THOMAS-SLAG OR r.ASIf CINDEK. '2:\~^ 



U8ed for steel-making, and steel was therefore a dear coiuiiuxlity. 

 In 1879 Messrs Thomas and Gilchrist discovered a method of 

 making steel which got rid of the phosphorus, so that steel can 

 now be made from inferior kinds of ironstone. The residt has 

 been an entire revolution in the steel trade, and so great a 

 cheapening of that article, that it is now used for making rails, 

 bridges, armour-plates, &c., for which purposes iron onlj'used to 

 be available. The method essentially consists in adding lime t< > 

 the pig-iron, and in lining the furnace, or " converter," as it is 

 called, with that material, instead of with bricks composed 

 largely of silica, as used to be done. On that account it is 

 called the " basic " process, as distinguished from the former 

 " acid " one. In the converter the pig-iron is raised to a very 

 high temperature, by a blast of air which is blown through the 

 molten mass to burn off some of the carbon. The converter is 

 a huge pear-shaped vessel mouthing upwards, but capable of 

 being turned upside down. While the blast is roaring through 

 the molten metal, a great flame issues from the upturned mouth, 

 presenting a brilliant and striking spectacle. The flame is due 

 not only to the combustion of carbon, but also to the combustion 

 of other metallic and non-metallic impurities, and at the close of 

 the operation, which lasts only for about a quarter of an hour, the 

 iron itself begins to be burned. 



Among the impurities that are burned in this way is phos- 

 phorus, which is converted into phosphoric acid. The phos- 

 phoric acid unites with lime, which is present in abundance, and 

 forms with it a kind of phosphate of lime. As soon as the blast 

 is discontinued, this phosphate, along with other oxidised im- 

 purities, rises as a scum to the surface, and constitutes what is 

 knoAvn as the basic or Thomas slag. The converter is then 

 slowly tilted round, and the floating slag is poured out into boxes 

 capable of holding two tons or more, and there it is allowed to 

 cool. The steel is thereafter cast into ingots, and the converter 

 is ready for another charge. When the slag is cold it is removed 

 from the boxes, and for some years it was cast aside as a waste 

 product, and its accumulation formed huge unsightly heaps in 

 the neighbourhood of the steel-works. It was known that this 

 refuse material contained a large amount of phosphoric acid, 

 and many attempts were made by chemists and others to extract 

 it; but although numerous patents were taken out by ex- 

 perimenters, none of the processes were able to be worked at a 

 profit. It was some time before it occurred to any one to try 

 the effect of simply grinding it to a fine powder, and apphdng it 

 directly as a manure to the soil. When that plan was tried it 

 was found in some instances to be a failure, and in others to be 

 attended with considerable success. These experiments will be 

 referred to immediately, but, in the first place, it will be as well 



