26 



Chemical Composition and Commercial Value of 



analysis is almost always contaminated with oxalate of magnesia, 

 and therefore make it a general practice to redissolve the par- 

 tially-washed precipitated phosphate of magnesia in ammonia, 

 and to throw it down a second time. The magnesia precipitate 

 must be washed with strong ammonia water. 



The lime precipitate generally continues a variable and often 

 altogether insignificant proportion of phosphate of iron and 

 alumina. In ordinary analyses, it is hardly necessary to take 

 any notice of the traces of phosphate of iron, which exist in good 

 white samples of bone-ash. For very minute analyses, I dissolve 

 the lime precipitate, after having been weighed, in hydrochloric 

 acid, precipitate the solution with ammonia, collect precipitate 

 on a small filter, wash and redissolve on filter, and precipitate a 

 second time in the cold. The phosphate of iron and alumina, 

 after washing, is free from lime. Its weight is determined, and 

 deducted from the first weight of the lime precipitate. In order 

 to obtain the phosphoric acid contained in the phosphate of iron 

 and alumina, the precipitate is dissolved in hydrochloric acid, a 

 little tartaric acid is added, then ammonia, and finally the phos- 

 phoric acid is determined as phosphate of magnesia. If necessary, 

 a separate determination of carbonic acid and sulphuric acid is 

 made in the bone-ash. 



As regards accuracy, this plan of analysis leaves nothing to 

 be desired. A proof of this is furnished in the subjoined analyses 

 of the same sample of bone-ash ; two of them were made by 

 myself, and two by my first assistant, Mr. Sibson. 



Though all ordinary care is taken in preparing a sample for 

 analysis, it is next to impossible to obtain a perfectly homogeneous 

 powder. The trifling discrepancies in the results of the four 

 separate analyses are due, perhaps, in a higher degree to this 

 circumstance than to the method of analysis. 



Composition of a Sample of Bone-ash. 



