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Mr. J. A. Ewing and Mr. W. Bosenhain. 



The experiments^ of which this is a preliminary account, have been 

 directed to examine the behaviour of the crystalline grains when the 

 metal is subjected to strain. 



For this purpose we have watched a polished surface under the micro- 

 scope while the metal was .gradually extended until it broke. By 

 arranging a small straining machine on the stage of the microscope, we 

 have been able to keep under continuous observation a particular 

 group of crystalline grains while the piece was being stretched, and 

 have obtained series of photographs showing the same group at various 

 stages in the process. Strips of annealed sheet iron, sheet copper, and 

 other metals have been examined in this way. We have also observed 

 the effects of strain on the polished surfaces of bars in a 50-ton testing 

 machine by means of a microscope hung from the bar itself, and have 

 further observed the effects of compression and of torsion. 



When a piece of iron or other metal exhibiting the usual granular 

 structure is stretched beyond its elastic limit a remarkable change 

 occurs in the appearance of the polished and etched surface, as seen 

 by the usual method of " vertical " illumination. A number of sharp 

 black lines appear on the faces of the crystalline grains : at first they 

 appear on a few grains only, and as the straining is continued they 

 appear on more and more grains. On each grain they are more or 

 less straight and parallel, but their directions are different on different 

 grains. At first, just as the yield-point of the material is passed, the 

 few lines which can be seen are for the most part transverse to the 

 direction of the pull. As the stretch becomes greater oblique systems 

 of lines on other grains come into view. 



The photograph, fig. 1 (Plate 1), taken from a strip of transformer 

 plate (rolled from Swedish iron and annealed after rolling), gives a 

 characteristic view of these lines as they appear after a moderate 

 amount of permanent stretching, but long before the iron has reached 

 its breaking limit. 



The appearance of each grain is so like that of a crevassed glacier, 

 that these dark lines might readily be taken for cracks. Against this, 

 however, was the consideration that an over-strained piece of iron 

 recovers its original elasticity after a period of rest, though, as we 

 found, the dark lines did not disappear when recovery took place, and 

 further that sharp lines of the same nature were not seen on the surface 

 of metal which was polished and etched after straining. 



The real character of the lines is apparent when the crystalline 

 constitution of each grain is considered. They are not cracks, but 

 slips along planes of cleavage or gliding planes. 



Fig. 2 is intended to represent a section through the upper part of 

 two contiguous surface grains, having cleavage or gliding planes as 

 indicated by the cross-hatching, AB being a portion of the polished 

 surface. When the metal is pulled beyond its elastic limit, in the 



