PHOSPHORUS FERTILIZERS 



125 



gives only a small amount of phosphates, yet a second extrac- 

 tion will give a further considerable quantity of phosphate. 

 Obviously, therefore, the citric acid test as designed by Wagner 

 affords no reliable indication of the value of the slags, since 

 the curve of solubility is cut at a purely arbitrary point, 

 which seems to possess no particular meaning. Tables 14 

 and 15, which are copied from Robertson's paper, show the 



TABLE 14. 

 DORMAN, LONG AND Co.'s BASIC SLAG. 



Total SiO 2 



17-10 per cent. 



TABLE 15. 

 BELL BROS.' SLAG. 



details of continuous -extraction of slags with citric acid. It 

 will be noticed that in one of these cases quoted above, there 

 is more phosphoric acid dissolved in the second citric acid 

 extraction than in the first. This is doubtless due to the 

 fact that the first citric acid extract is weakened by the pre- 

 sence of much lime. In order to make quite sure that the 

 fluorine was the real cause of this difficulty, Robertson took 



