216 SOILS AND MANURES 



Phosphoric acid, properly so-called, is the hydrate H 3 P0 4 , 

 but the term is often applied to the oxide P 2 0s. 



The relation of these various compounds to each other 

 may be illustrated as follows : 



H 2 0) H 2 0) CaO) CaO) 



P 2 5 H 2 0[P 2 5 CaO P 2 5 H 2 OlP 2 5 CaOfP 2 5 

 H 3 0) H 2 0] CaO) CaO) 



Phosphorus Phosphoric Monocalcic Dicalcic Tricalcic 



pentoxide. acid. phosphate. phosphate. phosphate. 



H 3 P0 4 CaH 4 (P0 4 ) 2 CaHP0 4 Ca 3 (P0 4 ) 2 



Manufacture of Superphosphates. It is unnecessary to 

 enter into technical details, but a brief outline of the 

 process is germane to the subject. The object is to con- 

 vert the insoluble tricalcic phosphate into the soluble 

 monocalcic form, and to avoid as far as possible, the 

 production of dicalcic phosphate and of phosphoric acid. 

 Dicalcic phosphate, being insoluble, is but little better 

 than the original substance. If much free phosphoric 

 acid be present the substance is apt to get into a moist 

 and sticky condition in which it is difficult to handle, and 

 which is favourable to reversion (p. 219). 



A weighed quantity of the finely ground raw (native) 

 phosphate is introduced into an apparatus called the 

 " mixer"; a measured quantity of cold dilute 1 sulphuric 

 acid is run in and agitated until the two are thoroughly 

 mixed together. The whole is then dropped into a pit 

 or den, as it is called, and allowed to remain until the 

 reaction is complete. 



The mixer generally consists of an oblong wooden box, 

 lined with lead, through the middle of which runs a 



1 The strength of the acid is a matter of considerable technical 

 importance. The " chamber acid " commonly used has a specific 

 gravity about 1'5 r@. 



