132 CHEMICAL MANUEES. 



bone dust and Algerian phosphate, as the drier to be added to the 

 superphosphate, depends on circumstances. If the superphosphate 

 is to be dispatched in a week or thereabouts, one remains satisfied 

 with 1 per cent of Algerian phosphate. On the other hand, if it 

 should not require to be dispatched before a month or six weeks, 



5 of bone dust and 0"O per cent of Algerian phosphate is added. 

 When the phosphate has to be kept longer, 1 per cent of bone dust 

 is simply added. The reason of this method of proceeding is, that 

 the addition of the phosphate tends to harden the superphosphate 

 in the heap so that it has to be screened again before dispatch, 

 or even passed through Carr's disintegrator. The new methods of 

 Lutjens for the working of superphosphates present real advant- 

 ages. But such installations are somewhat costly ; moreover, they 

 are only used where high strength superphosphates are required or 

 when agriculture requires pulverulent products capable of being 

 spread by the drill. It is necessary, in that case, that the super- 

 phosphate contain an important proportion of free phosphoric acid 

 intended to dissolve the raw phosphate added as mentioned above. 

 A Florida superphosphate of 18 per cent, for example, should not 

 contain more than 1 to 5 per cent of free phosphoric acid if it be 

 not desired to add raw phosphate thereto ; if on the other hand 



1 per cent of Algerian phosphate be added thereto, the free acid 

 may be increased to 7 to 8 per cent. Certain foreign countries 

 stipulate for superphosphates of very high strength. Thus Scandi- 

 navia stipulates for 20 per cent, which is nonsense, as products of 

 this kind run relatively dearer than 15 to 18, which are current 

 types. 



Storing (Preservation) of SiqjerjjJiosjjhate — Betrogradation [Be- 

 duction) of Pliosj^lioric Acid. — Superphosphate keeps well from one 

 season to another when the phosphate from which it was made does 

 not contain more than 2 per cent of sesquioxides, but it is not so if 

 kept longer. The phosphoric acid of the superphosphate commences 

 to retrograde (" reduce ") if the raw phosphate was not pure. 

 Betrogradation sets in much sooner if the raw phosphate contained 

 more than 2 per cent of sesquioxides, or if mistakes have been made 

 in its manufacture. Eetrogradation (" reduction ") occurs under the 

 influence of different causes, physical and chemical. The heat and 

 pressure of the superphosphate heap appear to be the direct pre- 

 determinating causes. Stored superphosphate forms, first of all, 

 packed layers. The granules of which it consists have only a slight 

 superficial contact, but as the thickness of the layer increases the 

 substance compresses in virtue of its own weight, afterwards in 

 virtue of the length of contact, so that finally the particles are 

 glued together to form a very compact mass. There is thus estab- 

 lished between each an exchange of chemical energy which means 

 decomposition. Experience shows that these changes are more 



