VI.—On the Extraction of pure Phosphoric Acid from Bones, and on a new and 
anomalous Phosphate of Magnesia. By Witu1aM Grecory, Lsg., M.D., 
F.R.S.E., Professor of Chemistry in the University of Edinburgh. 
[Read 3d March 1845. ] 
I.—On the Preparation of Pure Phosphoric Acid. 
Tue usual methods of obtaining pure phosphoric acid by the oxidation, with 
nitric acid, or by combustion, of pure phosphorus, are well known ; but, although 
they yield a pure product, yet, as the phosphorus must be prepared from phos- 
phoric acid, it is obvious that we shall derive a great advantage from any method 
of purifying easily and cheaply the phosphoric acid from bones, instead of first 
reducing it to phosphorus, and then re-oxidizing it. In practice, phosphorus is 
made from the superphosphate of lime, and it is from the same salt that phos- 
phoric acid may be most economically prepared. 
Two processes, already given for this purpose, are worthy of notice. It is to 
be borne in mind, that the superphosphate of lime is the soluble compound ob- 
tained by acting on burnt bones with sulphuric acid and water, and filtering to 
separate the sulphate of lime. 
In the first process, the solution of superphosphate is neutralized with am- 
monia or carbonate of ammonia, which precipitates all the lime in the solution, with 
about one-fourth of the phosphoric acid, as bone phosphate; while three-fourths of 
the acid are converted into phosphate of ammonia. The filtered liquid being eva- 
porated, deposits crystals of that salt, which, when purified, are decomposed by 
heat in a platinum crucible; the ammonia and a great part of the water being 
expelled, while the phosphoric acid, with one equivalent of water, or metaphospho- 
ric acid, isleft. The objections to this process are the following :—The salt, when 
heated, melts, spirts much in boiling, becomes viscid, and froths up to a most 
inconvenient degree, requiring vessels of platinum of a large size for small quan- 
tities of material. Secondly, a very high and long continued heat is required to 
expel all the ammonia; and at that temperature, a portion of phosphorus is re- 
duced by the hydrogen of the ammonia, and corrodes the platinum, leaving a 
blue stain of phosphuret of platinum. Besides this, the purest phosphate of am- 
monia often contains a trace of organic matter, which causes the glass of phos- 
phoric acid, thus prepared, to be disfigured by carbonaceous particles. At least 
I have always seen black particles in the phosphoric acid made by this process. 
It is obvious, that if carbon be present, we have an additional source of reduced 
phosphorus; and if the black particles are phosphuret of platinum, then they 
