stokes] TRIMETAPHOSPHIMIC ACID. 91 
barium salt were, as he admits, not satisfactory, while the analyses 
of the silver salt were made with a crude preparation, for the purity 
of which there is no guaranty. I have myself found that barium trimeta- 
phosphimate varies considerably in composition, contains water, and is 
hygroscopic. Sodium trimetaphosphiinate is the only well-defined 
substance I have been able to obtain by decomposing the chloronitride 
by alcoholic soda, and the salt is so characteristic as hardly to be mis- 
taken. It seems quite possible, therefore, that Gladstone actually had 
crude trimetaphosphimic acid in hand and failed to recognize its true 
nature only because of the unfortunate selection of salts which are 
amorphous, hydrated, and difficult to obtain pure. 
In a paper of much later date Mente 1 described several acids 
obtained by the successive action of ammonium carbamate and water 
on phosphorus oxychloride. Among them is one to which he gave the 
PO.OH 
name diimidodiphosphoric acid and the formula NH < > NH, and which 
PO.OH 
he regards as identical with Gladstone's pyrophosphodiamic acid. 
If Mente's formula is correct, it is the second or dimetaphosphimic 
acid. I have been unable to repeat Mente's work, and his data are too 
meager to admit of a positive conclusion, either as to its molecular 
weight or even its empirical composition, but it appears not to be 
identical with trimetaphosphimic acid. 
Constitution of trimetaphosphimic acid. — Although trimetaphosphimic 
acid is an uncrystallizable and unstable body, it forms stable salts, 
several of which crystallize in characteristic forms. Three atoms of 
hydrogen are replaceable by alkali metals, while silver is able to replace 
either 3 or 6. The following are the most noteworthy : 
P:iN 3 06H3Na3+4HaO — Rhombic prisms. 
P:iN30 6 H3Na3+H.iO— Slender prisms. 
P3N 3 O t; H3(NH 4 )3+H 2 0— Scales. 
Pa^CHiNaBa+UHjO— Rhombohedra. 
P3N30 l ;H i Ag3 — Monoclinic prisms. 
PNiOtjAgg — Two forms, white and red. 
The first, third, and fifth serve to identify the acid. The tertiary 
silver salt, being anhydrous and easily obtained pure, establishes its 
empirical formula. Those salts which contain water of crystallization 
do not lose it completely at any temperature short of decomposition, 
leaving open the question whether the acid may not have the formula 
P 3 N 3 7 H 8 . 
The constitution of trimetaphosphimic acid depends on that of the 
chloronitride P 3 N 3 C1«. The structural formula of the latter has not yet 
been definitely established, but the following data are available. It is 
reasonably certain that in the chloronitride phosphorus atoms aw 
united by nitrogen atoms. Its formation from phosphorus pentachlo 
» Ann. Chein. (Liebig), Vol. CCXLVII, 1888, pp. 239, 244. 
