134 PHOSPHATIC MANURES [chap 



phoric acid is almost wholly combined as di-calcium 

 phosphate, it is to that extent soluble in the dilute citric 

 acid solution above described. The fine state of division 

 of this manure and the form of combination in which 

 the phosphoric acid exists render it very available to 

 the plant, so that it is a good phosphatic manure for 

 use on light soils deficient in lime, though it may be 

 doubted whether a mode of manufacture which first 

 involves solution of the phosphoric acid by means of 

 sulphuric acid and then neutralisation and precipitation 

 by lime is not essentially uneconomical. 



Small quantities of various forms of precipitated 

 phosphate come on the market from time to time : 

 these are bye-products in the manufacture of gelatine, 

 when the bones are treated with hydrochloric or 

 sulphuric acid to dissolve out all the earthy matter, and 

 the resulting solution of phosphoric acid is neutralised 

 with milk of lime. When the initial solution has been 

 effected by sulphuric acid the product is sometimes sold 

 as "phosphatic gypsum," since it consists largely of 

 gypsum formed by the reaction of the sulphuric acid 

 and the lime. These fertilisers are very good sources 

 of phosphoric acid, which is combined in them in the 

 form of di-calcium phosphate ; they form excellent 

 phosphatic fertilisers for light soils, being easily avail- 

 able and neutral. 



Wiborg PJwsphate is the product of the heating of 

 apatite, occurring as a waste material mixed with 

 felspar in the preparation of certain Swedish iron ores, 

 with sodium carbonate in the proportions of 30 soda 

 to 100 apatite containing 17 per cent, of felspar. The 

 result is a phosphate to which Nilson ascribes the formula 

 2NaoO. loCaO. 3P2O5, which is completely soluble in 

 ammonium citrate solution and has proved to be 

 particularly valuable on the peaty soils poor in phos- 



