174 



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



systems of lines running in a generally straight and parallel fashion 

 over it. The direction of the parallel lines changes from grain to 

 grain. Thus these lines serve to mark out one grain from another in 

 a metal which, although polished before straining, has not been etched 

 to develop the boundaries. As straining proceeds, the lines become 

 more and more numerous and emphatic, and two, three, or four 

 systems appear on each grain. 



The nature of these lines has been described in the authors' paper of 

 March 16. They are slips along cleavage or gliding planes in the 

 crystals. The effect of each slip is to develop a step on the polished 

 face. The short inclined surface forming this step looks black under 

 vertical illumination, but shines out brightly when oblique light of 

 a suitable incidence is used. These slip bands, as they were named in 

 the previous paper, are thus seen as narrow dark or bright bands, 

 accordingly to the nature of the lighting. 



The authors have developed slip-bands in iron, copper, gold, silver, 

 platinum, lead, tin, bismuth, cadmium, aluminium, nickel, as well as 

 steel, brass, gun-metal, and various other alloys. So far as the observa- 

 tions go, they occur in all metals. 



The slip-bands are in themselves an evidence of crystalline structure, 

 and, further, they show how such a structure is consistent with plas- 

 ticity, and how it persists after plastic strain has occurred. The 

 " flow " or non-elastic strain of a metal occurs through numerous finite 

 slips taking place on the cleavage or gliding surfaces in each of the 

 crystalline grains of which the metal is an aggregate. The elementary 

 pieces which slip on one another retain their primitive crystalline 

 character. 



Further, if the movement of the pieces with respect to each other 

 in any one grain is a movement of translation only, their orientation 

 should remain uniform in each grain. 



That this is actually the case is demonstrated by examining speci- 

 mens of metal which had been violently deformed without any subse- 

 quent annealing or heating. In metal that has been rolled or hammered 

 in the cold state, or deformed by tension or compression or strain of 

 any kind, however severely, the grains are still seen where a surface is 

 polished and etched. Their form is much changed by the strain which 

 the piece has undergone. But the fact that they have retained their 

 crystalline structure is demonstrated when, after polishing, the piece 

 is subjected to a slight additional strain of any kind, for the effect of 

 this additional strain is to develop slips of the same general character 

 as before. Further evidence to the same effect is given by the fact 

 that etching the polished surface of a very severely strained piece 

 develops geometrical pits, which are similar and similarly oriented 

 over the face of each grain, notwithstanding the great distortion which 

 the grain has suffered as a whole. The effects of oblique lighting in 



