STOKING OF SUPERPHOSPHATE. 133 



rapid in moist hot superphosphate than in dried cooled superphos- 

 phate. The crystalUzation of the sulphate of lime, unfinished in 

 superphosphate, stored hastily, is completed in the superphosphate 

 heap. The superphosphate of lime, combining with the sulpho 

 sesquioxides, forms once more sulphate of lime ; free syrupy phos- 

 phoric acid acts on the silicates, etc. Now, all these reactions give 

 place to a disengagement of heat, consequently the matter expands 

 and tension is produced. Each of these reactions occur at a fixed 

 temperature, which it is impossible to gauge directly owing to the 

 isolating influence of the sulphate of lime. Finally, agglutination 

 is still further induced by the rarefaction of the air between the 

 granules cooled in the heap. For a difference of 10° C. (18° F.) the 

 difference is 3*5 percent. It has been observed that the phosphoric 

 acid of the superphosphate does not commence to retrograde 

 ("reduce") until the moment when the particles agglutinate, i.e. 

 when it is subjected to a fixed pressure. It is, therefore, pressure 

 which causes retrogradation by destroying the friable compounds 

 of the superphosphate. The part played by temperature has also 

 been determined by direct experiments. These show that it may 

 rise to 100° C. without hurting pulverulent friable superphosphate, 

 and up to 50° C. for agglutinated superphosphates higher tempera- 

 tures are only injurious under pressure. Smetham, in examining 

 "the influence of the oxides of iron and alumina on the retrogradation 

 of phosphates, observed an essential difference in the action of the 

 two oxides, as had been observed for a long time in actual practice. 

 Whilst 1 part of oxide of iron will cause 2 parts of phosphate of 

 lime to retrograde, the oxide of alumina can only retrograde its own 

 weight of phosphate (theoretically it retrogrades 3 times its own 

 weight). In Florida phosphate, Pebbles and Eiver phosphate, the 

 greater part of the sesquioxides are present as alumina. The ratio 

 of the alumina to the iron is, on an average, 1 to O'l. According 

 to the same authority, iron forms two insoluble compounds, the 

 mono-di-ferri phosphate and the di-tri-ferri phosphate ; alumina only 

 forms one compound. The free ortho phosphoric acid would appear to 

 be rendered insoluble in the soil more rapidly by iron and alumina 

 compounds than monocalcic phosphate, as previously determined by 

 Gerlach ; the latter also remarks that phosphoric acid, combined 

 with oxide of iron and alumina, does not dissolve in carbonic acid 

 water, contrary to w^hat occurs with the acid combined with lime 

 or magnesia. Phosphates of iron and alumina are only very 

 slightly soluble in the solutions of organic acids ; phosphate of iron 

 is almost insoluble therein. As the moisture in the soil is not more 

 acid than this artificial solution, it may certainly be taken for 

 granted that the phosphates of sesquioxides remain insoluble in the 

 soil. When retrogradation starts nothing can stop it ; it pursues 

 a very rapid course, even when the cohesion of the matter is 



