108 The Phosphates of America. 



with diluted sulphuric acid, are all dislodged. The bases become 

 sulphates. The phosphoric anhydride combines with water and 

 remains in the mass as free phosphoric acid, while the carbonic, 

 fluoric, and part of the silicic acids, go off as vapor, the two latter 

 generally combined in the very poisonous form of silicon tetra- 

 fluoride. In the manufacture of fertilizers, however, as at present 

 carried out, the object is not to produce free phosphoric acid, but 

 " acid phosphate " since, as we have already seen, this latter salt is 

 quite soluble in water, and therefore can fulfil all the conditions 

 that are deemed by some authorities to be essential in a plant food. 

 It hence follows that the substitution of the bases must not be 

 complete, but must be only carried to a sufficient point to displace 

 all foreign acids, and to saturate two out of the three molecules of 

 the lime combined with phosphoric anhydride. 



Lest this proposition should appear too complicated, we may 

 endeavor to make it more clear by an example, in which we shall 

 assume that we are called upon to deal with a Florida phosphate, 

 the composition of which has been determined by chemical analy- 

 sis to be as follows : 



Moisture and organic matter 3.90 



Tribasic phosphate of lime 79.40 (equal to 36.43 P 2 O 5 ) 



Carbonate of lime 5.48 



Phosphates of iron and alumina. . . 3.00 (equal to about 1.50 P 2 O 5 ) 



Carbonate of magnesia 0.72 



Sulphate and fluoride of lime 3.20 



Sandy matters, silicates, etc 4.30 



100 



The sulphuric acid known as "chamber acid" when measured 

 with Beaume's hydrometer at a temperature of 60 F., contains the 

 following percentages of sulphuric anhydride (SO 3 ) and pure mono- 

 hydrate (H 3 SO 4 ) : 



Degrees Beaume Percentage of Percentage of 



at 60 F. SO 3 (Anhydride) H^SO* (Monohydrate). 



48 48.70 59.63 



49 49.80 61.00 



50 51.00 62.47 



51 52.20 63.94 



52 53.50 65.53 



53 54.90 67.30 



54 56.00 68.60 



55... 57.10 69.94 



