THE STRUCTURE OF METALS 101 



influence of impurities on the structure. The most easily 

 recognised impurities are those which are not to be regarded 

 as true constituents of the alloys but rather as foreign matter 

 mechanically entangled. Dross in badly made brass is of this 

 character and some other metallic oxides often occur as 

 mechanical impurities. Thus molten aluminium becomes 

 covered with a peculiarly tough and resistant film or pellicle 

 of alumina which is not readily eliminated in the preparation 

 of aluminium alloys by fusion. Remelting is frequently 

 necessary to remove these films. Crystalline stannic oxide 

 remains obstinately entangled in molten tin bronze which has 

 not been sufficiently protected against oxidation and naturally 

 is a cause of brittleness. A slightly different position is occupied 

 by the slag and sulphides found in iron and steel, the impurities 

 in this case being liquid instead of solid at the moment of 

 entanglement in the molten metal. Masses of silicate slag, 

 drawn out into fibres in the direction of rolling, are char- 

 acteristic of wrought-iron bars, whilst oval globules of grey 

 manganese sulphide are found in mild steel, as in the middle 

 of the field in fig. 5. In the absence of manganese, however, 

 the sulphur in steel is present as ferrous sulphide, which has 

 much less tendency to agglomerate into such oval masses and 

 is commonly met with in the far more dangerous form of 

 thin films separating neighbouring crystal grains. Steel 

 containing ferrous sulphide is invariably red-short so that 

 microscopic cracks are developed in it during rolling. 



Passing now to those impurities which are truly alloyed 

 with the metals under examination it is evident that elements 

 which become associated homogeneously with one or the 

 other of the primary constituents cannot be immediately 

 detected by the microscopical method, although occasionally 

 their presence may bring about some perceptible change in 

 the character of the crystals. For example, manganese is 

 miscible with iron and manganese carbide with iron carbide, 

 so that the structure of a mild steel is unchanged by the 

 introduction of manganese. On the other hand, when the 

 manganese is very much increased in quantity, as in certain 

 rich varieties of pig-iron, the increased coarseness of the 

 carbide crystals due to its presence gives a characteristic 

 aspect, both to the etched sections and to the fractured surface, 

 although no new structural constituent has made its appearance. 



