
are therefore almost wholly wanting. Presumably the phosphorus 
atoms in the nucleus P 4 N 4 are united by nitrogen atoms, but whether 
bhey constitute a ring of 8 atoms can not be decided definitely in the 
absence of intermediate decomposition products. It is quite likely that 
3he reason of the greater stability of the acid may be found in steric 
considerations, and in this connection it is interesting to observe that 
while P 4 N 4 Q 8 H 8 is much more stable than P 3 N 3 O fi H 6 , the reverse is true 
)f the chloronitrides, for while P 4 N 4 C1 8 also resists the action of boiling 
vater and can be distilled with steam, it is perceptibly easier to sapon- 
ify than P 3 N 3 C1 6 . If we assume the formulas 
PCI* PC1 2 = N 
/\ / \ 
N N N PCI 
I! I and || || 
C1>P PCI, C1P N 
\S \ / 
N N = PC1, 
3r the chloronitrides, and 
PO.OH HO.PO— NH 
/\ ' / \ 
HN NH and HN PO.OH 
II II 
HO.PO PO.OH HO.PO NH 
\/ / 
NH NH — PO.OH 
>r the acids, it is obvious that the nitrile configuration is more 
:able in a ring of 6 than in one of 8 atoms, while for the group — Nil 
-PO(OH) — NH — the reverse is true. 
