10 



Chemical Composition and Commercial Value of 



and is used in Germany for the making of superphosphate, but 

 as yet it has not found its way into England. 



Another species of phosphorite, distinguished by the name of 

 osteolith, was discovered some years ago near Hanau, in 

 Germany. It occurs in thin seams in a volcanic rock, is soft 

 and almost white, and very rich in phosphate of lime. Unfor- 

 tunately the seams of this osteolith are not of sufficient thickness 

 to repay the cost of excavation, consequently no practical appli- 

 cation has been made of it as yet. 



3. CAMBRIDGESHIRE COPROLITES. 



The phosphatic nodules of the lower chalk are known in 

 commerce under the name of Cambridgeshire coprolites. Their 

 physical character and the localities where they are found are so 

 well known that 1 need not dwell on these points. 



The composition of different average samples, obtained by 

 reducing to powder several tons, varies to some extent. In some 

 more carbonate of lime and insoluble siliceous matter occur 

 than in others, and consequently the amount of phosphate of lime 

 in different samples fluctuates to a certain degree. 



In commercial analyses it is usual to determine the amount of 

 phosphate of lime or bone-earth by digesting the finely-powdered 

 coprolites in hydrochloric acid, filtering off the solution from the 

 insoluble siliceous matter, and precipitating the phosphates with 

 ammonia. However careful the analyst may be to employ 

 ammonia perfectly free from carbonic acid, and to avoid contact 

 with the air, some carbonate of lime invariably falls down with 

 the precipitated phosphates. It is necessary, therefore, to redis- 

 solve the latter in hydrochloric acid, and to throw them down a 

 second time with ammonia. If this be neglected, the amount of 

 phosphates in coprolites, and in all other phosphatic substances 

 containing carbonate of lime, is determined too high. 



By way of example, the following commercial analyses of 

 powdered Cambridgeshire coprolites are given : 



General Composition of Cambridgeshire Coprolites. 



