J. F. Tocher 1:35 



a jwiori. The difficultv arises when the composition of the phosphates is 

 unknown and when other interfering substances are present. Since the 

 nature of the substances present are unknown, equations expressing the 

 law of mass action cannot be written down in these cases. 



The following table (Table VI) shows the average compo.sition of 

 some commercial slags as given by Collins {Chemical Fertilisers, p. 122). 



Table VI. Composition of Slags. 



12 3 4 5 



Total P-A IL'-IJO 20-49 O-O'J 17-57 19-35 



SiUca 17-(i9 10-12 13-49 7-77 12-12 



Lime 3S-02 46-81 40-43 52-22 44-75 



Magnesia 4-24 2-92 5-01 1-94 0-11 



Mansanese o.xide ... 7-39 4-38 5-41 9-37 4-68 



Iron" 12-89 9-98 13-83 8-13 9-10 



These latter tables (Tables V and VI) show in a general wa)' the 

 differences between slags and mineral pho.sphates. These two classes of 

 fertihsers contain non-jihosphatic residues differing in chemical com- 

 position, and residues which are common to both in different proportions. 

 The results of tliis section show that citric solubility is merely a special 

 case of the law which has been proved to hold for the solubihty of a 

 definite chemical substance in dilute acids and it can always be stated 

 a prion when the conditions are known for a definite substance in a 

 definite dilution. When, however, we pass from a single substance to 

 mixtures of varying composition citric solubility cannot be descriptive 

 ■of available jihosphate of definite composition. The reason for this lies 



(1) in the unknown changes which take place in the initial molecular 

 concentration of the citric acid, due to the formation of calcium and 

 other citrates from the carbonates and hydrates present in the fertiliser, 



(2) in the unknown changes which take place on agitating phosphatic 

 fertihsers of varying composition and (.3) in the known effects produced 

 by the presence of fluorides and of silica. 



V. EXPERIMENTS IN WHICH THE VOLUME OF FLUID AND THE WEICiHT 

 OF MINERAL PHOSPHATE ARE BOTH CONSTA.XT. THE VARYIN(i 

 FACTOR BEINC4 ACID CONCENTRATION. 



We shall now consider condition (2) namely, where the amount of 

 mineral phosphate (m.^) and the volume of fluid (iii.^) are both constant, 

 i.e. jWg/Wg = constant, while the amount of citric acid is varied. 



The following series of experiments was carried out with a constant 

 weight of mineral pho.sphate (5 grams) in varying concentrations of citric 

 acid in a constant volume of 500 c.c. The undernoted results (Table VII) 



Journ. of Agrio. Sci. xii 10 



