20 



(2) There may be insufficient potash ; this may be sup- 

 plied by addition of kainit, 20 per cent, potash salts, 

 etc. 

 All basic slags, however, do not behave alike. Examination 

 shows that they fall into two great groups : those in the making 

 of which fluorspar was used : and those to which no fluorspar 

 was added. Field experience shows that the fluorspar slags are 

 often less effective than the others : chemical examination indicates 

 that they contain some of their phosphate in the form of fluorapa- 

 tite, a substance having little, if any, value to plants. The 

 slags free from fluorspar, on the other hand, contain some, if not 

 all, of their phosphate in the form of silico-phosphate, which is 

 of very considerable value to plants. Mr. Page has developed a 

 method for ascertaining the amount of fluorine in slags, from 

 which can be calculated the maximum value for the quantity of 

 fluorapatite present. Some of the results are : — 



(1) Total phosphoric oxide (P 2 O s ) multiplied by 2.18 to 



convert into the equivalent quantity of tricalcic phosphate 

 (Ca 3 (POJ a ). 



(2) Percentage of the total phosphoric oxide (P 2 O s ) which 



is soluble in the official 2% citric acid solution. 



(3) Calculated from fluorine present, assuming all to be in 



form of fluorapatite. 



(4) The remaining phosphate. 



The slags are arranged in order of total phosphate and there- 

 fore approximately in order of price. Reference to the last column 

 shows, however, that they differ considerably in their content of 

 effective phosphates. Thus slags 2 and 3 are rated equal by the 

 ordinary analysis and might be offered at the same price by a 

 merchant acting in perfectly good faith and honesty. In the field 

 tests No. 3 is less effective than No. 2. Mr. Page's method shows 

 that it may contain most of its phosphorus in the non-effective 

 form of fluorapatite, while No. 2 contains all its phosphates in 

 the effective forms. The citric solubility test discriminates 

 between these slags but its indications are not always very clear. 

 The fluorine method promises to be more helpful. 



