196 AGRICULTURAL ANALYSIS 



ing the reactions of this body as to make theories of its constitu- 

 tion largely visionary. But the existence in definite crystalline 

 form of this salt shows that it is not merely an intimate mechan- 

 ical mixture, but a true molecular form. As a type of the sup- 

 posed arrangement of its particles, the graphic formula proposed 

 by Kormann may be consulted ; viz., 



O Ca , 



PO (X 

 \ /Ca 

 (X 



6 



Nca 

 PO O/ 



\ I 

 O Ca 1 



The crystals of this salt, as may be seen by inspection of the 

 analytical data, contain other bodies than calcium, oxygen and 

 phosphorus. It would be of interest to push the investigation 

 of their constitution further and see if crystals of pure tetracal- 

 cium phosphate could be obtained, and under what conditions 

 they would be contaminated by other metallic oxids. Usually, 

 by the color of the crystals, it will be easy to determine some- 

 thing of the nature, if not the extent, of the contamination. 



167. Solubility of Phosphatic Slags. The high agricultural 

 value of basic slags led to an early study of their solubility in 

 ammonium citrate, citric acid, and other organic solutions. Even 

 finely ground mineral phosphates and bones are soluble to some 

 extent in ammonium citrate, as was pointed out as long ago as 

 i882. 47 The most common solvents for basic slags are 

 ammonium citrate and citric acid. The ammonium citrate should 

 be the same as that used for the determination of reverted phos- 

 phoric acid and the citric acid solution commonly used contains 

 five grams in a hundred cubic centimeters. The slags of dif- 

 ferent origin and even of different age vary greatly in respect of 

 the quantity of soluble matter they contain. It is believed, how- 

 ever, that a very fair idea of the agricultural value of a slag 

 47 Wiley, Journal of Analytical and Applied Chemistry, 1889, 3 : 413. 



