24 Chemical Composition and Commercial Value of 



ash, which yielded an amount of phosphoric acid corresponding 1 

 to 82*59 of tribasic phosphate of lime, I obtained 82*48 per cent, 

 of phosphate by precipitation. 



But unless these precautions are carefully observed and the 

 phosphates are washed with strong amrnonia-water, discordant 

 results are obtained. At the best, the method of determining 

 the amount of phosphate of lime by precipitation is liable to 

 furnish results that cannot be relied upon. In proof of these 

 statements the following instances may be cited : 



Two separate determinations in bone-ash gave 75*84 per cent, 

 and 73*29 per cent, of bone-earth by the ordinary method of 

 precipitation. 



In another sample, I obtained, by throwing down the phos- 

 phates once, 79*03 per cent. 



By re-dissolving the phosphates in acid and precipitating a 

 second time, 76*21 per cent, were obtained. 



On analysing the 79*03 per cent, I obtained 34*95 per cent, 

 of phosphoric acid, corresponding to 75*72 of tribasic phosphate ; 

 whilst the precipitated phosphates in the second determination 

 (76*21 per cent.) furnished 34*89 per cent, of phosphoric acid, 

 corresponding to 75*59 per cent, of tribasic phosphate. 



A sample of animal charcoal gave 



Phosphates precipitated once 77-46 



Re-dissolved and precipitated a second time .. .. 73*63 

 Again re-dissolved and thrown down a third time .. 72*96 



A direct determination of phosphoric acid in the same sample 

 of animal charcoal furnished 33*34 per cent., which, calculated 

 as tribasic phosphate of lime, gives 72*23. 



These results thus show that the amount of bone-earth is stated 

 too high if the phosphates are thrown down only once ; that a 

 tolerably near approach to truth is obtained if the phosphates 

 are re-dissolved in acid and thrown down a second time ; and, 

 that lastly, perfectly accordant results are obtained if the phos- 

 phoric acid is determined, and from it the amount of tribasic 

 phosphate is calculated. 



Since bone-ash, animal charcoal, coprolites, &c., are chiefly 

 purchased for the sake of the phosphoric acid which they con- 

 tain, all possible care should be bestowed to obtain an accurate 

 determination of that constituent which mainly regulates their 

 commercial value. 



In purchases of bone-ash, the dealer generally guarantees a 

 certain amount of phosphate of lime or bone-earth ; but since a 

 question may arise as to the precise meaning of these terms, I 

 would suggest, as far more satifactory both to the purchaser and 

 the honest dealer, that the seller should guarantee the percent- 

 age of phosphoric acid ; or, to render the change in present 



