Phosphatic Materials used for Agricultural Purposes. 23 



the phosphoric acid as tribasic phosphate of lime, the ammonia 

 precipitate, which is still erroneously assumed by many to be 

 8 C O -f- 3 P O 5 , is recalculated as tribasic phosphate of lime. 

 This practice deserves to be strongly condemned, for it leads to 

 wrong- results, giving invariably the amount of tribasic phosphate 

 much higher than it is in reality. Great discrepancies in the 

 determinations of phosphate of lime in bone-ash, &c., by different 

 analysts, are a source of constant annoyance and frequent disputes 

 between seller and buyer. As long as the practice prevails of 

 ascertaining the phosphates simply by precipitation, such dis- 

 crepancies must remain matters of almost daily occurrence. 



It is, of course, a much more expeditious plan to determine 

 the phosphates by precipitation than to ascertain correctly the 

 amount of phosphoric acid ; but if we consider the difference 

 that an error of 3 or 4 per cent, of phosphate of lime will make in 

 the value of a ship's cargo, we shall admit that accuracy ought not 

 to be sacrificed to expedition. There is, indeed, ground to fear 

 that analyses are carried out in a too commercial nay, often a too 

 interested spirit, such as is calculated to bring analytical che- 

 mistry into disrepute. It is therefore the duty of all desirous of 

 carrying out analytical investigations in a manner consistent 

 with truth to raise a strong opposition against the mode in which 

 more especially commercial analyses of bone-ash and animal 

 charcoal are frequently executed at present. 



I have myself repeatedly analysed the ammonia precipitate 

 from bone-ash, and arrived at the conclusion that, under 

 the most favourable circumstances, it never contains less than 

 3 equivalents of lime for 1 equivalent of phosphoric acid. 

 Generally, however, it contains an additional quantity of lime, 

 or, more correctly speaking, carbonate of lime, for I find it 

 extremely difficult to prevent more or less carbonate of lime 

 from falling down with the phosphates when precipitating the 

 latter with ammonia. This is especially the case when the pre- 

 cipitation is effected in a hot solution. Notwithstanding the 

 entire absence of carbonic acid in the ammonia used for preci- 

 pitation, and the observance of every precaution to exclude the 

 air from the precipitated phosphates, some additional lime beyond 

 the proportion required to combine with phosphoric acid to form 

 tribasic phosphates is invariably found in the precipitate obtained 

 from a boiling-hot solution, even after the precipitate has been 

 re-dissolved and thrown down again a second or third time. If, on 

 the contrary, the precipitation is effected in the cold, and the phos- 

 phates are re-dissolved in acid after washing with ammonia- 

 water, and thrown down again a second time with pure ammonia 

 from a dilute cold solution, I find their composition agrees closely 

 with tribasic phosphate of lime. Thus from a sample of bone- 



