THE STRUCTURE OF METALS 197 



Time also plays a part in the deformation of the softer 

 metals but the mechanism of the process is quite different. 

 The bending of a stick of sealing-wax, for example, is not 

 accompanied by any obvious change in microscopic appearance ; 

 but even in the case of the softest metals, such as lead, the 

 structure is altered. Lead, in fact, is a convenient metal for 

 the study of the process. A smooth surface is prepared and 

 the metal is deformed, say by lightly bending between the 

 fingers. Examination under the microscope shows at once 

 that a change has taken place, the originally smooth surface 

 being crossed by very numerous lines arranged in parallel 

 groups ; unlike the systems of lines common to amorphous 

 and crystalline materials, to which reference has been made 

 above, these systems of microscopic lines do not bear any 

 necessary relation to the direction of the deforming force. On 

 the other hand, they are very evidently related to the crystalline 

 structure. Fig. 1 represents a surface of lead after bending ; 

 it will be seen that a close parallelism is preserved by the lines 

 in each crystal but that their direction changes abruptly from 

 one crystal to its neighbour. 



The lines thus developed have been termed "slip-bands" 

 by Ewing and Rosenhain l and the name has been generally 

 adopted. They are parallel with the cleavages of the metallic 

 crystals and their direction in any one grain seen under the 

 microscope depends on the crystalline orientation of that grain. 

 It has been found possible to show, by the direct examination 

 of a cross-section, after protecting the marked surface by 

 depositing a thick layer of copper on it by electrolysis, that 

 each line is really a minute step and that the surface on one 

 side of a line is at a different level from that on the other. 2 

 The same conclusion may be reached by illuminating the 

 specimen obliquely and rotating the stage of the microscope; 

 it is then obvious that the lines disappear in certain positions 

 and flash out again on reaching such a position that they reflect 

 the incident beam into the tube of the microscope. As the lines 

 in any one grain flash out simultaneously whilst they are inde- 

 pendent of those in neighbouring grains, their dependence on 

 orientation is clear. 



These facts furnish the explanation of slip-bands. They are 



1 Phil. Trans. 1889, 193 A, 353. 



2 W. Rosenhain, Journ. Iron and Steel Inst. 1906, ii. 189. 



