84 MANUFACTURE OF FERTILIZING MATERIALS 



can retrograde a molecule of phosphoric acid, from 

 which it follows that one part of MgO can render 

 3.55 per cent of phosphoric acid insoluble. If 

 one uses, for example, twenty-nine parts of potash 

 with 2.05 per cent of free magnesia, percentage 



2x05 X29 

 controlled by estimation, the : -=0.59 



parts MgO suffices to combine 0.59x3.55=2 

 parts of phosphoric acid; as an actual fact, only 

 1.4 of insoluble was obtained, which proves that 

 the magnesia did not exert all its action. The 

 cause lies in the slight solubility of magnesia, 

 and in the fact that the salts of potash combine 

 partially with the precipitate formed, so that a 

 part of the phosphoric acid of this latter remains 

 in solution. 



The use of nitrate of soda in compound ma- 

 nures is rather restricted; it is used in making 

 nitrophosphate, sometimes in nitrophosphate of 

 ammonia. It is found that nitrate of soda and 

 sulphate of ammonia are incompatible, and that 

 in fact it is better to use these manures sepa- 

 rately. Besides, mixtures of superphosphate 

 and nitrate sometimes enter into spontaneous 

 combustion in the bags. This is caused by the 

 superphosphate fresh from the mixing "den" 

 being mixed with the nitrate and bagged up 

 before it has had time to cool. Cold superphos- 

 phate, however damp, does not act on nitrate 

 of soda, unless in very warm weather. They 

 are no longer objects of terror to the manufao- 



