The Crystalline Structure of Metals. 



173 



bismuth, from which the still molten metal has been poured away. 

 Another striking example of this structure is seen in steel containing 

 about 4J per cent, of silicon. The fractured ingot of this material 

 exhibits large crystals, and by deeply etching a polished surface 

 Mr. Stead has obtained a beautiful development of the regularly 

 oriented elements of which the crystalline grains are built up* on a 

 scale so large as to require but little magnification. 



The authors have obtained much evidence that this structure is 

 typical of metals generally. Probably under no condition does any 

 metal cease to be crystalline. 



The crystalline character of wrought-iron bars or plates is seen when 

 the polished surface is etched, not merely by the general appearance of 

 the grains under oblique light, but by the development of geometrical 

 pits on the surface. These pits have a definite orientation over each 

 grain, and the orientation changes from one grain to another. Usually 

 in the purest commercial iron their outline is that of plane sections of 

 .a cube, but occasionally they are apparently plane sections of an octa- 

 hedron. In some instances isolated and comparatively large pits only 

 .are seen ; in others nearly the whole surface of a grain, when viewed 

 under a magnification of 1000 or 2000 diameters, is found to be 

 covered with small as well as large pits, geometrically similar and simi- 

 larly oriented. Photographs of these are given in the paper. 



For the purpose of producing smooth surfaces in the more fusible 

 metals, without polishing, the metal was poured in a molten state on a 

 plate of smooth glass. The surface produced in this way shows well 

 the boundaries between the grains, and in some cases it also exhibits 

 the crystalline character of the grains in a remarkable way by means 

 of geometrical pits, which are apparently formed on the surface in con- 

 sequence of the presence of small bubbles of air or, more probably, of 

 gas given out from the metal itself during solidification. Cadmium 

 shows these particularly well, and they are to be observed also in tin and 

 .zinc. These air-pits are seen, under 1000 diameters, to be negative 

 crystals, similar and similarly oriented on each grain, and, in cadmium, 

 to have outlines which suggest that they are sections of hexagonal 

 prisms. Their characteristics are exhibited in the photographs, which 

 also show how the boundaries between the grains are emphasised by 

 the collection there of air or of gas given off by the metal during 

 solidification. The true boundary is merely the trace of a surface on 

 a plane, but it may be broadened out in this way into a wide shallow 

 channel. 



The effects of strain have been examined in many metals, using sur- 

 faces prepared either by polishing or by casting against a smooth plate. 

 When any metal is strained beyond its elastic limit in any way, the 

 surface of each crystalline grain becomes marked by one or more 

 * ' Journal of the Iron and Steel Institute,' 1898. 



