Pltoi'phoric Acid in Rocks of Igneous Origin, 295 



It was thought that phosphoric acid, if present, would be in combi- 

 nation with a portion of the alumina ; and as the phosphate of that 

 earth is readily soluble in dilute mineral acids, while the silicate 

 offers great resistance to these agents, mere digestion with acid would 

 suffice to extract the whole, or the greater part of the phosphate, 

 which could be afterwards precipitated by an alkali, and examined. 



With this view, 1000 grains of the clay were boiled during seve- 

 ral hours in a flask with a quantity of pure dilute hydrochloric acid ; 

 a large bulk of distilled water was then added, and the whole allowed 

 to rest until perfectly clear. The acid liquid was then carefully de- 

 canted from the undissolved clay, evaporated in a porcelain basin to 

 a small bulk, and precipitated by a slight excess of pure ammonia. 

 The scanty reddish precipitate obtained, which consisted chiefly of 

 alumina and oxide of iron, was collected upon a little filter, thoroughly 

 washed with distilled water, dried, and ignited. It was next reduced 

 to fine powder, and mixed with an equal weight of pure silica in a 

 finely divided state, and six times as much anhydrous carbonate of 

 soda. This mixture was heated to fusion in a platinum crucible. 

 When cold, the melted mass was acted upon by boiling water, and 

 the soluble and highly alkaline portion separated by a filter from the 

 insoluble silicate of alumina. The solution was mixed with excess 

 of nitric acid, evaporated to dryness, water added, and the product 

 filtered. The liquid thus obtained was divided into two portions ; 

 one of these was carefully neutralized by a little ammonia, and mixed 

 with a few drops of nitrate of silver ; a distinct yellow precipitate 

 appeared which was freely soluble in dilute nitric and acetic acids. 

 The nitric acid was mixed with excess of ammonia and some hydro- 

 chlorate of ammonia, and a few drops of solution of sulphate of mag- 

 nesia added. After a short interval, a crystalline, granular, white 

 precipitate, the ammonio-magnesian phosphate, made its appearance, 

 which increased in quantity by agitation. 



This experiment, which demonstrates the presence of a small 

 quantity of phosphoric acid in the clay in a most unequivocal man- 

 ner, was several times repeated with a like result. The purity of 

 the acids, carbonate of soda, and other materials employed, were 

 rigorously tested, and filtration through paper of the original acid 

 liquid purposely avoided, lest a trace of earthy phosphate should 

 have been dissolved from the paper. 



The porcelain-clay is extracted from the disintegrated granite by 

 mere washing with water, and subsidence ; and the water of the dis- 

 trict in which it is found, is, in all probability, exceedingly pure. 

 It was thought worth while, however, to examine in the same man- 

 ner the decomposed rock which had not been subjected to any artifi- 

 cial treatment, and a specimen taken by myself from the quarry was 

 chosen for the purpose. The result shewed the presence of phos- 

 phoric acid as in the clay, and apparently to about the same extent, 

 allowance being made for the quartz-grains, mica, &c. 



