364 PROFESSOR J. A. EWING AND MR. W. ROSENHAIN 



as soon as plastic deformation takes place. They have been observed in .specimens of 

 platinum, gold, silver, copper, lead, zinc, tin, cadmium, bismuth, aluminium, iron, and 

 nickel, as well as in steel, brass, bronze, and other alloys. In the case of iron it AMIS 

 proved definitely that under tensile stress the bands appear as soon as the yield point is 

 passed. For this experiment a flat was polished on the side of a test-piece, which \\ as 

 strained in a 50-ton testing-machine, and the surface was kept under observation, 

 during the application of the stress, by means of a microscope hung from the bar 

 itself. 



Slip-bands are developed by all kinds of strain involving permanent deformation of 

 the piece. A microscopic inspection of the surface after straining does not enable us 

 to detect whether the deformation has been caused by tension, compression, bending, 

 or torsion, but the appearance depends very much on the amount of strain that has 

 taken place. The more severely the specimen has been strained the greater is the 

 number of slip-bands developed. After slight straining there is generally only one 

 system of bands to be seen on each crystalline grain, but as many as four such systems 

 intersecting one another may come into view after severe straining. Fig. 23 (Plate 21) 

 is a photograph of strained lead, showing four systems of slip-bands. 



The general appearance of the slip-bands is different in different metals. In lead 

 they are particularly straight, even under extreme magnification, as is illustrated in 

 fig. 24. In silver there is a tendency to more wavy outlines, as appears in fig. 25, 

 Plate 22, but in gold the lines are as straight as those in lead. In iron the lines tend, 

 as a rule, to be rather wavy, and to fork and branch. Examples of slips in this 

 metal have already been given in figs. 12, 14, and 16 ; others have been given in the 

 preliminary notice referred to above.* 



In gold, lead, and other metals showing straight slip-bauds it is easy to distinguish 

 well-marked steps where intersecting systems cross. Fig. 24 is a characteristic example. 



In several specimens of lead, prepared by casting against glass, a peculiarity was 

 noticed which forms an apparent exception to the statement made above that slip- 

 bands which shine out under oblique light at one particular incidence do not reappear 

 when the incidence is changed (i.e., the specimen rotated) by 180. The specimens in 

 question were examined under oblique light with an objective of 16 millims. focus, 

 giving a magnification of 100 diameters. On rotating the stage carrying the specimen 

 the slip-bands on one crystalline grain were found to shine out strongly in two posi- 

 tions, very nearly 180 apart. Under this low magnification it seemed that identical 

 bands were visible in both positions ; but on applying a power of 2000 diameters with 

 a combination of " vertical " light and an oblique beam of grazing incidence from an 

 arc lamp it was seen that there were really two systems of parallel slip-bands on the 

 same crystalline grain, and that only one of them was picked out by the oblique light. 

 In the other system of slip-bands the slope of the cleavage surfaces exposed by the 

 slips was inclined away from the source of light, and consequently remained dark. The 



* ' Roy. Soc. Proc.,' vol. 65, Plates 1-5. 



