IV.] BASIC SLAG 129 



soil in Wiltshire and a heavy clay in Durham with 

 basic slag finely ground on the one hand and on the 

 other dissolved by treatment with sulphuric acid, com- 

 pared with superphosphate and other forms of undis- 

 solved phosphate. The results were highly favourable 

 and showed that the slag was comparable with super- 

 phosphate as a source of phosphoric acid to the crop, 

 being much superior to the other insoluble phosphates 

 tried. About the same time as Wrightson and Munro's 

 experiments, basic slag, which was known in Germany as 

 Thomas slag or Thomas phosphate, after one of the 

 inventors, attracted considerable attention there ; many 

 experiments were made with it and turned out so success- 

 ful that it rapidly grew into considerable demand for 

 agricultural purposes. Indeed, so much more quickly 

 was basic slag taken up in Germany, that a considerable 

 export trade from Great Britain at once grew up, and 

 even at the present day of the 300,000 tons annually 

 made in Great Britain, about 150,000 tons are exported 

 to Germany. 



Basic slag, basic cinder, or Thomas phosphate 

 powder (the two latter names are little used in the 

 United Kingdom nowadays) comes into the market 

 as a dense black powder, so finely ground that four-fifths 

 of it will pass through a fine brass wire sieve carrying 

 100 meshes to the inch, which is found to pass only 

 particles having a smaller diameter than 0-2 mm. A 

 small but varying amount of free quicklime is present ; 

 from 2 to 10 per cent, may be obtained from fresh 

 samples by careful extraction with pure carbon dioxide 

 free water. Both free iron and magnetic oxide of iron 

 are present, and can be separated from the bulk by 

 means of a magnet ; this test, together with the presence 

 of free lime, the density of the material, and the evolu- 

 tion of a little sulphuretted hydrogen on treatment with 



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