MANURES AND MANURING 1 5 97 



phate may be present either as the anhydrous salt or as g3Tpsum. In or 

 dinary superphosphates the former predominates, usually almost exclu- 

 sively ; where however the superphosphate has been prepared from bones 

 and with limited quantities of sulphuric acid, the calcium sulphate is then 

 nearl}' all in the form of gypsum. Monocalcium phosphate is commonly 

 CaH^ (P04)2 Hg O, but the anhydrous salt is not rare and has a very 

 definite effect on the character of the manure. The same applies to di- 

 calcium phosphate. 



2) Superphosphate also contains a certain amount of moisture, 

 its so-called " liquid phase " which consists essentially of a solution of 

 phosphoric acid and phosphate of Ume, and which largely controls the 

 mechanical condition of the manure. 



3) The investic^ations in this connection have made it possible to 

 define exactly what is meant by " true moisture " and " free phosphoric 

 acid ", terms which have been used somewhat loosely up to the present. 

 An examination of the relationship between the chemical and physical 

 properties of the superphosphate have shown that the state of division is 

 intimately connected with the measure of its Hquid phase and still more 

 directly with its true moisture, or in other words, with the water present 

 in a liquid form as distinct from water of crystallisation. Further the 

 degree of fineness of the superphosphate also varies with the amount 

 of free phosphoric acid in the fertilizer. Two methods are suggested for 

 the determination of true moisture, both being based on the solubility 

 of the free water in cold alchohol. This alchohol extract may also be used 

 for estimating free phosphoric acid. 



4) The investigation in to the conditions of the manufacturing pro- 

 ces as affecting the constitution of the product has shown that the state 

 of hydration in which mono-and dicalcium phosphate are deposited and 

 the content of free phosphoric acid in the fertilizer are regulated by the 

 temperature curve of the reaction and by the dilution of the sulphuric 

 acid employed. Both free phosphoric acid and true moisture (taken as 

 the difference between total moisture and that retained as water of crystal- 

 lisation) increase with the temperature. Other conditions being equal 

 the free phosphoric acid content also varies with the strength of the sul- 

 phuric used. 



5) In the industrial process, calcium sulphate is first deposited 

 in the anhydrous form or rarely in the semi-hydrated form (2 CaS4 OHg O) ; 

 gypsum, when present, may be looked upon as a secondary product 

 resulting from the hydration of the two above compounds. Mono and di- 

 calcium phosphates on the other hand are deposited as anhydrous or hy- 

 drated salts according to the temperature and composition of the liquid 

 in which they are formed, and these two related factors govern the me- 

 chanical condition of the resulting superphosphate.. 



Once the relationship has been established between the temperature 

 curve of the reaction and the properties of the substance produced, it is 

 easy to organize means of regulating the process by adjusting the heat of 

 the reagents, the quantity of acid used, the degree of fineness to which 



