IV.] BASIC SLAC^ 127 



until 80 per cent, passes through a sieve with 100 

 meshes to the inch, the African phosphates being much 

 easier to grind than those from Florida, and then it is 

 mixed with about 70 per cent, of chamber acid of a 

 specific gravity 1-52, according to the composition of 

 the rock. The mass takes a day or two to cool, is 

 disintegrated and thrown into heaps, after which it 

 must be crushed again before it is bagged. About 

 half a ton of acid is used in making a ton of super. 



The next great development in regard to phos- 

 phatic manures came from a very unexpected source — 

 the introduction of basic slag as a waste product in 

 steel-making. A great many ores of iron, notably those 

 found in the Cleveland district of North Yorkshire, 

 contain considerable quantities of phosphates, and in 

 the process of smelting in the ordinary blast furnace 

 much of the phosphorus passes into the iron. As far 

 as the product of the blast furnace goes — the cast iron 

 — the presence of phosphorus does no particular harm, 

 but as soon as the cast iron has to be converted into 

 steel it becomes highly objectionable. With the general 

 introduction of mild steel, obtained cheaply by the 

 Bessemer process of blowing air through the molten 

 cast iron until all the carbon and silicon are burnt out of 

 it, and then adding just enough of an iron rich in carbon 

 to get back to the proportion of carbon and iron which 

 forms steel, the Middlesborough iron made from the 

 Cleveland ores was at a serious disadvantage, since it 

 contained a considerable amount of phosphorus which 

 could not be removed in the Bessemer process. After 

 much research two chemists, Thomas and Gilchrist, 

 invented a process for removing the phosphorus in the 

 Bessemer process, and so obtained a phosphorus free 

 steel from the impure Middlesborough iron. Their 

 plan was to line the " converter," the great vessel con- 



