128 PHOSPHATIC MANURES [chap. 



taining the molten iron through which the blast of 

 air was forced, with a " basic " lining composed of lime 

 and magnesia, instead of the previous acid lining of 

 bricks composed mainly of silica. Lime was also added 

 to the converter, and when the oxidation due to the 

 blast of air takes place in the molten metal, in presence 

 of the lime the phosphorus oxidises as well as the carbon 

 and silicon, because the resulting phosphoric acid is 

 at once taken up by the bases present and so removed 

 from the action, instead of being immediately reduced 

 again by the molten iron. Under these conditions the 

 resulting slag, or molten impurities derived from the 

 iron, which is " basic " from the excess of lime instead 

 of " acid " as usual from excess of silica, contains con- 

 siderable quantities of phosphoric acid, ranging from 

 12 up to as much as 23 per cent. At the present time 

 the Bessemer has largely been replaced by the " open 

 hearth " process of making steel, but as the principle 

 is the same — the oxidation of the impurities in the 

 iron by a current of air — it can similarly be carried 

 out in the presence of lime with the production of a 

 "basic slag" containing phosphoric acid. 



For some time after Thomas and Gilchrist had intro- 

 duced their process in 1879, ^^o use was made of the basic 

 ilag, though it was known to contain so much phosphoric 

 acid, and it accumulated in the usual mounds near the 

 steel furnaces. Various methods were tried for extract- 

 ing the phosphoric acid or bringing it into a soluble 

 form, though without any success ; but early in the 

 'eighties it began to be found that the only thing 

 necessary to make the basic slag available as a manure 

 was to grind it to an extremely fine powder. In this 

 country the value of fine ground basic slag was first 

 brought to light by Wrightson and Munro in 1885, 

 who carried out a series of experiments, on a chalk 



