78 CHEMICAL MANURES. 



The silicic fluoride makes itself felt by a penetrating acrid odour ; it 

 decomposes in presence of water into gaseous hydrofluosilic acid 

 which volatilizes, and into orthosilicic acid 



3SiF, + 4Hp = 2H,SiF,, + SiO, + •2R.p 



Gaseous hydrofluoric acid acts on sflicates to render them soluble : — 



Al.,(SiO..)=^ + 6HF ^ Al.F, + 3H.,SiO, 

 Ai:,(SiO;;)=^ + ISHF -^ A1.,F, + 3SiF, + 9H.,0 

 CaSiO.3 + 2HF = CaF.. + H.^SiO.^ 

 CaSiO;3 + 6HF = CaF: + SiF^ 3^,0 



Ost observed that in the "dissolving" process two-thirds of the fluorine- 

 contained in the phosphates escapes as gas, whilst one-third remains, 

 as calcium fluoride; but according to Klippert 50 per cent of the 

 fluorine is disengaged as gas, 30 per cent remaining undecomposed, 

 and 20 per cent absorbed mechanically by the mass as SiF,.H^,.. 

 Fossil bones contain up to 16 per cent of CaF^. 



Silicates of Lime and Alumina. — The silicates CaSiO.,, Al^fSiOj.^ 

 are partly soluble in sulphuric acid and partly insoluble. The lime 

 silicates in Algerian phosphate are decomposable by sulj^huric 

 acid, the silica being precipitated as a gelatinous thick swollen mass, 

 and can thus enclose a certain amount of insoluble phosphate of 

 lime which escapes being rendered soluble. The undecomposed 

 silicates of alumina absorb acid mechanically and are gradually de- 

 composed by it. 



Oxides of Iron and Alumina. — The presence of oxides of iron 

 and alumina in raw phosphates has serious drawbacks. It may 

 in some cases prevent their use in superphosphate manufacture. 

 The iron is present in different forms, more often as oxide combined 

 with the phosphate, more rarely as free oxide or as protoxide (ferrous 

 oxide j. It also occurs as sulphide FeS- either infinitely divided, as 

 in Tennessee phosphate, which contains as much as 4 per cent in 

 the river pebbles of the Carol inas, in certain Belgian phosphates,. 

 or as lamellae in the phosphates of Podolia. But sulphide of iron 

 is generally present in too small a quantity to cause serious mishaps.. 

 Phosphates containing protoxide of iron, ferrous oxide FeO, have a 

 greyish-black or a bluish-grey colour like certain Florida, Tennessee, 

 and Carolina river pebbles. In the process of "dissolving" phos- 

 phates the iron compounds are decomposed more or less rapidly 

 and completely according to the form in which they occur, accord- 

 ing to the density (? compactness) of the phosphates, the amount 

 and the state of concentration of the acid used, and finally according- 

 to the extent to which the "mixing"' becomes heated during the: 

 re-action. 



Decomposition takes place thus : — 



3FeP0, + 3H,S0, Ii (FeP0,2H,P0j + Fe,(SOj, 



