212 SOIL ITS ANALYSIS. 



must be evaporated to dryness, and the residue heated strongly in a 

 platinum capsule. After the dissipation and decomjtosition of the 

 ammoniacal salts, there remains watery phosphoric acid, distinguish- 

 able by its powerful acid reaction, its sirupy consistence, and its 

 fixity. 



By way of example, I shall give the results obtained in an analysis 

 of this kind : 



From the acid liquor, ammonia threw down of: 

 Phosphates and metallic oxides 

 These gave of sulpfiate of lime 

 Equivalent to lime ...... 



Hydrosiilphate of ammonia caused a precipitate, 

 which, calcined, gave of metallic oxides 

 Lime and metallic oxides together 

 Difference due to phosphoric acid . . 2.75D 



The analysis for phosphoric acid may be simplified by employing 

 a process conceived by IM. IJerthier, and which is founded upon the 

 strong afTmity of this acid for the peroxide of iron and the insolu- 

 bility of the phosphate of the peroxide of iron in dilute acetic acid. 

 If to a fluid containing at once phosphoric acid, lime, pero.\ide of 

 iron, alumina, and n»agnesia in solution, ammonia be added, the pre- 

 cipitate will contain the whole of the phosphoric acid. The acid 

 will be in great part combined in the state of phosphate of iron, if the 

 peroxide of iron be in quantity more than sufficient to neutralize it, 

 a condition which must be frequently expected in an arable soil ; 

 however, to make sure of this point it is well to add a certain quantity 

 of the peroxide of iron to the soil which is to be analyzed. Besides 

 the phosphate of iron, the precipitate may contain phosphate of lime, 

 phosphate; of ulumina, and certainly ammoniacal magnesian phos- 

 phate. Finally, with these phosphates will be found associated 

 alui'nina and oxide of iron, the latter especially if it has been intro- 

 duced in excess. The precipitate collected upon a filter and wash- 

 ed, must then be treated with dilute acetic acid, whiclj will dissolve 

 the lime, the magnesia, and the excess of the oxides of iron and 

 alumina, and tliere will remain phosphate of iron or phosphate uf 

 alumina, because the latter salt is as insoluble as the former in acetic 

 acid. Whenever the precipitate in question, therefore, leaves a 

 residue which is insoluble in vinegar, the presence of phosphoric 

 acid may be inferred ; this residue may consist of basic phosphates 

 of iron or alumina, or of a mixture of the two salts, and no great 

 error will be committed if one hundred parts of this residue, calcined, 

 be assumed as representirig lifty ol" phosphoric acid. 



The presence of silica in the precipitate ins»)lnble in acetic acid 

 may, however, lead to error. To make sure that the precipitate is 

 formed by a phosi)hate it must be redissolved in hydroc-hloric acid, 

 and the acid solution evaporated to dryness, so as to reiuler the silica, 

 which may exist in it, iivsoluble. By treatiuii the residue with hy 

 drochlonc acid again, the phosphates alone will be dissolved. Thi 



