30 PROFESSOR GREGORY ON AN ANOMALOUS 
36.67 per cent. of magnesia. In three analyses, the per-centage of magnesia in 
the new salt was found to be 16.78, 16.92, and 15.94. In the last of these I 
pushed the washing further than in the first two, and I did the same in three 
more analyses, which yielded 15.12, 15.34, and 15.12, per cent. of magnesia. The 
mean of the six analyses, all made with portions of salt prepared at different 
times, is 15.87 per cent. of magnesia. As the salt lost no weight by ignition, it 
contained no water, and therefore was composed of magnesia and phosphoric 
acid alone: the absence of lime being previously ascertained. Its composition is 
therefore, 
Magnesia, : : , : 15.87 
Phosphoric acid, : ; ; : , 84.13 
The only formula which at all approaches to this composition is 3 P, 0, + 2 Mg0, 
according to which, it is an acid sesquiphosphate of magnesia. The composition, 
calculated according to this formula, is, 
Magnesia, : : : : 16.18 
Phosphoric acid, : : ; 4 83.82 
Considering the imperfection of the means for determining the amount of 
magnesia in analyses with precision, there can, I think, be no doubt, that the 
above formula expresses, empirically, the composition of the salt in question. 
But what is the rational formula of this salt? As far as I know, there are 
no known sesquiphosphates of protoxides. Indeed, the very characteristic of 
the three known classes of phosphates, is their tendency to form salts with 1, 2, 
and 3 equivalents of base for 1 of phosphoric acid. 
Metaphosphoric, or monobasic phosphoric acid, forms salts of the general for- 
mula. P, 0,, MQ; and if our salt is to be classed as a metaphosphate, it must be 
one with the very singular formula 2 (P, 0,, Mg 0) +P 0,; in which 2 eq. of 
metaphosphate of magnesia are combined with 1 eq. of anhydrous phosphoric acid, 
which has either entered the radical of the acid, without increasing its neutralising 
or assumed a neutral character. 
Pyrophosphoric acid, or bibasic phosphoric acid, forms salts of the general 
formula, P, 0.,2 M0, or P, 0 M Me If our salt be a pyrophosphate, and the tem- 
Pd 2“5) HO pyrop 
perature appears favourable to its being so, its formula must be (P, 0,,2Mg 0)+ 
2 P, 0,, in which 1 eq. pyrophosphate of magnesia is combined with 2 eq. of 
anhydrous phosphoric acid, in one of the characters alluded to above. 
Lastly, common or tribasic phosphoric acid forms salts of the general for- 
‘ 2M0 M0 
mula, P,0,,3M0; P, 0, | Ho oF Pe 9; ‘2 10° Lf our salt belong to this class, 
its formula will be P, 0, ae +P, 0,. Here, 1 eq. of phosphoric acid, act- 
a) 
