476 BOARD OF AGRICULTURE. 



be taken. I will, therefore, give a brief outline of the plan — com- 

 monly known to chemists as the Molybdic acid plan of determining 

 phosphoric acid— which yields accurate results. 



Not less than 100 grains, or better 200 grains, of the dried and 

 finely-powdered soil are digested for an hour or thereabouts with 

 three or four ounces of moderately strong nitric acid. The acid 

 solution is then passed through a filter, and together with the 

 washings from the insoluble portion of the soil left on the filter is 

 evaporated to a small bulk ; thus getting rid of the greater part 

 of the acid employed for effecting the solution. During evapora- 

 tion a large excess of molybdate of ammonia is added to the solu- 

 tion, care being taken to keep it strongly acid. 



If there be much phosphoric acid in the soil, a bright yellow 

 precipitate, consisting of molybdic and phosphoric acid, makes its 

 appearance at once ; if traces only be present, the yellow precipi- 

 tate appears only on the concentration of the liquid, after the great 

 excess of nitric acid tas been expelled by evaporation. The yellow 

 precipitate containing the whole of the phosphoric acid present in 

 the soil, molybdic acid, together with a little silica, and frequently 

 some oxide of iron, is thrown on a filter an(i washed with a solu- 

 tion of molybdate of ammonia rendered strongly acid by nitric 

 acid, until a drop of the washings passing through the filter ceases 

 to show a reaction of iron with yellow prussiate Of potash solution. 

 It is then 'dissolved on the filter in an excess of ammonia, and the 

 ammoniacal liquid precipitated with an ammoniacal solution of 

 sulphate of magnesia, which throws down the phosphoric acid as 

 phosphate of magnesia and ammonia. After standing at rest for 

 about twelve hours, the magnesia precipitate is collected on a small 

 filter and washed clean with strong ammonia-water. Together 

 with the phosphoric acid, traces of silica,- and generally also traces 

 of oxide of iron, are thrown down with the magnesia precipitate. 

 In order to separate these impurities the precipitate is dissolved in 

 a few drops of hydrochloric acid, and the acid solution carefully 

 evaporated to complete drj^ness. The hard, dried residue is again 

 made acid with muriatic acid, a little water is then added, and the 

 liquid passed through a small filter, on which are left insoluble 

 traces of the silica originally thrown down witli magnesia, A 

 few drops of citric acid having been added to the acid solution, 

 with a view of keeping any traces of iron in solution, strong am- 

 monia is finally added, which throws down a second time phos- 

 phate of magnesia and ammonia, now free from silica and oxide of 



