198 CHEMICAL MANUEES. 



stood if one thinks of the great difficulties which they had to sur- 

 mount, from a technical point of view, to reduce the slag to a tine 

 powder, and to obtain with a material at that time of very varyin^: 

 strength a marketable product of uniform strength. When, later 

 on, the solubility in citric acid was adopted as the method of 

 determining the fertilizing value of basic slag in powder, it was again 

 Hoyermann who, working from his own data, suggested the addition 

 of silica in the converter, as a means of considerably increasing the 

 solubility of the phosphoric acid. It is thus that, through appar- 

 ently insurmountable difficulties, basic slag became a precious- 

 source of phosphoric acid for agriculture. Its comparative cheap- 

 ness, its content of lime and silica, and the good results which they 

 give on meadows, peaty soils, sandy soils poor in lime, have caused 

 basic slag to be used in all intensive culture countries. It no longer 

 forms, as previously, a useless and cumbersome ballast, but a pro- 

 duct of great fertilizing value, the consumption of which increases 

 from year to year. 



Origin of Basic Slag. — Up to a comparatively recent epoch, good 

 steel could only be obtained by using ores exempt or almost exempt 

 from phosphorus. A proportion of 0-25 of phosphorus sufficed to 

 render the iron brittle in the cold. These sort of ores had become 

 more and more rare, whilst there existed abundant deposits of 

 phosphorous ore. The attention of metallurgists was, therefore, 

 iDound to turn in the direction of the latter, and it was necessary to 

 try to utilize them. 



It is to two young Englishmen, Thomas Gilchrist (?) and Percy 

 Gilchrist, to whom in 1879 the honour of this discovery, which was 

 to revolutionize the manufacture of steel, is due. It did not enter 

 into ordinary practice until after five or six years of efforts, varied 

 tentatives and numerous and delicate trials. It is now the basis of 

 the manufacture of the greater part of steel. The Thomas process, 

 as it is called, has a double advantage : it enables an excellent steel 

 free from phosphorus to be obtained whilst utilizing the phosphorus- 

 ore ; and on the other hand it gives as a secondary product a fertil- 

 izing material, the use of which, in agriculture, has assumed a 

 rapid and considerable extension. Let us now examine rapidly the 

 manufacture of the cast-iron, then that of the steel which yields the 

 slag. The ore conveyed to the ironworks is smelted in blast 

 furnaces which reach to about 65 feet in height by 20 feet in width : 

 it is there laid alternately with layers of coke, and there is added, 

 according to the nature of the mineral, calcareous or silicious matter 

 which forms what is called the castine or erbue. The object of this 

 lining is to deprive the mineral of any argillaceous or calcareous 

 gangue present, and to obtain finally, in consequence of the de-oxida- 

 tion and of the partial carburation of the ore, as a useful product 

 cast-iron, and as residue, a lighter substance floating on the top 



