ON PREPARATION OF PURE PHOSPHORIC ACID. 179 
be kept up for 15 minutes, the whole of the magnesia sepa- 
rates in the form of a powder, which is quite insoluble in 
acids or water ; when cold, the mass is digested with water, 
which dissolves the phosphoric acid, leaving the phosphate 
of magnesia as a heavy, fine, snow-white powder, of a faint 
silky lustre. The filtered liquid is free from every trace 
of magnesia, and may be considered as pure phosphoric 
acid. If the bones contain chloride of sodium, a trace of 
phosphate of soda will remain. In that case the burnt and 
powdered bones ought to be boiled with water, to remove 
any soluble chloride. Dr. Gregory recommends this pro- 
cess as a simple one for obtaining pure phosphoric acid. It 
is necessary to keep up the heat of about 600° for 12 — 15 
minutes, but not to go beyond that temperature, because 
a stronger heat would again, no doubt, produce the glass 
formed in the first fusion, which as we have seen is entirely 
soluble in water. The acid is easily tested for magnesia 
by diluting and supersaturating with ammonia; when, if 
magnesia be present, it very soon appears as ammoniaco- 
magnesian phosphate. The phosphate of magnesia thus 
separated is a peculiar and anomalous salt. From six 
analyses Dr. Gregory induces the formula 2MgO-f 3PO0 5 , 
that is, an acid sesquiphosphate of magnesia, according to 
the older notions of phosphoric acid. This composition can- 
not be reconciled with any of the three generally admitted 
modifications of phosphoric acid; and it would seem to in- 
dicate, either that another modification exists, or that the 
theory of phosphoric acid and the phosphates at present ad- 
mitted is erroneous. Dr. Gregory is engaged in the ex- 
amination of this point. The insolubility of the phosphate 
is very great, and presents a great obstacle ; for boiling 
nitric acid, aqua regia, are almost entirely without action 
upon it; and it is, he believes, the only salt of magnesia 
insoluble in acids. — Ibid from, Med. Gaz. 
