DIGESTION APPARATUS FOR REVERTED PHOSPHATES 115 



dissolved rock a quantity of common salt chemically equivalent 

 to its iron content. Ammonium sulfate also helps to hold the 

 phosphoric acid water-soluble. Reversion of the phosphoric acid 

 is quite certain to take place in those products where the solvent 

 action of the sulfuric acid has not been complete. 84 Especially 

 is this the case when there are still substances present which can 

 be attacked by the acid calcium phosphate. This action is illus- 

 trated by the following equation : 



CaH 4 (PO 4 ) 2 +Ca 3 (PO 4 ) 2 -f8H 2 O=4CaHPO 4 (H 2 O) 2 . 



In this case the undissolved tricalcium phosphate is attacked by 

 the acid monocalcium phosphate with the production of a com- 

 pound insoluble in water. 



103. Influence of Movement. The influence of time and tem- 

 perature of digestion, and of variations in the composition of 

 the ammonium citrate on the quantity of phosphoric acid dis- 

 solved by that reagent, has been pointed out. Of great impor- 

 tance also in the process is the character of the movement to 

 which the materials are subjected during the digestion. For 

 this reason various mechanical devices have been constructed to 

 secure uniformity of solution. Inasmuch as the temperature fac- 

 tor must also be faithfully observed, the best of these devices are 

 so arranged as to admit of a uniform motion within a bath of 

 water kept at the desired temperature which, by the association 

 method, is 65. 



104. Digestion Apparatus for Eeverted Phosphates. The diges- 

 tion apparatus used by Huston consists of two wheels 25 centi- 

 meters in diameter, mounted on the same axis, having a clear 

 space of four and one-half centimeters between them. 85 In the 

 periphery of each wheel are cut 12 notches, which are to re- 

 ceive the posts bearing the rings through which the necks of the 

 flasks pass. The posts are held in place by nuts which are screwed 

 down on the faces of the wheel. Should it become necessary to 



84 Riimpler, Kaufliche Dungestoffe und ihre Anwendung, 4th Edition, 

 1897 : 85. 



85 Indiana Agricultural Experiment Station, Bulletin 54, 1895 : 4. 



