116 
CONTRIBUTIONS TO CHEMISTRY AND MINERALOGY. 
[bull. 167. 
fact that a solution of freshly precipitated magnesium pyrophosphate in 
excess of cold acetic acid is quantitatively precipitated on boiling, in a 
permanently insoluble form, while magnesium orthophosphate remains 
dissolved. The above weak acetic solution is mixed with magnesium 
acetate in excess, the precipitate dissolved by adding about one- fifth 
volume of strong acetic acid, and boiled a few minutes. The precipitated 
granular magnesium pyrophosphate is dissolved in nitric acid, in the 
manner described for the separation of the decomposition products of 
trimetaphosphimic acid, converted into silver salt and then into sodium 
salt. This, after several precipitations by alcohol, is recrystallized from 
water. A careful determination of the yield of pyrophosphoric acid in two 
experiments gave 13.5 and 16 per cent of the theoretical. For analysis 
the sodium salt was converted into silver pyrophosphate, which gave: 
P . 
Ag 
Calculated for 
Ag 4 P 2 7 . 
Found. 
10.24 
71. 27 
10.35 
70. 74 
Decomposition of Sodium Trimetaphosphimate by Acetic Acid. 
As before pointed out, this salt is not decomposed by acetic acid in 
the cold, or on short heating. If, however, its solution is heated for 
two or three hours with 3D per cent acetic acid and magnesium acetate, 
under the conditions mentioned under the magnesium salt, a granular 
precipitate is obtained, consisting of a mixture or double salt of magne- 
sium pyrophosphate and imidodiphosphate, which does not perceptibly 
redissolve on cooling, while the liquid contains much orthophosphoric 
acid. The sodium pyrophosphate, separated from this by the above 
method, showed the characteristic form. For analysis it was converted 
into the silver salt, and gave: 
p .. 
Ag 
Calculated for 
Ag 4 P,0 7 . 
10.24 
71.27 
Found. 
10.43 
70. 33 
3. ON TETRAMETAPHOSPHIMIC ACID. 
Trimetaphosphimic acid and its decomposition products have been 
described in the preceding pages. Some account will now be given of the 
next higher member of the series, tetrametaphosphimic acid, P 4 N 4 8 HJ 
which results from the decomposition by water of tetraphosphonitrilic 
chloride, P 4 N 4 01 8 . The molecular weight of the chloride having been 
established with certainty, that of the acid follows directly. Although 
the free acid crystallizes with 2 molecules of water, the silver salts and 
