STORING OF SUPEEPHOSPHATE. 129 



core. The scraping machine enables superphosphates, of a quality 

 hitherto unknown, to be made cheaply." Dr. Eiemann, a master 

 of superphosphate manufacture, says : " We have only O'O to O'T 

 per cent of insoluble P0O5 in Florida superphosphate. As a general 

 rule we obtain an increase in the phosphate rendered soluble, and 

 we have not observed retrogradation in Florida superphosphate 

 even after several months' storage." 



Superheated Sujjerphosjjhates. — Crispo draws attention to the 

 .changes which occur in overheated superphosphates. After a 

 certain temperature orthophosphoric acid is changed into meta- 

 phosphoric acid. Experiments on this point have shown that 

 orthophosphoric acid, heated to 105° C. during three hours, is con- 

 verted into metaphosphoric acid to the extent of 10 per cent, whilst 

 if heated to 200' C. it is converted to the extent of 90 per cent, by 

 the loss of a certain amount of combined water. If the soluble 

 phosphoric acid by the citrate method be determined in over- 

 heated superphosphates, it is to be observed that metaphosphoric 

 is not precipitated by magnesia mixture, and that only the ortho- 

 phosphoric acid is found in the precipitate. If the determination 

 be made by the molybdate of ammonia method, the sum total of 

 the two phosphates is found, the nitric acid bringing the meta back 

 into the ortho form. Crispo considers metaphosphoric acid of less 

 Talue than the ortho. However, experiments made in Germany 

 have shown that under the first of these two forms phosphoric acid 

 lias given good results. Experiments at the Agronomical Station of 

 GomlDlouse, by Gregoire and Hendrich, have shown : — 



1. That the alterations undergone by the dried superphosphate 

 in no way diminish the fertilizing value of that product. 



2. That desiccation at a temperature of 165° C. has appreciably 

 increased the activity of the phosphoric acid. 



3. That the metaphosphate and the pyrophosphate of lime re- 

 ■spectively, the products of dehydrating mono and bicalcic phos- 

 phates, are. without fertilizing value. In pot culture experiments in 

 the Liege laboratory, each of the pots containing 4, kilos (8-8 lb.) of 

 soil, Ligouro oats gave, on an average, according to the phosphates 

 used : — 



t:able xlil— results of experiments with ordinary super- 

 phosphate AND superphosphate DRIED AT DIFFERENT 



temperatures on ligouro oats. 



Grain and Proportional 



Phosphate used. straw gr. yield. 

 Without phosphoric acid . . . 12-3 100 



Ordinary superphosphate . . . 28-8 234 



Superphosphate dried, slightly calcined . 26*9 219 



Metaphosphate 19-3 157 



.Superphosphate di-ied at 160° C. . . 28-8 234 



?> 



27-6 224 



