CH. VIII J Basic Slag 147 



are found in the drainage-water: practically the whole 

 of the superphosphate added to the Rothamsted soils 

 during the past 60 years and not taken by the crop still 

 remains in the soil. It is sometimes described as an acid 

 manure; the statement may at one time have been 

 correct but it is now misleading, a well-made sample 

 contains no more than 3 or 4 per cent, of acidity. There 

 is no evidence that it causes the soil to become acid : the 

 Broadbalk plot manured with superphosphate does not 

 lose its lime any more quickly than the corresponding 

 plot without superphosphate. It has no bad effect on 

 the texture of the soil, on the contrary it not infre- 

 quently causes an improvement. 



Superphosphate ought always to be in good mechani- 

 cal condition so 'that it can readily be drilled. 



Basic slag ( formerly called basic cinder, Thomas'' 

 phosphate powder, etc.) 



Basic slag is a by-product in the manufacture of 

 steel. The first stages lead to the production of pig iron 

 which contains quantities of silicon, carbon, sulphur, 

 phosphorus, etc. In the second stage these substances 

 have to be largely removed, and this is done by heating 

 the molten metal to a high temperature in presence of 

 air, when they are aU oxidised. 



Two processes are in use for effecting this oxidation. 



The older one, adopted in 1879, is the Bessemer pro- 

 cess ; the molten pig iron is run into a pear-shaped vessel 

 known as the converter, and air is blown through it. 

 The changes to be brought about require a high tem- 

 perature, which is mainly obtained by the combustion 

 of the phosphorus, silicon, etc., contained in the pig iron 

 itself. It is therefore to the advantage of the steel maker 



102 



