ox THE CRYSTALLINE STRUCTURE OF LEAD. 
231 
slip-lines follow the sides of the pits in such a manner that the plane of slip is 
parallel to a hexagonal face; in fig. 9 this face and the slip-lines show up bright. 
Owing to the difference of level in the surface due to the pits, all of it could not he 
brought into focus together—this accounts for the somewhat blurred patches in the 
photographs. From these and other observations it may be concluded that lead 
tends to slip along planes perpendicular to the octahedral axes of the crystals, and 
there would, therefore, be at least four possible directions in which slip could occur. 
Four systems of slip-lines have already been noted in strained lead by Ewjng and 
Rosenhaix. 
When a single crystal is strained in tension, the slip along the gliding planes does 
not take place to an ecpial extent right across the specimen. The surface, originally 
fairly plane, becomes slightly undulated as the stress is applied, the undulations 
running diagonally across the specimen at an angle of about 45° to the direction of 
pull. The general character of the slip-lines can be best examined upon a plane 
unetched surface not broken up by pits. Such a surface exists on the upper side of 
the crystal, namely, the side in contact with the air when cast as described above 
and not afterwards etched. If we examine the slip-lines formed upon it when 
strained in tension, we find that the undulations are caused by the slip-lines being 
larger and more numerous along certain areas. When the specimen is re-etched 
after straining, the undulations are still visible as diagonal bands, which appear 
either slightly brighter or darker than the surrounding material, according to the 
illumination used. 
Fig. 9 shows part of such an etched surface at a magnification of 45 diameters. 
A band of pits, more brightly illuminated than tlie rest, runs across the centre of 
the figure, and similar, though less distinct, bands can also be traced running parallel 
to it. The following j^oints can be noticed: (l) that there is no distinct boundary 
between the light and dark j^ortions, the change from one to the other l)eing gradual; 
(2) the bands do not exhibit the same illumination ; (3) the sides of the etched pits 
upon the bright parts are not cpiite parallel to those upon the darker ones. 
Such slight differences of orientation occurring after a single crystal has been 
strained are evidently not due to a re-crystallisation. Professor Ewing has 
suggested to the author that it is cpiite possible to account for them if we consider 
that the strain is not homoc'eneous. It has been stated above that the number of 
O 
slip-lines was not the same all over the strained crystal, but that certain parts 
showed more signs of slijj than others. From this we would gather that when once 
slip occurs in a certain part it tends to go on there rather than in other parts of the 
specimen. Hence these parts get more or less distorted from their original shape, 
while other parts of the same crystal contiguous to them either have not changed 
at aU, or have done so in a less degree. It may readily occur that the contiguous 
portions, which were originally in parallel orientation, become relatively inclined 
through the distortion of the material between—the material between behaving as a 
