238 
AIK. J. C. W. IIUAIFREY OX THE EFFECTS OF STRAIN 
relation existed between the etched pits on either side of them. All these 
characteristics pointed to their not being twin boundaries. 
One experiment directly bearing on this point consisted in examining the slip- 
lines which were formed as the process of straining went on. If the action ■^^'as 
one of successive twinning, when one twin had formed the slip-lines should, as the 
strain })roceeded, form in a new direction over its surface, and hence, as more and 
more strain was given, numerous systems of slip-lines running in various directions 
should appear, so grouped as to exhibit the twin character of the crystals produced 
by the earlier part of the straining. Experiments were, therefore, made to see 
whether the slip-lines whicli were formed during the application of the strain gave 
any such indication of a change of orientation in any part of the crystal, and thus 
to find out whether re-crystallisation was a direct and immediate result of the strain, 
or happened after the strain liad l^een given. 
As was mentioned on jj. 231, a plane unetched surface, such as that obtained from 
the surface of the casting in contact with the air dining solidification, should be 
used in studying the direction of the slip-lines. A specimen was, therefore, cut from 
an unetched single crystal, and the spiky projections were cut away from the under 
side. The specimen was strained by bending nearly double and re-straightening. 
It was known from previous experiments that after such a strain the lead always 
gave evidence of re-crystallisation, but, on examining the specimen under the micro¬ 
scope, it was found that the slip dines extended in parallel systenis all over the 
surface of the crystal. An area was marked by scratching upon the surface with 
a steel point, and was photographed. This is shown in fig. 25 (Plate 6), and although 
the scratches have produced a certain amount of local displacement of the slip-lines, 
it is obvious tliat these all run in directions which are uniform over the whole strained 
area. The surface was now etched, and tlie same area was again photographed 
(fig. 26). Numerous patches of different orientation are seen to have appeared which 
have no apparent relation with the sli])-lines in fig. 25. It is clear, thei'efore, that 
these })atches have been developed subse(pient to the strain, and not in the process 
of straining. 
In tlie next experiment the specimen was strained first of all in a similar manner 
to the above, tbat is to say, by bending. This developed simple uniform marking by 
slip-lines. The specimen was then allowed to rest for about five minutes, and was 
not etched. A further strain was then applied Ijy tension. When this was applied, 
it was seen that upon the outlying parts the slip-lines ran in directions uniform with 
those already formed, but upon tlie middle area, whicli had already been sei'erely 
strained, they ran in numerous directions which were parallel over certain small 
jiatches, but bore no apparent I'elation to the original direction. It was clear from 
this that over the part which was severely strained to begin with, the interval of 
rest had caused crystals to form difiering from the original orientation, their 
existence being manifested by the new directions which the sliji-lines assumed ivhen 
