136 CHEMICAL MANUEES. 



3. Another experiment was made ^vith a Florida phosj^hate^ 

 with 35-49 per cent P0O5 and 2-25 per cent of sesquioxides. When 

 "dissolved" it gave 18-98 per cent of soluble P0O-, and 0-95 per 

 cent of insoluble. Schucht's method was applied in three ways,, 

 using a Schucht's instrument constructed by Kohler and Martini 

 of Berlin, by which a higher temperature and pressure could be 

 applied. The following are the results : — 



(a) Pressure and average temperatures as above : ■! mm. (? atmos- 

 pheres) of p)ressure at 40"" C, 17-09 per cent soluble P.>0-, 1-85 

 insoluble ; (b) increased pressure and average temperature : 7 mm. 

 (? atmospheres) of pressure at 40^ C, 16-67 per cent soluble phos- 

 phates, and 1-91 per cent insoluble; (c) increased temperature and 

 pressure : 7 mm. (? atmospheres) pressure at 70° C, 16-72 per cent 

 soluble PoO., and 2-28 per cent insoluble. It will be seen that 

 retrogradation increases with the temperature and pressure. This- 

 superphosphate, though piled in a heap 16 ft. high, did not retro- 

 grade even in its lower part, which Dr. Grueber attributes specially 

 to the storing of the superphosphate, after complete cooling, by Dr. 

 Lutjens' scraping and cooling appliance. The fact that the cooled 

 superphosphate did not retrograde is not sui-prising, because as far 

 back as 1870 this important fact was observed in the case of super- 

 phosphates made from Lahn phosphorite, very hable to retrograde, 

 that the more rapidly it was cooled the less the soluble phosphoric 

 acid retrograded. Dr. Grueber concludes that, in order to explain 

 retrogradation, decisive results will onlv be obtained bv working on 

 the large scale ^^-ith supei-phosphates to which different materials 

 have been added. With the data so obtained, the treatment of 

 phosphates could be modified according to their composition and 

 the manner in which they behave. 



Dr. W'^. Paj'san considers retrogi'adation is a rather exceptional 

 phenomenon.! He supports his view by personal experiences with 

 Algerian phosphates with 0-5 of sesquioxides, and Peace Eiver 

 Pebbles with 2-5 per cent of sesquioxides. The solubihzation of the 

 first was always pushed in a constant manner to 0*75 per cent of 

 insoluble, that of the second to 1 per cent. He never found retro- 

 gradation in either even after long storing. He considers that in 

 valuing phosphates up to now too much importance is attached to 

 the percentage of sesquioxides. To prove it he quotes trial mixings 

 he made with this end in view. It was a case of three Tennessee 

 phosphates (a), (o) and (c) which are characterized from this point 

 of view by containing 79 per cent phosphate of lime, 2-36 per cent 

 oxide of iron, and 2-24 per cent of alumina, say a total of 4-6 per 

 cent of sesquioxides. The results are given in the following 

 table : — 



1 See note 1, p. 14-2. 



