178 ON PREPARATION OF PURE PHOSPHORIC ACID. 
ART. XLVIIL-- ON THE PREPARATION OF PURE PHOSPHORIC 
ACID. 
By Dr. Gregory. 
Dr. Gregory states that he has not found the alcohol 
process for separation of the phosphate of magnesia, in the 
preparation of phosphoric acid (described in his recent work, 
p. 110,) to answer. He recommends the following as pre- 
ferable: — 
Having removed the lime by means of sulphuric acid, 
and evaporated the filtered liquid (filtering again if any 
sulphate of lime separates during the evaporation) to the 
consistence of syrup, a few drops of sulphuric acid are 
added, to make sure that no lime remains. Of course, if 
turbidity ensues, the lime has not been completely separated 
before; and the addition of sulphuric acid, filtration after 
adding some water, and evaporation, must be repeated till 
the syrupy liquid continues quite clear when sulphuric acid 
is added. This liquid now contains only the phosphoric 
acid, the magnesia, and the excess of sulphuric acid. It is 
concentrated, and heated in a covered platinum crucible 
until the whole of the sulphuric acid has been expelled, 
and the residue has acquired a low red heat. On cooling, 
it forms a glass, perfectly colourless and transparent, which 
contains only phosphoric acid and the magnesia of the bones. 
This glass when boiled with water dissolves rather slowly, 
but completely. When the solution is again concentrated 
in a capsule of platinum until most of the water is expelled, 
and the temperature rises to between 595° and 600° Fahr., 
it suddenly, while hot, becomes turbid, from the separation 
of a peculiar phosphate of magnesia. At the same moment 
the phosphoric acid begins to crystallize like the granular 
sugar deposited in honey, a form ascribed by Peligot to the 
bibasic or pyrophosphoric acid. If the same temperature 
