STOKES.] 
HIGHER METAPIIOfcPHIMIC ACIDS. 
147 
filtrate is concentrated to about one-half with addition of about 5 parts 
solid sodium acetate, whereby sodium triimidotetraphosphate separates, 
in the form of flat prisms or plates, which are washed with a little sat- 
urated sodium acetate solution and purified by dissolving in water and 
precipitating by alcohol, or by adding solid sodium acetate to their 
hot solution. On further concentrating the filtrate, large plates are 
often obtained, which give, with silver nitrate and nitric acid, a crys- 
talline precipitate of the characteristic silver diimidotriphosphate, 
I > 3 N208H 4 Ag3. No satisfactory and certain method can be given for 
the separation of these two acids. 
The sodium tetrametaphosphimate thus obtained usually has the form 
of spindles, but, when well developed, consists of brilliant double 
pyramids. It had not been observed at the time of writing the paper 
on tetrametaphosphimic acid. It is obviously an acid sodium salt, as 
it is formed only in the presence of acetic acid or a limited amount of 
a stronger acid. It was also obtained directly from the original sample 
of the acid, but was not analyzed, as it is difficult to obtain a sufficient 
quantity un contaminated by the free acid. It is moderately soluble 
in w T ater, but almost insoluble in solutions of sodium salts and is highly 
characteristic of this acid. The crude salt was dissolved in ammonia, 
with the addition of some ammonium nitrate, and precipitated by nitric 
acid as I^^OkH^NH^. The precipitation is almost quantitative. 
This pure salt was dissolved in ammonia, largely diluted, and, after 
acidifyiug with nitric acid, precipitated by an excess of silver nitrate 
in the iorm of the characteristic P^OgH^Ag^ This gave: 
P . 
N. 
Ag 
Calculated for 
P 4 N 4 8 H 4 Ag 4 . 
16.68 
7. 55 
58.03 
Found. 
16.88 
7.56 
57.51 
The free acid was obtained by decomposing the silver salt with 
lydrochloric acid in the characteristic form of difficultly soluble needles, 
t gave : 
Calculated for 
r 4 N 4 () 8 II 8 i 2H 2 0. 
Found. 
p 
35.22 
15.94 
35. 08 
15.75 
N 
A comparison of the acid potassium, sodium, and ammonium salts 
id neutral ammonium salt with those prepared from a sample of 
trametaphosphimic acid from tetraphosphonitrilic chloride showed 
