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Mr. J. A. Ewing and Mr. W. Eosenhain. 



fig. 8. The crystals in metals are generally cubical, but the angle at 

 which the intersecting systems of bands cross depends on the inclina- 

 tion of the polished surface to the planes of cleavage. Occasionally a 

 third system of bands may be seen. 



As straining proceeds the originally smooth surface of the specimen 

 becomes roughened by the surface grains changing in their relative 

 levels and also becoming more or less inclined, as well as more or less 

 stepped. All this happens in consequence of the slips which they and 

 their neighbours undergo. To this is due the dull appearance which 

 an originally bright surface assumes when the metal is overstrained. 

 Under the microscope the strained surface is seen to be full of ups and 

 downs, and a continuous alteration in focus • is required to trace the 

 system of bands, even over the face of a single grain. 



When the experiment is made with a polished but unetched specimen 

 the slip bands appear equally well. The boundaries of the grains are 

 invisible before straining ; but they can be distinguished as the strain 

 proceeds, for the slip bands form a cross-hatching which serves to mark 

 out the surface of each grain. To strain a polished but unetched 

 specimen reveals in a striking way the granular character of the 

 structure.* 



Figs. 5, 6, and 7 are selected from a series of photographs showing 

 under a magnification of 140 diameters, the same group of crystalline 

 grains in a specimen of soft wrought iron at various stages of straining 

 by pull. The arrows show the direction in which the pull was applied. 



Fig. 5 shows the group before straining began. Fig. 6 is the same 

 group after the strain had been carried some way past the yield point. 

 Fig. 7 is the same group after the piece had suffered considerable 

 further extension in the same direction. Comparison of the three will 

 show how the grains change their shape in consequence of the slips 

 which occur in them, and also how the faces of the grains become tilted 

 and altered in relative level. 



Fig 8 is another sample of iron strained by pull. The specimen in 

 this case was a bar of Swedish iron, in which a comparatively large 

 crystalline structure had been developed by annealing for some hours at 

 700° C. The photograph was taken after the bar had been broken in 

 the testing machine, and shows with a magnification of 400 diameters 

 a portion of the surface not far from the place of fracture. The large 

 grain which appears to the left in fig. 8 measured 0*16 mm. in the 

 direction of the arrows before straining, and was extended by the 

 strain in that direction to about 0*2 mm. The slip bands upon it are 

 on the average 1/400 mm. apart. This applies to both of the two 

 systems of bands which appear in the photograph. The apparent 



* The fact that the crystalline structure is revealed when a specimen with a 

 polished unetched surface is strained has already been pointed out by Charpy 

 (' Comptes Eendus,' vol. 123, 1896, p. 225). 



