362 PROFESSOR .T. A. K\V1N<: AM) MR. W. ROSENHAIN 



The appearance of the grains after straining so closely resembles that of a crevassed 

 glacier that the black lines might be taken for cracks. But from the outset it \\as 

 clear that they could not be actual fissures. A piece of strained iron, when it has 

 been allowed to rest for a time, or has been heated to 100 C.,* recovers its original 

 elasticity and more than its original strength, yet the dark lines do not disappear 

 under such treatment. Further, if a specimen showing these lines be re-polished the 

 lines disappear, and even light etching does not reproduce lines of the same nature. 



The real character of the lines is apparent when the crystalline constitution of each 

 grain is considered. They are not cracks but steps in the surface. These steps are due 

 to slips along the cleavage or gliding planes of the crystals, t 



The diagram, fig. 15, is intended to represent a section through the upper part of 

 two contiguous surface grains, having cleavage or gliding places as indicated by the 

 dotted lines, A B being a portion of the polished surface, C being the junction 

 between the two grains. 



Fig. 15. 



Before straining. 



After straining. 



When the metal is strained beyond its elastic limit, as say by a pull in the direction 

 of the arrows, yielding takes place by finite amounts of slips at a limited number of 

 places, in the manner shown at a, b, c, d, e. This exposes short portions of inclined 

 cleavage or gliding surfaces, and when viewed in the microscope under normally 

 incident light these surfaces appear black because they return no light to the 

 microscope. They consequently show as dark lines or narrow bands extending over 

 the 'polished surface in directions which depend on the intersection of the polished 

 surface with the surfaces of slip. 



The correctness of this view is demonstrated when these bands are examined under 

 oblique light. When the light is incident at only a small angle to the polished 

 surface, that surface appears for the most part dark ; but here and there a system of 

 the parallel bands shines out brilliantly in consequence of the short cleavage surfaces 

 which constitute the bands having the proper inclination for reflecting light into the 

 microscope. The groups of bright bands which are seen under oblique light are 



* See J. Mum "On the Recovery of Iron from Overstrain," 'Phil. Trans.,' A, 1899. 

 t See 'Roy. Soc. Proc.,' March 16, 1899 (vol. 65, p. 85), where the authors have published a preliminary 

 account of some of these ol>se: various. 



