1902.] Effects of Strain on the Crystalline Structure of Lead. 463 



simple tension. The slip lines (the formation of which has already 

 been described by Ewing and Kosenhain) were found to occur in such 

 a manner that the plane of slip was parallel to an octahedral face of 

 the etched pits. As the specimen was strained, the surface, originally 

 plane, became slightly undulated, the undulations running diagonally 

 across it, at an angle of about 45° to the direction of pull. When 

 the specimen was re-etched after straining these undulations were still 

 visible as slight changes of orientation. These undulations are appa- 

 rently due to the strain not being uniform throughout the specimen. 

 Professor Ewing has suggested to the author how a non-homogeneous 

 strain can give rise to differences in orientation in a single isolated 

 crystal. When slip occurs in any part, it tends to go on there rather 

 than in other parts of the specimen. This tends to localise the strain 

 in certain parts, while other parts of the same crystal contiguous to 

 them have not changed their shape, or have done so in a less degree. 

 It may thus happen that parts which were originally in parallel 

 orientation become relatively displaced through the distortion of the 

 material between, this acting as a strained wedge, and hence differences 

 of orientation may arise within a single crystal. 



The next experiments deal with the recrystallisation which is 

 observed to go on in an isolated crystal of lead when it has been 

 strained. When a single uniformly oriented crystal is in any way 

 severely strained, it is seen upon re-etching that the orientation no 

 longer remains uniform, but that the crystal has become broken up 

 into numerous small patches of various new orientations. Many of 

 the patches thus formed consist of two or more parts which bear a 

 twin relation to one another. Change of this kind is more readily pro- 

 duced by compression than by tension, but it is produced by tensile 

 strain when that is sufficiently severe. 



The effect of subjecting large strained crystals to moderate tempera- 

 tures (up to 100° C.) is next described, and it is shown that such cook- 

 ing will produce further changes in the specimen. If the originally 

 uniformly oriented crystal exhibits any sign of recrystallisation upon 

 etching immediately after straining, cooking will produce a far greater 

 amount of change, the recrystallised areas extending outwards into the 

 parts of the crystal which before cooking had retained their original 

 orientation. Cooking also produces a recrystallisation in specimens 

 which show no change immediately after straining, but a certain 

 amount of straining is always necessary before any change can be 

 produced. 



Series of photographs are given showing the gradual change which 

 proceeds when a single crystal is cooked after being strained. From 

 these it is seen that a process of recrystallisation continues up to a 

 certain point, but that beyond that point, further cooking at the same 

 temperature produces no further change. A series of photographs is 



