42 PYROMETALLURGY. [ill. 



color. The number of measures used corresponds exactly to 

 1 gm. of iron. The iron-ore to be tested is dissolved in mu- 

 riatic acid, and any peroxide it may contain is reduced to 

 protoxide by adding crystallized sulphite of soda. 



To this test it may be objected that it is difficult to prepare 

 the permanganate with any degree of uniformity, and that if 

 an excess of sulphate be added, the test-liquid would probably 

 not indicate the amount of iron with exactness. 



Carbon in Cast, Steel, and Bar-iron. — Karsten has endea- 

 vored to determine the limits in the amount of carbon, which 

 separate cast-iron, steel, and bar-iron from each other, pro- 

 ceeding on the assumption that their characteristic properties 

 are due to their content of carbon. He first determined the 

 carbon in a single cast-iron by various methods, from which it 

 appears that combustion with a mixture of chlorate of potassa 

 and chromate of lead, or separation by chloride of copper or 

 chloride of silver, yielded the best results. In the white iron 

 from sparry ore, the amount of carbon was 5.586. When iron 

 contains as little as 2.3 per cent, carbon, it still exhibits the 

 properties of cast-iron, especially its precipitation of graphite 

 (making gray iron) when cooled slowly. It is not forgeable 

 when containing 2 per cent., and this property seems to begin 

 with a percentage of 1.9, when it forms steel. The steel is 

 not, however, capable of being welded, and is barely capable 

 of it when the proportion is reduced to 1.75. A percentage 

 of 1.4 to 1.5 indicates the maximum of combined strength and 

 hardness. When the quantity is reduced to 0.5 it is a very 

 soft steel, and forms the proper line of demarcation between 

 steel and bar-iron. These limits are higher with a purer 

 iron, and lower when it contains silicium, phosphorus, and 

 sulphur. 



On the protection of iron from oxidation by coating it, see 

 Journ. Fr. Inst. (3) xix. 209. 



Reduction of Iron- ores. — Sir F. C. Knowles' patent for re- 

 ducing iron-ores consists in heating pure ores in retorts, and 

 passing into them carbohydrogen from the coking of bitumin- 

 ous coal, or carbonic oxide from the combustion of coals. The 



