CH. viii] Grades of Slag 149 



. 1 . Bessemer slag, containing phosphoric acid equiva- 

 lent to 40 per cent, or more of tricalcic phosphate, and 

 largely soluble in 2 per cent, citric acid; usually 80 per 

 cent, of the total is guaranteed to be soluble. 



2. Basic open hearth slag containing less phosphoric 

 acid, equivalent to 15 to 31 per csnt. tricalcic phos- 

 phate largely soluble (80 per cent.) in 2 per cent, citric 

 acid, the first pourings being richer than the last. 



3. Basic open hearth slag made by the use of lime 

 and fluorspar, containing as much phosphate as the 

 poorer members of the preceding class but only slightly 

 soluble (20 per cent, or less) in 2 per cent, citric acid. 



When basic slag was first obtained in the Bessemer 

 converters in 1879 its fertilising properties were not 

 recognised; not till John Wrightson in 1884 and 1885 

 made his field experiments at Ferryhill and at Downton, 

 and Paul Wagner in 1885 began his systematic pot 

 experiments at Darmstadt, were agriculturists aware of 

 its value. It gradually came into use and within 4 or 5 

 years could profitably be adulterated with mineral 

 phosphates, to detect which Wagner devised the well- 

 known citric acid test that, with certain modifications, 

 has remained in force ever since. 



The more modern open hearth slags have been tested 

 in Northumberland by Gilchrist, in Essex by Scott 

 Robertson, in Devon by Dutton and at Saxmundham 

 by Oldershaw. The second class have proved substan- 

 tially equal in fertiliser value to the old Bessemer slags. 

 The third class have also proved much more effective 

 than was first assumed from their low solubility in citric 

 acid. Where the growing season has been sufficiently 

 long these slags are approximately as useful as the others 

 in spite of their low solubility. Where the growing 



