47 ) 



VI. — On the Extraction of pure Phosphoonc Acid from, Bones, and on a neiu and 

 anomalous Phosphate of Magnesia. By William Gregory, Esq., M.D., 

 E.R.S.E., Professor of Chemistry in the University of Edinburgh. 



[Read 3d March 1845.] 



I. — On the Preparation of Pure Phosphoric Acid. 



The usual methods of obtaining pure phosphoric acid by the oxidation, Avith 

 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 easUy and cheaj^ly 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, ah-eady 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 bm-nt 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, is left. The objections to this process ai-e the folloAving : — 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 



